Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/30/1996
Publication Date: N/A
Citation: Interpretive Summary: Exposing first-calf cows to mature bulls for 2 h on Day 30 appears to induce a physiological change in the hypothalamo-hypophyseal axis that results in increase in LH concentrations. The stimulus(I) for this physiological effect maybe related to pheromonal cues. Continuous or intermittent exposure maintains elevated LH concentrations presumably by altering GnRH release, the mechanism for this response is not clear. However, the physiological action of the bull to increase LH frequency and concentrations does not appear to be a major factor that reduces the postpartum anestrous interval in first-calf cows because even in the face of elevated LH concentrations and LH pulse frequency interval to estrus was not reduced in intermittently exposed cows as it was in cows continuously exposed to bulls. Exteroceptive cues, other than pheromones may be involved in the biostimulatory effect of bulls on postpartum cows.
Technical Abstract: The objective of this study was to determine if patterns of LH secretion are acutely or chronically affected by the presence of mature bulls. Cows were assigned randomly to be either continuously exposed to (BE; n=20) or isolated from bulls at calving (NE; n=32), or exposed continuously (NEBE; n=10) or intermittently (BEI; n=21) to bulls beginning on Day 30. BEI cows were exposed to bulls for 2 h every 3rd d for 18 d. More (P=.07) BE and NEBE (75%) cows showed estrus by the end of the experiment than BEI and NE cows (48%). Interval to estrus was longer (P<.05) in BEI and NE cows (95.6 +/- 6.1 d) than in BE and NEBE cows (75.9 +/- 6.1 d). Baseline LH and amplitude and duration of LH peaks during the first 6 h after 2 h of bull exposure did not differ (P>.10) among treatments on Day 30 postpartum. However, mean LH and LH pulse frequency were higher (P=.06) for NEBE and BEI cows than in NE cows. Baseline LH and amplitude and peaklength interval of LH peaks did not differ (P>.10) over the seven sampling periods among NEBE, BEI, or NE cows. Mean LH and LH pulse frequency were higher (P<.05) in NEBE and BEI cows than NE cows on each sampling period. Exposing first-calf suckled beef cows to bulls on Day 30 postpartum increased mean LH concentrations by increasing pulse frequency within a short period after 2-h exposure. Thereafter, mean LH concentrations were higher in cows that were either continuously or intermittently exposed to bulls. Although mean LH and LH pulse frequency in NEBE and BEI cows were similar, intervals to estrus after bull exposure differed between treatments. The mechanism whereby bulls alter postpartum interval to estrus appears to involve factors other than just LH.