Genetic analysis of milk yield traits in first lactation of dairy cattle of Hamedan province

Estimation of variance components is necessary for precise planning of animal breeding programs. The present study was conducted to estimate genetic parameters of milk yield traits in the first parity of Holstein cattle in Hamedan province.
First parity records of 4027 lactating cows from 148 herds, collected by Agricultural Jahad Institute of Hamedan province, were used for this study. (Co)variance components of milk yield traits were estimated by bivariate analyses of mixed animal models based on Average Information algorithm of Restricted Maximum Likelihood (AI-REML). In these models the fixed effects were herd-year-season of calving and Holstein percentage (for all traits) and calving age (for 305 d milk yield, fat corrected milk and fat yield traits). The animal additive genetic effect was considered as the random effect in the model.
Heritabilities of 305 d milk yield (MY), fat corrected milk yield (FCM), fat percentage (F), protein percentage (P), fat yield (FY) and protein yield (PY) at the first parity were estimated 0.16, 0.17, 0.25, 0.18, 0.20 and 0.14, and their genetic trends were 2.510 and 2.010 kg/yr, -0.001 and -0.000 %/yr and 0.068 and 0.022 kg/yr, respectively. The estimated genetic correlations between MY and FCM, F, P, FY and PY traits were 0.86, -0.27, -0.25, 0.60 and 0.90, respectively. Genetic correlations of FCM with F, P, FY and PY were estimated 0.25, 0.25, 0.94 and 0.94, respectively.
Regarding the higher heritability of FCM than MY and also its high and positive genetic correlations with milk contents, it seems that selection based on FCM is better than the MY.