Effect of synchronized breeding on genetic evaluations of fertility traits in dairy cattle.

Discipline: fertility; Keywords: dairy cattle, fertility, genetic evaluations, hormonal synchronization protocols.

Oestrus detection has become more difficult over the years due to decreases in the oestrus expression of high-producing dairy cows, and increased herd sizes and animal densities. Through the use of hormonal synchronization protocols, also known as timed artificial insemination protocols, it is possible to alleviate some of the challenges associated with oestrus detection. However, timed artificial insemination masks cows’ fertility performance, resulting in a possibly biased comparison of treated animals and innately fertile animals. Consequently, genetically inferior and superior cows show similar phenotypes, making it difficult to distinguish between them. As genetic programs rely on the collection of accurate phenotypic data, phenotypes collected on treated animals likely add bias to genetic evaluations. In the study by Dr C. Lynch and colleagues, to assess the effect of timed artificial insemination, the rank correlation of bulls for a given trait using only timed artificial insemination records were compared with the same trait using only heat detection records. The authors published their results in the Journal of Dairy Sciences, Volume 104 of 2021, page 11820 to 11831. The title being: Effect of synchronized breeding on genetic evaluations of fertility traits in dairy cattle.

A total of 270,434 records from 192,539 animals split across heifers, first and second parity cows were analyzed for the traits: calving to first service, first service to conception, and days open.

The results showed large re-ranking across all traits and parities between bulls compared based on either having only timed artificial insemination records or only heat detection records, suggesting that a bias does indeed exist. Large re-ranking was also observed for both the heat detection and timed artificial insemination groups among the top 100 bulls in the control group, which included all records. Furthermore, breeding method was added to the model to assess its effect on bull ranking. However, there were only minor changes in the rank correlations between scenario groups. Therefore, more complex methods to account for the apparent bias created by timed artificial insemination should be investigated; for this, the method by which these data are collected needs to be improved through creating a standardized way of recording breeding codes.

Although the results of the study suggest the presence of bias within current fertility evaluations, additional research is required to confirm the findings here, including evaluating high-reliability bulls specifically, to determine if the levels of re-ranking remain. It is further suggested that future studies should also aim to understand the potential genetic differences between the fertility traits split via management technology, possibly in a multiple-trait analysis.