|Cushman, Robert - Bob|
Submitted to: Theriogenology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/15/2015
Publication Date: 9/3/2015
Publication URL: http://handle.nal.usda.gov/10113/61418
Citation: McNeel, A.K., Cushman, R.A. 2015. Influence of puberty and antral follicle count on calving day in crossbred beef heifers. Theriogenology. 84:1061-1066.
Interpretive Summary: Identification of quality replacement beef heifers can be difficult and costly for many cattle producers. Combining assisted reproductive technologies may offer a means of maximizing a producer’s return on investment and help improve the sustainability of beef production. In this experiment we evaluated the reproductive tract of 95 cross-bred beef heifers once a month for the three months immediately preceding the breeding season. Heifers were randomly assigned to either the Control or PGF (prostaglandin F2 alpha) treatment group, with PGF heifers receiving a single injection of 25 mg prostaglandin immediately following their May exam (approximately 13 months of age). Heifers were then managed according to standard heifer management protocols for the US Meat Animal Research Center. The effects of treatment and reproductive tract development were assessed on production data for the following production year. Our data demonstrate that the use of a single injection of prostaglandin is not sufficient to shorten the overall calving date in the PGF group compared to the Control group. Furthermore, these findings indicate that differences in the number of germ cells within an animal’s ovaries are associated with an earlier calving day.
Technical Abstract: The ability of a cow to produce a calf every 12 months beginning at 24 months of age is one of the primary factors contributing to the long-term profitability of a cow-calf operation. Cows that calve early in the calving season have the greatest productive lifespan, but which factors contribute the most to early calving remain uncertain. Age at puberty and ovarian antral follicle count are two factors that may contribute to the reproductive success of replacement heifers. Accurate identification of pubertal replacement heifers can be time-consuming and costly for most cow-calf operations. Assessing the development of the reproductive tract by ultrasonography (including antral follicle counts), and when coupled with estrus synchronization may maximize the return on investment for the producer. The objective of this study was to evaluate the effects of 1) age at puberty on calving day during the first calving season, 2) number of antral follicle counts during pre-breeding examination on first calving day, and 3) a single injection of PGF2a prior to the start of the first breeding season on production performance in the first year following treatment. Our results indicate that those yearling heifers pubertal by two months prior to the start of the breeding season were more likely to be pregnant than non-pubertal animals (P=0.02). There was a negative correlation between antral follicle count and subsequent first calving day (P=0.042) demonstrating that heifers with more antral follicles calve earlier in the calving season. Those animals calving within the first 21 days of the calving season possessed more antral follicles during the pre-breeding examination than those animals calving in the last 21 days of the calving season (P=0.0292). Treatment with prostaglandin resulted in an earlier calving day during the first 21 days of the calving season (P=0.0142). These findings indicate that while it is possible to shift calving day using a single injection of prostaglandin, the effects are confined to the first 21 days of the season. Furthermore, selection of replacement heifers for greater antral follicle counts may be a more effective way of maximizing the number of animals calving early in the calving season.