THE EFFECTS OF PARITY NUMBER, SEASON AND YEAR OF CALVING OF SUDANESE ZEBU CATTLE (BUTANA) ON THE LACTATION CURVE AND MILK YIELD
The present study was conducted to investigate the effects of parity number, season and year of calving of Sudanese Zebu cattle (Butana) on the lactation curve and milk yield. A Wood's model (1967) was adopted for the description of the curve, it is a gamma function utilized for regression of milk yield on time lapse post-partum. The regression equation is presented by (Y(n) = an b e -cn ); where: Y(n) is the total milk yield for n th week, a, is the initial milk yield and is considered as a factor which could influence the height of the curve across time but has no effect on the curve. b is the rate of increase of milk yield pre-peak and is considered as the linear constant that measures the average slope of the curve during the increase phase. c is the rate of decrease of milk post-peak, a linear constant that describes the rate of change of the slope of the curve during the decline phase and determines the slope of the curve during this phase. Records of 178 cows were taken from the fifth days of lactation till 30 weeks from the year 1994 - 2001. The records were grouped according to parity (till eight parities), season of calving (dry and wet summer and winter) and year of calving. The results revealed that effect of parity on initial milk yield, although significant, but variable. The peak week, persistency and rate of increase of milk pre-peak were the highest (P<0.01) in parity 1 compared to other parities. However, rate of decrease post-peak was not affected by parity number. Peak yield and total yield increased steadily from parity one to parity 6 then decreased. Calving weight increased significantly (P<0.01) from 1 to 8. Season of calving was shown to have a significant effect on initial milk yield, a, peak week and persistency where, a, was the highest (P<0.01) in wet summer than winter and dry summer and hence was increased to the maximum peak during wet summer with shorter persistency around the peak compared to dry summer and winter. Year of calving significantly affected the rate of decrease post-peak, c, peak yield, weekly and total milk yields. It was shown that cows that calved in year 1997 and 2000 had the lowest (P<0.01) rate of decrease in milk yield, weekly and total yields.