Toward improved calf health in French dairy cattle breeds: Genetic parameters for calf survival across key periods.

Shifts in the dairy cattle industry have placed greater emphasis on breeding healthier animals. While adult cow health-related traits have historically been more studied than calf health-related traits, largely due to greater data availability, the significant impact of the latter on dairy production, especially calf survival, has gained increased attention. Several countries have already assessed the feasibility of implementing a genomic evaluation for this trait, whereas in France, only a genomic evaluation for perinatal mortality currently exists in dairy cattle breeds. Hence, implementing a routine genomic evaluation may help improve calf survival. In this context, this study first aimed at estimating genetic parameters for calf survival across 2 key subperiods (between d 3-30 and between d 31-365 where mortality is mainly associated with gastrointestinal diseases and with respiratory diseases, respectively) and across the entire calf survival period (d 3-365) in 9 French dairy cattle breeds. Second, we investigated whether calf survival should be divided into 2 subperiods to more precisely discriminate the causes of mortality or be considered as a single period. Third, genetic trends for calf survival traits were explored. Accordingly, linear animal models were used to estimate heritability and genetic correlations between calf survival traits. The analyses included up to 12 million pedigree records and between 4,750 and 8,296,055 records collected from 2010 to 2024, depending on the breed. Estimated breeding values of bulls obtained from genomic evaluations for calf survival were used to assess genetic trends. The 3 calf survival traits showed low heritabilities (h2 = 0.006-0.031) but displayed large genetic standard deviations (SD) making genomic selection for these traits possible. High genetic correlations were found between the calf survival subperiods, with most estimates near 0.80, indicating close genetic backgrounds. To better discriminate the causes of mortality, we propose combining both d3-30 and d31-365 in an index synthesis. Genetic trends were favorable after 2010 in the 3 major breeds (Holstein, Montbéliarde, and Normande), but no change was observed in the others. Future work will estimate genetic correlations between calf survival traits and traits under selection, thereby paving the way for including calf survival in French dairy cattle breeds' breeding programs to help decrease juvenile mortality.