Genetic analysis of female fertility traits in Simmental cattle

Female fertility is an economically important trait in dairy and dual-purpose cattle, but genetic improvement is hindered by its low heritability and complex biological background, especially in Simmental populations. This study aimed to identify the most appropriate fixed-effect model structure for estimating genetic parameters for five female fertility traits in Polish Simmental cows and to estimate variance components within that framework. The traits examined were: interval from first to last insemination (FLI), non-return rate within 56 days after first insemination (NR56), interval from calving to first insemination (CFI), days open (DO), and calving interval (CI), across the first three parities. The dataset comprised 16 860 calving records from 10 041 cows born between 2002 and 2021 that calved in 197 herds. The data included records from the first three calvings of cows. Approximately 50% of the cows contributed records from two or three calvings. Four candidate models differing in fixed-effect and age covariate specification were compared using information criteria; model selection addressed the covariate structure within each analytical framework (linear mixed model for continuous traits; logistic regression for NR56) and does not imply identification of a universally optimal statistical model. Variance components were estimated using a five-trait repeatability linear-threshold animal model incorporating random additive genetic and permanent environmental effects. Fertility traits defined before conception (FLI, NR56) were most favourable in first-parity cows, which required fewer inseminations and had shorter FLI. Postcalving interval traits decreased with advancing parity: CFI shortened by approximately 5 days, DO by over 10 days, and CI by approximately 11 days. Heritability estimates were low for all traits: FLI 0.013 (SE = 0.006), NR56 0.022 (SE = 0.008), CFI 0.028 (SE = 0.009), DO 0.028 (SE = 0.008), and CI 0.029 (SE = 0.008). Repeatability ranged from 0.09 (CFI) to 0.22 (FLI). Most genetic correlations were moderate to high. The strongest positive correlation was between DO and CI (0.96, SE = 0.015), indicating near-genetic equivalence of these two traits. NR56 was negatively correlated with all interval traits; the strongest negative correlation was with FLI (-0.87, SE = 0.073), reflecting that higher early conception success is associated with shorter insemination intervals. The genetic correlation between NR56 and CFI was weak and imprecisely estimated (-0.35, SE = 0.248). These results constitute the first multiparity, repeatability-model genetic parameter estimates for female fertility in Simmental cattle and provide a reference for national genetic evaluations of this dual-purpose breed.