Relationship of anogenital distance with fertility in nulliparous Holstein heifers

摘要

Anogenital distance (AGD), defined as the distancefrom the center of the anus to the base of the clitoris,in lactating dairy cows of first and second parity,has been reported to be inversely related to fertilityand moderately heritable. Thus, AGD may be a usefulreproductive phenotype for future genetic selectionto improve fertility. The objectives of this study wereto (1) characterize AGD in nulliparous dairy heifers;and (2) determine if the inverse relationship betweenAGD and fertility, found in lactating dairy cows, is alsoevident in nulliparous heifers. We measured AGD in1,692 Holstein heifers from 16 herds in Western Canada(Alberta and British Columbia) and one herd in theUnited States (Washington State). Data were analyzedusing MEANS, UNIVARIATE, LOGISTIC, ROC,GLIMMIX, and LIFETEST procedures of SAS (SASInstitute Inc.). Mean (±standard deviation) age atAGD measurement was 13.9 ± 1.5 mo, and AGD wasnormally distributed with a mean of 107.3 ± 10.5 mm,ranging from 69 to 142 mm. With every 1-mm increasein AGD, the predicted probability of pregnancy was reducedby 1.9%. Receiver operating characteristic curveanalysis was used to determine the optimum thresholdAGD that predicted the probability of pregnancy. Basedon the optimum threshold AGD, data from heifers werecategorized into short (≤110 mm) and long (>110 mm)AGD groups, and associations between AGD groupsand fertility measures were determined. Heifers withshort AGD required fewer services per conception (1.5vs. 1.7) than heifers with long AGD. Consequently, heiferswith short AGD conceived earlier (448.4 vs. 454.3d) and had greater pregnancy to first AI than thosewith long AGD (58.3 vs. 49.6%). Moreover, heifers withlong AGD had reduced hazard (hazard ratio of 0.59)for pregnancy up to 450 d of life compared with thosewith short AGD. In summary, AGD was normally distributedand highly variable in the population. In addition,an inverse relationship between AGD and fertilitymeasures in nulliparous heifers was evident, confirmingan earlier report of a similar relationship in lactatingdairy cows. These findings strengthen the potential forAGD to be used as a fertility trait and managementtool in future selection programs.