Influence of direction of body weight change on pattern of gonadotropin secretion in ovariectomized beef heifers of equivalent body weight

Our hypothesis was that alterations in pattern of gonadotropin secretion induced by level of dietary energy intake are dependent on direction of BW change. Beef heifers were assigned to one of two treatments 21 d after ovariectomy: 1) increasing followed by decreasing BW (I-D; n = 9) or 2) decreasing followed by increasing BW (D-I; n = 9). Heifers assigned to the I-D treatment received 15.4 Mcal ME/d (HE) from wk 0 to 8 followed by 4.3 Mcal ME/d (LE) from wk 9 to 16. Heifers assigned to the D-I treatment received the LE diet for the initial 8 wk followed by the HE diet for the subsequent 8 wk. At 2-wk intervals blood samples were collected at 10-min intervals for 11 h to determine the pattern of secretion and(or) concentration of gonadotropins in circulation (LH and FSH) and pituitary responsiveness to administration of 750 ng and 50 microgram of LHRH. Frequency of LH pulses increased (P < .05) in a quadratic fashion with increasing ADG in heifers in both treatment groups. Amplitude of LH pulses decreased (P < .05) in a linear fashion with increasing ADG in heifers in both treatment groups and magnitude of this decline was greater (P < .05) in heifers receiving the D-I treatment. Amplitude of response to 750 ng of LHRH decreased (P < .05) in a linear fashion with increasing ADG in heifers receiving the D-I treatment but was unrelated to changes in ADG (P > .05) in heifers receiving the I-D treatment. When heifers in both treatment groups attained similar BW (P > .05; wk 14), amplitude of LH pulses, responsiveness to 750 ng of LHRH and mean concentration of FSH in serum were higher (P < .05) in heifers receiving the I-D than in heifers receiving the D-I treatment. Thus, we accept the hypothesis that alterations in pattern of gonadotropin secretion in heifers fed diets with a low energy content are dependent on direction of BW change.