Changes to the genetic evaluation of fertility in Irish dairy cattle

Abstract

In Ireland, the current genetic evaluation of dairy cattle includes a joint fertility and survival module. It is a multiple trait animal model combining 23 traits. Calving interval is utilized as the primary fertility trait, and this aligns with the definition used in many countries with predominantly all year-round calving systems. It is also widely accepted within the scientific community as providing an accurate measure of genetic merit for fertility. However, around half of the herds in Ireland display seasonal calving patterns, with this trend steadily increasing, and is actively promoted by advocates for a low-cost industry. Calving interval has been found to be an inappropriate selection criterion for fertility in seasonal systems, as early calving and likely more fertile cows often have very long calving intervals, because they are withheld from mating, to avoid them calving too early in the following calving season. The aim of this work was to assess the feasibility and benefits of defining more seasonally orientated phenotypes for the genetic evaluation of fertility for those herds that are classed as seasonal calving. The results from the following steps of work are presented. Firstly, it was necessary to develop a robust set of criteria to differentiate seasonal from non-seasonal herds. This was complicated by the fact that many herds are dynamic across years in the extent to which they restrict calvings to within a season. The approach taken in this study treats a non-seasonal classification as the default for any cow fertility record. Fertility records were classified into the seasonal category if the animal’s contemporary group met certain seasonality criteria. Secondly, the definitions of fertility phenotypes in a seasonal system were investigated and are presented. Based on the results, the following traits are recommended for inclusion in a new fertility evaluation: age at first calving as a heifer fertility trait in both seasonal and non-seasonal herds, timing of conception and calving season day for animals in seasonal calving herds and calving interval and number of services in non-seasonal herds. These traits would be evaluated together in a five-trait repeated record animal model. Thirdly, the variance component estimates for the five-trait model are presented. Lastly, results from a prototype genetic evaluation using the new model with alternative seasonality inclusion thresholds are presented. In summary, based on the obtained results, recommendations are presented for the genetic evaluation of fertility in Ireland.