Genetic relationships between twinning rate and 305-days milk production traits in Italian Holsteins

: Twinning is associated with metabolic and reproductive disorders in dairy cows and health issues in calves, with unavoidable effect on herd profitability. To explore the feasibility of selecting against twinning or stabilizing its occurrence, it is crucial to evaluate the genetic relationship with key production traits to avoid any unintended negative consequences. The aim of the present study was to estimate the genetic and phenotypic correlations of twinning rate (TR) with 305-d milk, protein, and fat yields, and protein and fat percentages in Italian Holstein cows. Data included 1,086,917 calving records from 514,702 cows in 924 herds spanning the period from 1992 to 2022. (Co)variance components and genetic parameters were estimated using linear-linear (LL) and threshold-linear (TL) models, which included parity as fixed effect and herd-year-season, permanent environmental, additive genetic, and the residual as random effects. The genetic correlations between TR and milk production traits were weak and consistent under both LL and TL approaches. Specifically, genetic correlations of TR with milk yield, protein yield, fat yield, protein percentage, and fat percentage estimated with the TL approach were 0.129, 0.147, 0.055, 0.028, and -0.081, respectively, and those estimated with the LL approach were 0.106, 0.125, 0.043, 0.031, and -0.069. The phenotypic correlations were weak too (-0.025 to 0.061). Whereas the genetic trend of TR showed a slight increase (1-3 SD units) toward the early 2000s followed by stability throughout most of the study period, the phenotypic trend was inconsistent, with a modest overall increase from 2.36% in 1992 to 2.54% in 2022. Altogether, these results suggest that past breeding decisions coupled with on-farm management practices may have unintentionally led to a progressive increase of TR in the Italian Holstein population. Nonetheless, the weak genetic correlations between TR and milk traits suggest that the incidence of multiple births could be potentially reduced or stabilized through targeted breeding programs without detrimental effects on production traits.