Investigating anogenital distance and antral follicle count as novel markers of fertility within a herd of cows with positive or negative genetic merit for fertility traits

Abstract

Using early-in-life markers of reproductive characteristics may enhance the speed and success of genetic improvement in fertility. We investigated 2 phenotypes that can be measured early in life and are moderately heritable to determine their association with traditional measures of reproductive success and genetic variation under a seasonal-calving, pasture-based system. Cows were bred to be divergent in the New Zealand Fertility Breeding Value, which estimates genetic merit for fertility. Cows consisted of 2 groups with an average positive (+5%) or negative (-5%) genetic merit for fertility traits and were expected to have large diversity in reproductive outcomes. Calves were genotyped at 41 ± 3.1 d of age (mean ± SD; n = 538), and antral follicle counts (AFC) were measured when they were postpubertal heifers before their first breeding (416 ± 15 d old; 92-d range; n = 520). The anogenital distance (AGD) was measured in 478 primiparous cows of this same population 50 to 60 d after the breeding start date when they were 881 ± 25 d old (145-d range). The AGD was shorter in animals with a positive genetic merit for fertility traits (based on parent averages). An indicator of herd reproductive success in a seasonal-calving system (recalving by 6 wk in lactation 2) was chosen for logistic regression with cross-validation, and if significant, a cut-off was calculated that categorized animals into groups. Both linear and quadratic regression was undertaken, and the model with the greatest sensitivity for detection of nonpregnant cows used. The AGD linear model was significant with a sensitivity of 64% and a specificity of 48%. This model resulted in a cut-off of 102 mm, which we used to classify cows as short (≤102 mm) or long (>102 mm) AGD animals. Primiparous cows with a short AGD were more likely to be pregnant within the first 3 and 6 wk of mating, and become pregnant as a primiparous cow, than those with a long AGD. The time from calving to conception was 20 d earlier in short AGD compared with long-AGD cows. None of the models tested for AFC were significant; therefore cows were categorized into 3 groups based on previous work in seasonal systems. However, associations between fertility phenotypes and AFC group were limited. Genomic regions of interest for AGD and AFC did not overlap, indicating phenotypes were genetically independent. Overall, AGD appears as a promising early marker of fertility in seasonal grazing systems.