Factors affecting side of ovulation in heifers and mares-A comparative study

Effect of side (left ovary, LO; right ovary, RO) and intraovarian presence of the regressing corpus luteum (CL) on the preovulatory follicle (PF) and ovulation were studied using ultrasonic- records of complete interovulatory intervals (IOI) for 500 IOI in heifers and 485 IOI in mares. Only IOI with a recording of side of a single PF and a single CL at each end of the IOI were used. In heifers, ovulation at the end of the IOI occurred more frequently (P < 0.0001) from RO (n = 285, 57%) than LO (n = 215, 43%). In mares, frequency of ovulation did not differ between RO (n = 250. 52%) and LO (n = 235, 48%). In heifers but not in mares, the number of predeviation follicles has been reported to be greater in RO than LO. In heifers, RO ovulation was more frequent when CL was in PF ovary (66%) than when CL was in opposite ovary (50%). There was no difference in side of ovulation (RO, 50%, LO, 50%). In mares, there were no significant main effects or interactions for frequency of ovulation involving side and intraovarian presence of the CL. On a species comparative basis, the present and reported results supported hypothesis 1 that a greater number of predeviation follicles in RO than in LO is associated with the predilection for RO ovulation. Results also supported hypothesis 2 that ovulation from RO in heifers is more frequent when RO contains the regressed CL than when the CL is in LO.