PS1-17. Reproductive Tract Score Outperforms Anogenital Distance for Predicting Pregnancy in Beef Heifers.

Abstract Anogenital distance (AGD) has been found to be inversely correlated to probability of pregnancy to artificial insemination (AI) in dairy cattle. Use of AGD as a pre-breeding fertility biomarker in beef cattle would be of great value, but its accuracy relative to reproductive tract score (RTS), the current industry standard, has not been tested at scale. Our objective was to compare AGD and RTS as predictors of timed-artificial insemination (TAI) pregnancy outcome in replacement beef heifers. A total of 1,262 Bos taurus commercial beef heifers across four U.S. states and 3 seasons (spring, summer, and fall) from 2023 to 2025 were enrolled. All heifers underwent controlled internal drug release (CIDR)-based estrus synchronization (ES) and received TAI. Body weight, AGD, and RTS were collected during ES, and TAI pregnancy status was recorded 35 to 45 days post-TAI by transrectal ultrasonography. Individual AGD was measured with a digital caliper from the center of the anus to the base of the clitoris. Heifer RTS was assessed on a 1–5 scale by uterine horn diameter and tone and palpable ovarian structures: 1=immature, < 20 mm, no tone, no palpable structures; 2=20–25 mm, no tone; 3=20–25 mm with slight tone; 4=30 mm with good tone; 5 ≥30 mm with good tone and a corpus luteum. Relationships between RTS, AGD, and pregnancy were measured with Pearson correlations. A generalized linear mixed model (PROC GLIMMIX) was fit to analyze pregnancy outcomes, with the fixed effects of AGD quartile, RTS, and their interaction, body weight as a co-variate, year and season within location as random effects. A parallel GLIMMIX model was fit for AGD with RTS as fixed effect, body weight as co-variate, and the same random effects as previous model. Overall TAI pregnancy rate was 46.2%; mean AGD was 87.9±12.7 mm, and mean RTS was 3.97±1.04. Reproductive tract score correlated with AGD (r = 0.27, P < 0.0001) and TAI pregnancy (r = 0.11, P = 0.0001), but AGD was not correlated with TAI pregnancy (r=–0.04, P = 0.21). There was an effect of RTS on TAI pregnancy (P = 0.0006), which increased from RTS 2 to 5; corresponding values were 39.3 ± 7.5%, 42.1 ± 5.6%, 51.1 ± 6.1%, and 60.7 ± 5.8% for RTS 2, 3, 4, and 5, respectively. The AGD quartile and the AGD × RTS interaction did not affect TAI pregnancy (P ≥ 0.50). In the parallel AGD model, body weight (P < 0.0001) but not RTS (P = 0.91) explained AGD variance, indicating the AGD–RTS association was mediated by body weight. In conclusion, RTS was a better predictor of AI pregnancy outcomes, and the AGD–RTS correlation was confounded by body weight. These data argue against AGD as a stand-alone fertility biomarker in replacement beef heifers and support continued use of RTS for pre-breeding selection.