317 Investigation of ovarian reserve parameters in Bos indicus cows

Abstract

Beef heifers that give birth within the first 21 d of their first calving season have greater lifetime productivity and longevity. A favorable relationship exists between the number of antral follicles detectable by ultrasonography and calving date in beef heifers, indicating greater lifetime fertility in cows with more antral follicles. In contrast, Bos indicus cows have more surface antral follicles than Bos taurus cows but exhibit no advantage in fertility. Prior studies demonstrated that despite having more antral follicles, Bos indicus cows have equal numbers of primordial follicles per ovary as Bos taurus cows. Bos indicus cows do have heavier ovaries with more stromal tissue than Bos taurus cows. This study served to further compare preantral follicle populations between Bos indicus and Bos taurus cows to test the specific hypothesis that number of primordial follicles per gram of ovarian tissue are decreased in Bos indicus cows compared with Bos taurus cows. Ovaries were collected from Bos indicus cows (n = 7) and Bos taurus cows (n = 42), weighed, fixed, and sectioned for histological evaluation to determine the number of primordial, primary, and secondary follicles per ovary. The total number of follicles per ovary was divided by the weight of the ovary to determine the number of follicles per gram of ovarian tissue. Data were analyzed using the GLM procedure of SAS (SAS Inst. Inc., Cary, NC) with species as a class effect. Ovaries from Bos indicus cows were heavier than ovaries from Bos taurus cows (12.6 ± 1.1 vs. 7.0 ± 0.5 g, respectively; P < 0.01). No difference was found between Bos indicus and Bos taurus cows in the number of primordial follicles per ovary (P = 0.64); however, Bos indicus cows had a greater number of primary and secondary follicles per ovary (P < 0.01). Upon being adjusted for ovarian weight, Bos indicus cows tended to have fewer primordial follicles per gram of ovary than Bos taurus cows (P = 0.06) but still had greater numbers of primary follicles per gram of ovary (P < 0.01). This suggests that the mechanisms controlling primordial follicle activation are different between Bos indicus and Bos taurus cows. From these results, we conclude that Bos indicus cows do not display enhanced fertility over Bos taurus cows because despite having more antral follicles, Bos indicus cows do not have larger ovarian reserves in comparison to the size of their ovaries. The USDA is an equal opportunity provider and employer.