165. Postnatal Nutritional Programming of the Ovarian Reserve in Beef Heifers.

Abstract Nutritional management of heifers during the first year of life can influence reproductive efficiency and longevity. It is well established that heifers are born with a finite number of follicles within their ovaries, and the ovarian reserve is depleted at a presumed constant rate over time. Nutritional management, however, has been demonstrated to influence follicle numbers in the ovary of both rodents and heifers. In rats, decreased nutrient intake during the peri-pubertal phase increased the number of primordial follicles in the ovaries. Research in developing heifers has evaluated the impact of altering nutrient intake and growth patterns during development on ovarian development and the ovarian reserve. Heifers managed on a stair-step growth pattern, with decreased nutrient intake post-weaning during the peri-pubertal development period, followed by an increase in nutrient intake, resulted in increased numbers of primordial follicles compared to heifers developed on a constant rate of gain. In New Mexico, heifers were developed on either a constant gain or a stair-step nutritional program and managed either in the drylot or grazing native range. Heifers developed on a stair-step system in the drylot and on native range had increased primordial follicles per histological section compared to constant gain drylot, with constant gain native range similar to all other treatments. Developing heifers on a stair-step nutritional program, regardless of management (drylot or native range), resulted in an increased ovarian reserve. Recent research in Wyoming used a similar 2 × 2 factorial arrangement, reported a significant treatment × year interaction, with stair-step heifers in year two having an increased number of primordial follicles (P = 0.05) compared to all other treatments in year one and two. Stair-step developed heifers had a greater (P = 0.03) number of primordial follicles compared to constant gain heifers. Across all studies, stair-step heifers did not have an increase in antral follicle count to coincide with their increased primordial follicle numbers. Recent research placed stair-step developed heifers into production to evaluate longevity, reporting stair-step heifers had improved herd survival. Overall, these results suggest nutritional management during critical stages of development can influence the size of the ovarian reserve and impact reproductive longevity. Physiological mechanisms controlling increased primordial follicle numbers and enhanced reproductive longevity in stair-step heifers, however, are not well understood. USDA is an equal opportunity provider and employer.