Assessing the impact of hormonal interventions on genetic evaluation of fertility in New Zealand

Abstract

Reproductive efficiency is crucial for productivity and profitability in the New Zealand seasonal dairy system. It requires compact calving patterns to align lactation with pasture availability. Achieving a high 6-week in-calf rate is essential for maintaining a 365-day calving interval and optimising farm efficiency. Hormonal reproductive interventions are often being used to manage anoestrus cows, synchronise oestrus in cycling cows and heifers, and improve AI submission rates. Accurate genetic evaluation of fertility requires, therefore, reliable recording of hormonal synchronisation interventions. The aim of this study was to assess if the current level of reporting of fertility interventions is adequate for inclusion in the national genetic evaluation of cow fertility, and to evaluate alternative approaches for incorporating synchronisation information into genetic evaluation. After extensive data filtering requiring consistent synchrony reporting and herd continuity, a high‑quality subset was created for variance component estimation. Adjusting fertility phenotypes for synchrony through either setting synchronised records to missing or applying a penalty substantially increased heritabilities, revealing additional additive genetic variation masked by intervention use. Synchronised cows consistently exhibited poorer fertility performance, with 1.5% lower 3‑week submission rates and 11% lower 6‑week in‑calf rates compared to non‑synchronised contemporaries. Use of synchronisation itself as a new trait showed heritability of 0.07–0.1, which is significantly higher than any fertility trait evaluated currently in New Zealand. Moderate to strong negative genetic correlations between synchrony use and fertility traits (ranging from -0.38 to -0.78) demonstrate that synchronisation captures biologically meaningful information about underlying reproductive merit. Overall, the study shows that intervention data can substantially improve fertility trait modelling and breeding value accuracy. However, only 7% of cows have treatment records available in recent years, which is not sufficient for implementation. Improved industry‑wide synchrony recording is essential for integrating these findings into routine genetic evaluations.