Genetics of lactation persistency and relationships with reproductive performance in Holsteins

The objectives of this study were (1) to estimate the heritability of two measures of persistency, (2) to quantify genetic correlations between persistency and 305-d milk yield, and (3) to estimate genetic and phenotypic relationships between lactation persistency and reproductive performance. A two-stage process was used; (1) the Multiple Trait Prediction procedure was used to predict 305-day lactation yields from individual milk, fat and protein test day yields, using Wilmink's lactation curve and (2) these estimates were then used as dependent variables in an animal model to calculate genetic parameters using a software package for multivariate mixed models via Gibbs sampling. Persistency of milk, fat, and protein were defined as (1) predicted yield on day 280 of lactation as a percentage of predicted yield on day 60 of lactation, and (2) the b-parameter of the Wilmink function for each yield trait. Reproductive traits included age at first insemination, non-return rate at 56 days as a virgin heifer and first calf heifer, calving ease, and calving interval. Phenotypically, as lactation number increased, and as yield increased, persistency decreased. Heritability of milk persistency ranged from 0.17 to 0.37. Heritability of the Wilmink b parameter for milk yield was much higher in second lactation compared to that in first or third lactation, and was generally more heritable than predicted milk yield on day 280 of lactation as a percentage of predicted yield on day 60 of lactation. Genetic correlations of persistency between lactations were moderate, with correlations between subsequent lactations being higher than between first and third lactation. The Wilmink b parameter for milk yield was selected as a measure for persistency for the remainder of the study because of its moderate heritability, ease of computation and its weak genetic correlation with 305-d yields. Heritabilities of reproductive traits were low (0.03 to 0.19). The highest heritability was found for AFS. Genetic correlations between heifer fertility traits and persistency in first lactation were favourable. Genetically, first lactation was more persistent if heifers were younger when they were bred as a heifer, and conceived quickly and easily. Longer calving interval between first and second calving was associated with higher persistency in first lactation. Relationships between persistency and heifer and cow fertility are clearly different and should be investigated further. Later peak daily milk yield was also associated with a longer calving interval. Phenotypic correlations between reproductive traits and persistency were weaker than their genetic counterparts.