Superovulatory response during the period of decrease in the number of ultrasonographically identified follicles in postpartum beef cows

The number of follicles more than 4 mm in size in the ovaries of 2 Japanese Black cows was determined by ultrasonography from 14 to 147 days after parturition. We found that the number of ultrasonographically identified follicles (uiFOL) with a diameter of 4 mm or more mark-edly decreased at about 92 days postpartum. We measured the number of uiFOL in 7 cows from 14 to 98 days postpartum and conducted superovulatory treatment with follicle stimulating hor-mone (FSH-P) at 95-98 days postpartum. The number of uiFOL decreased similarly at about 92 days after parturition, and that of recovered and transferable embryos was 17.7 ± 5.3 and 11.9 ± 4.4, respectively. The results suggest that it is preferable to start superovulatory treatment in the period when the number of uiFOL decreases. It is also important to examine whether these results can be applied to other periods excepting the period (92-98 days postpartum) in which the uiFOL de-creases in number. We measured both the number of uiFOL at the beginning of superovulatory treatment and that of recovered transferable embryos in 10 cows at more than 120 days postpartum. As a result, the coefficiency of correlation (r) obtained was 0.319, indicating that there was no correlation between the number of uiFOL and that of transferable embryos. This suggested that the number of uiFOL at the time of superovulatory treatment did not influence the yield of transferable embryos, except for the period (92-98 days postpartum) in which the uiFOL decreases in number.