Assessing fertility: Development of a selection index for fertility in Italian Jersey cattle

Selection for increased milk yield in dairy cattle has led to a decline in fertility due to unfavorable genetic correlation. However, fertility remains crucial in dairy farming for maintaining a regular productive cycle, ensuring continuous milk supply, and reducing costs related to nonproductive cows. For this reason, the aim of this study was to estimate genetic parameters for fertility traits and to assess their genetic correlation with production traits in the Italian Jersey cattle breed. We considered days open (DO) as a fertility trait, and kg/305-d milk (MY), protein (PY), and fat yields (FY) as production traits, respectively. After data editing, the final dataset included 33,053 DO records from 16,667 cows, restricted to values between 25 and 400 d. The mean (± SD) phenotypic value for DO was 121 ± 71 d. Variance components and h² estimates for DO were obtained using a single-trait animal model, whereas genetic correlations were estimated with a multiple-trait animal model. The models accounted for herd, permanent environment, and animal as random effects, whereas parity and month-year were considered as fixed effects. A pedigree-based relationship matrix was used in the model. Estimated h² (± SE) for DO was low (0.04 ± 0.01), and unfavorable genetic correlations (± SE) were observed with MY (0.16 ± 0.08), PY (0.12 ± 0.09), and FY (0.23 ± 0.09). Genetic trends revealed a consistent increase in production traits, whereas the trend of DO showed a greater variability over time. Despite the low h², these results highlight the potential to improve fertility through selection and support the inclusion of reproductive traits in breeding programs aimed at achieving a more balanced and sustainable genetic progress in Italian Jersey cattle.