2014•Unpublished venueRequires access
Evaluation of the dairy fertility genetic evaluation model using selection index theory
Katarzyna Stachowicz, Gemma Jenkins, N. A. Mensah Dennis, P.R. Amer, Jeremy R. Bryant, S. Meier
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Abstract
Cow fertility is important in the New Zealand seasonal calving system but intense selection on production traits has resulted in a decline in dairy cow fertility. To prevent further decline, PM21 (percentage of cows mated in the first 21 days of mating) and CR42 (calving rate in the first 42 days of calving) are used in the genetic evaluation of bulls for fertility. These traits are binary - a cow either meets these critical reproduction deadlines or she is classified as a late breeder or calver. However, the earlier a cow successfully breeds the more valuable she is to the production system, and this is not adequately captured with a binary trait. A preliminary study using data from the National Herds Fertility Study database indicated that re-defining the CR42 binary trait to a continuous trait could increase the accuracy of fertility trait estimates. As well as this, substituting the CR42 binary trait for its continuous equivalent (calving season day, CSD) can improve the heritability of the trait meaning that more genetic variation is captured. Additionally, other measures of fertility, such as heifer calving season day, showed promise for inclusion in the fertility model. The study reported in this paper used selection index modelling to identify the best models for predicting true fertility by comparing models with either the binary or continuous equivalents of the current fertility traits, replacing milk with protein yield or protein percent as a predictor trait in the fertility model and by considering the impact of inclusion of heifer fertility trait. The results of this study agree with, and build on, previous preliminary findings, that: 1) the continuous version of CR42 was a more accurate predictor of true fertility than the currently used binary trait; 2) including milk production traits in the fertility model has minimal impact on model accuracy, and 3) that adding heifer calving increased the accuracy of fertility trait estimates.