Multiple-trait multiple country genetic evaluation of fertility traits in dairy cattle

Female fertility is one of the most economically important traits for the dairy cattle industry. Because of a few decades of selection mainly for higher milk production, many dairy farms around the world suffer from the consequences of female fertility loss. The aims of this thesis were to study the international genetic evaluation of female fertility traits, to quantify the bias of analyzing multiple female fertility traits per country with the current method in use for international genetic evaluations (MACE), to study the implementation of a new method (MT-MACE) for the analysis of multiple traits per country, and also to study the effect of across country selection for milk yield on the international genetic evaluation of female fertility traits, when MT-MACE is applied. Female fertility traits are low heritable, each describing a part of the female fertility complex. International genetic evaluation is useful for improving the accuracy of female fertility evaluations and making the across country comparisons of bulls possible. Despite the low heritability values of female fertility traits, the estimated across country genetic correlations were moderate to high, making the international genetic evaluations feasible for female fertility traits. Results for female fertility traits showed that including multiple traits per country in a MACE analysis would lead to considerable bias. This bias is due to ignoring covariances from multiple-trait national models. Avoiding bias by performing several unbiased MACE analyses, each including one trait per country is not advantageous, because it is computationally prohibitive and it does not make an optimal use of the available data. MT-MACE was applied to female fertility data, which led to higher reliabilities compared to the (unbiased) MACE evaluations. The reliability gains were larger for foreign bulls and the Top 100 bulls in each country-trait. The results showed that when the within country selection for milk yield is already taken care of by the multiple-trait national models, the across country selection for milk yield did not make significant bias in the international evaluation of female fertility traits.