Artificial insemination conception rates in primiparous, suckled cows exposed to the biostimulatory effect of bulls before and during a GnRH-based estrus synchronization protocol

¹ Montana State University-Bozeman, Bozeman 59717

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

	Abstract

The objective was to evaluate whether exposing primiparous, suckled beef cows to the biostimulatory effect of bulls alters breeding performance associated with an estrus synchronization protocol that included GnRH followed 7 d later by PGF₂ and fixed-time AI (TAI). This is a composite analysis of 3 experiments that evaluated: 1) effects of bull exposure at different days after calving (Yr 1); 2) biostimulatory effects of bull excretory products (Yr 2); and, 3) biostimulatory effects of familiar and unfamiliar bulls (Yr 3), on resumption of ovarian cycling activity. In all studies, cows were exposed (biostimulated; n = 94) or not exposed (non-biostimulated; n = 67) to bulls or excretory products of bulls for at least 60 d before the start of the estrus synchronization protocol. Average calving day did not differ among years and was 52 ± 5 d. Year did not affect proportions of biostimulated and non-biostimulated cows that were cycling at the start of the estrus synchronization protocol; however, a greater (P < 0.001) proportion of biostimulated than non-biostimulated cows were cycling at this time. In each year, cows were given GnRH followed by PGF₂ 7 d later. Cows were observed for estrus twice daily (am:pm) after PGF₂. Cows that exhibited estrus before 54, 60, and 64 h after PGF₂ were inseminated by AI 12 h later in Yr 1, 2, and 3, respectively. Cows that failed to show estrus were given GnRH and TAI at 62, 72, and 72 h after PGF₂ in Yr 1, 2, and 3, respectively. Conception rates were determined by transrectal ultrasonography 35 d after TAI in each year. Percentages of cows that exhibited estrus after PGF₂ and before TAI, interval from PGF₂ to estrus, and percentages of cows inseminated 12 h after estrus or at TAI did not differ between biostimulated and non-biostimulated cows and were 51%, 54.7 ± 7.3 h, 35%, and 65%, respectively. Conception rates for cows bred by AI 12 h after estrus did not differ between biostimulated and non-biostimulated cows; however, TAI conception rate was greater (P < 0.05) for biostimulated cows (57.6%) than for non-biostimulated cows (35.6%). We conclude that fixed-time AI conception rates in an estrus synchronization protocol that includes GnRH followed 7 d later by PGF₂ may be improved by the biostimulatory effect of bulls in postpartum, primiparous cows.

Key Words: biostimulation, bovine, estrus synchronization, fixed-time AI, postpartum, conception rate

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