ACUTE AND CHRONIC CHANGES IN LUTEINIZING-HORMONE SECRETION AND POSTPARTUM INTERVAL TO ESTRUS IN FIRST-CALF SUCKLED BEEF-COWS EXPOSED CONTINUOUSLY OR INTERMITTENTLY TO MATURE BULLS

The objective of this study was to determine whether patterns of LH se cretion are acutely or chronically affected by the presence of mature bulls in postpartum first-calf suckled beef cows exposed to bulls eith er continuously or intermittently beginning on d 30 after birth. Cross bred cows were assigned randomly to be either continuously exposed to (BE; n = 20) or isolated from bulls (NE; n = 32) at calving, or expose d continuously (NEBE; n = 10) or intermittently (BEI; n = 21) to bulls beginning on d 30. The BEI cows were exposed to bulls for 2 h every 3 rd d for 18 d. Ten cows from the NEBE, BEI, and NE treatments only wer e fitted with indwelling jugular catheters. Intensive blood sampling o f NEBE and BEI cows began within 45 min after they were placed with bu lls for 2 h on d 30, and for BEI cows on each d after 2 h of exposure. Samples were collected at 15-min intervals for 6 h beginning on d 30 for NEBE, BEI, and NE cows; sampling continued at 3-d intervals until d 48. Samples were assayed for LH by RIA. Cows were observed twice dai ly (am:pm) for estrus. More (P = .07) BE and NEBE (75%) cows showed es trus by the end of the study than BEI and NE cows (48%). Interval to e strus 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 inte rpeak interval of LH peaks during the first 6 h after 2 h of bull expo sure did not differ (P > .10) among treatments on d 30 after birth. Ho wever, mean LH and LH pulse frequency were higher (P = .06) for NEBE a nd BEI cows than in NE cows. Baseline LH and amplitude and duration of LH peaks did not differ (P > .10) over the seven sampling periods amo ng NEBE, BEI, or NE cows. Mean LH and LH pulse frequency were higher ( P < .05) in NEBE and BEI cows than in NE cows on each sampling period. Exposing first-calf suckled beef cows to bulls on d 30 after birth in creased mean LH concentrations by increasing pulse frequency within a short period after a 2-h exposure. Thereafter, mean LH concentrations were higher in cows that were either continuously or intermittently ex posed to bulls. Although mean LH and LH pulse frequency in NEBE and BE I cows were similar, intervals to estrus after bull exposure differed between treatments. The mechanism whereby bulls alter postpartum inter val to estrus seems to involve other factors that may be related to bu t not directly linked with LH secretion.