Animal Science Department


Date of this Version



Vraspir, R. A. 2014. Effects of pubertal status and number of estrous cycles prior to breeding in beef heifers, average daily gain on reproductive performance and comparison of two fixed time AI estrus synchronization protocols.


A THESIS Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Master of Science, Major: Animal Science, Under the Supervision of Professors Richard N. Funston and Jennifer R. Wood. Lincoln, NE: August, 2014

Copyright (c) 2014 Rebecca A.Vraspir


Reproductive success is the most important factor in beef cattle production and is affected by timing of pubertal onset in heifers, and reproductive biotechnologies utilized. Three studies were conducted to evaluate the effects of pubertal status, ADG, and 2 fixed time AI protocols on reproductive success in beef heifers.

In the first study, heifers that were pubertal prior to breeding had a greater AI and overall pregnancy rate; produced more calves born within the first 21 d of the calving season; and weaned older, heavier calves than non-pubertal heifers. Additionally, heifers that had ≥2 estrous cycles, prior to the first breeding season, had a greater second season pregnancy rate than those heifers that had 0 or 1 estrous cycles prior to the first breeding season.

In the second study, as ADG increased leading up to breeding the odds of attaining puberty increased for Gudmundsen Sandhills Laboratory heifers. For West Central Research and Extension Center heifers, as ADG increased the odds of puberty attainment decreased. Odds of pregnancy were affected by BW gain and pubertal status interaction; however, pubertal status had the greatest influence on increasing the odds of pregnancy.

In the third study, 2 progestin-based fixed time AI protocols, MGA and 14-d CIDR, were compared to evaluate pregnancy rates. Fixed time AI pregnancy rate and final pregnancy rate was similar between MGA and 14-d CIDR. An economic analysis suggested synchronizing heifers with MGA was a more cost-effective treatment.

Advisors: Richard N. Funston and Jennifer R. Wood