INFLUENCE OF STAGE OF LACTATION, EXOGENOUS LUTEINIZING-HORMONE-RELEASING HORMONE, AND SUCKLING ON ESTRUS, POSITIVE FEEDBACK OF LUTEINIZING-HORMONE, AND OVULATION IN SOWS TREATED WITH ESTROGEN

The overall objective of this study was to investigate the LH response and occurrence of estrus and ovulation in sows treated with estradiol benzoate (EB) at mid- (d 14) or late (d 28) lactation. Experiment 1 tested the hypothesis that hourly pulses of LHRH given to sows treated with EB would cause a greater proportion to exhibit LH surges. Sows (n = 32) at mid- or late lactation were given (i. m.) corn oil (CO) or EB (10 mug/kg BW) at 0 h and no pulses or hourly pulses (100 ng/pulse) of the LHRH agonist des-GLY10,[D-ALA6]-LHRH ethylamide (LHRH-A) from 54 to 96 h, the anticipated period of positive feedback of LH. Blood samples for determination of estradiol-17beta (E2) and LH were collected every 6 h from -12 to 120 h from sows treated with EB. Estrus was checked daily with a mature boar, and ovulation was assessed by measuring progesterone in blood 1 wk after estrus induced by EB or 2 wk after weaning at 34 +/- 1.4 d postpartum. Lactational estrus occurred in 0 of 16 and 15 of 16 sows given CO and EB, respectively; however, no sow ovulated in response to EB. Concentrations of LH were lower ( P < .01) during mid- than during late lactation, and LHRH-A pulses had no effect on secretion of LH and E2. The LH concentrations during the pretreatment (-12 to 0 h), negative feedback (6 to 54 h), and positive feedback (60 to 96 h) periods at mid-lactation (.32 +/- .03, .22 +/- .01, and .54 +/- .08 ng/mL, respectively) differed (P < .01) from values at late lactation (.80 +/- .06, .49 +/- .02, and 1.47 +/- .17, respectively). Preovulatory-like surges of LH occurred in one of eight (peak 2.1 ng/mL) and seven of eight (peak 2.9 +/- 1 ng/mL) sows given EB at mid- and late lactation, respectively. Experiment 2 tested the hypothesis that the hypothalamus and pituitary of sows weaned 96 h before they were given EB would show a greater responsiveness than those weaned and given EB simultaneously. Sows (n = 16) at mid- or late lactation were treated with EB (10 mug/kg BW; i.m.), and one-half of the sows were suckled until the moment of EB treatment, whereas the other half had their litters weaned 96 h before treatment with EB. All sows showed estrus within 72 h after EB, but at mid-lactation, more sows whose litters had been weaned 4 d earlier had LH surges than sows whose litters were weaned at EB treatment (3 of 4, 2.2 +/- .37 ng/mL vs 1 of 4, 1.58 +/- .28). At late lactation there was no difference in the proportion of sows that had LH surges (3 of 4, 2.53 +/- .74 vs 3 of 4, 2.93 +/- 1). These results indicate that 1) estrus induced by EB occurs similarly at mid- and late lactation, 2) a smaller number of surges of LH occurs at mid- than late lactation, apparently because of inadequate releasable pools of pituitary LH, and 3) suckling prevents ovulation following a LH surge induced by EB in lactating sows.