Estimation of lactation curve parameters in Iranian Holstein dairy cows by Wilmink function

In this study, we used 16752 test- day records from 3068 cows in the first parity during 1997 to 2010. All records collected from Jehad griculture organization in Khorasan Razavi Province. Estimation of genetic parameters was performed by Wombat software with univariate and bivariate procedures. Wilmink’s function was fitted to describe the lactation curve and associated production characteristics. Results showed that heritability for lactation curve parameters ranged from 0.05 (peak time) to 0.20 (total 305-day milk yield). The estimated heritability rates were low, and this shows that lactation curve parameters largely are influenced by environmental factors. Genetic correlations for lactation curve parameters ranged from –0.47 (0.01) (peak Time and increasing slop in lactation curve) to 0.95 (0.06) (peak milk and initial production level). A large positive genetic correlation found between 305-day and peak milk yield. Therefore, direct selection for total lactation yield would be expected to result in lactations with a higher peak production in Iranian Holstein cows. This shows that Wilmink exponential function can be used for estimation of lactation curve parameters in Iranian Holstein cows.