Period of dominance of the ovulatory follicle influences embryo quality in lactating dairy cows
Length of dominance of the ovulatory follicle and exposure to oestradiol (OE2) during proestrus can affect fertility. Lactating cows had their oestrous cycle pre-synchronized and were subjected to one of the four synchronization treatments. Cows in the oestrus detection (OD) treatment received GnRH on day 6 of the oestrous cycle, PGF2α 7 days later, and were inseminated at detected oestrus. The remaining cows were subjected to the Ovsynch (OVS) protocol (day 0 GnRH, day 7 PGF2α, day 9 GnRH, and timed artificial insemination (AI) 12 h later) starting on day 3 (OVS3) or day 6 (OVS6 and OVS6E) of the oestrous cycle. Cows in the OVS6E treatment received an injection of 0.5 mg oestradiol cypionate 36 h before AI. Ovaries were examined by ultrasonography and blood was sampled for progesterone and OE2 concentrations. Uteri were flushed 6 days after AI and recovered embryos–oocytes evaluated. Diameter of the ovulatory follicle at AI differed (P<0.01) among treatments, and it was the largest for OVS3 cows, which also had extended (P<0.01) length of follicular dominance. During proestrus, OD and OVS6E cows had increased (P<0.01) OE2 concentrations. Fertilization was not altered by treatments, and maximum fertilization was achieved when the number of accessory spermatozoa was >7. Proportions of viable embryos in relation to embryos and embryos–oocytes recovered were smaller for OVS3 cows (P<0.01) than the other treatments, and embryos from OVS3 cows also had fewer (P<0.01) blastomeres and tended (P=0.09) to have a lower proportion of live blastomeres. Extending the period of follicle dominance did not alter fertilization but reduced (P<0.001) embryo quality. Embryo quality was compromised even when the dominance of the ovulatory follicle was extended by only 1.5 days.