Effect of ovulatory follicle size on luteal function, pregnancy rate, and late embryonic/fetal mortality in beef cattle
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Previous reports indicate that ovulation of small dominant follicles resulted in reduced pregnancy rates compared ovulation of large follicles in cattle. A reciprocal embryo transfer approach was used to differentiate between oocyte competence and uterine environment factors that affect establishment of pregnancy following induced single ovulations of small follicles. Embryos from donor cows that ovulated a small follicle ([less than] 12.5 mm) were transferred into recipient cows that ovulated a large follicle ([equal to] 12.5 mm) and vice versa resulting in the following treatment groups: small to large (S-L; primary effects of oocyte quality; n = 111), large to small (L-S; primary effects of uterine environment; n = 122), small to small (S-S; negative control; n = 71), and large to large (L-L; positive control; n = 50). The probability of recovering a fertilized and live embryo 7 d after breeding increased as the diameter of the ovulatory follicle increased (p = 0.01). As ovulatory follicle diameter and serum concentrations of progesterone at embryo transfer increased in the recipient cow, the probability of pregnancy increased (p = 0.05 and [lesser than] 0.001, respectively); however neither follicle size nor progesterone at ET in the donor cow was significant (p [greater than] 0.3). In summary, ovulatory diameter at GnRH2 was positively associated with recovery of a live embryo (possibly indicating improved oocyte competence and (or) an early uterine environment that was more conducive to embryonic/fetal development in cows that ovulated a large follicle). Pregnancy establishment following embryo transfer was related to the uterine environment established by the ovulatory follicle independent of oocyte quality.