Abstract:
Milk yield recorded at a weekly interval throughout a lactation period of 40 weeks was utilized to model lactation curve of Sahiwal cattle. A gamma-type function was used to predict the shape of curves for different parities and calving seasons. The co-efficiants for the initial yield (alpha), increase towards peak (beta), and decline after peak (ceta), were estimated for 1084 lactations. The effects of year, season of calving and parity on these co-efficients were studied in a fixed effect model. About 36% of l-te lactations were atypical/abnormal (having negative beta or positive ceta). The overall estimates (in 681 normal lactations) for the alpha (in the log form), beta (per log of week), cera (per week), and peak yield were 1.87, .202, 0.29, and 8.23 kg. Peak yield was obtained around 7th week of lactation. Out of the three parameters estimated, initial milk yield and decline from, peak differed (P<O.Ol) among different parities while increase towards peak was statistically similar. Peak yield among parities was also different (P ~().o(j:• among different parities while increase towards peak was statistically similar. Peak yield among parities was also different (P<0.01). Parameters were also different for the two seasons. Among parities, co-efficients were different for cows calving for the first time as compared to later parity cows. Highest peak yield (8. 96 kg) was estimated for fifth parity cows while first parity cows had the minimum peak yield (7 .54 kg). Lactation curves of first and later parity cows differed in shape and other attributes as well. Very high percentage of atypical/abnormal curves need further investigation.
