The effect of melatonin treatment on wool growth and thyroxine secretion in sheep

Melatonin implants are commonly used to control sheep reproduction during the winter solstice, but their effect on wool growth is unclear. In this study, wool growth was measured in 17 Assaf (carpet wool), 21 Rasa Aragonesa (medium wool) and 20 Merino (fine wool) ewes after inserting melatonin implants in January, February and April, and 13 ewes were used as control. All ewes were previously ovariectomised and treated with oestradiol implants. The mean wool fibre diameter was measured at the beginning of the study (1 January), after shearing (1 June), and at the end of the study (31 August). Wool growth was lowest in April for ewes implanted on 1 January. Plasma thyroxine (T4) concentrations differed significantly among breeds, except when peaked in June. In controls ewes, T4 levels were significantly lower in July. Fibre diameter was affected by breed but not melatonin treatment. Inserting the implants in spring appeared to influence wool production. At the same time, early treatment during the winter solstice could have a negative effect on wool growth in spring, except for fibre diameter. The differences in wool growth and fibre diameter among breeds remained constant throughout the year.