Superovulatory response in lactating cows with different follicular dynamics.

To determine the influence of a dominant follicle on the superovulatory response in midlate lactating cows, the dynamics of follicular growth and the development of corpora lutea and embryos were studied. Real-time B-mode ultrasonography with a 7.5 MHz rectal linear-array transducer was used to scan the ovaries daily, from 8-12 days before superovulation until day 7 post-estrus, when embryos were recovered. On the first day of superovulation cows were classified as belonging to either a 'dominant' group (n = 12) having a 'dominant follicle', i.e. > 9 mm in diameter in growing phase or stable for < 4 days, or a 'non-dominant' group (n = 11) those carrying a 'non-dominant follicle', i.e. a follicle in a regressing phase or with the same diameter for > 4 days. Cows in the 'dominant' group had significantly higher numbers of 4-6 and 7-10 mm follicles than cows in the 'non-dominant' group on the third and the fifth day of superovulation. The total number of follicles (> 4 mm) preovulation and the number of CL day 7 postovulation was higher in the 'dominant' group than in the 'non-dominant' group. There were no differences between groups regarding the total number of embryos or transferable embryos recovered. The results indicate that superovulation in mid-late lactation is not negatively affected by the presence of a 'dominant follicle'. The criteria to define 'dominant' and 'non-dominant' follicles might be redefined when applied to lactating cows, whose response to superovulation seems to be under the influence of many other factors.