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Postweaning nutritional programming of ovarian development in beef heifers

O. L. Amundson, T. H. Fountain, E. L. Larimore, B. N. Richardson, A. K. McNeel, E. C. Wright, D. H. Keisler, R. A. Cushman, G. A. Perry, H. C. Freetly
Journal of animal science 2015 v.93 no.11 pp. 5232-5239
alfalfa hay, beef cattle, body weight, breeding, calves, cattle feeding, compensatory growth, corn silage, cortex, crossbreds, genes, heifers, histology, leptin, longevity, low calorie diet, messenger RNA, nutritional intervention, ovarian development, ovarian follicles, ovariectomy, overfeeding, reverse transcriptase polymerase chain reaction, small cereal grains, weaning
From weaning to breeding, 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. Overfeeding heifers decreases the number of calves weaned, while peri-pubertal caloric restriction increases primordial follicle numbers in the developing ovary. 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), Slit2, 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 8 mo of age, 6 heifers were ovariectomized. The remaining 24 heifers were divided into 2 treatment groups (n = 12 heifers/treatment). Heifers in the Stair-Step treatment received 67.4 g DM / BW '"kg" '^0.75, and heifers in Control treatment were offered 97.9 g DM / BW '"kg" '^0.75 for 84 d and ovariectomized. At 84 d, the remaining heifers were fed for an additional 83 d, Stair-Step heifers received 118.9 g DM / BW '"kg" '^0.75 and Control heifers received 97.9 g DM / BW '"kg" '^0.75. 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 > 0.10) did not differ between treatments. Plasma leptin concentrations were greater in Control than 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. Ovarian ROBO4 and Slit2 mRNA abundance increased with age (P = 0.001), and increased more rapidly in Stair-Step heifers between 11- and 13 mo of age. Abundance of AMH mRNA did not change due to dietary treatment or age (P > 0.10). In conclusion, developing heifers on a Stair-Step compensatory growth scheme increased the size of the ovarian reserve prior to the onset of breeding, which may have beneficial effects on increasing reproductive lifespan as well as decreasing development costs.