Genetic evaluation of lactation curve parameters and the estimation of inbreeding effect on them in Holstein cows

The aim of this study was to estimate the genetic parameters of lactation curve traits (milk yield at beginning of lactation, the rate to reach peak yield, a parameter related to maximum milk yield, the changes in curve shape after reaching maximum yield, peak time, peak milk, persistency) was fitted by Rook function and the inbreeding effect on these parameters for Holstein cows in Iran. For this purpose, the test-day milk yield records of the first three lactations that were collected by the Animal Breeding Center of Iran from 1983 to 2017, were used. The Rook function parameters were calculated for all animals and the peak time, peak milk and persistency were estimated from the curve parameters. The estimation of variance components and genetic parameters of traits were performed by single and two- trait analysis via Gibbs sampling method using of Gibbs3f90 software. The effect of inbreeding was negative on the most of the traits; especially those had a higher heritability and also decreased milk production persistency. The heritability range of the aforementioned traits varied from 0.0003 to 0.086 and the highest heritability was related to persistency trait (0.086) in second lactation and peak milk (0.067) in third lactation. In general, the parameters of the lactation curve had a low heritability, so genetic selection would not show favorable results and changes in environmental and management conditions could be more effective for improving these traits.