FOLLICULAR DYNAMICS PRIOR TO AND DURING SUPEROVULATION IN HEIFERS

The present ultrasonographic study examined the relationship between c ertain follicular parameters and the superovulatory response in gonado tropin-stimulated heifers. Thirty heifers received a total of 35 mg FS H twice daily for 4 d and 0.75 mg cloprostenol were given to induce lu teolysis and estrus at 72 h after the initial FSH injection. Transrect al ultrasonography was performed once daily from 1 or 2 d before the i nitial FSH injection and until the day of estrus. The number of small (2 to 4 mm), medium (5 to 9 mm), and large (greater than or equal to 1 0 mm) size follicles as well as the diameter af the large follicles we re recorded. Embryos were recovered non-surgically 6 or 7 d after estr us, and the number of corpora lutea was determined by palpation per re ctum, Heifers with >2 or less than or equal to 2 corpora lutea were cl assified as responders or nonresponders, respectively. All follicular categories were affected by treatment with FSH in the responders (P<0. 0001), while in the nonresponders only small follicles and the total n umber of follicles showed a change after treatment (P<0.05). All folli cular categories were different between the 2 groups (P<0.005). There was no effect of corpus luteum location (ipsilateral vs contralateral) on any of the follicular categories (P>0.05). The number of large fol licles and the sum of medium and large follicles were positively corre lated (r=O.43 and r=0.54, respectively; P<0.05) with the number of cor pora lutea palpated on the day of embryo recovery (6 to 7 d after estr us). In conclusion, there was an effect of the day relative to initiat ion of FSH treatment on all follicular categories in heifers respondin g positively to superovulation, and there was no effect of side (left or right ovary) or of corpus luteum diameter (ipsilateral or contralat eral).