GONADOTROPIN-RELEASING HORMONE-INDUCED SECRETION OF LUTEINIZING-HORMONE IN POSTPARTUM BEEF HEIFERS MAINTAINED ON 2 PLANES OF NUTRITION BEFORE AND AFTER BREEDING

An experiment was conducted to examine the effect of pre- and postbree ding nutrition on GnRH-induced LH release in beef heifers on d 3 and 1 4 of the subsequent postpartum period. Treatment groups consisted of h eifers fed high (H; n = 12) and low (L; n = 12) planes of nutrition fo r 204 d before breeding. Each group was further subdivided to receive either high or low planes of nutrition after breeding in a 2 x 2 facto rial arrangement of treatments (H-H, H-L, L-H, and L-L). On d 3 and 14 postpartum, heifers were injected with 100 mug of GnRH (i.v.), and bl ood was collected via jugular venipuncture at 15-min intervals for 2.5 h and at 30-min intervals for an additional 2.5 h for LH analysis. He ifers fed a high level of nutrition throughout gestation (H-H and L-H) had a greater (P < .05) mean cumulative serum concentration of LH (ng LH.mL-1.min) in response to GnRH on d 3 than did those fed a lower le vel of nutrition. On d 14, mean cumulative serum concentration of LH i n the H-H group was greater (P < 0.5) than that of the other three gro ups. These data indicate that postbreeding nutritional status signific antly influenced pituitary responsiveness to GnRH on d 3 and that resp onse to GnRH on d 14 was greatly enhanced by maintaining heifers on a high plane of nutrition both before and after breeding. In addition, t he negative effect of low prebreeding nutrition on GnRH-induced LH sec retion on d 14 was not overcome by increasing the level of nutrition a fter breeding.