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University of Wyoming, Laramie 82071 and and Colorado State University, Fort Collins 80523
4 Address reprint requests to: T. G. Dunn, Div. Anim. Sci., Univ. of Wyoming, Box 3354, Univ. Station, Laramie 82071.
Abstract
Levels of luteinizing hormone (LH), prolactin, progesterone, cortisol, estrone and estradiol-17β were determined during the postpartum anestrous period in samples collected from six second-calf, crossbred, suckled beef cows. Blood samples were collected at 6-h intervals beginning 2 wk before expected parturition and continuing until 10 d after first postpartum estrus. Samples were also collected at 15-min intervals for 4 h once weekly. The postpartum interval was 48 ± 19 d (mean ± SE). Serum progesterone levels fell from a mean of 2.1 ± 1.2 ng/ml 2 d prepartum to basal levels of .1 ± .1 ng/ml 6 d postpartum and remained at this level until just before estrus. An elevation (1.3 ± .1 ng/ml) in serum levels of progesterone beginning 4.6 ± .1 d and ending .6 ± .1 before estrus was observed in four cows. Mean levels of estradiol-17β concentrations remained at these levels until they increased to 10 ± 3 pg/ml just before first estrus. Mean levels of LH were .7, 1.1, .4 and .3 ng/ml for wk 1 to 2, 3 to 4, 5 to 6 and 7 to 8 before estrus, respectively, with wk 1 to 2 and 3 to 4 being higher (P<.05) than wk 5 to 6 and 7 to 8. The frequency of LH peaks in serum tended to be greater (P<.1) during the second week preceding estrus than it was at any other time during the postpartum period. Serum levels of prolactin were characterized by excessive variation (CV = 103.5%) between cows. Mean prolactin levels, calculated over 2-wk periods, were 26.4, 28.4 and 24.6 ng/ml for wk 1 to 2, 3 to 4, and 5 to 6 before estrus, respectively. Time before estrus did not affect prolactin levels. A critical step in regulation of the length of the postpartum interval appears to be the onset of episodes of LH secretion 4 to 8 wk before estrus. Mean levels of LH and frequency of LH peaks were greatest 2 wk preceding estrus; therefore, it is possible that the increase in LH at this time acts as a priming step to initiate the occurrence of the pre-estrous progesterone peak.
1 Research supported in part by CSRS Grant # WS-999 "Reproductive Endocrinology of the Postpartum, Suckled Beef Female" (T.G.D.). The research is a contribution to regional research project W-112 "Reproductive Performance of Domestic Ruminants."
2 Div. Anim. Sci., Univ. of Wyoming, Box 3354, Univ. Station, Laramie 82071.
3 Present address: Texas A&M Univ. Research and Extension Center, Star Route 2, Box 43C, Beeville 78102.
5 Present address: Dept. of Comparative Medicine, Bowman Grey School of Medicine, Wake-Forest Univ., Winston-Salem, NC 27103.
6 Dept. of Physiol. and Biophys., Colorado State Univ., Ft. Collins 80523.
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