Nutritional and biochemical factors impacting reproduction in replacement heifers and puberty in bull calves

Three experiments evaluated alternative heifer development using dormant, standing forages with strategic supplementation. In Exp. 1, heifers were developed in a dry lot (DL) or while grazing corn residue (CR). Heifers developed on CR gained less weight between weaning and breeding than DL heifers and fewer were pubertal before breeding. Fewer CR heifers conceived to AI, but final pregnancy rate was similar. Neither calf production nor second season rebreeding were affected by treatment. A second experiment evaluated heifer development on CR compared to grazing dormant winter range (WR). Grazing CR reduced post weaning ADG and the percentage of heifers pubertal by breeding. Neither AI conception nor final pregnancy rates were affected by treatment. Grazing CR during development tended to increase calving difficulty. Second season AI conception and final pregnancy rates were similar. A third study explored periconceptual nutrition of heifers developed on WR. Supplementation did not affect heifer reproduction. Supplemented heifers gave birth to heavier calves; however weaning weight was similar. Estrous synchronization was or was not performed in heifers using prostaglandin F2alpha (PGF). Fewer estrous synchronized heifers became pregnant, but more pregnant PGF heifers calved in the first 21 days of the season. Thus, PGF dams gave birth earlier than non-PGF heifers and steer calves from PGF dams were heavier at weaning. Experiment 4 assessed a gonadotropin-releasing hormone receptor (GnRHR) in bull testis and sperm, via quantitative RT-PCR. The GnRHR was expressed in bovine testis tissue from 158 to 363 days of age. Expression of a GnRHR was detected in bovine sperm. These data indicate heifer development using standing dormant forage does not reduce reproductive efficiency. Periconceptual nutrition may not be beneficial for heifers developed grazing WR. Estrous synchronization with PGF improves calving synchrony. A GnRHR is expressed in the bovine testis and mature spermatozoa.;Keywords. GnRH receptor, heifer development, nutrition, reproduction.