Postweaning nutritional programming of ovarian development in beef heifers

Peripubertal caloric restriction increases primordial follicle numbers at breeding, which may improve reproductive potential. Our hypothesis was that feed restriction was changing primordial follicle number through stimulation of follicle formation via leptin, roundabout axon guidance receptor, homolog 4 (ROBO4), or SLIT2 or through inhibition of follicle activation via anti-Mullerian hormone (AMH). Heifers (n = 30) were fed a ration consisting of 30% alfalfa hay, 69.8% corn silage, and 0.2% salt as DM. Heifers received the control diet for 42 d before an initial 6 heifers were ovariectomized at 8 mo of age. The remaining 24 heifers were divided into 2 treatment groups. Controls were offered 97.9 g DM/kg BW0.75 over the entire feeding period. Stair-step heifers received 67.4 g DM/kg BW0.75 for 84 d. Following the 84-d restriction, heifers were stepped up to receive 118.9 g DM/kg BW0.75 over a 15-d period and were held at this feeding level 68 d. At the end of the feed restriction (11 mo of age), ovaries were collected from 6 heifers per treatment, and at the end of the refeeding period (13 mo of age), ovaries were collected from 6 heifers per treatment. Plasma leptin concentrations were greater in control heifers than in stair-step heifers at 11 mo of age (P < 0.0001). In histological sections, stair-step heifers had more primordial follicles (P = 0.03) than control heifers at 13 mo of age. There was no difference in secondary or antral follicle numbers between dietary treatment groups or ages. Relative abundance of ROBO4 mRNA in ovarian cortex of control heifers was greater at 13 mo than at 11 mo or before feed restriction (8 mo; P = 0.01). Relative abundance of ROBO4 mRNA in stair-step heifers at 13 mo was greater than before feed restriction (P = 0.02) and at 11 mo did not differ from 8 or 13 mo (P = 0.70). Relative abundance of SLIT2 mRNA in the ovarian cortex followed a similar pattern, being greater in stair-step heifers at 11 mo compared with control heifers (P = 0.001). At 13 mo, SLIT2 mRNA did not differ between treatments (P = 0.30). Abundance of AMH mRNA in the ovarian cortex did not change due to dietary treatment or age (P > 0.10). In conclusion, developing heifers on a stair-step compensatory growth scheme resulted in larger ovarian reserve before the onset of breeding, which may have beneficial effects on increasing reproductive lifespan.