Validation of trans-rectal ultrasonography for counting preovulatory follicles in weaned sows

The present study investigated the accuracy of trans-rectal ultrasonography (TRU) for assessing the exact number of preovulatory follicles (POFs, with a diameter from 6 to 10 mm) present in the ovaries of weaned sows. The ovaries of 63 hormonally treated (1500 IU of eCG) weaned sows were checked with TRU (7.5-MHz multiple scan angle transducer) in two successive scanning sessions performed at 26-27 and 30-31 h after the beginning of oestrus signs, and the maximum number of POFs were counted. Sows were subjected to laparoscopy (LAP) immediately after the last TRU scan to confirm the number of POFs. The differences (mean ± S.D.) in the number of POFs counted with TRU and LAP on each ovary were analyzed as a whole and after sorting the ovaries into three classes according to the number of POFs visualized by LAP: (1) less than 7; (2) from 7 to 13; and (3) more than 13. A significant correlation (P < 0.01) was found between TRU and LAP for both the whole data set (126 ovaries) and in each of the three ovarian classes. Despite the significant correlation, TRU underestimated the number of POFs by 1.40 ± 1.67 compared with LAP (P < 0.001). However, the underestimation varied among the ovarian classes. This difference was not significant (P > 0.05) in class 1 and was significant (P < 0.001) in classes 2 (1.11 ± 1.30 less POFs than counted by TRU) and 3 (3.19 ± 1.54 less POFs than counted by TRU). In conclusion, TRU is a valuable tool to count the number of POFs present in the ovaries of weaned sows, but a certain degree of underestimation should be expected when the number of POFs is large.