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

摘要

Using early-in-life markers of reproductive characteristicsmay enhance the speed and success of geneticimprovement in fertility. We investigated 2 phenotypesthat can be measured early in life and are moderatelyheritable to determine their association with traditionalmeasures of reproductive success and genetic variationunder a seasonal-calving, pasture-based system. Cowswere bred to be divergent in the New Zealand FertilityBreeding Value, which estimates genetic merit forfertility. Cows consisted of 2 groups with an averagepositive (+5%) or negative (−5%) genetic merit forfertility traits and were expected to have large diversityin reproductive outcomes. Calves were genotyped at41 ± 3.1 d of age (mean ± SD; n = 538), and antralfollicle counts (AFC) were measured when they werepostpubertal 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 thissame population 50 to 60 d after the breeding startdate when they were 881 ± 25 d old (145-d range). TheAGD was shorter in animals with a positive geneticmerit for fertility traits (based on parent averages). Anindicator of herd reproductive success in a seasonalcalvingsystem (recalving by 6 wk in lactation 2) waschosen for logistic regression with cross-validation, andif significant, a cut-off was calculated that categorizedanimals into groups. Both linear and quadratic regressionwas undertaken, and the model with the greatestsensitivity for detection of nonpregnant cows used. TheAGD linear model was significant with a sensitivity of64% and a specificity of 48%. This model resulted ina cut-off of 102 mm, which we used to classify cows asshort (≤102 mm) or long (>102 mm) AGD animals.Primiparous cows with a short AGD were more likelyto be pregnant within the first 3 and 6 wk of mating,and become pregnant as a primiparous cow, than thosewith a long AGD. The time from calving to conceptionwas 20 d earlier in short AGD compared withlong-AGD cows. None of the models tested for AFCwere significant; therefore cows were categorized into3 groups based on previous work in seasonal systems.However, associations between fertility phenotypes andAFC group were limited. Genomic regions of interest forAGD and AFC did not overlap, indicating phenotypeswere genetically independent. Overall, AGD appears asa promising early marker of fertility in seasonal grazingsystems.