Multiple birth in Austrian Holsteins: genetic parameters and impacts on calving, health, fertility, and milk production traits.
Multiple birth (MB) is associated with adverse effects on cow fertility and calf survival, thereby contributing to economic losses for dairy farmers; however, it is not currently included in routine genetic evaluations or selection indices in cattle worldwide. This study aimed to (i) estimate the h2 and repeatability of MB, (ii) assess its genetic and phenotypic associations with calving performance, postpartum health, fertility, and milk production traits, and (iii) quantify its impact on these traits in Austrian Holstein population. After data editing, the analyses used 678,877 records belonging to 262,192 cows across 2,350 dairy farms spanning the period from 2000 to 2024. The h2 and repeatability of MB were estimated using both single-trait linear and threshold repeatability animal models, whereas genetic and environmental correlations between MB and the other traits were estimated using bivariate models within a Bayesian framework implemented via Gibbs sampling. The effects of MB on functional and production traits were quantified using linear and logistic mixed regression models, according to the specific trait. The h2 of MB was low (linear model: 0.017, threshold model: 0.076) as well as repeatability (linear model: 0.044, threshold model: 0.217). Estimated genetic correlations of MB with 305-d milk, protein, and fat yields were weak (∼0.10). Multiple birth was weakly genetically correlated with maternal gestation length (-0.195), maternal calving ease (0.095), and maternal stillbirth (0.150). The interval from calving to first service, the interval from first to last service, the non-return rate at 56 d after first service, days open, and calving interval were all weakly genetically associated with MB. Fertility disorders showed the strongest genetic correlation with MB (0.308), whereas cystic ovaries showed a weak genetic correlation (0.068). Multiple birth was associated with shorter gestation length (-5 d), higher risk of stillbirth and fertility disorders, longer calving interval (16 d), and lower 305-d milk (-67 kg) and fat yield (-7 kg) compared with singleton birth. The phenotypic trend indicated that MB increased slightly from 3.01 to 3.34% between 2000 and 2024, whereas the genetic trend remained largely stable throughout the study period, suggesting that past selection decisions had little or no effect on the genetic merit for MB. Overall, the findings of the present study indicate that selection aimed at reducing or stabilizing MB would not substantially impair milk production traits and may provide indirect benefits for fertility and health traits in the long term.