Plasma luteinizing hormone concentrations in cows given repeated treatments or three different doses of gonadotropin releasing hormone.

We hypothesized that: (i) repeated GnRH treatments would increase the magnitude and duration of the LH surge and would increase progesterone (P4) concentrations after ovulation; and (ii) the release of pituitary LH would be greater in response to larger doses of GnRH. In Experiment 1, ovary-intact cows were given an intravaginal P4 (1.9g) insert (CIDR) for 10 d and 500 microg cloprostenol (PGF) at CIDR removal to synchronize estrus. On Days 7 or 8 after estrus, cows received two PGF treatments (12h apart) and 100 microg GnRH at 36 (Control), 36 and 38 (GnRH38), or 36 and 40h (GnRH40) after the first PGF. Mean plasma LH concentration (ng/mL) was greater (P<0.05) in GnRH38 (8.8+/-1.1) than in Control (5.1+/-1.3), with that in GnRH40 (5.8+/-1.3) being intermediate. Although the duration (h) of the LH surge was longer in GnRH40 (8.0+/-0.4) than in either GnRH38 (P<0.05; 7.0+/-0.3) or Control (P<0.09; 7.1+/-0.4), mean postovulatory P4 (ng/mL) was greater (P<0.01) in Control (4.2+/-0.7) than in GnRH38 (2.9+/-0.6) or GnRH40 (3.0+/-0.7) cows. In Experiment 2, ovariectomized cows were given a CIDR for 10 d and 2mg of estradiol cypionate im at CIDR insertion. Thirty-six hours after CIDR removal, cows received, 50, 100, or 250 microg of GnRH. Cows given 250 microg GnRH released more LH (9.4+/-1.4ng/mL) than those given 50 or 100 microg (6.1+/-1.3 and 5.4+/-1.4ng/mL, respectively), and had an LH surge of longer duration than those given 50 microg (6.8+/-0.4 vs. 5.1+/-0.3h). In summary, ovary-intact cows in the GnRH38 group had greater mean and peak LH concentrations, but subsequent plasma P4 concentrations were lower than in Control cows. Ovariectomized cows given 250 microg GnRH had a greater pituitary release of LH.