Patterns of tonic luteinizing hormone release and ovulation frequency in suckled anestrous beef cows following varying intervals of temporary weaning

Abstract

Thirty postpartum Brahman crossbred cows were utilized to determine the effects of varying intervals of temporary weaning on tonic LH secretion and ovulation. Cows were assigned randomly on day 17–21 postpartum to one of five groups: 1) Suckled Ad libitum, 2) 48-hr weaned, 3) 72-hr weaned, 4) 96-hr weaned, or 5) 144-hr weaned. The mean maximal rise in LH pulse frequency due to weaning occurred within 2 days and averaged 221 percent of time 0 values. The frequency of LH pulses was greater (P<.06) in weaned than in suckled controls. This temporal increase was self-limiting, displaying an acute rise followed by a variable rate of decline in all groups. However, pulse frequency remained elevated relative to suckled controls for the longest period of time for weaning durations of 96 and 144 hr (P<.10). In 48-hr and 72-hr weaned cows, a rapid reversal of the initial increase in LH pulse frequency was observed following calf return. A significant linear regression (y = 1.9 ± .64x; P<.03) described the increase in LH pulse frequency that occurred in cows which ovulated following weaning. Nonovulators were sensitive to calf return and responded by exhibiting a linear decline (y = 2.87 − .43x; P<.04) in LH pulse frequency following this event. The amplitude of LH pulses increased (P<.02) during the period after calf return in ovulators, but did not change in nonovulators. Percentage ovulating by day 10 increased (P<.05) with increased weaning duration past 72 hr. We conclude that calf return before 96 hr markedly attenuates weaning-induced increases in LH secretion and ovulation.