Repeatability of antral follicle count during early gestation in dairy cattle

Introduction and Objectives. The number of ovarian antral follicles ≥3 mm in diameter (antral follicle count, AFC) in cattle is highly repeatable in individuals, while being highly variable among animals [1]. Furthermore, AFC is positively associated with ovarian function and fertility in cattle [2]. However, the mean numbers of follicles per month gradually decline from month 7 to month 9 of pregnancy [3]. The ability to phenotypically classify cows based on AFC with a single examination is highly desirable, hence the objective of this study was to investigate if AFC was influenced by pregnancy status and stage of gestation. Materials and Methods. Transrectal ultrasonography of the reproductive tract was performed as part of the routine reproductive management in a commercial dairy farm on 47 pregnant dairy cows, aged from 1.3 to 4.9 years. Examinations took place before pregnancy establishment (on a random day of the cycle or at the time of artificial insemination, AI±5 days), on the day of pregnancy diagnosis (Day 27-39) and on the day fetal sex assessment (Day 58-68). Each ultrasonography was recorded and subsequently analyzed to determine AFC. The correlations between AFC during different stages of gestation and between AFC and age, parity, body condition score (BCS) were analyzed using ANOVA. Results. Antral follicle count ranged from 5 to 39 with a mean (±SEM) of 13.21±1.02 and was not influenced by age, parity or days from calving; AFC was not related to pregnancy status, day of gestation or body condition score. Conclusions. Antral follicle count is a phenotypic biomarker that can be reliably assessed during early gestation in cattle. Cows with high or low numbers of follicles can be reliably identified through transrectal ultrasonography performed on a single examination on a random day during the first trimester of gestation. The variation of AFC within animal during later stages of gestation remains to be determined. References. [1] Burns DS, Jimenez-Krassel F, Ireland JL, Knight PG, Ireland JJ. Numbers of antral follicles during follicular waves in cattle: evidence for high variation among animals, very high repeatability in individuals, and an inverse association with serum follicle-stimulating hormone concentrations. Biol Reprod. 2005;73:54-62. [2] Ireland JJ, Smith GW, Scheetz D, Jimenez-Krassel F, Folger JK, Ireland JL, et al. Does size matter in females? An overview of the impact of the high variation in the ovarian reserve on ovarian function and fertility, utility of anti-Müllerian hormone as a diagnostic marker for fertility and causes of variation in the ovarian reserve in cattle. Reprod Fertil Dev. 2011;23:1-14. [3] Ginther OJ, Kot K, Kulick LJ, Martin S, Wiltbank MC. Relationships between FSH and ovarian follicular waves during the last six months of pregnancy in cattle. J Reprod Fertil. 1996;108:271-9. Acknowledments. Funded by the Italian Ministry of University and Research, Rita Levi Montalcini Grant 2010.