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This Article Full Text (PDF) All Versions of this Article: jas.2008-1728v1 87/6/1971    most recent Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Cushman, R. A. Articles by Freetly, H. C. Search for Related Content PubMed PubMed Citation Articles by Cushman, R. A. Articles by Freetly, H. C.

Evaluation of antral follicle count and ovarian morphology in crossbred beef cows: Investigation of influence of stage of the estrous cycle, age, and birth weight

R. A. Cushman^*, M. F. Allan^*, L. A. Kuehn^*, W. M. Snelling^*, A. S. Cupp and H. C. Freetly^*

^* USDA, ARS, Roman L. Hruska U.S. Meat Animal Research Center, P.O. Box 166, Clay Center, NE 68933 Department of Animal Science, University of Nebraska-Lincoln, Lincoln, Nebraska 68583-0908

Bob.Cushman{at}ars.usda.gov

Abstract

Depletion of the ovarian reserve is associated with reproductive senescence in mammalian females, and there is a positive relationship between the size of the ovarian reserve and the number of antral follicles on the surface of the ovary. Therefore, we conducted a series of experiments to investigate the influence of stage of the estrous cycle, age, and birth weight on antral follicle counts (AFC) in beef cows and heifers. Pairs of ovaries were collected from crossbred beef cows at slaughter (n = 72) or at necropsy (n = 333; 0 to 11 yr of age); all visible antral follicles were counted, the ovaries were weighed, and stage of the estrous cycle was estimated based on ovarian morphology. There was no influence of estimated stage of the estrous cycle on AFC (P = 0.36). There was a small but positive effect of birth weight on AFC [AFC = -1.7 + 0.31(birth weight); P = 0.007, r-squared = 0.05]. When antral follicle counts were regressed on age, there was a quadratic effect of age such that AFC increased until 5 yr of age and decreased thereafter [AFC = 12.9 + 9.0(yr) – 0.86(yr²); P < 0.001, r-squared = 0.22]. In a third experiment, crossbred beef heifers (n = 406; 353 to 463 d of age) at 3 locations were subjected to ovarian ultrasonography on unknown day of the estrous cycle. Heifers were classified as low AFC (<15 follicle, n = 84) or high AFC (>24 follicles, n = 178). While estimated stage of the estrous cycle did not influence AFC (P = 0.62), heifers classified as low AFC had smaller ovaries (P = 0.001), lower birth weight (P = 0.003), and a decreased heifer pregnancy rate (P = 0.05) when compared to heifers in the high AFC group. From these results, we conclude that AFC in beef cows and heifers is influenced by birth weight and age but not by stage of the estrous cycle. In beef cows, the number of antral follicles increases to 5 yr of age and then begins to decline. This may indicate that a decrease in fertility due to decline of the ovarian reserve may begin earlier than previously thought in beef cows.

Key Words: age • antral follicle count • beef cow • birth weight • fertility