Simple SummaryThe inclusion of plant-derived products, such as herbs or essential oils, in the feed can improve the production of animals. Moreover, these photogenic products can fight against parasites in an animal’s digestive system. The scope of the present study was to observe the impact of a feed supplement containing carvacrol and limonene on the performance and parasitological status of sheep. The feed supplement exerted an advantageous effect on the metabolic processes of lactating ewes after 42 days of supplementation, while the average live weight and average daily weight gain of their twin suckling lambs increased by the termination of the study. In another experiment, fattening lambs fed the same supplement showed a decreased faecal egg count of gastrointestinal nematodes. Additionally, no differences in live weight, average daily gain, or mean number of Haemonchus contortus nematodes in the abomasum were observed. The results highlighted that carvacrol and limonene in the feed of lactating ewes effectively increased the weight gain of their twin suckling lambs, presumably due to the ewes’ energy status improvement. At the same time, further studies are needed to elucidate the effects of carvacrol and limonene against gastrointestinal parasites.AbstractThere is currently worldwide interest in phytogenic feed supplements (PFSs) because they can lead to improved animal production. The scope of the present study was to observe the impact of a feed supplement containing carvacrol (CAR) and limonene (LIM) on the performance and parasitological status of sheep. The feed supplement decreased the plasma levels of β-hydroxybutyrate (p< 0.001), triglycerides (p= 0.014), nonesterified fatty acids (p= 0.021), and fructosamine (p= 0.002) in lactating ewes after 42 days of supplementation, while the average live weight (p= 0.002) and average daily weight gain (p= 0.001) of their twin suckling lambs increased significantly by the end of the study. In another experiment, fattening lambs fed the same supplement showed a decrease in fecal egg number of gastrointestinal nematodes (p= 0.02) but no differences in live weight, average daily gain, or mean number ofHaemonchus contortusnematodes in the abomasum. The results highlighted that the inclusion of carvacrol and limonene in the feed of lactating ewes effectively increased the weight gain of the suckling lambs, presumably due to the ewes’ improved energy, but further studies are needed to elucidate the effects of carvacrol and limonene against gastrointestinal parasites.