Follicular dynamics, estradiol-17[beta] concentrations, and luteinizing hormone release following norgestomet implant insertion during estrus synchronization with melengestrol acetate

The objective of this experiment was to determine whether norgestomet implant insertion following melengestrol acetate (MGA) administration altered LH pulse frequency and follicular dynamics. Multiparous Angus cows were randomly assigned to receive MGA (.5 mgcow⁻¹d⁻¹ ; MGA; n = 14) for 18 d or to receive MGA (.5 mgcow·⁻¹ ld⁻¹; MGA-N; n = 11) for 15 d and a norgestomet implant for 4 d beginning on d 15. Ultrasound was used to record images of each ovary in cows beginning on d 8 of MGA administration. On d 16, serial blood samples were collected from all cows in replicate one (MGA, n=6; MGA-N, n=6) for quantification ofLH pulse frequency. A persistent, dominant follicle was identified in all cows on d 8 ofMGA administration. Forty-three percent and 64% (P > .10) of MGA and MGA-N cows, respectively, initiated a new wave of follicular development during treatment that was the source of the ovulatory follicle. Pulse frequency of LH did not differ between MGA and MGA-N cows or between cows that ovulated a persistent (PERSIST) follicle and those that ovulated a follicle from a new follicular wave (NEW). Growth rate of the ovulatory follicle for the 7 d preceding ovulation was greater in PERSIST than in NEW cows (P < .01). Diameter of the owlatory follicle on the day preceding ovulation was greater in PERSIST cows than in NEW cows (P < .01). In conclusion, MGA administration caused a persistent follicle to develop, but that follicle was unable to be regressed consistently by supplemental norgestomet administration.