INFLUENCE OF ENVIRONMENT AND HEREDITY ON THE SHAPE OF LACTATION CURVES IN SAHIWAL COWS

The shape of the lactation curve of Sahiwal cows was estimated by fitting a gamma function to 2034 lactations made by 681 cows at two farms in Indo-Gangetic plains of Northern India. The persistency of lactation yield was estimated by three methods: P1, the coefficient of variation among weekly yields; P2, ratio of lactation yield to peak yield, and P3, from the gamma function.The gamma function fitted to the weekly yields explained 75-9 of the variation. A least-squares analysis of different traits associated with lactation curve shape indicated significant influence of parity, period and season of calving on the lactation curve. The lactation yield, peak yield and daily yield up to the peak were highest for winter calvers, while persistency was highest for monsoon calvers. The milk yield traits showed an increase up to the second or third lactation, while the persistency decreased from the first to eighth lactation with increase in parity order. The lactation curve was also more fiat in the first lactation than later. The lactation yield and persistency increased with increase in age at calving independent of parity order. The lactation yield, peak yield, persistency and daily yield up to the peak were positively correlated with service period, lactation length and calving interval.The heritability and repeatability estimates of different traits, genetic and phenotypic correlations of lactation milk yield with different persistency measures indicated that P2 is a better measure of persistency. The peak yield could be used as a criterion of selection in early lactation to bring about improvement in lactation yield and persistency. © 1979, Cambridge University Press. All rights reserved.