Romney rams selected for high wool staple strength grew 25% more wool than rams selected for low staple strength when fed near maintenance rations during a 10 week winter experiment. Wool growth responses to supplements of energy (6% of diet as tallow), protein (9% of diet as bloodmeal) and methionine (1.2 g/d by intraperitoneal injection) were similar in the two staple strength selection lines. Tallow increased the estimated supply of metabolic energy by 53% but was without effect on wool growth. Bloodmeal increased dietary protein by 82%, dietary energy by 9% and sulphur supplies by 15% and produced 38% more wool. Methionine injections also increased sulphur supplies by 13% but had no effect on energy or protein supplies and the wool growth response (35%) was similar to that of bloodmeal. The finding that methionine supplies limited wool growth is consistent with other studies in sheep on low planes of nutrition. A component of wool growth responses was an increase in minimum mean fibre diameter. It accounted for most differences in staple strength between treatments but comparison at the same minimum mean fibre diameter indicated that the methionine supplement increased staple strength through an additional mechanism. Staple strength responses to nutritional treatments were similar in the two staple strength selection lines. It is concluded that the greater wool growth of the high staple strength selection line accounted for the majority of its staple strength advantage over the low strength line, and that there may be an effect of methionine on wool strength that is additional to its effect on fibre diameter. Because nutritional and genetic effects on staple strength were independent, they can be exploited concurrently to reduce the incidence of tender wools.
Proceedings of the New Zealand Society of Animal Production, Volume 53, , 271-274, 1993
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