Induction of precocious puberty in heifers I: enhanced secretion of luteinizing hormone.

In beef heifers weaned between 3 and 4 mo of age and fed a high-concentrate diet, approximately 50% reach puberty before 300 d of age (precocious puberty). The objectives of this experiment were 1) to determine whether precocious puberty could be induced experimentally by weaning heifers early and feeding a high-concentrate diet, and 2) to determine the dynamics of secretion of LH associated with precocious puberty. Crossbred Angus and Simmental heifer calves were weaned at 73 +/- 3 d of age and 115 +/- 3 kg of BW and fed a high-concentrate (60% corn; HI, n = 9) or control diet (30% corn; CONT, n = 9). Heifers were fed individually, and target BW gains were 1.50 and 0.75 kg/d for the HI and CONT treatments, respectively. Heifers were weighed every 2 wk. Blood samples were collected weekly and assayed for progesterone concentration to determine age at puberty. Serial blood samples were collected at 20-min intervals for 24 h at mean ages of 102, 130, 158, 172, 190, 203, 217, 231, and 259 d and assayed for LH concentration to evaluate the dynamics of secretion of LH. Heifers fed the HI diet exhibited greater BW gain (P < 0.01) than CONT heifers (1.27 +/- 0.05 vs. 0.85 +/- 0.05 kg/d, respectively). As a result, BW in the HI treatment was greater (P < 0.01) than in the CONT treatment by 188 d of age and remained different through the end of the experiment. Precocious puberty occurred in 8 of 9 heifers fed the HI diet and 0 of 9 heifers fed the CONT diet. Age at puberty was reduced in the HI (P < 0.01) compared with the CONT heifers (262 +/- 10 vs. 368 +/- 10 d of age, respectively). Body weight at puberty was also reduced in the HI (P < 0.05) compared with the CONT treatment (327 +/- 17 vs. 403 +/- 23 kg, respectively). Heifers attaining puberty during the experiment continued with subsequent luteal phases as evidenced by cyclic patterns of progesterone concentrations. Frequency of pulses of LH (pulses/24 h) increased with age (P < 0.01) for both treatments. Heifers in the HI treatment exhibited a greater number of pulses of LH (P < 0.01) than those in the CONT treatment by 190 d of age and in all subsequent collection periods (treatment x age, P < 0.05). Mean LH concentrations also increased with age (P < 0.01) for both treatments but did not differ between treatments. In conclusion, precocious puberty induced by early weaning and feeding of a high-concentrate diet is preceded by increasing frequency of pulses of LH.