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Abstract : |
Genetic aspects of the lactation curve of Iranian Holstein heifers were studied with the use of Wood`s incomplete gamma function (yt = atb(exp)-ct). Data comprised 179,460 monthly test day milk records from 17,946 Iranian Holstein dairy heifers calving between 1986 and 2001 and distributed in 287 herds. Wood`s incomplete gamma function parameters (a, b and c) were initially estimated for individual cows using SAS programme and subsequently along with some production characteristics including peak time (b/c), peak yield (a(b/c)b(exp)-b), persistency (-(b+1)Logec) and 305 day lactation milk yield were subjected to a multivariate Animal Model to estimate genetic parameters of seven traits using MTC programme. Based on the results, it was revealed that heritability estimates for the parameters of Wood`s incomplete gamma function (0.1, 0.03 and 0.05 for a, b and c, respectively), peak time (0.1) and persistency (0.08) were low indicating that these traits were substantially influenced by the environmental factors and as a result additive genetic variation between animals for these traits was not remarkable suggesting that direct selection may not meaningfully change the shape of the lactation curve. In this study, the heritability estimates of peak yield (0.28) and 305 day milk yield (0.29) were found to be approximately the same., |
