Intraovarian factors associated with switching of a future dominant follicle to a subordinate follicle during induced luteolysis in heifers

The factors involved in the switching of a future dominant follicle (DF) to subordinate status were studied (n = 42) by induction of luteolysis with PGF2α (hour 0) when the largest follicle (F1) in follicular wave 2 was 7.0 or 8.5 mm. Combined for 7.0- and 8.5-mm groups, the frequency of switching was greater (P < 0.01) when F1 and CL were ipsilateral (10 of 28, 36%) than when contralateral (0 of 14). The frequency of switching in the ipsilateral relationship was greater (P < 0.002) when F1 and CL were adjacent (<3 mm apart; 10 of 17) than when separated (0 of 11). The difference in diameter between F1 and F2 was less (P < 0.005) when switching occurred (0.4 ± 0.1 mm) than when switching did not occur (ipsilateral, 1.3 ± 0.2 mm; contralateral, 1.1 ± 0.2 mm). Treatment at hour 0 when F1 was 7.0 mm and ipsilateral to the CL resulted in smaller diameter (P < 0.001) of F1 at hour 12 (7.6 ± 0.2 mm) than when treatment was not given (8.3 ± 0.1 mm). The hypotheses were supported that (1) switching from a future DF to a future subordinate is functionally related to luteolysis, and (2) factors involved in switching include an ipsilateral relationship between the largest follicle and CL and close intraovarian proximity of the follicle to the regressing CL. It is proposed that switching of the future DF to subordinate status during spontaneous luteolysis accounts for the reported greater frequency of the contralateral than ipsilateral relationships between the preovulatory follicle and CL in three-wave interovulatory intervals.