SUPEROVULATORY RESPONSE IN LACTATING COWS WITH DIFFERENT FOLLICULAR DYNAMICS

To determine the influence of a dominant follicle on the superovulator y response in midlate lactating cows, the dynamics of follicular growt h and the development of corpora lutea and embryos were studied. Real- time B-mode ultrasonography with a 7.5 MHz rectal linear-array transdu cer was used to scan the ovaries daily, from 8-12 days before superovu lation until day 7 post-estrus, when embryos were recovered. On the fi rst 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-dominan t' group (n = 11) those carrying a 'non-dominant follicle', i.e. a fol licle in a regressing phase or with the same diameter for > 4 days. Co ws 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 superovulat ion in mid-late lactation is not negatively affected by the presence o f a 'dominant follicle'. The criteria to define 'dominant' and 'non-do minant' follicles might be redefined when applied to lactating cows, w hose response to superovulation seems to be under the influence of man y other factors.