Effects of Nutritional Level and Biological Type on Gonadotropin-releasing Hormone-induced Luteinizing Hormone Release and Plasma Progesterone, Estrone and Estradiol Concentrations in Pre- and Post-partum Beef Heifers1, 2

Forty-six beef heifers (16 to 23 mo) of two biological types (small = Red Poll-sired, large = Charolais-sired) were individually fed from d 90 of gestation through parturition to evaluate the effects of nutritional restriction on plasma LH and steroid hormone concentrations. Heifers were allotted to one of two nutritional treatments to achieve a BW reduction (loss, fed at 1% of BW/d) or to maintain initial BW (maintenance, fed 1.5% of BW/d) to parturition. Gonadotropin-releasing hormone (100 lag) was injected i.m. three times during gestation (d 130; d 200; d 270) and twice after parturition (d 1 to 14; d 23 to 36). Blood samples were collected at 20-min intervals after GnRH for 4 h. Maternal BW change from d 90 to parturition differed (P < .01) between loss and maintenance heifers. Mean plasma progesterone concentrations were greater (P < .05) at d 130 and 270 of gestation in small than in large heifers and were greater (P < .01) at d 23 to 36 postpartum in maintenance than in loss heifers. Mean concentrations of estrone and estradiol were greater (P < .05) in large than in small heifers at d 200 of gestation. Mean plasma LH concentrations following GnRH injection were greater (P < .01) in loss than in maintenance heifers at 200 and 270 d of gestation. Metabolizable and retained energy were related inversely to LH release during mid and late gestation. Change in body condition score from 90 to 270 d gestation and age of dam also accounted for a significant proportion of the variation in LH response to GnRH at d 270 of gestation. Although nutritional restriction during gestation increased pituitary LH release in response to GnRH prior to parturition, the increase was not related to changes in plasma progesterone or estrogen concentration. Intake and retention of energy during gestation appear to be related inversely to GnRH-induced LH release before parturition in beef heifers. (