On August 12, 2025, calving traits calculated by CDCB for Holstein and Brown Swiss will undergo both a phenotypic and genetic base change. This comes after the routine five-year base change was delayed for the April 2025 evaluations following unanticipated results. Further investigation has validated the initial calculations.
In the calving ease and stillbirth evaluations, a sire-maternal grandsire threshold model is used, where outputs are expressed in terms of probabilities or frequencies of difficult calvings (or stillbirth). In this model, the observed calving difficulty (or stillbirth) is understood as a combination of two factors: genetics (which we aim to improve through selection) and environment (management factors such as feeding and gestation practices). Environmental effects are incorporated into the final PTA because, for example, sires’ daughters calving in a “poor” environment will generally experience more difficult calvings, while daughters calving in a “good” environment will tend to have easier births.
In the Holstein breed, the average frequency of difficult calvings on first-parity cows from bulls born in 2020 (reported as Sire Calving Ease, SCE) is 1.36%, compared to 2.29% for bulls born in 2015. This is reflected as the phenotypic base for the trait. As this average approaches zero, PTA variability has decreased as a consequence. The low incidence of reported dystocic calvings naturally constrains PTAs since values below 0% are biologically impossible. Now is not the time to become lax in selection criteria for SCE, but producers should not be alarmed if minimal variability is seen across bulls in the population.
This difference in SCE is partly due to genetic progress and partly due to management improvements, such as better animal handling and increased use of sexed semen. The combined effect of the genetic and phenotypic base changes will result in a reduction in PTAs (indicating easier calvings) of approximately 0.8% in Holsteins. The opposite trend will be observed in Brown Swiss, where PTAs will reflect an increase in difficult calving of about 0.6%.
The calving traits differ from most other traits that CDCB evaluates because they are categorical and not continuous – a cow either experiences dystocia or not; there is nothing in between. To account for this, they are evaluated with sire/maternal grandsire (MGS) threshold models. The application of a breed-specific phenotypic base also makes these traits unique.
For more details and further explanation, see HERE.
