EFFECT OF EXOGENOUS MELATONIN AND PLANE OF NUTRITION AFTER WEANING ONESTROUS ACTIVIN, ENDOCRINE STATUS AND OVULATION RATE IN SALZ EWES LAMBING IN THE SEASONAL ANESTRUS

Forty-nine Spanish Salt ewes lambing in the second fortnight of March (20 March+/-1.5 d) were used to determine the effects of exogenous mel atonin and postweaning nutrition on endocrine status, date of first es trus and ovulation rate. Experimental design was a factorial defined b y 2 postweaning planes of nutrition, 1.80 (high) and 1.35 (low) times the maintenance requirements, and treatment with a single 18-mg subcut aneous implant of melatonin (M) 32 d after lambing or no treatment con trol (C). Mean weaning to first estrus interval was shorter in treated than in control ewes (50.8+/-4.2 vs 87.6+/-6.3 d; P<0.01). Considerin g both the treated and control animals together, the ratio between mea n night and daytime plasma melatonin levels was significantly correlat ed with the implant insertion-first estrus interval on Day 5 (0.67; P< 0.01) and Day 35 (0.63; P<0.05) after implantation. Melatonin implants induced a significant increase of mean LH concentrations at Days 14 a nd 33 after implantation (P<0.01) without any significant influence of plane of nutrition. Ovulation rate was higher for treated than contro l ewes in the second estrus (P<0.05). An interaction between plane of nutrition and exogenous melatonin on ovulation rate at the second cycl e after weaning was detected (P<0.01), being close to the significance in the first, fourth and fifth cycles (P<0.1). These results suggest that exogenous melatonin in April may be an effective way of advancing the breeding season and enhancing ovulation rate associated with a lo w rather than a high plane of nutrition.