Influence of the length of progestagen treatment and the time of oestradiol benzoate application on the ovulatory follicle size and ovulation time in anoestrous and cyclic beef cows.

Previous research from our laboratory in beef cattle suggests that the pre-ovulatory follicle size, maturity and subsequent susceptibility to gonadotropin are influenced by the length of progestagen treatment in artificial insemination programme in beef cows. To test this hypothesis, two experiments were conducted. In experiment 1, 35 anoestrous beef cows received an intravaginal sponge containing 200 mg of medroxyprogesterone acetate. The treatment lasted for 7 (n = 12), 8 (n = 11) or 9 (n = 12) days. Half of the animals in each group were injected with 0.7 mg of oestradiol benzoate (EB) at device removal (0 h) and the other half 24 h later. In experiment 2, 38 cycling beef cows were treated with the same protocols as in experiment 1. Ultrasound examinations were performed to determine the follicular diameter at device removal (dominant follicle), interval to ovulation and ovulatory follicle diameter. The dominant follicle of anoestrous cows with progestagen for 7 days (8.4 ± 1.6 mm) resulted smaller (p < 0.05) than the cows treated for 8 (10.5 ± 1.6 mm) and 9 days (10.6 ± 1.2 mm). However, regardless of the length of the treatments, ovulation time after device removal was longer (p < 0.05) when EB was injected 24 h after withdrawal than at 0 h in anoestrous cows (EB0 = 52.7 ± 4.0 h; EB24 = 70.8 ± 6.2 h) and in cyclic cows (EB0 = 50.0 ± 21.0 h; EB24 = 73.0 ± 20.0 h). In anoestrous cows, the treatment with progestagen for 9 days and EB at 24 h increased the diameter of the ovarian follicle (p = 0.033) but did not affect the diameter of the ovulatory follicle in cyclic cows. In conclusion, increasing the length of progestagen treatment for 8 or 9 days compared to 7 days increased the diameter of the dominant follicle, in anoestrous and cyclic beef cows. Oestradiol benzoate administered at device removal resulted in a shorter interval from device removal to ovulation compared with EB injection 24 h after the end of a progestagen treatment.