Putative urinary pheromone of bulls involved with breeding performance of primiparous beef cows in a progestin-based estrous synchronization protocol

¹ Department of animal and Range Sciences, Montana State University, Bozeman 59717

^* To whom correspondence should be addressed. E-mail: jgb{at}montana.edu.

	Abstract

The objective was to determine if factors associated with the biostimulatory effect of bulls alter breeding performance of primiparous, suckled beef cows using a progestin-based estrous synchronization protocol. We tested the hypotheses that the estrous synchronization response and AI pregnancy rates differ among cows exposed to bulls, continuously exposed to bull urine, and exposed to fence-line contact of bulls, or cows not exposed to bulls or bull urine. Data were collected from 3 experiments (Exp.) performed over consecutive years. Cows were assigned to: bull exposure (BE; n = 26) or no bull exposure (NE; n = 25) in Exp. 1; bull urine exposure (BUE; n = 19) or steer urine exposure (SUE; n = 19) in Exp. 2; and, fence-line contact with bulls (BFL; n = 26) or no bull exposure (NE; n = 26) in Exp. 3. Synchronization protocols in each experiment included the use of controlled internal drug release device (CIDR; d -10), PGF₂ (d -3), and GnRH and fixed-time AI (TAI; d 0). Cows that were observed in estrus by 60 h after PGF₂ were inseminated 12 h later. Cows not observed in estrus by 60 h after PGF₂ were TAI at 72 h and given GnRH (100 µg). Pregnancy was determined by ultrasonography 35 d after TAI. In Exp. 1, 2, and 3 cows were exposed directly to bulls, bull urine, or bull fence-line contact for 35, 64, and 42 d, respectively. Data were analyzed between treatments within each experiment. Proportion of estrous cycling cows did not differ between treatments at the start of each experiment; however, more (P < 0.05) BE and BFL cows were estrous cycling at the start of the estrous synchronization protocol than NE cows in Exp. 1 and 3. Proportion of cows that showed estrus and interval to estrus after PGF₂ did not differ between treatments in Exp. 1 and 3. However, in Exp. 2, more BUE cows tended (P = 0.09) to have shorter intervals to estrus and exhibit estrus after PGF₂ than SUE cows. Overall AI pregnancy rates were greater (P < 0.05) for BE and BUE cows than NE and SUE cows in Exp. 1 and 2, respectively. There was no difference in AI pregnancy rates between BFL and NE cows in Exp. 3. Presence of bulls and exposure to bull urine appeared to improve breeding performance of primiparous beef cows using a progestin-based estrous synchronization protocol, whereas, fence-line bull exposure is insufficient to cause this biostimulatory effect. We propose that a novel urinary pheromone of bulls may be responsible for the enhancement of fertility in the postpartum cow.

Key Words: biostimulation, bovine, breeding performance, CIDR, pheromone, postpartum