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

Abstract

The objective of this study was to determine whether patterns of LH secretion are acutely or chronically affected by the presence of mature bulls in postpartum first-calf suckled beef cows exposed to bulls either continuously or intermittently beginning on d 30 after birth. Crossbred cows were assigned randomly to be either continuously exposed to (BE; n = 20) or isolated from bulls (NE; n = 32) at calving, or exposed 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 3rd d for 18 d. Ten cows from the NEBE, BEI, and NE treatments only were fitted with indwelling jugular catheters. Intensive blood sampling of NEBE and BEI cows began within 45 min after they were placed with bulls for 2 h on d 30, and for BEI cows on each d after 2 h 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 daily (am:pm) for estrus. More (P = .07) BE and NEBE (75%) cows showed estrus by the end of the study 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 interpeak interval of LH peaks during the first 6 h after 2 h of bull exposure did not differ (P > .10) among treatments on d 30 after birth. 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 duration 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 in NE cows on each sampling period. Exposing first-calf suckled beef cows to bulls on d 30 after birth increased 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 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 seems to involve other factors that may be related to but not directly linked with LH secretion.