Ovulation control in heifers with prostaglandin and luteinising hormone releasing hormone analogue under different conditions of management

Abstract

As a preliminary, dose response studies were carried out in 48 two-year-old Hereford cross Friesian heifers to establish that an intramuscular injection of 10 to 20 μg of the luteinising hormone releasing hormone analogue ICI 118,630 at days 8 to 10 of the cycle resulted in a peak in plasma luteinising hormone concentrations two hours later. Then two groups of 10 two-year-old Hereford cross heifers were treated with two intramuscular injections of cloprostenol (PG1 and PG2) 11 days apart, one group being further treated with 20 μg ICI 118,630 72 hours after PG2. Blood samples were taken from all animals immediately before PG2, four hourly from 48 to 80 hours after PG2 and a single sample 13 days after PG2; additional half hourly samples were taken in the luteinising hormone releasing hormone treated animals from 72 to 76 hours after PG2. Four hourly rectal examinations of uterus and ovaries were carried out for all animals from 68 to 144 hours after PG2. Plasma assays established concentrations of luteinising hormone, follicle stimulating hormone, progesterone and total cholesterol, and rectal examinations the ovulation patterns. The experiment was carried out twice; once during high dietary β-carotene availability in June and again during low β-carotene availability in December. No significant advantage in terms of reducing group ovulation spread was achieved with the luteinising hormone releasing hormone analogue in either of the experiments. In the December experiment β-carotene, progesterone and total cholesterol plasma concentrations were significantly lower than in the June experiment; ovulatory disorders and variation in the timing of ovulation increased and follicles were subjectively judged to be smaller. Luteinising hormone and follicle stimulating hormone plasma concentrations however were higher in the December experiment compared with those in June.