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Post-weaning nutritional programming of ovarian development in beef heifers
From weaning to breeding, the nutritional management of replacement females is critical to their lifetime productivity. Traditionally, cereal grains have been used to develop replacement heifers to enter the breeding system at a younger age. However, overfeeding heifers decreased number of calves weaned, while peri-pubertal caloric restriction increased primordial follicle numbers in the developing ovary. As the mechanisms by which this increase in primordial follicle number occurs are unknown, the objective of this study was to determine the timing of changes in primordial follicle numbers and evaluate the influence of Leptin, roundabout axon guidance receptor, homolog 4 (ROBO4) and Anti-Müllerian Hormone (AMH) genes on formation and activation of primordial follicles in beef heifers. Crossbred heifers (n=30) were fed a control ration consisting of 30% alfalfa hay, 69.8% corn silage and 0.2% salt as dry matter. At eight months of age, six heifers were ovariectomized. The remaining 24 heifers were divided into two treatment groups (n=12 heifers/diet). Heifers in the Stair-Step treatment (n=6) received 157 kcal ME/BW , and heifers in Control treatment (n=6) were offered 228 kcal ME/BW for 84 d and ovariectomized. At 84 d, the remaining heifers (n=6/diet) were fed for an additional 83 d, Stair-Step heifers received 277 kcal ME/BW and Control heifers received 228 kcal ME/BW . Heifers were ovariectomized at the end of the feeding period. Pieces of ovarian cortex were fixed in paraformaldehyde for histology or snap frozen for real-time RT-PCR. Body weight (P=0.13) did not differ between treatments. Serum leptin concentrations (P=0.01) were greater in Control heifers and ovarian ROBO4 mRNA abundance (P=0.05) was greater in Stair-Step heifers at 11 months of age. There was a tendency for AMH mRNA abundance (P=0.09) to be greater in the ovaries of Stair-Step heifers at 11 months of age. In histological sections, control heifers had more primary follicles (P=0.03) at 11 months of age, while Stair-Step heifers had more primordial follicles (P=0.04) at 13 months of age. There was no difference in secondary or antral follicle numbers between treatment groups or time points. In conclusion, developing heifers on a Stair-Step compensatory growth scheme decreased development costs and decreased the depletion of the ovarian reserve prior to the onset of breeding, which may have beneficial effects on increasing reproductive lifespan. USDA is an equal opportunity provider and employer.
Keywords:
heifer, developmental programming, primordial follicle