Associations of routinely collected biometric measurements with pregnancy outcome following fixed-time artificial insemination in Spanish autochthonous beef heifers

Context Performance-testing programs routinely collect bodyweight and linear morphometric measurements, but their usefulness for assessing reproductive readiness in autochthonous beef heifers remains unclear. Aims This study evaluated the associations of age, bodyweight, and linear morphometric measurements with pregnancy, following fixed-time artificial insemination (FTAI) in three Spanish autochthonous beef breeds and determined whether morphometric traits provided additional information beyond bodyweight. Methods Retrospective data were analysed from 102 heifers managed under standardised conditions, including 34 Retinta, 29 Berrenda en Negro, and 39 Berrenda en Colorado. Age, bodyweight, height at the withers, height at the rump, total body length and chest circumference were recorded at initial and prebreeding assessments. Pregnancy was diagnosed 40 days after FTAI, and non-pregnant heifers underwent a second FTAI. Associations with pregnancy following the first FTAI and cumulative pregnancy were evaluated using breed-adjusted logistic regression. Model discrimination was described using the apparent area under the receiver operating characteristic curve (AUC). Key results Pregnancy rates following the first FTAI and after completion of the reproductive program were 55.9% and 81.4% respectively. Bodyweight was positively associated with pregnancy following the first FTAI at both the initial (odds ratio (OR) = 1.22 per 10 kg; 95% confidence interval (CI): 1.08–1.37; P < 0.001) and prebreeding assessments (OR = 1.16; 95% CI: 1.06–1.27; P = 0.002). Bodyweight was also associated with cumulative pregnancy at the initial (OR = 1.23; 95% CI: 1.07–1.42; P = 0.003) and prebreeding assessments (OR = 1.23; 95% CI: 1.09–1.39; P < 0.001). Apparent AUC values for these models ranged from 0.684 to 0.759. Age and linear morphometric measurements were not significantly associated with either primary reproductive outcome, and morphometric traits provided no significant additional information beyond bodyweight. Conclusions Bodyweight was consistently associated with pregnancy outcome, whereas the evaluated linear morphometric measurements were not. However, the modest apparent discrimination and absence of external validation preclude interpreting bodyweight as a standalone predictor. Implications Routinely recorded bodyweight may contribute to a broader assessment of reproductive readiness, but it should be combined with direct indicators of body condition and reproductive maturity.