RESUMEN
Background: Antral follicle count (AFC, follicles e"3 mm in diameter) varies between 5 and 50 follicles in among Bos taurus cattle. AFC is highly repeatable within animals, and is correlated with the total number of follicles in ovaries. Cattle with low AFC have higher circulating concentrations of FSH and LH, but lower concentrations of AMH, progesterone and androgens than animals with high AFC. Review: The cause of the inherently high variation in the number of follicles is unknown but it is reasonable to speculate that maternal nutrition during gestation, at the time of ovarian development in their foetuses, may impact oogonia proliferation and thus follicle numbers postnatally. We have recently started to test this hypothesis by restricting nutrition of beef heifers to 0.6 of their maintenance energy requirements, from shortly before conception to the end of the first trimester of pregnancy (period encompassing the peak in oocyte numbers in foetuses). Results show that calves born to nutritionally restricted mothers have a 60% lower peak, minimum and mean AFC during follicular waves compared with calves born to mothers fed control diets. Interestingly these calves from restricted mothers also had higher blood pressure compared to controls. Additional studies also provide evidence to suggest that fertility may be compromised in animals with low AFC due to effects on oocytes, progesterone and the endometrium compared to animals with high AFC. To test this hypothesis we performed ovarian ultrasonography on 306 dairy cows during the first wave of follicular development 1 to 4 months post partum and recorded their reproductive performances during the breeding season; cows with a high AFC had higher pregnancy rates, shorter calving to conception intervals and received fewer services during the breeding season compared to cows with a low AFC. Conclusion: Recent studies suggest that the numbers of follicles in follicle waves, and in total in the ovaries, may now be an important consideration for further efforts to understand and manipulate fertility. Since ovarian follicle numbers are reflective of the environment during foetal development when all the organ systems are developing, altered follicle numbers may also be reflective of other developmental difference in organs that may have more profound effects on the animals health and welfare beyond effects on reproduction and fertility.