Effect of dietary energy and protein on bovine follicular dynamics and embryo production in vitro: Associations with the ovarian insulin-like growth factor system

Heifers were assigned either low or high (HE) levels of energy intake and l ow or high concentrations of dietary crude protein. The effect of these die ts on the plasma concentrations of insulin, insulin-like growth factor (ICF )-I, and urea on follicular growth and early embryo development is describe d. We propose that the observed dietary-induced changes in the ovarian IGF system increase bioavailability of intrafollicular IGF, thus increasing the sensitivity of follicles to FSH. These changes, in combination with increa sed peripheral concentrations of insulin and IGF-I in heifers,offered the H E diet, contribute to the observed increase in growth: rate of the dominant follicle. In contrast to follicular growth, increased nutrient supply decr eased oocyte quality, due in part to increased plasma urea concentrations. Clearly a number of mechanisms are involved in mediating the effects of die tary energy and protein on ovarian function, and the formulation of diets d esigned to optimize cattle fertility must consider the divergent effects of nutrient supply on follicular growth and oocyte quality.