EFFECT OF EXOGENOUS MELATONIN AND PLANE OF NUTRITION AFTER WEANING ON ESTROUS 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 melatonin and postweaning nutrition on endocrine status, date of first estrus and ovulation rate. Experimental design was a factorial defined by 2 postweaning planes of nutrition, 1.80 (high) and 1.35 (low) times the maintenance requirements, and treatment with a single 18-mg subcutaneous implant of melatonin (M) 32 d after lambing or no treatment control (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). Considering both the treated and control animals together, the ratio between mean night and daytime plasma melatonin levels was significantly correlated 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 and 33 after implantation (P<0.01) without any significant influence of plane of nutrition. Ovulation rate was higher for treated than control ewes in the second estrus (P<0.05). An interaction between plane of nutrition and exogenous melatonin on ovulation rate at the second cycle 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 low rather than a high plane of nutrition.