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

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


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 = 77 (SEM 4)%, Control group = 44 (SEM 7)%; p<0.0001), and overall lambing percentage (Melatonin group = 109 (SEM 1)%, Control group = 59 (SEM 2)%; p<0.0001). Treatment differences were especially pronounced in 2005 and 2006, when annual rainfall was exceptionally low; ewes in the Control group had the lowest lambing rates those years. Lambing rates and overall lambing percentage were positively correlated (p<0.05) with the amount of annual rainfall but the correlation coefficients were higher in the Control than Melatonin group.
CONCLUSIONS: Melatonin implants are an effective means of improving lamb production of Merino ewes, especially in harsh environments where low annual rainfall limits the availability of food. When melatonin treatment was used, however, the responses of flocks on individual farms were difficult to predict because within a year, responses did not occur on all farms.
KEY WORDS: Sheep, melatonin, rainfall, between-year variation, inter-farm variation

Access to the full text of this article is available:

through another providers website:

If you're a member or subscriber and believe you should have access:



Register for an account

Request new password

The whole of the literary matter of the New Zealand Veterinary Journal is copyright Taylor and Francis, Downloading this article signifies agreement with the terms and conditions of electronic access.