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)

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