Melatonin treatment prior to the normal breeding season increases fetal number in United Kingdom sheep flocks

Litter size determined ultrasonographically at 45–90 days after mating in eight groups of commercial sheep (n = 2609) treated with slow release melatonin implants immediately prior to the natural breeding season showed a 19% treatment effect averaged over the eight groups, from 148% to 167% (P < 0.01) compared with controls (n = 2499). Control groups had a litter size ranging from 124% to 194% thereby representing most commercial production systems in the UK sheep industry. This is the first large scale study of slow release melatonin implants in commercial UK flocks to demonstrate a significant positive effect on litter size when used before the natural breeding season. The data indicate that increased productivity and profitability could be achieved while maintaining desirable purebred characteristics.