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Item Factors associated with the biostimulatory effect of bulls on resumption of ovarian cycling activity and breeding performance of first-calf suckled beef cows(Montana State University - Bozeman, College of Agriculture, 2005) Tauck, Shaun Austin; Chairperson, Graduate Committee: James G. Berardinelli.The objective of this research was to evaluate factors associated with the biostimulatory effect of bulls on the resumption of ovarian cycling activity and breeding performance of first-calf suckled beef cows. In Experiment 1, we tested the hypotheses that short-term (30 d) bull exposure before the breeding season does not alter: 1) the proportion of cows that resumed cycling activity; 2) the proportion of cows that responded to estrous synchronization (ES); and, 3) AI and overall pregnancy rates. Resumption of ovarian cycling activity was measured by changes in progesterone patterns at 3 d intervals from the start of the experiment to the start of the breeding season. Cows were synchronized for estrus using an ES protocol that included CIDR, PGF2á, GnRH and time AI (TAI). Breeding performance was measured by: estrous response after PGF2á, and AI and overall pregnancy rates. We found that short-term bull exposure increased the proportion of cows that; were cycling by the end of the exposure period, and were pregnant from AI. Experiment 2 tested the hypothesis that exposure to bull urine does not alter: 1) the interval from exposure to resumption of ovarian cycling activity; 2) the proportion of cows that resumed cycling activity; 3) the proportion of cows that responded to ES; and, 4) AI pregnancy rates. Exposure of cows to bull urine did not alter; the interval from exposure to resumption of ovarian cycling activity, the proportion of cows cycling before the breeding season, and the proportion of cows that responded to ES. However, AI pregnancy rates were improved by exposing cows to bull urine before the breeding season. We conclude that the short-term physical presence of bulls, but not long-term continuous exposure to bull urine, reduced the interval from exposure to the resumption of ovarian cycling activity and increased the proportion of cows cycling before the breeding season. However, short-term exposure to bulls or longterm exposure to bull urine, before the breeding season appeared to enhance breeding performance of first-calf suckled beef cows using an ES protocol that included CIDR, PGF2á, GnRH and TAI.