A Genetic Study on Some Milk Production Characteristics in Holstein Cows Using Gompertz Function

In this study, genetic parameters and trends were estimated for milk yield at peak time (YM), peak time (TM) and total lactation milk yield (YL) using 46,420 monthly test day records belonging to 5,323 first lactation Holstein cow in Mashhad’s dairy farms. The number of herds was 64 and cows calved during 1994-2007. Total number of sires and dams were 660 and 4,685, respectively. Gompertz non-linear function was utilized to describe the shape of the lactation curve. In order to genetic analysis of the traits, a multivariate animal model was used in which fixed effect of contemporary group of herd- year – calving season – sperm type, some covariables, as well as random additive genetic effect were included. Restricted maximum likelihood statistical method was used to estimate variance and covariance components. Heritability of milk yield at peak time, peak time and total lactation milk yield were found to be 0.159, 0.129 and 0.190, respectively. Genetic and environmental correlations between YM and TM were 0.830 and 0.515, between YM and LY were 0.978 and 0.938, and between TM and YL were 0.920 and 0.689, respectively. The results indicated that there were positive and significant (P<0.05) genetic trends for YL 4.664 kgy- 1), YM (0.017 kgy-1) and TM (0.130 dy-1).