Effects of an agonist of gonadotrophin releasing hormone in cattle. I. Hormone concentrations and oestrous cycle length

Four trials were completed to study the effects of a single intramuscular injection of 5 μg of an agonist of gonadotrophin releasing hormone (Hoe 766) on plasma concentrations of LH and progesterone, and on oestrous cycle length in normally cycling dairy cows.The first trial (four cows) showed that a mid-cycle injection of Hoe 766 temporarily increases plasma LH from less than 5 ng/ml to over 20 ng/ml within 2.5 h. Average plasma progesterone concentrations ranged from 4.8 to 7.0 ng/ml compared to 3.3 ng/ml in the control animal.The second trial (22 cows) showed that an injection of Hoe 766 on Cycle Day 3, 6 or 9 (Oestrus = Cycle Day 0) increased average plasma progesterone concentrations during Cycle Days 13, 14 and 15 by 1.2 ng/ml. Each of three cows injected on Cycle Day 16 maintained plasma concentrations above 3.9 ng/ml until Cycle Day 19 and corpus luteum (CL) size was maintained until Cycle Day 21. Except for the group of cows injected on Cycle Day 3, all other groups had temporarily reduced concentrations of plasma progesterone when sampled 24 h after Hoe 766 administration.The third trial (216 cows) showed that a single injection of Hoe 766 made between Cycle Day 1 and 10 did not alter oestrous cycle length (21.5 vs 21.3 days). In contrast, in the fourth trial (371 cows), a single injection of Hoe 766 between Cycle Days 12 to 16 altered the distribution of cycle lengths of 17–29 days, the average cycle length and the incidence of ovulation without detected oestrus. Compared to matched control cows, fewer Hoe 766-treated cows were detected in oestrus (73.9% vs 90%), or had cycle lengths of less than 20 days (4.7% vs 22.2%). These effects were most pronounced among cows injected on Cycle Day 16 when only 51.7% were detected in oestrus and their average cycle length was 24.1 days.These effects were not due to the formation of a secondary CL. Rather, the injection of Hoe 766 stimulated CL function and appeared to prevent or delay normal luteolysis when administered from Cycle Day 12.