Gonadotropin-releasing hormone-induced secretion of luteinizing hormone in postpartum beef heifers maintained on two planes of nutrition before and after breeding.

An experiment was conducted to examine the effect of pre- and postbreeding nutrition on GnRH-induced LH release in beef heifers on d 3 and 14 of the subsequent postpartum period. Treatment groups consisted of heifers fed high (H; n = 12) and low (L; n = 12) planes of nutrition for 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 factorial arrangement of treatments (H-H, H-L, L-H, and L-L). On d 3 and 14 postpartum, heifers were injected with 100 micrograms of GnRH (i.v.), and blood 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. Heifers 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 level of nutrition. On d 14, mean cumulative serum concentration of LH in the H-H group was greater (P < .05) than that of the other three groups. These data indicate that postbreeding nutritional status significantly influenced pituitary responsiveness to GnRH on d 3 and that response to GnRH on d 14 was greatly enhanced by maintaining heifers on a high plane of nutrition both before and after breeding.(ABSTRACT TRUNCATED AT 250 WORDS)