ESTROUS ACTIVITY, OVULATION RATE AND PLASMA MELATONIN CONCENTRATIONS IN RASA-ARAGONESA EWES MAINTAINED AT 2 DIFFERENT AND CONSTANT BODY CONDITION SCORE LEVELS AND IMPLANTED OR REIMPLANTED WITH MELATONIN

Forty-seven adult and non-pregnant Rasa Aragonesa ewes were used to st udy the effect of two constant body condition score levels on reproduc tive parameters in ewes implanted in early April or reimplanted 49 day s later with melatonin. The duration of the experiment was 9 months (2 7 November 1992-3 September 1993), and was designed as a factorial stu dy defined by two constant body condition (BC) scores, high (H; 2.75 o r greater) or low (L; 2.50 or less) and the implant on 8 April (M) or reimplant (2M) with melatonin (18 mg, Melovine(TM)). Oestrus was detec ted daily using vasectomized rams and ovulation rate was measured by l aparoscopy 6 days after positive identification. Blood samples were co llected twice weekly from each ewe and assayed for progesterone. In ad dition, samples were collected from 20 ewes (five in each treatment gr oup) every 2 h and hourly for 24 h on Days 27 and 84 after first impla ntation, respectively, and assayed for melatonin. Onset of seasonal an oestrus was significantly influenced by the BC score (February 16 +/- 8 and March 15 +/- 5 for H and L groups; P < 0.01), but neither of the factors considered in the study influenced the interval between the f irst implantation (8 April) and the first detected oestrus (54 +/- 7 d ays, 63 +/- 8 days, 64 +/- 10 days and 66 +/- 5 days for HM, LM, H2M a nd L2M groups, respectively). Ovulation rate in the third cycle detect ed from 1 month after first implant insertion was significantly influe nced by the BC score level (1.78 vs. 1.44 corpora lutea for H and L gr oups; P < 0.05). However, ovulation rate in the first and second cycle s was slightly lower in the H group (1.46 vs. 1.64 and 1.48 vs. 1.56 c orpora lutea, respectively), suggesting a positive effect in the short term of the melatonin implants in ewes with a moderately low BC score . No significant effect of the reimplant on ovulation rate was detecte d. Plasma melatonin concentrations on Day 84 were not different betwee n groups of ewes receiving one or two implants. Furthermore, and excep t for nocturnal levels in the high BC score group, ewes that received one implant showed significantly higher melatonin concentrations on Da y 84 than on Day 27 after implantation. It is concluded that (i) inter val between melatonin implantation and first oestrus is not influenced by the BC score level; (ii) the reimplant does not improve reproducti ve parameters, although exogenous melatonin seems to overcome the posi tive effect of a high BC score in the early breeding season; (iii) one implant is adequate to maintain high plasma melatonin levels up to 3 months after implantation.