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Title: 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

Author
item Cushman, Robert - Bob
item Allan, Mark
item Kuehn, Larry
item Snelling, Warren
item CUPP, ANDREA - University Of Nebraska
item Freetly, Harvey

Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/16/2009
Publication Date: 6/1/2009
Citation: Cushman, R.A., Allan, M.F., Kuehn, L.A., Snelling, W.M., Cupp, A.S., Freetly, H.C. 2009. 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. Journal of Animal Science. 87(6):1971-1980.

Interpretive Summary: Depletion of follicles in the ovary is associated with reproductive aging and decline in fertility in mammalian females. The present study tested the hypotheses that a low birth weight would result in smaller ovaries and fewer antral follicles within the ovarian cortex at birth and that antral follicle counts (AFC) decrease as beef cows age. Pairs of ovaries were collected from 333 crossbred beef cows (0 to 11 years of age) at necropsy, all visible antral follicles were counted, and the ovaries were weighed. There was a small but positive effect of birth weight on AFC. Antral follicle counts and total ovarian weight (TOW), defined as the sum of the weight in grams of the two ovaries, were regressed on age. There was a quadratic effect of age on AFC such that AFC increased until 5 years of age and decreased thereafter. The change in TOW was also quadratic. There was a positive relationship between follicle count and ovarian weight that may indicate that a larger ovarian cortical mass is able to support the growth of a greater number of follicles. Within an animal, there was a strong positive relationship between the number of follicles in the left ovary and the number of follicles in the right ovary, suggesting that when using ultrasonography to assess follicle numbers, one may be able to save time and labor by only examining one ovary. In a subsequent experiment, crossbred beef heifers (n = 406; 353 to 463 d of age) at three locations were subjected to ovarian ultrasonography on unknown d 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, heifers classified as low AFC had smaller ovaries, lower birth weight, and a decreased heifer pregnancy rate 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 suggests that a decrease in fertility due to decline of the ovarian reserve may begin earlier than previously thought in beef cows.

Technical 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 yrs 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-square = 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(yr**2); P < 0.001, r-square = 0.22]. In a third experiment, crossbred beef heifers (n = 406; 353 to 463 d of age) at three locations were subjected to ovarian ultrasonography on unknown d 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 suggests that a decrease in fertility due to decline of the ovarian reserve may begin earlier than previously thought in beef cows.