Estimation of Genetic Parameters for Milk Trait at Different Milking Times for Holstein Cows of Khorasan Razavi Using a Random Regression Test Day Model

In this research in order to estimate genetic parameters for milk yield at different milking times for Holstein cows of Razavi Khorasan، a total of 180،462 monthly test day records belonging to first lactation cows calved from 2000 to 2009 were utilized. The records were analysed by a random regression test day model. In the model، the fixed effects of contemporary group of geographical location – herd – year of recording – production season – sperm type، linear covariables of Holstein gene percentage، age at recording، as well as additive genetic and permanent random effects were included. For morning (M1) and night (M3) milking times، maximum additive genetic variances were found in month eight (0. 808 and 0. 659، respectively) while the minimum value (0. 399 and 0. 319، respectively) was found for both milking times in month two of the lactation curve. Maximum and minimum additive genetic variances for noon (M2) milking times were observed in months ten (0. 732) and two (0. 278)، respectively. Permanent and residual variance components were the greatest values at the end of the lactation course. Maximum heritability estimates of M1، M2 and M3 were found to be 0. 143 (month eight)، 0. 137 (month nine) and 0. 139 (months eight and nine)، respectively. For all milking times، minimum additive genetic correlations were found between month one and month eight and ranged from 0. 216 to 0. 285، while the maximum values were observed between month six and month seven and ranged from 0. 993 to 0. 996. For all milking times، permanent environment correlations were maximum between months five and six (ranged from 0. 992 to 0. 993) and minimum between months one and ten (ranged from 0. 411 to 0. 445).