Four experiments were conducted to examine effects of GnRH administered at estrus on various reproductive characteristics in repeat-breeding dairy cows (eligible for third service). In Exp. 1, cows (n = 8 per group) received (i.m.) either saline or 50, 100, or 250 micrograms of GnRH at 12 h after onset of estrus. There was a positive linear (P < .05) effect of dose on concentrations of LH, but not of FSH, in serum at 2 h after the injection. Concentrations of progesterone (P4) were higher (P = .10) through 16 d after estrus in nonpregnant cows that returned to estrus 18 to 24 d after GnRH treatment than in nonpregnant cows given saline. Likewise, among cows with elevated concentrations of P4 through 30 d after estrus (diagnosed pregnant by increased concentrations of pregnancy-specific protein B; PSPB), P4 was higher (P = .10) in those given GnRH than in those given saline. In Exp. 2, concentrations of LH, FSH, and estradiol-17 beta (E2) were similar among 12 cows during the periestrual period before they received (i.m.) 100 micrograms of GnRH or saline at estrus. Concentrations of P4 after estrus were increased 2 d earlier (P < .05) in GnRH- than in saline-treated cows. Among cows with elevated concentrations of P4 and PSPB through 40 d after AI, P4 was higher (P < .05) in those given GnRH than in those given saline. In Exp. 1 and 2, a greater proportion (43 vs 14%; P = .07) of cows given GnRH was pregnant 42 to 56 d after estrus than of those given saline, but calving rate was similar (27 vs 14%). In Exp. 3, average number of LH pulses per 8 h was similar between saline- and GnRH-treated cows on d 1, 3, and 8, but overall concentrations of LH were reduced (P < .05) in GnRH-treated cows. Concentrations and number of pulses of FSH were increased (P < .05) on d 8 after treatment with GnRH. Concentrations of P4 were increased earlier (P < .05) after estrus in GnRH-treated (69 +/- 12 h) than in saline-treated cows (126 +/- 12 h), with higher (P < .05) concentrations of P4 on d 4 to 8. In Exp. 4, eight cows received either 100 micrograms of GnRH or saline at 12 h after onset of estrus (d 0) in a cross-over design.(ABSTRACT TRUNCATED AT 400 WORDS)

Association of the calf with its dam prolongs the postpartum interval to first ovulation in intact and mastectomized cows indicating that tactile stimulus of the teat and (or) udder is not necessary. Therefore, we conducted the following experiment to determine whether individual calves allowed restricted contact with their mastectomized dam would delay the onset of first postpartum ovulation. Mastectomized cows (n = 21) were allotted randomly and equally to three groups in which cows were 1) allowed unrestricted access to their calves, with unlimited tactile, olfactory, visual, and auditory stimuli (calf present); 2) removed permanently from their calves 24 h after parturition and provided no further association with their calves (calf removed); and 3) remained with their calves for 24 h and then the cow-calf pair was restricted to a pen where tactile contact of the calf was limited to the head and neck region of its dam (calf restricted). Seven udder-intact cows (control) remained with their calves and were allowed to nurse ad libitum. Postpartum intervals to ovulation and first estrus for the calf-restricted cows and calf-removed cows were similar but shorter (P < .05) than those for the calf-present and udder-intact groups. We conclude that restricting tactile stimuli of the calf to the head and neck area of its dam failed to prolong postpartum anovulation in the mastectomized dam and that a cow-calf nursing orientation with tactile stimuli to the inguinal area, but not limited to the teat and(or) udder, was sufficient to prolong anovulation.

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