Effects of melatonin treatment on the attainment of puberty in heifers

This experiment determined the effect of treatment of 3- to 4-mo-old heifer calves with exogenous melatonin, in early summer, on the attainment of puberty the following spring. Hereford heifers, born between February 22, and March 16, were paired by age and BW and assigned at random within pair to receive an ear implant of melatonin (MEL, n = 12) or to serve as controls (C, n = 12). The implant period was 5 wk, starting June 10 at a mean age of 105 +/- 5 d and 134.4 +/- 3.5 kg BW. Puberty was evaluated the following year on four occasions: January 26, March 19, May 2, and June 1. On each date, rectal palpation of the ovaries was performed, blood samples were taken to assess concentrations of progesterone (P4), and BW was determined. Puberty was considered to have occurred when concentrations of P4 were greater than or equal to .5 ng/mL concomitant with a palpable corpus luteum (CL), or when concentrations of P4 were greater than or equal to 1 ng/mL and a CL was present on the next date. At implant removal, concentrations of melatonin in plasma collected at 1000 to 1400 were 69.5 +/- 6.3 and 16.7 +/- 2.4 pg/mL for MEL and C heifers, respectively. On that and subsequent occasions, neither BW nor ADG differed between MEL and C heifers. Treatment with melatonin increased the incidence of heifers pubertal by the second observation date (P < .05). The cumulative proportions of MEL and C heifers pubertal at each date were as follows: 1/12 vs 0/12, 7/12 vs 2/12, 7/12 vs 2/12, and 10/12 vs 6/12, respectively. Body weight, ADG, and BW:age ratio at confirmation of puberty did not differ between MEL and C animals, indicating that the effect of melatonin on puberty was not mediated by growth/nutritional factors. In conclusion, treatment with melatonin for 5 wk early in their first summer hastened puberty in heifers born in late winter.