Sixty-four normally cycling Holstein heifers were randomly assigned into four equal groups (three treatment and one control). Each of the three treatment groups received 0·5 mg of melengestrol acetate (MGA®) daily and a single injection of either oestradiol cypionate (ECP®), human chorionic gonadotrophin (hcg) or pituitary luteinizing hormone (plh) between Days 22 and 26 following oestrus or Days 8 to 12 of MGA feeding. The control animals received neither MGA nor an injection. Heifers from each treatment group were spayed (overiectomized) on Days 2, 5, 8 and 11 from the injection and control heifers were spayed on Days 1, 4, 7 and 10 from oestrus. The corpora lutea (cl) and the largest follicles on the ovaries of each heifer were observed for size and colour.
Oestrus was observed in only the ECP-treated group within 24 hr following the injections (nine of sixteen animals). Ovulation occurred following the injection in all of the sixteen animals in the hcg group and in fifteen of the sixteen animals in each of the ECP and plh groups. All the control heifers ovulated following oestrus.
Normal cl were present in all the treated heifers spayed by Day 5. On subsequent days, increasing numbers of heifers had cl which were regressing.
The percentages of heifers spayed on Day 8 and Day 11 after injection with regressing cl were: 0% and 50% for the ECP group; 25% and 100% for the hcg group; and 75% and 50% for the plh group. All the heifers in the control group had normally developing cl regardless of the day of spaying.
The follicles from those animals which had regressing cl were generally larger than those from control heifers.
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