Luteinizing hormone secretion after withdrawal of exogenous progestogen in heifers fed three levels of dietary energy.

As experiment was conducted with 15 Angus X Hereford heifers that were 16 mo of age at the time the experiment was initiated. The heifers were assigned randomly in equal numbers to be fed low (L), maintenance (M) or high (H) energy diets for 191 d. Overall average weight for heifers at the time the experiment was initiated was 294 +/- 6 kg. All heifers were exhibiting estrous cycles at regular intervals and estrus was synchronized using a progestogen implant (norgestomet) in combination with an injection of estradiol valerate and norgestomet before initiation of dietary treatments. Dietary treatments started at time of implant removal and this day was designated as d 0. Blood samples were collected every other day from d 150 to 160 and progesterone concentrations were quantified to evaluate estrous cycle activity. In order to resynchronize the reproductive state, all heifers were retreated with norgestomet implants and injected with the estradiol valerate and norgestomet combination on d 160. Implants were removed on d 170. Serial blood samples were collected for 4-h periods at 20-min intervals starting at 0, 10, 20 and 30 h after implant removal and luteinizing hormone (LH) concentrations were quantified. Progesterone was quantified in samples collected at daily intervals (170 to 191 d) to evaluate whether a normal estrous cycle occurred subsequent to removal of the implant. Four of five heifers in each of the L and M treatment groups were not cycling, whereas all heifers in the H treatment were cycling at the time of progestogen implantation on d 160.(ABSTRACT TRUNCATED AT 250 WORDS)