Effects of annual rainfall and farm on lamb production after treatment with melatonin implants in Merino sheep: A 4-year study

Abstract AIM: To determine the effects of annual rainfall and farm on the efficacy of melatonin implants in improving lamb production in Merino sheep in Spain. METHODS: A study was conducted on 3,871 Merino sheep on six farms over a 4-year period (2004–2007). Melatonin implants were inserted during the second half of February or early March (winter) (Melatonin group) or not (Control group). Multinomial logistic regression was used to determine the effects of melatonin, farm and year, and their interactions, on reproductive outcomes. Regression analysis was used to examine the relationship between annual rainfall and the percentage of ewes lambing, percentage of lambs born to ewes lambing, and overall lambing percentage, for each year and treatment group within farm. RESULTS: Annual rainfall, farm and treatment with melatonin, and their interactions, had a significant effect on the reproductive performance of ewes (p<0.001). Treatment with melatonin increased the percentage of ewes lambing (Melatonin group...