As a legume cover crop, sunn hemp can fix atmospheric nitrogen that is available over time to succeeding crops as it decomposes. With high fertilizer prices and sustainability concerns with synthetic soil amendments, the potential use of sunn hemp to provide nitrogen to crops such as cotton, corn, and rice has prompted research to determine N availability from a sunn hemp cover crop (Balkcom and Reeves 2005; Chung et al. 2000; Sangakkara et al. 2004; Schomberg et al. 2007). Nitrogen production with sunn hemp varies depending on many factors; however, reported N values in sunn hemp range between 110 and 160 kg ha-1 (Balkcom and Reeves 2005; Mansoer et al. 1997; Marshall et al. 2001). In most investigations, sunn hemp nitrogen content equals or exceeds N content of traditional winter legume cover

With nitrogen release during winter months reported at approximately 75 kg ha-1, remaining N may still be utilized by spring planted crops (Reeves et al. 1996). Due to winter nitrogen loss from leaching and utilization, however, an alternative scenario mentioned previously for sunn hemp use is to employ the crop as a late summer cover prior to grain cover crop or winter cash crop planting in the fall (Balkcom et al. 2011; Schomberg et al. 2007). Released nitrogen from sunn hemp residue would be available to the subsequent crop

control, nematode and weed suppression, and N accumulation before frost occurs.

(Balkcom and Reeves 2005; Mansoer et al. 1997).

2011; Creamer and Baldwin 2000; Schomberg et al. 2007).

while minimizing N losses during winter months.

**4.1 Sunn hemp cover crops**

crops (Reeves 1994).

1990; Reeves 1994; Wagger 1989). Traditional fall-seeded legume cover crops have limited biomass production prior to cold temperatures, which can limit N accumulation and availability to subsequent crops. Utilization of tropical legumes, such as sunn hemp, may allow for greater biomass production and N accumulation during the available growing season between fall harvest and onset of winter in temperate climates (Mansoer et al. 1997).

#### **3.2 Weed control with legume cover crops**

Weed control obtained through legume cover crops has been well researched for a variety of cash crops (Caamal-Maldonado et al. 2001; DeGregoria and Ashley 1986; Teasdale 1988). Common legume covers have been shown to suppress growth of many species like pigweed (*Amaranthus* spp.), foxtail (*Setaria* spp.), and morningglory (*Ipomoeae* spp.) (Collins et al. 2007; Teasdale 1988; White et al. 1989). Either as a ground cover or through allelopathy, as noted in subterranean clover (*Trifolium subterraneum* L.), weed control achieved through legume cover crops has the potential to reduce early-season herbicide use in agricultural systems (Hartwig and Ammon 2002; Leather 1983; Mosjidis and Wehtje 2011). However, in most climates, the inability to produce high biomass with winter legumes, coupled with rapid decomposition, weed suppression is largely obtained during active cover growth and just after cover crop termination (Reddy 2001; Teasdale 1996). The use of sunn hemp immediately behind an early harvest summer cash crop like corn (*Zea mays* L.) may allow for extended post-harvest weed control through increased biomass production as well as slower decomposition rates in comparison to some other legume choices (Cherr et al. 2006; Cobo et al. 2002; Mansoer et al. 1997).

#### **4. Sunn hemp**

Sunn hemp, or Indian hemp, has become an important crop in regions such as India and Brazil that have climates well suited to the tropical, herbaceous annual (Bhardwaj et al. 2005; Duke 1981) (Figure 2). Typically utilized as a green manure due to its nitrogen accumulation, sunn hemp is also grown as a fiber crop; it can also be grown for forage since this *Crotalaria* species is nontoxic to animals (Rotar and Joy 1983). As a vigorously growing, relatively drought tolerant plant species, sunn hemp has been shown to thrive in a variety of soil types and with variable rainfall, but it is still most successful in tropical or subtropical environments (Wang et al. 2002b).

The utilization of this species in cooler, temperate climates, such as those found in the continental United States (US), began in the early 1930's in response to sunn hemp's potential as a green manure and for suppression of root-knot nematodes (Cook and White 1996; Cook et al. 1998; Dempsey 1975). At the onset of World War II, increased demands for rope fiber drew more attention to sunn hemp as an alternative for imported cordage material (Cook and White 1996; Wilson et al. 1965). During the 1950's and 1960's, US research placed particular emphasis on sunn hemp production as a quickly-renewed source of fiber for paper materials (Nelson et al. 1961). Although most attention for nonwood fiber sources has been concentrated on kenaf (*Hibiscus cannabinus* L.), some research continues to identify sunn hemp as a potential source that can be produced in the US, particularly in Hawaii, southern Texas, and south Florida (Cook and Scott 1998; Webber and Bledsoe 1993). In temperate regions of the US, however, more recent research has evaluated the cover crop potential of sunn hemp as a frost-terminated, late-summer alternative to winter legume covers such as clover and vetch or between crop harvest and cereal cover crop planting (Balkcom and Reeves 2005; Mansoer et al. 1997).

#### **4.1 Sunn hemp cover crops**

104 Weed Control

1990; Reeves 1994; Wagger 1989). Traditional fall-seeded legume cover crops have limited biomass production prior to cold temperatures, which can limit N accumulation and availability to subsequent crops. Utilization of tropical legumes, such as sunn hemp, may allow for greater biomass production and N accumulation during the available growing season between fall harvest and onset of winter in temperate climates (Mansoer et al. 1997).

Weed control obtained through legume cover crops has been well researched for a variety of cash crops (Caamal-Maldonado et al. 2001; DeGregoria and Ashley 1986; Teasdale 1988). Common legume covers have been shown to suppress growth of many species like pigweed (*Amaranthus* spp.), foxtail (*Setaria* spp.), and morningglory (*Ipomoeae* spp.) (Collins et al. 2007; Teasdale 1988; White et al. 1989). Either as a ground cover or through allelopathy, as noted in subterranean clover (*Trifolium subterraneum* L.), weed control achieved through legume cover crops has the potential to reduce early-season herbicide use in agricultural systems (Hartwig and Ammon 2002; Leather 1983; Mosjidis and Wehtje 2011). However, in most climates, the inability to produce high biomass with winter legumes, coupled with rapid decomposition, weed suppression is largely obtained during active cover growth and just after cover crop termination (Reddy 2001; Teasdale 1996). The use of sunn hemp immediately behind an early harvest summer cash crop like corn (*Zea mays* L.) may allow for extended post-harvest weed control through increased biomass production as well as slower decomposition rates in comparison to some other legume choices (Cherr et al. 2006;

Sunn hemp, or Indian hemp, has become an important crop in regions such as India and Brazil that have climates well suited to the tropical, herbaceous annual (Bhardwaj et al. 2005; Duke 1981) (Figure 2). Typically utilized as a green manure due to its nitrogen accumulation, sunn hemp is also grown as a fiber crop; it can also be grown for forage since this *Crotalaria* species is nontoxic to animals (Rotar and Joy 1983). As a vigorously growing, relatively drought tolerant plant species, sunn hemp has been shown to thrive in a variety of soil types and with variable rainfall, but it is still most successful in tropical or subtropical

The utilization of this species in cooler, temperate climates, such as those found in the continental United States (US), began in the early 1930's in response to sunn hemp's potential as a green manure and for suppression of root-knot nematodes (Cook and White 1996; Cook et al. 1998; Dempsey 1975). At the onset of World War II, increased demands for rope fiber drew more attention to sunn hemp as an alternative for imported cordage material (Cook and White 1996; Wilson et al. 1965). During the 1950's and 1960's, US research placed particular emphasis on sunn hemp production as a quickly-renewed source of fiber for paper materials (Nelson et al. 1961). Although most attention for nonwood fiber sources has been concentrated on kenaf (*Hibiscus cannabinus* L.), some research continues to identify sunn hemp as a potential source that can be produced in the US, particularly in Hawaii, southern Texas, and south Florida (Cook and Scott 1998; Webber and Bledsoe 1993). In temperate regions of the US, however, more recent research has evaluated the cover crop potential of sunn hemp as a frost-terminated, late-summer alternative to winter legume

**3.2 Weed control with legume cover crops**

Cobo et al. 2002; Mansoer et al. 1997).

environments (Wang et al. 2002b).

**4. Sunn hemp**

Sunn hemp is readily used as a rotational crop in tropical regions with cash crops such as rice, cotton, and corn (Purseglove 1974). Its value as a cover crop is due to its biomass production, N accumulation, reduced pests and pathogen infestation, and weed suppression achieved when planted (Wang et al. 2002b). While sunn hemp is not winter hardy, its adaptability to various soil types and precipitation amounts has allowed sunn hemp to be grown in temperate regions as a green manure (Dempsey 1975). In the US, sunn hemp was used in this manner during the 1930's until reduced seed availability caused interest in the crop to diminish (Cook and White 1996). Recently, sunn hemp has again received attention as a potential alternative to winter annual legume cover crops in these temperate climates (Balkcom and Reeves 2005; Mansoer et al. 1997). Sunn hemp's vigorous growth and nitrogen production can provide needed ground cover to control erosion and an N source to succeeding cash crops. To fully utilize sunn hemp nitrogen release, investigations have also been conducted to determine the suitability of sunn hemp grown as a late summer cover crop between harvest and winter planting of cash crop or cereal cover crop (Balkcom et al. 2011; Creamer and Baldwin 2000; Schomberg et al. 2007).

The rapid growth of sunn hemp in a relatively short period of time allows for a relatively high amount of biomass production prior to the onset of cool temperatures in mild climates across the southeastern US. Previous research has reported sunn hemp biomass to average between 1 and 9 Mg ha-1 in 45 to 90 days after planting, respectively (Mansoer et al. 1997; Morris et al. 1986; Schomberg et al. 2007; Reeves et al. 1996; Yadvinder et al. 1992). Although environmental conditions affect potential biomass production, substantial amounts of biomass can be achieved under typical late-summer and autumn conditions to aid in erosion control, nematode and weed suppression, and N accumulation before frost occurs.

As a legume cover crop, sunn hemp can fix atmospheric nitrogen that is available over time to succeeding crops as it decomposes. With high fertilizer prices and sustainability concerns with synthetic soil amendments, the potential use of sunn hemp to provide nitrogen to crops such as cotton, corn, and rice has prompted research to determine N availability from a sunn hemp cover crop (Balkcom and Reeves 2005; Chung et al. 2000; Sangakkara et al. 2004; Schomberg et al. 2007). Nitrogen production with sunn hemp varies depending on many factors; however, reported N values in sunn hemp range between 110 and 160 kg ha-1 (Balkcom and Reeves 2005; Mansoer et al. 1997; Marshall et al. 2001). In most investigations, sunn hemp nitrogen content equals or exceeds N content of traditional winter legume cover crops (Reeves 1994).

With nitrogen release during winter months reported at approximately 75 kg ha-1, remaining N may still be utilized by spring planted crops (Reeves et al. 1996). Due to winter nitrogen loss from leaching and utilization, however, an alternative scenario mentioned previously for sunn hemp use is to employ the crop as a late summer cover prior to grain cover crop or winter cash crop planting in the fall (Balkcom et al. 2011; Schomberg et al. 2007). Released nitrogen from sunn hemp residue would be available to the subsequent crop while minimizing N losses during winter months.

hemp.

suppression by *C. juncea* will continue.

production in sunn hemp stands.

**4.4 Breeding and seed availability**

**4.3 Weed control in sunn hemp production**

will be critical for successful production in the future.

Utilization of Sunn Hemp for Cover Crops and Weed Control in Temperate Climates 107

In contrast, weed suppression specifically by sunn hemp cover crops has been minimally investigated and only recently has it received more attention. General comments concerning the potential of sunn hemp to suppress weed species have been reported in several studies (Reeves et al. 1996; SARE 2007). Weed control by sunn hemp has been mostly attributed to vigorous plant growth and rapid shading of the ground (Duke 1981). In fact, Mosjidis and Wehtje (2011) demonstrated that there was a progressive reduction in weed biomass as a sunn hemp stand increased up to 100 plants/m2. Furthermore, recent research has suggested allelopathic compounds released from sunn hemp also cause weed suppression (Adler and Chase 2007; Collins et al. 2007; Leather and Forrence 1990; Price et al. 2008). More research is necessary to determine the extent of allelochemical functions in sunn

Significant weed control can be achieved under moderate to high levels of sunn hemp (Mosjidis and Wehtje 2011; SARE 2007; Severino and Christoffoleti 2004). However, several weed species, such as nutsedge (*Cyperus* spp.), morningglory (*Ipomoea* spp.), and bermudagrass (*Cynodon dactylon*), are capable of thriving in a sunn hemp stand (Chaudhury et al. 2007; Collins et al. 2007; McKee et al. 1946). It is expected that, as sunn hemp utilization for cover crops and weed control grows, research efforts to fully understand weed

Little research has been conducted to determine weed control strategies in sunn hemp production. *C. juncea* grown as a green manure or cover crop typically does not require extensive weed management but production of sunn hemp for seed production may benefit from additional weed control practices. Due to sunn hemp's rapid growth and possible allelopathic effects, it can be easily established and can out-compete neighboring weed species. In fact, areas that rely on hand removal of weed species in agricultural productions generally do not employ this practice in sunn hemp grown for fiber, since weed competition is minimal (Chaudhury et al. 2007). However, in regions that utilize herbicides for weed control, early season weed suppression with herbicide applications may increase seed

At present, no herbicides are labelled for use in sunn hemp production, but research by Mosjidis and Wehtje (2011) identified a preemergent herbicide, pendimethalin, as a potential treatment that would provide effective weed control during establishment. This research also found that sunn hemp could also tolerate 2,4-DB as a postemergent application (Mosjidis and Wehtje 2011). Imazethapyr was also determined to be safe for use in sunn hemp and effective against yellow nutsedge (*Cyperus esculentus* L.), which is not well suppressed by sunn hemp (Collins et al. 2007; Mosjidis and Wehtje 2011). With advancements in sunn hemp breeding and increased utilization of this species in a broader geographical range, continued research efforts to determine additional herbicide programs

As a native tropical species, sunn hemp use in temperate regions faces some challenges. Although sunn hemp experiences rapid vegetative growth in a short time frame, viable seed

Fig. 2. The vigorous growth of sunn hemp cultivars developed at Auburn University, Alabama, can be seen here, 70 days after planting (Photo by J.A. Mosjidis).

#### **4.2 Weed suppression by sunn hemp**

It has been noted by many researchers that *C. juncea* can suppress populations of pests such as nematodes and vigorous weed species (Collins et al. 2007; Fassuliotis and Skucas 1969; McSorley et al. 1994; Taylor 1985; Wang et al. 2001). Much of the pest control potential of sunn hemp has focused primarily on nematode control of such species as *Meloidogyne* spp., *Rotylenchulus reniformis*, *Radopholus similis,* and *Heterodera glycines* (Birchfield and Bristline 1956; Desaeger and Rao 2000; Good et al. 1965; Marla et al. 2008; Robinson et al. 1998; Wang et al. 2004). Although some questions remain concerning suppression by sunn hemp for specific nematode species, extensive research has provided considerable knowledge as to how *C. juncea* reduces certain nematode populations (Halbrendt 1996; Kloepper et al. 1991; LaMondia 1996; Rodriguez-Kabana 1994; Wang et al. 2002b).

In contrast, weed suppression specifically by sunn hemp cover crops has been minimally investigated and only recently has it received more attention. General comments concerning the potential of sunn hemp to suppress weed species have been reported in several studies (Reeves et al. 1996; SARE 2007). Weed control by sunn hemp has been mostly attributed to vigorous plant growth and rapid shading of the ground (Duke 1981). In fact, Mosjidis and Wehtje (2011) demonstrated that there was a progressive reduction in weed biomass as a sunn hemp stand increased up to 100 plants/m2. Furthermore, recent research has suggested allelopathic compounds released from sunn hemp also cause weed suppression (Adler and Chase 2007; Collins et al. 2007; Leather and Forrence 1990; Price et al. 2008). More research is necessary to determine the extent of allelochemical functions in sunn hemp.

Significant weed control can be achieved under moderate to high levels of sunn hemp (Mosjidis and Wehtje 2011; SARE 2007; Severino and Christoffoleti 2004). However, several weed species, such as nutsedge (*Cyperus* spp.), morningglory (*Ipomoea* spp.), and bermudagrass (*Cynodon dactylon*), are capable of thriving in a sunn hemp stand (Chaudhury et al. 2007; Collins et al. 2007; McKee et al. 1946). It is expected that, as sunn hemp utilization for cover crops and weed control grows, research efforts to fully understand weed suppression by *C. juncea* will continue.

#### **4.3 Weed control in sunn hemp production**

106 Weed Control

Fig. 2. The vigorous growth of sunn hemp cultivars developed at Auburn University,

It has been noted by many researchers that *C. juncea* can suppress populations of pests such as nematodes and vigorous weed species (Collins et al. 2007; Fassuliotis and Skucas 1969; McSorley et al. 1994; Taylor 1985; Wang et al. 2001). Much of the pest control potential of sunn hemp has focused primarily on nematode control of such species as *Meloidogyne* spp., *Rotylenchulus reniformis*, *Radopholus similis,* and *Heterodera glycines* (Birchfield and Bristline 1956; Desaeger and Rao 2000; Good et al. 1965; Marla et al. 2008; Robinson et al. 1998; Wang et al. 2004). Although some questions remain concerning suppression by sunn hemp for specific nematode species, extensive research has provided considerable knowledge as to how *C. juncea* reduces certain nematode populations (Halbrendt 1996; Kloepper et al. 1991;

Alabama, can be seen here, 70 days after planting (Photo by J.A. Mosjidis).

LaMondia 1996; Rodriguez-Kabana 1994; Wang et al. 2002b).

**4.2 Weed suppression by sunn hemp**

Little research has been conducted to determine weed control strategies in sunn hemp production. *C. juncea* grown as a green manure or cover crop typically does not require extensive weed management but production of sunn hemp for seed production may benefit from additional weed control practices. Due to sunn hemp's rapid growth and possible allelopathic effects, it can be easily established and can out-compete neighboring weed species. In fact, areas that rely on hand removal of weed species in agricultural productions generally do not employ this practice in sunn hemp grown for fiber, since weed competition is minimal (Chaudhury et al. 2007). However, in regions that utilize herbicides for weed control, early season weed suppression with herbicide applications may increase seed production in sunn hemp stands.

At present, no herbicides are labelled for use in sunn hemp production, but research by Mosjidis and Wehtje (2011) identified a preemergent herbicide, pendimethalin, as a potential treatment that would provide effective weed control during establishment. This research also found that sunn hemp could also tolerate 2,4-DB as a postemergent application (Mosjidis and Wehtje 2011). Imazethapyr was also determined to be safe for use in sunn hemp and effective against yellow nutsedge (*Cyperus esculentus* L.), which is not well suppressed by sunn hemp (Collins et al. 2007; Mosjidis and Wehtje 2011). With advancements in sunn hemp breeding and increased utilization of this species in a broader geographical range, continued research efforts to determine additional herbicide programs will be critical for successful production in the future.

#### **4.4 Breeding and seed availability**

As a native tropical species, sunn hemp use in temperate regions faces some challenges. Although sunn hemp experiences rapid vegetative growth in a short time frame, viable seed

Utilization of Sunn Hemp for Cover Crops and Weed Control in Temperate Climates 109

set (Chaudhury et al. 2007). In cooler climates above this latitude, temperatures below optimal usually occur before sunn hemp seed can mature. The recent development of cultivar, 'AU Golden' and 'AU Durbin' has been shown to produce viable seed in these temperate regions; research is on-going to determine best management practices for

The progression of agricultural systems towards more sustainable, yet high yielding production has required researchers to identify numerous alternative weed management practices that can be employed along with traditional weed control tactics. The use of sunn hemp as a cover crop, either as a substitute for winter annual legumes or as a late summer cover between harvest and winter crops, delivers effective weed control while providing ground cover and a nitrogen source for subsequent crops. Continued research with sunn hemp and crops similar to this species may provide even more benefits to weed control

Adler, M.J., and C.A. Chase. 2007. A comparative analysis of the allelopathic potential of

Balkcom, K.S. and D.W. Reeves. 2005. Sunn hemp utilized as a legume cover crop for corn

Balkcom, K.S., H. Schomberg, D.W. Reeves, and A. Clark. 2007. Managing cover crops in

Balkcom, K.S., J.M. Massey, J.A. Mosjidis, A.J. Price, and S.F. Enloe. 2011. Planting date and

Bhardwaj, H.L., C.L. Webber, and G.S. Sakamoto. 2005. Cultivation of kenaf and sunn hemp in the mid-Atlantic United States. Industrial Crops and Products 22: 151-155. Birchfield, W. and F. Bristline. 1956. Cover crop in relation to the burrowing nematode,

Bugg, R.L. and J.D. Dutcher. 1989. Warm-season cover crops for pecan orchards:

Caamal-Maldonado, J.A., J.J. Jimenez-Osornio, A. Torres-Barragan, and A.L. Anaya. 2001.

Chaudhury, J., D.P. Singh, and S.K. Hazra. 2007. Sunn Hemp (*Crotalaria juncea* L.). Central

Cherr, C.M., J.M.S. Scholberg, and R. McSorley. 2006. Green manure as nitrogen source for sweet corn in a warm-temperate environment. Agronomy Journal 98: 1173-1180.

cover crop. International Journal of Agronomy 2011: 1-8.

*Radopholus similis*. Plant Disease Reporter 40: 398-399.

cropping systems. Agronomy Journal 93: 27-36.

http://assamagribusiness.nic.in/Sunnhemp.pdf

leguminous summer cover crops: cowpea, sunn hemp and velvetbean. HortScience

conservation tillage systems. *In* A. Clark (ed.). *Managing Cover Crops Profitably.*

seeding rate effects on sunn hemp biomass and nitrogen production for a winter

horticultural and entomological implications. Biology, Agriculture, and

The use of allelopathic legume cover and mulch species for weed control in

Research Institute for Jute and Allied Fibres (ICAR). available at:

implementing these sunn hemp cultivars (Balkcom et al. 2011; Mosjidis 2007, 2010).

**5. Conclusions**

efforts in the future.

42: 289-293.

production. Agronomy Journal 97: 26-31.

SARE. College Park, MD: 44-61.

Horticulture 6: 123-148.

**6. References**

production typically requires a longer season than can be achieved before winter conditions in temperate regions (Li et al. 2009)(sunn hemp seed pictured in Figure 3). The lack of seed production outside of tropical and subtropical climates severely limits seed availability to producers in cooler climates. Moreover, with seed costs ranging from \$90 to \$130 (US) per hectare, implementation of *C. juncea* as a cover crop can be an expensive task for growers (Li et al. 2009; Petcher 2009).

Fig. 3. Sunn hemp seed production can yield 450 to 1000 kg of seed per hectare (Photo by J.A. Mosjidis).

A good deal of breeding research has been conducted in countries throughout the world that grow sunn hemp for fiber production (Kundu 1964; Ram and Singh 2011). In India, particularly, cultivars are developed for high fiber yield and resistance to wilt diseases (Chaudhury et al. 2007). Most commonly used in this region is 'Kharif sunn' or 'K-12' which produce high yield of good quality fiber (Chaudhury et al. 2007). Other important cultivars in India include 'SS-11' and 'T-6' which is a day-neutral variety, while most varieties are short day plants (Chaudhury et al. 2007). In other regions of sunn hemp production, varieties such as 'Somerset' in South Africa and 'KRC-1' in Brazil are commonly used.

Due to elevated seed costs and limited seed production in the US, sunn hemp breeding research has focused on developing a *C. juncea* cultivar that can produce seeds in climatic conditions prevalent in the southeastern US. For successful seed production in traditional sunn hemp regions, characteristics of seed production locations should typically be below 24° N latitude, not fall below 10° C, have ample sunlight and not receive rainfall during fruit set (Chaudhury et al. 2007). In cooler climates above this latitude, temperatures below optimal usually occur before sunn hemp seed can mature. The recent development of cultivar, 'AU Golden' and 'AU Durbin' has been shown to produce viable seed in these temperate regions; research is on-going to determine best management practices for implementing these sunn hemp cultivars (Balkcom et al. 2011; Mosjidis 2007, 2010).

#### **5. Conclusions**

108 Weed Control

production typically requires a longer season than can be achieved before winter conditions in temperate regions (Li et al. 2009)(sunn hemp seed pictured in Figure 3). The lack of seed production outside of tropical and subtropical climates severely limits seed availability to producers in cooler climates. Moreover, with seed costs ranging from \$90 to \$130 (US) per hectare, implementation of *C. juncea* as a cover crop can be an expensive task for growers (Li

Fig. 3. Sunn hemp seed production can yield 450 to 1000 kg of seed per hectare (Photo by

A good deal of breeding research has been conducted in countries throughout the world that grow sunn hemp for fiber production (Kundu 1964; Ram and Singh 2011). In India, particularly, cultivars are developed for high fiber yield and resistance to wilt diseases (Chaudhury et al. 2007). Most commonly used in this region is 'Kharif sunn' or 'K-12' which produce high yield of good quality fiber (Chaudhury et al. 2007). Other important cultivars in India include 'SS-11' and 'T-6' which is a day-neutral variety, while most varieties are short day plants (Chaudhury et al. 2007). In other regions of sunn hemp production, varieties such as 'Somerset' in South Africa and 'KRC-1' in Brazil are commonly used.

Due to elevated seed costs and limited seed production in the US, sunn hemp breeding research has focused on developing a *C. juncea* cultivar that can produce seeds in climatic conditions prevalent in the southeastern US. For successful seed production in traditional sunn hemp regions, characteristics of seed production locations should typically be below 24° N latitude, not fall below 10° C, have ample sunlight and not receive rainfall during fruit

et al. 2009; Petcher 2009).

J.A. Mosjidis).

The progression of agricultural systems towards more sustainable, yet high yielding production has required researchers to identify numerous alternative weed management practices that can be employed along with traditional weed control tactics. The use of sunn hemp as a cover crop, either as a substitute for winter annual legumes or as a late summer cover between harvest and winter crops, delivers effective weed control while providing ground cover and a nitrogen source for subsequent crops. Continued research with sunn hemp and crops similar to this species may provide even more benefits to weed control efforts in the future.

### **6. References**

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Karlen, D.L. and J.W. Doran. 1991. Cover crop management effects on soybean and corn

Kloepper, J.W., R. Rodriguez-Kabana, J.A. McInroy, and D.J. Collins. 1991. Analysis of

Kundu, B.C. 1964. Sunn-hemp in India. Proceedings of the Soil and Crop Society of Florida.

LaMondia, J.A. 1996. Trap crops and population management of *Globoder tabacum tabacum*.

Leather, G.R. 1983. Weed control using allelopathic crop plants. Journal of Chemical Ecology

Leather, G.R. and L.E. Forrence. 1990. Sunn hemp is allelopathic to leafy spurge.

Li, Y., Q. Wang, W. Klassen, and E.A. Hanlon. 2009. Sunn hemp-A cover crop in Florida.

Lindemann, W.C. and C.R. Glover. 2003. Nitrogen fixation by legumes. New Mexico State

Marla, S.R., R.N. Huettel, and J. Mosjidis. 2008. Evaluation of *Crotalaria juncea* populations as

Mansoer, Z., D.W. Reeves, and C.W. Wood. 1997. Sustainability of sunn hemp as an

Marshall, A.J., R.N. Gallaher, A. Higuera, and R.S. Tubbs. 2001. Seeding rate for fall planted

Marshall, A.J., R.N. Gallaher, K.H. Wang, and R. McSorley. 2002. Partitioning of dry matter

Conservation Tillage Conference for Sustainable Agriculture 25: 310-313. McKee, R., G.E. Ritchey, J.L. Stephens, and H.W. Johnson. 1946. Crotalaria culture and

hosts and antagonistic crops to manage *Meloidogyne incognita* and *Rotylenchulus*

alternative late-summer legume cover crop. Soil Science Society of America Journal

sunn hemp. Proceedings Annual American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America meeting. Charlotte, N.C.

and minerals in sunn hemp. *In* E. van Santen (ed.) *Making Conservation Tillage Conventional: Building a Future on 25 Years of Research.* Proceedings Annual Southern

utilization. United States Department of Agriculture Farmer's Bulletin Number

University of Florida Extension bulletin SL 306. Available at:

University Cooperative Extension Guide A-129. available at:

Proceedings and Progress Reports of the Leafy Spurge Symposium. Gillette, WY.

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Alternative Agriculture 6: 71-82.

Journal of Nematology 28: 238-243.

http://edis.ifas.ufl.edu/pdffiles/TR/TR00300.pdf

http://aces.nmsu.edu/pubs/\_a/a-129.pdf.

*reniformis*. Nematropica 38: 155-161.

332-337.

seeded legume cover crops for no-tillage corn in the humid East. Agronomy

*Triticum aestivum* cover crop on herbicide efficacy in *Oryza sativa*. Weed Science 47:

growth and nitrogen dynamics in an on-farm study. American Journal of

populations and physiological characterization of microorganisms in rhizospheres of plants with antagonistic properties to phytopathogenic nematodes. Plant and

Chung, R., C. Wang, C.W. Wang, and Y.P. Wang. 2000. Influence of organic matter and

Collins, A.S., C.A. Chase, W.M. Stall, and C.M. Hutchinson. 2007. Competitiveness of three

Cobo, J.G., E. Barrios, D.C.L, Kass and R.J. Thomas. 2002. Decomposition and nutrient

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**1. Introduction**

the absence of resource competition (Einhellig 1994a).

systems, as well as organic production.

**6**

*USA*

*1Auburn University*

*2United States Department of Agriculture*

**Allelopathic Weed Suppression**

**Through the Use of Cover Crops**

Jessica Kelton1, Andrew J. Price2 and Jorge Mosjidis1

There has long been observed an inhibitive response by plant species to certain neighboring plants. The Greek philosopher and botanist, Theophrastus, noted this effect from cabbage as early as 300 BC (Willis 1985). Since that time, others have documented similar plant interactions. In 1937, Austrian botanist, Hans Molisch, described this phenomenon as allelopathy, which he determined to be the result of biochemical interactions between plants (Molisch 1937; Putnam and Duke 1978). When first described, allelopathy referred to both deleterious and beneficial interactions between species; since that time, however, allelopathy has been applied to only adverse plant interactions, rather than to both. First described by a Roman scholar during the first century, black walnut (*Juglans nigra* L.) has long served as the common example of allelopathic effects with its ability to inhibit growth of surrounding plants either through decaying leaves or nuts or from the tree itself (Weir et al. 2004). Researchers have continued to examine allelopathy and the mechanism for biochemical inhibition, which was initially scrutinized by many since differentiation between this effect and plant competition remained uncertain (Weir et al. 2004). Subsequent bioassays involving specific chemical compounds extracted from plants have confirmed that certain species do, in fact, produce biochemicals that can inhibit plant germination and growth in

With confirmation of allelopathy, many investigations have been conducted in order to determine how best to utilize this effect for possible weed control in agricultural settings (Khanh et al. 2005; Olofsdotter 2001; Weston 1996). The ability to inhibit weed growth through the implementation of cover crops into a crop rotation has been a focal point for this research for several reasons. In addition to weed suppression and control through allelopathy, as well as a mulching effect, cover crops provide substantial environmental benefits such as reduced erosion and water runoff (Price et al. 2006; Truman et al. 2003). Moreover, cover crops are readily available and easily adapted to many agricultural situations. Because of these many benefits, including natural weed suppression through allelopathy, the use of cover crops has become a vital component of sustainable agriculture

Ensuring sufficient food and fiber production for future generations can be hampered by limited options for weed control, particularly in developing countries where yields are

## **Allelopathic Weed Suppression Through the Use of Cover Crops**

Jessica Kelton1, Andrew J. Price2 and Jorge Mosjidis1 *1Auburn University 2United States Department of Agriculture USA*

#### **1. Introduction**

114 Weed Control

Wang, K., B.S. Sipes, and D.P. Schmitt. 2002b. *Crotalaria* as a cover crop for nematode

Wang, K.-H., R. McSorley, A.J. Marshall, and R.N. Gallaher. 2004. Nematode community

Wang, Q., Y. Li, and W. Klassen. 2005. Influence of summer cover crops on conservation of

Webber, C.L. and V.K. Bledsoe. 1993. Kenaf production, harvesting and products. *In* J. Janick

White, R.H., A.D. Worsham, and U. Blum. 1989. Allelopathic potential of legume debris and

Wilson, F.D., T.E. Summers, J.F. Joyner, D.W. Fishler, and C.C. Seale. 1965. 'Everglades 41'

Yadvinder, S., S. Bihay, and C.S. Khind. 1992. Nutrient transformations in soil amended

Yenish, J.P., A.D. Worsham, and A.C. York. 1996. Cover crops for herbicide replacement on

and J.E. Simon (eds.). New Crops. Wiley, New York, NY: 416-421.

and seed. Florida Agricultural Experiment Station Circular S168.

with green manures. Advances in Soil Science 20: 237-309.

no-tillage corn (*Zea mays*). Weed Technology 10: 815-821.

changes associated with decomposition of *Crotalaria juncea* amendment in

soil water and nutrients in a subtropical area. Journal of Soil and Water

and 'Everglades 71', two new cultivars of kenaf (*Hibiscus cannabinus* L.) for the fiber

management: a review. Nematropica 32: 35-57.

litterbags. Applied Soil Ecology 27: 31-45.

aqueous extracts. Weed Science 37: 674-679.

Conservation 60: 58-63.

There has long been observed an inhibitive response by plant species to certain neighboring plants. The Greek philosopher and botanist, Theophrastus, noted this effect from cabbage as early as 300 BC (Willis 1985). Since that time, others have documented similar plant interactions. In 1937, Austrian botanist, Hans Molisch, described this phenomenon as allelopathy, which he determined to be the result of biochemical interactions between plants (Molisch 1937; Putnam and Duke 1978). When first described, allelopathy referred to both deleterious and beneficial interactions between species; since that time, however, allelopathy has been applied to only adverse plant interactions, rather than to both. First described by a Roman scholar during the first century, black walnut (*Juglans nigra* L.) has long served as the common example of allelopathic effects with its ability to inhibit growth of surrounding plants either through decaying leaves or nuts or from the tree itself (Weir et al. 2004). Researchers have continued to examine allelopathy and the mechanism for biochemical inhibition, which was initially scrutinized by many since differentiation between this effect and plant competition remained uncertain (Weir et al. 2004). Subsequent bioassays involving specific chemical compounds extracted from plants have confirmed that certain species do, in fact, produce biochemicals that can inhibit plant germination and growth in the absence of resource competition (Einhellig 1994a).

With confirmation of allelopathy, many investigations have been conducted in order to determine how best to utilize this effect for possible weed control in agricultural settings (Khanh et al. 2005; Olofsdotter 2001; Weston 1996). The ability to inhibit weed growth through the implementation of cover crops into a crop rotation has been a focal point for this research for several reasons. In addition to weed suppression and control through allelopathy, as well as a mulching effect, cover crops provide substantial environmental benefits such as reduced erosion and water runoff (Price et al. 2006; Truman et al. 2003). Moreover, cover crops are readily available and easily adapted to many agricultural situations. Because of these many benefits, including natural weed suppression through allelopathy, the use of cover crops has become a vital component of sustainable agriculture systems, as well as organic production.

Ensuring sufficient food and fiber production for future generations can be hampered by limited options for weed control, particularly in developing countries where yields are

Allelopathic Weed Suppression Through the Use of Cover Crops 117

Like most allelochemicals, phenolic acids are secondary plant compounds typified by a hydroxylated aromatic ring structure. To date, a number of phenolic acids have been determined to have allelopathic properties and have been measured in extracts from a variety of plant species (Figure 1). Species which have been noted to produce phenolic acids include: rice (*Oryza sativa* L.), wheat (*Triticum aestivum* L.), mango (*Mangifera indica* L.), and spotted knapweed (*Centaurea stoebe* L.) (Bais et al. 2003; Chung et al. 2002; El-Rokiek et al. 2010; Fitter 2003). Many species, such as rice, contain multiple phenolic compounds along with other allelopathic compounds. In two studies, researchers isolated nine individual phenolic acids from rice hull extracts and 14 different phenolic acids from buffalograss [*Buchloe dactyloides* (Nutt.) Engelm] (Chung et al. 2002; Wu et al. 1998). At this time, however, it is not clear to what degree individual allelochemicals interact to produce plant inhibition. Some reports show a synergistic effect when allelochemicals are in a mixture, while other studies indicate decreased plant inhibition in the presence of a mixture when

compared to individual chemical inhibition (Chung et al. 2002; Einhellig 1996).

Caffeic acid Coumaric acid Vanillic acid

Fig. 1. Phenolic acids identified in many plant species, such as oat (*Avena sativa* L.) and rice

Syringic acid *p*-hydrobenzoic Ferulic acid

Although modes of action for allelopathic chemicals are not readily understood for each identified allelochemical, phenolic acids have been the focus of many studies designed to establish the basis of their allelopathy (Putnam 1985). Early research with phenolic acids indicated that some phenolic acids could function though increasing cell membrane permeability, thus affecting ion transport and metabolism (Glass and Dunlop 1974). More recent studies report disruption of cell division and malformed cellular structures in plants

(*Oryza sativa* L.), have been found to have allelopathic properties.

**3.1 Phenolic acids**

reduced by up to 25% by weed competition. Identifying and describing sustainable weed control measures that can be implemented to reduce weed pressure in a number of settings can help safeguard the productivity of agriculture. Therefore, the objectives of this chapter are to describe the fundamentals of allelopathy and how to utilize allelopathic compounds for weed control through cover crop use. The chapter also highlights many of the identified biochemicals, their structures, and the respective cover crops in which they are found. Lastly, we describe the degree of allelopathic potential for a number of cover crops, as determined by laboratory testing.

#### **2. Production and release of allelopathic compounds**

Allelochemicals enter the environment from plants in a number of ways, such as plant degradation, volatilization, leaching from plant leaves, and from root exudation (Bertin et al. 2003; Weir et al. 2004). During active plant growth, particularly in early growth stages or during periods of stress, root exudation, either through diffusion, ion channels, or vesicle transport, is the primary method for release of many organic and inorganic compounds into the rhizosphere (Battey and Blackbourn 1993; Uren 2000). These compounds serve a multitude of functions such as improving nutrient uptake, root lubrication, plant growth regulation, microorganism defense, and waste removal (Bertin et al. 2003; Fan et al. 1997; Uren 2000).

A large proportion of identified allelochemicals are noted to be secondary compounds formed during photosynthetic processes (Einhellig 1994b; Swain 1977). Since many allelopathic chemicals appear to perform no primary metabolic functions, although some compounds such as cinnamic acid and salicylic acid do serve other functions within a plant, it is unclear at this point as to what regulates the release of these compounds (Einhellig 1994a). Many environmental plant stressors have been observed to increase allelochemical release but not necessarily chemical production (Bertin et al. 2003; Inderjit and Weston 2003; Sterling et al. 1987). Plant stressors such as elevated temperature, reduced water availability, and herbivory may cause increased allelochemical release; however, a definitive correlation between environmental factors and allelopathic compounds has yet to be made (Bertin et al. 2003; Pramanik et al. 2000). Continued research directed at isolating and identifying individual root exudates while manipulating environmental stress factors may help to increase our understanding of allelochemical release into the rhizosphere.

#### **3. Allelopathic compounds**

Many allelochemicals have been identified since experiments began to isolate and determine allelopathic potentials of plant compounds. Compounds that have been identified thus far include a variety of chemical classes such as phenolic acids, coumarins, benzoquinones, terpenoids, glucosinolates, and tannins (Chung et al. 2002; Putnam and Duke 1978; Seigler 1996; Swain 1977; Vyvyan 2002). These and other allelochemicals are found in many plant species from woody to herbaceous plants, grasses and broadleaves, weeds and crops. There are many details left to be determined such as regulation and production stimuli and mode of action for inhibition. It is also not readily understood to what extent allelopathic compounds interact with each other and other chemical compounds within the rhizosphere to inhibit surrounding plants. The following sections present several of the structural classes of recognized allelochemicals as well as specific compounds within each group.

#### **3.1 Phenolic acids**

116 Weed Control

reduced by up to 25% by weed competition. Identifying and describing sustainable weed control measures that can be implemented to reduce weed pressure in a number of settings can help safeguard the productivity of agriculture. Therefore, the objectives of this chapter are to describe the fundamentals of allelopathy and how to utilize allelopathic compounds for weed control through cover crop use. The chapter also highlights many of the identified biochemicals, their structures, and the respective cover crops in which they are found. Lastly, we describe the degree of allelopathic potential for a number of cover crops, as

Allelochemicals enter the environment from plants in a number of ways, such as plant degradation, volatilization, leaching from plant leaves, and from root exudation (Bertin et al. 2003; Weir et al. 2004). During active plant growth, particularly in early growth stages or during periods of stress, root exudation, either through diffusion, ion channels, or vesicle transport, is the primary method for release of many organic and inorganic compounds into the rhizosphere (Battey and Blackbourn 1993; Uren 2000). These compounds serve a multitude of functions such as improving nutrient uptake, root lubrication, plant growth regulation, microorganism defense, and waste removal (Bertin et al. 2003; Fan et al. 1997; Uren 2000).

A large proportion of identified allelochemicals are noted to be secondary compounds formed during photosynthetic processes (Einhellig 1994b; Swain 1977). Since many allelopathic chemicals appear to perform no primary metabolic functions, although some compounds such as cinnamic acid and salicylic acid do serve other functions within a plant, it is unclear at this point as to what regulates the release of these compounds (Einhellig 1994a). Many environmental plant stressors have been observed to increase allelochemical release but not necessarily chemical production (Bertin et al. 2003; Inderjit and Weston 2003; Sterling et al. 1987). Plant stressors such as elevated temperature, reduced water availability, and herbivory may cause increased allelochemical release; however, a definitive correlation between environmental factors and allelopathic compounds has yet to be made (Bertin et al. 2003; Pramanik et al. 2000). Continued research directed at isolating and identifying individual root exudates while manipulating environmental stress factors may help to

Many allelochemicals have been identified since experiments began to isolate and determine allelopathic potentials of plant compounds. Compounds that have been identified thus far include a variety of chemical classes such as phenolic acids, coumarins, benzoquinones, terpenoids, glucosinolates, and tannins (Chung et al. 2002; Putnam and Duke 1978; Seigler 1996; Swain 1977; Vyvyan 2002). These and other allelochemicals are found in many plant species from woody to herbaceous plants, grasses and broadleaves, weeds and crops. There are many details left to be determined such as regulation and production stimuli and mode of action for inhibition. It is also not readily understood to what extent allelopathic compounds interact with each other and other chemical compounds within the rhizosphere to inhibit surrounding plants. The following sections present several of the structural classes

increase our understanding of allelochemical release into the rhizosphere.

of recognized allelochemicals as well as specific compounds within each group.

determined by laboratory testing.

**3. Allelopathic compounds**

**2. Production and release of allelopathic compounds**

Like most allelochemicals, phenolic acids are secondary plant compounds typified by a hydroxylated aromatic ring structure. To date, a number of phenolic acids have been determined to have allelopathic properties and have been measured in extracts from a variety of plant species (Figure 1). Species which have been noted to produce phenolic acids include: rice (*Oryza sativa* L.), wheat (*Triticum aestivum* L.), mango (*Mangifera indica* L.), and spotted knapweed (*Centaurea stoebe* L.) (Bais et al. 2003; Chung et al. 2002; El-Rokiek et al. 2010; Fitter 2003). Many species, such as rice, contain multiple phenolic compounds along with other allelopathic compounds. In two studies, researchers isolated nine individual phenolic acids from rice hull extracts and 14 different phenolic acids from buffalograss [*Buchloe dactyloides* (Nutt.) Engelm] (Chung et al. 2002; Wu et al. 1998). At this time, however, it is not clear to what degree individual allelochemicals interact to produce plant inhibition. Some reports show a synergistic effect when allelochemicals are in a mixture, while other studies indicate decreased plant inhibition in the presence of a mixture when compared to individual chemical inhibition (Chung et al. 2002; Einhellig 1996).

Fig. 1. Phenolic acids identified in many plant species, such as oat (*Avena sativa* L.) and rice (*Oryza sativa* L.), have been found to have allelopathic properties.

Although modes of action for allelopathic chemicals are not readily understood for each identified allelochemical, phenolic acids have been the focus of many studies designed to establish the basis of their allelopathy (Putnam 1985). Early research with phenolic acids indicated that some phenolic acids could function though increasing cell membrane permeability, thus affecting ion transport and metabolism (Glass and Dunlop 1974). More recent studies report disruption of cell division and malformed cellular structures in plants

Allelopathic Weed Suppression Through the Use of Cover Crops 119

and Paya 1996; Maddi et al. 1992; Razavi 2011). The broad activity of these compounds has made pharmaceutical use difficult due to the potential for non-target activity. Although allelopathic research has yet to indicate that the broad spectrum activity of coumarins could limit future use of these compounds for weed control, this may require further investigation

Gluconapin Glucobrassicin Progoitrin

Fig. 2. Glucosinolates, allelopathic compounds known to be produced by plants in the Brassicaceae family as well as other families, are produced in both the root and shoot

Glucoiberin Glucotropaeolin Glucoraphenin

Neoglucobrassicin

Gluconasturtin

as research moves forward.

regions of plants.

Sinigrin

exposed to phenolic acids (Li et al. 2010). Reduced respiration and reduced photosynthetic rates, due to decreased photosynthetic products such as chlorophyll, have also been reported in the presence of phenolic acids (Patterson 1981; Yu et al. 2003). Other studies have cited altered plant enzymatic functions, inhibited protein synthesis, and inactivated plant hormones as inhibitory mechanisms from these allelochemicals (Batish et al. 2008; Li et al. 2010). Each mechanism of plant inhibition can lead to the reduced growth and/or death of an exposed plant; however, it is likely multiple functions within a plant are being affected simultaneously due to the mixture of allelochemicals released from a plant species. Despite the extensive research with phenolic acids, target sites for allelochemical activity within affected plant species remain to be determined for many phenolic compounds.

#### **3.2 Glucosinolates**

Glucosinolates occur in many plant species, but are widely known to be produced by species within the Brassicaceae family (Figure 2) (Haramoto and Gallandt 2005; Malik et al. 2008; Mithen 2001). Members of this family include: wild radish (*R*a*phanus raphanistrum* L.), white mustard (*Sinapis alba* L.), turnip (*Brassica campestris* L.), and rapeseed (*Brassica napus*  L.). Glucosinolates, secondary metabolites containing sulfur and nitrogen, are enzymatically hydrolyzed by myrosinase in the presence of water to form isothiocynates, the active allelochemicals (Haramoto and Gallandt 2005; Norsworthy and Meehan 2005; Petersen et al. 2001; Price et al. 2005). Previous research examining extracts from glucosinolate-producing plant species have shown inhibition of other species through reduced germination, reduced seedling emergence and reduced size, as well as delayed seed germination (Al-Khatib et al. 1997; Brown and Morra 1996; Malik et al. 2008; Norsworthy et al. 2007; Wolf et al. 1984). Although specific modes of action have not been thoroughly investigated for each compound, it is evident that some plant species are able to tolerate these allelochemicals more readily than other species (Norsworthy and Meehan 2005). Some suggest that seed size variability plays a role in determining inhibitory effects of these allelochemicals; however, this may not be the only determinant for tolerance to these compounds (Haramoto and Gallandt 2005; Westoby et al. 1996). Future research with these allelopathic compounds will likely seek to answer this question, along with identifying the mode of action for plant inhibition, in order to utilize these compounds more effectively in agricultural production.

#### **3.3 Coumarins**

Coumarin compounds (Figure 3) are found in a range of plant species, particularly from the Apiaceae, Asteraceae and Fabaceae families (Razavi 2011). Coumarins and their derivatives have been identified in plants such as lettuce (*Lactuca sativa* L.), wild oat (*Avena sativa* L.), sweet vernalgrass (*Anthoxanthum odoratum* L.), and a number of other species (Abenavoli et al. 2004; Razavi 2011). Like many other allelochemicals, coumarins have been found to inhibit plant growth by reduced seedling germination and reduced root and shoot growth, likely with interference in photosynthesis, respiration, nutrient uptake and metabolism (Abenavoli et al. 2001; Abenavoli et al. 2004; Razavi et al 2010; Yamamoto 2008).

In addition to plant inhibition, biological activity of coumarins includes antibacterial, nematicidal, antifungal, and insecticidal activity; moreover, pharmacological activity of coumarins has been commonly noted in a number of instances with specific compounds functioning to reduce edema and inflammation (Casley-Smith and Casley-Smith 1992; Hoult

exposed to phenolic acids (Li et al. 2010). Reduced respiration and reduced photosynthetic rates, due to decreased photosynthetic products such as chlorophyll, have also been reported in the presence of phenolic acids (Patterson 1981; Yu et al. 2003). Other studies have cited altered plant enzymatic functions, inhibited protein synthesis, and inactivated plant hormones as inhibitory mechanisms from these allelochemicals (Batish et al. 2008; Li et al. 2010). Each mechanism of plant inhibition can lead to the reduced growth and/or death of an exposed plant; however, it is likely multiple functions within a plant are being affected simultaneously due to the mixture of allelochemicals released from a plant species. Despite the extensive research with phenolic acids, target sites for allelochemical activity within

Glucosinolates occur in many plant species, but are widely known to be produced by species within the Brassicaceae family (Figure 2) (Haramoto and Gallandt 2005; Malik et al. 2008; Mithen 2001). Members of this family include: wild radish (*R*a*phanus raphanistrum* L.), white mustard (*Sinapis alba* L.), turnip (*Brassica campestris* L.), and rapeseed (*Brassica napus*  L.). Glucosinolates, secondary metabolites containing sulfur and nitrogen, are enzymatically hydrolyzed by myrosinase in the presence of water to form isothiocynates, the active allelochemicals (Haramoto and Gallandt 2005; Norsworthy and Meehan 2005; Petersen et al. 2001; Price et al. 2005). Previous research examining extracts from glucosinolate-producing plant species have shown inhibition of other species through reduced germination, reduced seedling emergence and reduced size, as well as delayed seed germination (Al-Khatib et al. 1997; Brown and Morra 1996; Malik et al. 2008; Norsworthy et al. 2007; Wolf et al. 1984). Although specific modes of action have not been thoroughly investigated for each compound, it is evident that some plant species are able to tolerate these allelochemicals more readily than other species (Norsworthy and Meehan 2005). Some suggest that seed size variability plays a role in determining inhibitory effects of these allelochemicals; however, this may not be the only determinant for tolerance to these compounds (Haramoto and Gallandt 2005; Westoby et al. 1996). Future research with these allelopathic compounds will likely seek to answer this question, along with identifying the mode of action for plant inhibition, in order to utilize these compounds more effectively in agricultural production.

Coumarin compounds (Figure 3) are found in a range of plant species, particularly from the Apiaceae, Asteraceae and Fabaceae families (Razavi 2011). Coumarins and their derivatives have been identified in plants such as lettuce (*Lactuca sativa* L.), wild oat (*Avena sativa* L.), sweet vernalgrass (*Anthoxanthum odoratum* L.), and a number of other species (Abenavoli et al. 2004; Razavi 2011). Like many other allelochemicals, coumarins have been found to inhibit plant growth by reduced seedling germination and reduced root and shoot growth, likely with interference in photosynthesis, respiration, nutrient uptake and metabolism

In addition to plant inhibition, biological activity of coumarins includes antibacterial, nematicidal, antifungal, and insecticidal activity; moreover, pharmacological activity of coumarins has been commonly noted in a number of instances with specific compounds functioning to reduce edema and inflammation (Casley-Smith and Casley-Smith 1992; Hoult

(Abenavoli et al. 2001; Abenavoli et al. 2004; Razavi et al 2010; Yamamoto 2008).

affected plant species remain to be determined for many phenolic compounds.

**3.2 Glucosinolates**

**3.3 Coumarins**

and Paya 1996; Maddi et al. 1992; Razavi 2011). The broad activity of these compounds has made pharmaceutical use difficult due to the potential for non-target activity. Although allelopathic research has yet to indicate that the broad spectrum activity of coumarins could limit future use of these compounds for weed control, this may require further investigation as research moves forward.

Fig. 2. Glucosinolates, allelopathic compounds known to be produced by plants in the Brassicaceae family as well as other families, are produced in both the root and shoot regions of plants.

Allelopathic Weed Suppression Through the Use of Cover Crops 121

From the sunflower plant (*Helianthus annuus* L.), several compounds have been identified as being allelopathic (Leather 1983; Vyvyan 2002). The heliannuols are classified as phenolic sesquiterpenes and are noted for allelopathic as well as pharmacological activity (Vyvyan 2002). In addition to having been isolated from the sunflower, similarly structured compounds have been detected in animal species as well (Harrison and Crews 1997). Most notable about heliannuolic compounds is their ability to suppress plant growth at relatively low concentrations. Although they have been shown to inhibit growth of many broadleaf weed species, heliannuols appear to have a stimulating effect upon monocotyleden species (Weidenhamer 1996; Vyvyan 2002). This aspect of heliannuol activity may prove difficult

Fig. 4. Compounds, such as DIMBOA, heliannuol A, and sorgoleone, continue to be studied

Sorgoleone

DIMBOA Heliannuol A

Benzoquinone compounds, primarily sorgoleone, isolated from sorghum [*Sorghum bicolor*  (L.) Moench], have also been determined to be highly allelopathic (Netzly et al. 1998). Research with this compound indicates plant growth inhibition is achieved through disruption of photosynthesis as well as reduced chlorophyll development (Einhellig and Souza 1992). Like some other compounds, sorgoleone exhibits selective activity with inhibition of many germinating seedlings but little activity against certain species such as morningglory (*Ipomoea spp.*) (Nimbal et al. 1996). Research conducted with sorghum root exudates compares sorgoleone activity to that of the herbicide, diuron, but has many target

for their allelopathic properties.

when developing weed control applications of these compounds.

#### **3.4 Other allelopathic compounds**

Many other allelochemicals have been detected in a wide range of species; however, a few compounds have been more widely researched. Classes of allelochemicals under thorough investigation, such as the benzoxazinoids, heliannuols, and benzoquinones, offer potential benefits for weed control in agricultural systems (Figure 4) (Macias et al. 2005; Vyvyan 2002). These classes, described briefly below, represent only a few of the many other compounds that may one day provide substantial weed suppression through allelopathy.

Benzoxazinoid compounds, identified in cereal grains such as wheat and rye, include DIBOA [2,4-dihydroxy-(2*H*)-1,4-benzoxazin-3(4*H*)-one] and DIMBOA [2,3-dihydroxy-7 methoxy-(2*H*)-1,4-benzoxazin-3(4*H*)-one] (Burgos and Talbert 2000; Macias et al. 2005). These compounds are easily degraded into other allelopathic forms, BOA (2 benzoxazolinone) and MBOA (7-methoxy-2-benzoxazolinone), within the soil and can diminish plant germination and growth (Barnes et al. 1987; Burgos and Talbert 2000). In light of the allelopathic properties of BOA and MBOA, it is now recognized that continued research efforts are needed to understand the role of breakdown products of initial allelochemicals in inhibiting plant growth (Macias et al. 2005).

Umbelliferone Scopoletin Scopolin

Imperatorin Psoralen Bergapten

Fig. 3. Coumarins and their subgroups have been identified as allelopathic compounds in

Many other allelochemicals have been detected in a wide range of species; however, a few compounds have been more widely researched. Classes of allelochemicals under thorough investigation, such as the benzoxazinoids, heliannuols, and benzoquinones, offer potential benefits for weed control in agricultural systems (Figure 4) (Macias et al. 2005; Vyvyan 2002). These classes, described briefly below, represent only a few of the many other compounds that may one day provide substantial weed suppression through

Benzoxazinoid compounds, identified in cereal grains such as wheat and rye, include DIBOA [2,4-dihydroxy-(2*H*)-1,4-benzoxazin-3(4*H*)-one] and DIMBOA [2,3-dihydroxy-7 methoxy-(2*H*)-1,4-benzoxazin-3(4*H*)-one] (Burgos and Talbert 2000; Macias et al. 2005). These compounds are easily degraded into other allelopathic forms, BOA (2 benzoxazolinone) and MBOA (7-methoxy-2-benzoxazolinone), within the soil and can diminish plant germination and growth (Barnes et al. 1987; Burgos and Talbert 2000). In light of the allelopathic properties of BOA and MBOA, it is now recognized that continued research efforts are needed to understand the role of breakdown products of initial

several plant families including Apiaceae and Fabaceae.

allelochemicals in inhibiting plant growth (Macias et al. 2005).

**3.4 Other allelopathic compounds**

allelopathy.

From the sunflower plant (*Helianthus annuus* L.), several compounds have been identified as being allelopathic (Leather 1983; Vyvyan 2002). The heliannuols are classified as phenolic sesquiterpenes and are noted for allelopathic as well as pharmacological activity (Vyvyan 2002). In addition to having been isolated from the sunflower, similarly structured compounds have been detected in animal species as well (Harrison and Crews 1997). Most notable about heliannuolic compounds is their ability to suppress plant growth at relatively low concentrations. Although they have been shown to inhibit growth of many broadleaf weed species, heliannuols appear to have a stimulating effect upon monocotyleden species (Weidenhamer 1996; Vyvyan 2002). This aspect of heliannuol activity may prove difficult when developing weed control applications of these compounds.

Fig. 4. Compounds, such as DIMBOA, heliannuol A, and sorgoleone, continue to be studied for their allelopathic properties.

Benzoquinone compounds, primarily sorgoleone, isolated from sorghum [*Sorghum bicolor*  (L.) Moench], have also been determined to be highly allelopathic (Netzly et al. 1998). Research with this compound indicates plant growth inhibition is achieved through disruption of photosynthesis as well as reduced chlorophyll development (Einhellig and Souza 1992). Like some other compounds, sorgoleone exhibits selective activity with inhibition of many germinating seedlings but little activity against certain species such as morningglory (*Ipomoea spp.*) (Nimbal et al. 1996). Research conducted with sorghum root exudates compares sorgoleone activity to that of the herbicide, diuron, but has many target

Allelopathic Weed Suppression Through the Use of Cover Crops 123

(*Raphanus sativus* L.), an indicator species, and cotton (*Gossypium hirsutum* L.) established levels of inhibition for radicle elongation by extracts from cover crops, primarily legumes

Legume cover crops have the ability to fix atmospheric nitrogen that potentially provides a nitrogen source to the subsequent crop without the need for additional fertilizer applications (Balkcom et al. 2007; Hartwig and Ammon 2002). Legume species such as vetch (*Vicia villosa* Roth), clover (*Trifolium spp.*), black medic (*Medicago lupulina* L.), and winter pea (*Pisum sativum* L.) are typically used as cover crops in agricultural production in the United States (Figure 5) (SARE 2007). Other legume crops beginning to be researched as possible choices for cover crops are sunn hemp (*Crotalaria juncea* L.) and white lupin (*Lupinus albus*  L.); however, their availability and use are not as widespread as the previously mentioned legumes. In addition to being a nitrogen source for primary crops, legume covers provide a weed control potential. Due to the rapid degradation of legume residue on the soil surface in comparison to cereal grain residue, weed control through a physical barrier may not last

Fig. 5. Legume cover crops, such as white lupin (in mixture with black oats), provide weed

Determining allelopathic effects of legume cover crop extracts concluded that legume covers did inhibit radish and cotton radicle elongation; however, cotton root exhibited less inhibition than that of radish for all included crops (Price et al. 2008) (Figure 6). In our research, hairy vetch had the greatest inhibition while winter pea had the least effect on germinating seedlings. It is important to note that different varieties of cover crops are

suppression and nitrogen benefits to the subsequent cash crop.

and cereal grains.

as long into the season as other cover crops.

sites (Nimbal et al. 1996; Rimando et al. 1998). Thus far, characteristics of sorgoleone show that it is a promising compound for development into a natural herbicide as an alternative to synthetic herbicides.

#### **4. Weed control through allelopathy**

Ongoing research into allelopathy seeks to better understand the mechanisms of allelopathy in order to make use of these naturally occurring weed suppressants within agricultural areas. Benefits offered by employing allelopathy as some form of weed control could aid in developing more sustainable agricultural systems for future generations (Einhellig 1994a). Current efforts focus primarily on natural herbicide production and cover crops. Although these concepts are being utilized to some degree, there remains a great deal of research to fully utilize the potential of allelopathy.

The role of naturally derived compounds, or synthetically produced mimics, for use as pesticides has been widely adopted, particularly for insect control. Several plant derived compounds, such as pyrethrum, neem, and nicotine, are important chemicals for insect control in many areas (Isman 2006). Herbicide potentials of isolated plant extracts have been indicated by a number of researchers but to date, few have been marketed. Synthetic compounds, such as cinmethylin, and mesotrione, were developed based upon plantderived allelochemicals, but release of subsequent plant-based herbicides has lagged (Lee et al. 1997; Macias et al. 2004; Secor 1994; Vyvyan 2002). Slow production and release of herbicides developed in this manner are most likely attributed to limited understanding of the modes of action for many identified allelochemicals. To date, a number of allelochemicals have been isolated and investigated to develop natural herbicides with these compounds. Understanding the mode of action for plant inhibition may aid in the development of new products for the market.

A great deal of research has been devoted to the use of cover crops for weed control. Until recently, however, the allelopathic potential of cover crops has received less attention due, in part, to the lack of knowledge about allelopathy in general. As the functions of allelopathic compounds are beginning to be understood, more focus is being given to the allelochemicals within cover crops. In agricultural settings, cover crops have been in use for a number of years as a ground cover to slow erosion and water runoff as well as to impede germination of weed seed by providing a physical barrier (Kaspar et al. 2001; Price et al. 2008; Sarrantonio and Gallandt 2003). The growing need for sustainable agricultural systems has necessitated increased cover crop research to better utilize these covers for effective weed control. As a result, recent investigations have sought to understand the role of allelopathy for weed suppression within various cover crops (Burgos and Talbert 2000; Khanh et al. 2005; Price et al. 2008; Walters and Young 2008).

#### **5. Allelopathic potential of cover crops**

Determining allelopathic potential of exudates of plant species can be difficult and time consuming to complete. Bioassays are generally conducted to identify allelopathic properties of compounds in order to differentiate between allelopathy and mulching effects. Our research has focused on determining the extent of allelopathic effects of available cover crops on weed species as well as crop species. Extract-agar bioassays conducted with radish

sites (Nimbal et al. 1996; Rimando et al. 1998). Thus far, characteristics of sorgoleone show that it is a promising compound for development into a natural herbicide as an alternative

Ongoing research into allelopathy seeks to better understand the mechanisms of allelopathy in order to make use of these naturally occurring weed suppressants within agricultural areas. Benefits offered by employing allelopathy as some form of weed control could aid in developing more sustainable agricultural systems for future generations (Einhellig 1994a). Current efforts focus primarily on natural herbicide production and cover crops. Although these concepts are being utilized to some degree, there remains a great deal of research to

The role of naturally derived compounds, or synthetically produced mimics, for use as pesticides has been widely adopted, particularly for insect control. Several plant derived compounds, such as pyrethrum, neem, and nicotine, are important chemicals for insect control in many areas (Isman 2006). Herbicide potentials of isolated plant extracts have been indicated by a number of researchers but to date, few have been marketed. Synthetic compounds, such as cinmethylin, and mesotrione, were developed based upon plantderived allelochemicals, but release of subsequent plant-based herbicides has lagged (Lee et al. 1997; Macias et al. 2004; Secor 1994; Vyvyan 2002). Slow production and release of herbicides developed in this manner are most likely attributed to limited understanding of the modes of action for many identified allelochemicals. To date, a number of allelochemicals have been isolated and investigated to develop natural herbicides with these compounds. Understanding the mode of action for plant inhibition may aid in the

A great deal of research has been devoted to the use of cover crops for weed control. Until recently, however, the allelopathic potential of cover crops has received less attention due, in part, to the lack of knowledge about allelopathy in general. As the functions of allelopathic compounds are beginning to be understood, more focus is being given to the allelochemicals within cover crops. In agricultural settings, cover crops have been in use for a number of years as a ground cover to slow erosion and water runoff as well as to impede germination of weed seed by providing a physical barrier (Kaspar et al. 2001; Price et al. 2008; Sarrantonio and Gallandt 2003). The growing need for sustainable agricultural systems has necessitated increased cover crop research to better utilize these covers for effective weed control. As a result, recent investigations have sought to understand the role of allelopathy for weed suppression within various cover crops (Burgos and Talbert 2000;

Determining allelopathic potential of exudates of plant species can be difficult and time consuming to complete. Bioassays are generally conducted to identify allelopathic properties of compounds in order to differentiate between allelopathy and mulching effects. Our research has focused on determining the extent of allelopathic effects of available cover crops on weed species as well as crop species. Extract-agar bioassays conducted with radish

to synthetic herbicides.

**4. Weed control through allelopathy**

fully utilize the potential of allelopathy.

development of new products for the market.

Khanh et al. 2005; Price et al. 2008; Walters and Young 2008).

**5. Allelopathic potential of cover crops**

(*Raphanus sativus* L.), an indicator species, and cotton (*Gossypium hirsutum* L.) established levels of inhibition for radicle elongation by extracts from cover crops, primarily legumes and cereal grains.

Legume cover crops have the ability to fix atmospheric nitrogen that potentially provides a nitrogen source to the subsequent crop without the need for additional fertilizer applications (Balkcom et al. 2007; Hartwig and Ammon 2002). Legume species such as vetch (*Vicia villosa* Roth), clover (*Trifolium spp.*), black medic (*Medicago lupulina* L.), and winter pea (*Pisum sativum* L.) are typically used as cover crops in agricultural production in the United States (Figure 5) (SARE 2007). Other legume crops beginning to be researched as possible choices for cover crops are sunn hemp (*Crotalaria juncea* L.) and white lupin (*Lupinus albus*  L.); however, their availability and use are not as widespread as the previously mentioned legumes. In addition to being a nitrogen source for primary crops, legume covers provide a weed control potential. Due to the rapid degradation of legume residue on the soil surface in comparison to cereal grain residue, weed control through a physical barrier may not last as long into the season as other cover crops.

Fig. 5. Legume cover crops, such as white lupin (in mixture with black oats), provide weed suppression and nitrogen benefits to the subsequent cash crop.

Determining allelopathic effects of legume cover crop extracts concluded that legume covers did inhibit radish and cotton radicle elongation; however, cotton root exhibited less inhibition than that of radish for all included crops (Price et al. 2008) (Figure 6). In our research, hairy vetch had the greatest inhibition while winter pea had the least effect on germinating seedlings. It is important to note that different varieties of cover crops are

Allelopathic Weed Suppression Through the Use of Cover Crops 125

Fig. 7. Cotton growing in rolled black oat residue. Cereal grain cover crops, like black oat and rye, can be utilized to achieve a large quantity of plant residue on the soil surface.

> Radish Radicle (mm) Cotton Radicle (mm)

Fig. 8. Radish and cotton radicle elongation is reduced by cereal grain cover crops.

Control Black oat Rye Triticale Wheat

10

15

20

25

30

35

available for use in agricultural systems and the varieties of one species may differ in level of allelopathy. Although under field conditions, allelopathic performance of these species may fluctuate, it is apparent that these cover crops can provide additional weed control measures over systems that do not include a cover crop.

Fig. 6. Legume cover crops affect radicle elongation of different plant species to varying degrees.

Cereal grain crops such as black oat (*Avena strigosa* Schreb), rye, triticale (*X Triticosecale*  Wittmack), and wheat, are utilized frequently in conservation systems as cover crops with effective ground cover and weed suppression (Figure 7). Rye is a commonly used cereal cover crop due to its ability to be sown later in the season while maintaining successful growth and its biomass production capability. With increased biomass on the soil surface, weed suppression will be increased as well. Cereal crops will also decay more slowly than more herbaceous plant species and provide some ground cover, and allelochemical release, further into the growing season. Additionally, rye has been noted to be less affected by plant diseases than other cover crops, and aids in reducing insect pests within a system (Wingard 1996).

Like legumes, cereal grain crop exudates in our study were able to significantly inhibit radicle elongation compared to the control (Figure 8). The disparity between radish and cotton radicle inhibition for each cover crop studied suggests that minimized interference with primary crops and increased weed suppression potential could be achieved with the use of cereal grain crops. These allelopathic effects, however, may be amplified or diminished depending on the field environment, plant stress levels, cover crop variety, and a number of other factors involved in determining allelochemical levels. Nevertheless, this research provides a base of allelopathic concentrations and impacts from various cover crops and may be an initial consideration when choosing a cover crop for inclusion in a system.

available for use in agricultural systems and the varieties of one species may differ in level of allelopathy. Although under field conditions, allelopathic performance of these species may fluctuate, it is apparent that these cover crops can provide additional weed control

Fig. 6. Legume cover crops affect radicle elongation of different plant species to varying

Cereal grain crops such as black oat (*Avena strigosa* Schreb), rye, triticale (*X Triticosecale*  Wittmack), and wheat, are utilized frequently in conservation systems as cover crops with effective ground cover and weed suppression (Figure 7). Rye is a commonly used cereal cover crop due to its ability to be sown later in the season while maintaining successful growth and its biomass production capability. With increased biomass on the soil surface, weed suppression will be increased as well. Cereal crops will also decay more slowly than more herbaceous plant species and provide some ground cover, and allelochemical release, further into the growing season. Additionally, rye has been noted to be less affected by plant diseases than other cover crops, and aids in reducing insect pests within a system (Wingard 1996).

Lupin Sunn

Hemp

Winter pea

Radish Radicle (mm) Cotton Radicle (mm)

Like legumes, cereal grain crop exudates in our study were able to significantly inhibit radicle elongation compared to the control (Figure 8). The disparity between radish and cotton radicle inhibition for each cover crop studied suggests that minimized interference with primary crops and increased weed suppression potential could be achieved with the use of cereal grain crops. These allelopathic effects, however, may be amplified or diminished depending on the field environment, plant stress levels, cover crop variety, and a number of other factors involved in determining allelochemical levels. Nevertheless, this research provides a base of allelopathic concentrations and impacts from various cover crops and may be an initial

consideration when choosing a cover crop for inclusion in a system.

measures over systems that do not include a cover crop.

degrees.

10

Control Black

medic

Crimson clover

Hairy vetch

15

20

25

30

35

Fig. 7. Cotton growing in rolled black oat residue. Cereal grain cover crops, like black oat and rye, can be utilized to achieve a large quantity of plant residue on the soil surface.

Fig. 8. Radish and cotton radicle elongation is reduced by cereal grain cover crops.

Allelopathic Weed Suppression Through the Use of Cover Crops 127

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#### **6. Conclusions**

The growing demand for sustainable agricultural systems requires that researchers reevaluate current production methods and inputs. To ensure continued productivity and potentially reduce synthetic herbicide requirements, allelopathy has become a focal point for research in the agricultural community. Although, many questions have yet to be resolved, the utilization of allelochemicals for weed suppression remains a promising avenue for reducing herbicide usage. Whether through the development of natural herbicides from isolated allelochemicals or through the application of cover crops with allelopathic properties, allelopathy will most likely be a factor in providing sustainable systems in the future.

#### **7. References**

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**Part 2**

**Integrated Chemical Weed Management**