Factors associated with the rate of growth of clean wool on grazing sheep

Abstract

In a grazing study that spanned 2.5 years, the rate of clean wool growth (Wi, g/day) in 4 week periods on Corriedale wether sheep was measured on each side of the animal with dyebands that were applied, alternately, at 2 week intervals. The observations (n = 224) on wool growth from 3 contiguous periods Wi = 1, 2 and 3 with the middle overlapping the other two periods, were related to either the daily rate of organic matter intake (OMI, g/day), the digestible organic matter intake (g DOMI/day), the hourly rate of OMI (IR, g OMI/h) or the hourly rate of digestible organic matter intake (DR, g DOMI/h) in linear or quadratic expressions. The intake data were collected every 4 weeks over the 3 days that correspond to days 6 to 8 of W1. The variance accounted for in each of W1, W2 and W3 increased in the order OMI<DOMI<IR<DR. DR accounted for 33.2% of the variance in W1 with a significant quadratic fit (P < 0.01) and 24.3% and 7.8% with linear fits against W2 and W3, respectively. A relationship that comprised five factors and accounted for 68% of the variance in W1 was developed by nonlinear regression. Three factors that were associated with nutritional aspects, the hourly rate of digestible organic matter intake (DR, g DOMI/h), the organic matter digestibility (OMD %) and the average organic matter intake in a grazing period (M, g OMI/grazing period) accounted for 54.9% of the variance in W1 while the other two factors that were associated with the physiological state of the animal, fleece-free liveweight change (C, g/day) and the fleece-free liveweight (L, kg) accounted for a further 13.1% of the variance of W1. The study suggests that factors associated with phenomena within a daily period may have important effects on wool growth. Management aspects are discussed.