The anogenital ratio as an indicator of reproductive performance in dairy heifers

The anogenital distance (AGD) is considered a marker for prenatal androgen exposure and fertility in multiple species including humans. In dairy cattle, it is described as the length between the center of the anus and the clitoral base (AGDc). However, in other species, the distance from the center of the anus to the dorsal commissure of the vulva (AGDv) is also considered to be a predictor for fertility traits, as well as the anogenital ratio (AGR, defined as [AGDv/AGDc]*100). The primary aim of the present study was to assess whether AGDv and AGR can be used as an indicator for reproductive performance in dairy heifers. Additionally, the relation between AGDv and AGDc and the correlation with other body measurements were explored. Data of 656 Holstein Friesian heifers at an age of 13.5 ± 1.08 months were analyzed. Respective means of 62.9 ± 8.20 mm (AGDv) and 107.6 ± 9.27 mm (AGDc) were recorded. The mean AGR ratio was calculated as 58.6 ± 6.75%, varying from 37.3 to 79.6%. The age of the heifers was not associated with any of the AGD measurements nor the ratio. Except for a very low correlation between heart girth and AGDc (r = 0.09, P < 0.05), both AGDs were largely uncorrelated with other body measurements. Linear regression models revealed that AGDc was not associated with any of the recorded fertility parameters. However, results revealed a negative association between AGDv and AGR and reproductive performance: heifers with a short AGDv and small AGR were younger at first AI (P ≤ 0.003) and at conception (P = 0.004). Based on ROC curve analyses, AGDv was the best indicator for pregnancy to first AI, with a threshold estimated at 65.3 mm. The pregnancy rate at first AI was 72.4% in heifers with a short AGDv (<65.3 mm, n = 413) compared to 61.7% in heifers with a long AGDv (≥65.3 mm, n = 243). Hence, short-AGDv heifers had 63% higher odds to conceive at first AI compared to their long-AGDv counterparts (P = 0.004). Additionally, an AGR threshold of 59,6% was determined: heifers with a small AGR (<59.6%) had 44% higher odds to be pregnant at first AI compared to heifers with an AGR ≥59.6%. Results of the present study suggest to consider AGDv and AGR as potential indicators for reproductive performance in dairy heifers. The latter implies that it is relevant to measure both AGDc and AGDv in future studies. The absence of correlation between body- and AGD-measurements furthermore suggests that AGD sizes are rather pre-than postnatally determined.