Sulfur amino acid supplementation increases wool production in sheep at low planes of nutrition but it is unclear whether there is any benefit of supplementation at planes of nutrition above maintenance and what implications this might have for wool quality characteristics. This experiment directly investigated the interaction between sulfur supplementation and plane of nutrition in terms of wool growth and fibre characteristics. Twenty-four Romney ewes, acclimatised in individual metabolism units over a 7-week pre-treatment period, were allocated to 1 of 4 treatment groups based on a 22 factorial arrangement. Groups were low (L) or high (H) intake (0.8 or 1.3 maintenance, respectively) with continuous intravenous infusion of either saline (–Cys) or cysteine (+Cys, 2 g/day). During the 3-month treatment period, measurements were obtained for liveweight, plasma cysteine concentration, wool sulfur concentration and output, clean wool growth, mean fibre diameter (MFD), length growth rate (LGR), colour, loose wool bulk, handle, and crimp frequency and character. Clean wool growth response (P < 0.05) to cysteine supplementation was greater for the L sheep (6.06 v. 4.31 g/100 cm2) than the H sheep (7.20 v. 6.13 g/100 cm2). The response to supplementation in LGR (P < 0.01) was similar in both H (14%) and L (20%) sheep. There was no response in MFD due to sulfur supplementation, although fibre diameter measurements made along the fibres suggest that there was a response in L but not H sheep (P < 0.1). Wool sulfur concentration and output increased as a result of cysteine supplementation but concentration increased more in L (30.6 v. 24.5 mg S/g; P < 0.01) than in H sheep (28.4 v. 26.2 mg S/g). Qualitative electrophoresis analyses suggested that the increase in wool sulfur was achieved primarily by an increase in ultra-high-sulfur proteins. Crimp frequency and character were both significantly (P < 0.01) enhanced by cysteine supplementation. It is concluded that cysteine supplementation, at feed intakes that commonly occur in the commercial situation, can produce a useful increase in wool growth. This growth increase is primarily accomplished by increasing length growth rate rather than fibre diameter, which should also improve the value of the wool fibre produced.

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