Follicle numbers are highly repeatable within individual animals but are inversely correlated with FSH concentrations and the proportion of good-quality embryos after ovarian stimulation in cattle

BACKGROUND: The significance of the high variation in numbers of follicles produced during reproductive cycles in humans and cattle is unknown. METHODS: We selected beef heifers with high (> or =25) or low (< or =15) numbers of ovarian follicles and determined the association with alterations in FSH and estradiol concentrations, as well as responsiveness to superstimulation and embryo quality. The variation in follicle numbers was also compared with oocyte quality in natural cycles using IVF and abattoir-sourced bovine ovaries. RESULTS: Results show that: (i) FSH was lower (P < 0.03) in animals with high compared with low follicle numbers per follicle wave; (ii) after superovulation, in the high versus low follicle number group, the number of oocytes/embryos recovered after insemination (10.6 +/- 2.7 versus 4.7 +/- 0.7) and the number of transferable embryos (5.4 +/- 1.3 versus 3.8 +/- 0.8) per animal were greater (P < 0.05), whereas the proportion of transferable embryos (50.7% versus 79.8%) was lower (P < 0.05); (iii) in unstimulated animals, the numbers of high-quality oocytes harvested and in-vitro fertilized oocytes developing into blastocysts were up to 4-fold greater (P < 0.05) for ovaries with high versus low numbers of follicles, but the proportions of oocytes developing into blastocysts were similar in the two groups. CONCLUSION: Phenotypic classification based on numbers of follicles may be useful to improve superovulation procedures. The lower proportion of transferable embryos following superovulation of ovaries with high numbers of follicles is probably not the result of differences in the quality of oocytes before superovulation.