DefinePK

Abstract

The main goal of the current work is to break the undesirable genetic linkage between milk yield (MY) and length of days open (DO) by using partial restricted selection indexes. Eight unrestricted selection indexes to increase MY and decrease the number of DO and the number of services per conception (NSC) were constructed. The sources of information were various combinations of yields of milk, fat (FY), and protein (PY) beside DO and NSC. Required phenotypic and genetic parameters were estimated from 2538 lactation records of 846 Holstein cows, offspring of 98 sires, and 588 dams, adopting a multi-trait animal model with repeated records. Positive genetic correlations (0.33 to 0.99) were obtained among productive traits. Lactation curve traits were lowly genetically inter-correlated (-0.16 to +0.24). Genetically, MY was more related to DO (0.85) than the NSC (0.58). Selection for aggregate genotype involving MY, DO and NSC would develop Holstein cows with abundant MY (643.5 to 994.8kg) and longer DO (11.3 to 21.3 days) and higher NSC (0.15 to 0.37 service). Due to the collapse of selection accuracy, complete restriction of full index failed to break the unfavorable genetic associations between MY and DO. Meanwhile, it is possible to alleviate the rate of deterioration in reproduction in high lactating Holstein cows using the partial restricted indexes in the case of accepting the barter between the gains from increasing MY by the loss from prolonging DO.Keywords:High yielding dairy cows; Productive traits; Lactation curve traits; Reproductive traits; Selection indexes; Expected gain