Comparison of genetic parameters of 305-day milk yield trait in dairy cows in Isfahan province by using different animal models

The purpose of this study was to compare different animal models to estimate genetic parameters of milk yield trait in dairy cows of Isfahan province. Variance components and genetic parameters included heritability and repeatability were estimated by using records of 21346 dairy cows from 2 herds collected during 20 years. Data analyses were undertaken using three different animal models. In the first model each lactation records separately were analyzed by univariate animal model. In the second and third models, the records of the first three lactations were analyzed using repeatability and a multivariate animal model. The numbers of records were 21346, 20828 and 12579 for the first 3 lactation, respectively. Heritability estimates of milk yield from univariate and multivariate analyses for lactation 1, 2 and 3 were 0.27, 0.20, 0.19 and 0.30, 0.23 and 0.22, respectively. The heritability and repeatability estimates of milk yield from repeatability model analysis were 0.24 and 0.49, respectively. Additive genetic and environmental correlations between first and second, first and third, and second and third lactations were 0.91, 0.86, 0.97 and 0.36, 0.31, 0.41, respectively. The corresponding estimates for phenotypic correlations were 0.50, 0.45 and 0.53, respectively. All estimates for additive genetic, environmental and phenotypic correlations were positive. Estimated additive genetic, environmental and phenotypic correlations showed that estimates between consecutive lactations (first and second, second and third) were larger than the estimates between nonconsecutive lactations (first and third). The results of this study showed that the multivariate model analysis is more suitable for estimating the genetic parameters of milk yield trait regardless high computational costs and has greater accuracy because of considering the effects of culling bias.