The effect of melatonin on lambing and litter size in milking ewes after advancing the breeding season with progestagen and PMSG followed by artificial insemination

To advance the breeding season in an indigenous sheep breed of Greece, 207 multiparous milking ewes were used and allocated randomly in two groups, gr oup A of 114 and group B of 93 ewes, during the anoestrous period (spring), All ewes received intravaginal sponges impregnated with progestagen (60 mg , MAP) for 14 days and were injected with PMSG (500 IU) on the day of spong e removal. Ewes in group A received melatonin implants (18 mg) 35 days befo re inserting the sponges. Artificial insemination was carried out into both groups 48 and 60 h after sponge removal. Lambing rate at second oestrous w as significantly higher in group A (60.4 vs. 32.6%, P<0.05). The overall la mbing rate during two oestrous cycles was significantly higher in group A t han in group B (83.3 vs. 68.8, P<0.05). The groups differed significantly i n the mean litter size per lambing ewe that conceived during the first oest rus after sponge removal (1.52 vs. 1.32). The total mean litter size per la mbing ewe was higher in group A than in group B (1.48 vs. 1.31) as a conseq uence of the difference in pregnancy rate. (C) 1998 Elsevier Science B.V. A ll rights reserved.