Coarse-woolled sheep breeds such as the Romney exhibit a pronounced annual cycle of wool growth with concomitant changes in fibre length growth rate and mean fibre diameter with maxima in summer and minima in winter. This cycle, which is entrained by seasonal changes in daylength via changes in circulating prolactin, parallels wool growth rate in Romney sheep. The cycle is also influenced by complex interactions between nutrient supply and hormones associated with reproductive status. A marked depression in wool production occurs during early pregnancy, which cannot be completely ameliorated by supplementary feeding, and does not directly involve prolactin. Additionally, in ewes, experimental and pharmacological increases and decreases in the prolactin concentration at parturition and during lactation are associated with increases and decreases, respectively, in spring and summer wool growth rate. A greater appreciation of the different mechanisms that temporally influence the wool growth cycle represents a potential means to change fleece characteristics. The development of sheep genotypes with a reduced amplitude of their wool growth cycle would significantly contribute to a reduction in fibre variability that can affect processing performance.

PE, Kendall, RMW Sumner, and AJ Pearson

Proceedings of the New Zealand Society of Animal Production, Volume 66, Napier, 144-148, 2006
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