WOOL AND LIVEWEIGHT RESPONSES TO NUTRITION BY MERINO SHEEP GENETICALLY SELECTED FOR HIGH OR LOW STAPLE STRENGTH

The biological mechanisms underlying differences in wool staple streng th were examined in 2 groups of Merino sheep that have been geneticall y selected for high or low staple strength, while holding fibre diamet er constant. The sheep were fed below maintenance for 87 days, and the n re-fed ad libitum for 63 days with a diet containing either 9% or 23 % crude protein, in a cross-over design, after which they returned to the paddock until shearing. The fleeces of the 2 groups differed in st aple strength (25.2 v. 17.5 N/ktex, P < 0.001) but were similar in mea n fibre diameter and clean fleece weight. However, the pattern of wool growth was different. When fed below maintenance, sheep from the soun d group grew more wool than sheep from the tender group (P < 0.05), bu t lost more liveweight (P < 0.01). During re-feeding, the sound sheep grew less wool than the tender sheep (P < 0.05), but gained more livew eight. The mean fibre diameter at the point of break was similar in bo th groups. Immediately after re-feeding, the fibre diameter increased more rapidly in the tender group than in the sound (P < 0.001), but a similar difference was observed between the high and low protein diets , with no effect on staple strength. The sound sheep had a lower stand ard deviation of fibre diameter than the tender sheep (P < 0.001), bot h in the whole fleece and in 3-weekly midside patches. The data indica ted that the variability of fibre diameter between fibres made a large r contribution than the variability along fibres to the difference bet ween the groups in overall variability of fibre diameter. The sheep we re then grazed together at pasture for a second year and again differe d in staple strength. In addition, the sound sheep grew less wool on g reen spring pastures and had a lower clean fleece weight (P < 0.05). T he differences in wool growth rates between sheep from the sound and t ender lines depended more on whether pasture was green than on the amo unt of pasture available. We conclude that the difference in staple st rength between the sound and tender groups was most closely associated with the variability between fibres in diameter, and was also affecte d by a difference in variation in diameter along the fibres. Staple st rength was not affected by the amount of wool at the point of break, o r by the rate of change in fibre diameter after feeding. The sheep in the sound group grew less wool than those in the tender group when on good nutrition.