Oestrous activity, ovulation rate and plasma melatonin concentrations in Rasa Aragonesa ewes maintained at two different and constant body condition score levels and implanted or reimplanted with melatonin

Abstract

Forty-seven adult and non-pregnant Rasa Aragonesa ewes were used to study the effect of two constant body condition score levels on reproductive parameters in ewes implanted in early April or reimplanted 49 days later with melatonin. The duration of the experiment was 9 months (27 November 1992 – 3 September 1993), and was designed as a factorial study defined by two constant body condition (BC) scores, high (H; 2.75 or greater) or low (L; 2.50 or less) and the implant on 8 April (M) or reimplant (2M) with melatonin (18 mg, Melovine™). Oestrus was detected daily using vasectomized rams and ovulation rate was measured by laparoscopy 6 days after positive identification. Blood samples were collected twice weekly from each ewe and assayed for progesterone. In addition, samples were collected from 20 ewes (five in each treatment group) every 2 h and hourly for 24 h on Days 27 and 84 after first implantation, respectively, and assayed for melatonin. Onset of seasonal anoestrus 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 first implantation (8 April) and the first detected oestrus (54 ± 7 days, 63 ± 8 days, 64 ± 10 days and 66 ± 5 days for HM, LM, H2M and L2M groups, respectively). Ovulation rate in the third cycle detected from 1 month after first implant insertion was significantly influenced by the BC score level (1.78 vs. 1.44 corpora lutea for H and L groups; P < 0.05). However, ovulation rate in the first and second cycles was slightly lower in the H group (1.46 vs. 1.64 and 1.48 vs. 1.56 corpora 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 detected. Plasma melatonin concentrations on Day 84 were not different between groups of ewes receiving one or two implants. Furthermore, and except for nocturnal levels in the high BC score group, ewes that received one implant showed significantly higher melatonin concentrations on Day 84 than on Day 27 after implantation. It is concluded that (i) interval between melatonin implantation and first oestrus is not influenced by the BC score level; (ii) the reimplant does not improve reproductive parameters, although exogenous melatonin seems to overcome the positive 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.