Relationship between ovarian weight and follicular population in heifers

The size of the ovary varies substantially among cattle. This variation may influence the potential of the ovary to produce follicles. In the present study, we examined whether a relationship exists between the weight of the ovary and the number of antral follicles >= 1 mm. Paired ovaries were obtained from Holstein x Japanese Black F1 heifers. Follicles were classified into three size categories (small: 1.0 - < 5.0 mm, medium: 5.0 - < 8.5 mm and large: ! 8.5 mm), and the number of follicles in each category was recorded. Large variations in the weight of ovaries and the number of follicles were observed among animals. Significant positive correlations (r >= 0.4, P < 0.001) were found between the weight of intact ovaries and the number of follicles in all three categories for the ovary contralateral to CL (OCC) and in the small follicles for the ovary ipsilateral to CL (OIC). Significant positive correlations (r > 0.4, P < 0.0001) were also observed between the weight of ovaries devoid of CL and follicles and the number of small and medium follicles in both OIC and OCC, indicating that the correlation is not due to the increase in ovarian weight associated with the increase in follicular number. Paired ovaries contained a similar number of small and medium follicles, and significant positive correlations were observed between them (r > 0.6, P < 0.0001). There were significant positive correlations between the weight of OCC and the number of small and medium follicles in paired ovaries (r > 0.4, P < 0.0001). These results suggest that 1) the weight of an ovary reflects the potential of the ovary to produce antral follicles, and 2) a rough estimation of follicular population might be possible by using the weight of the ovary contralateral to CL in heifers.