Failure of melatonin implants to alter onset of puberty in gilts

Forty crossbred gilts (Hampshire × Yorkshire × Landrace × Duroc) weighing 55.4 ± 0.5 kg at 4.2 ± 0.1 months of age received either melatonin implants (N = 20) or served as controls (N = 20). Gilts were housed in a temperature controlled room (21°C) with a light (L):dark (D) cycle of 10L:14D. Initially, four implants (12 mg melatonin each, Wildlife Pharmaceuticals, Ft. Collins, CO) were placed in one ear subcutaneously. At 2-week intervals, five, six, seven or eight implants were placed in ears of the gilts (one implant per 11.8 kg body weight). Throughout the trial, blood samples were collected twice weekly by jugular venipuncture. All gilts were exposed to mature boars for 15 min on alternate days. Age of puberty was determined from serum concentrations of progesterone and visual observations of estrus. Serum concentrations of melatonin were elevated (5–10 fold) throughout the trial in those gilts that received melatonin implants. Average daily gain (P = 0.51) and age at puberty (P = 0.57) were similar between gilts that received melatonin implants or no implants. Even though gilts continued to receive melatonin after they attained puberty, elevated serum concentrations of melatonin did not alter the length of subsequent estrous cycles (P = 0.90). These data indicate that elevation of serum concentrations of melatonin via implants did not alter the onset of puberty or length of subsequent estrous cycles.