Efficiency of conversion of food to wool. III. Wool production of ewes selected for high clean wool weight and of random control ewes on restricted and unrestricted food intakes in pens

Abstract

The clean wool production (W), gross energy intake (I), and body weight (B) of individually fed medium Peppin ewes representative of two groups, one (selected, S) genetically superior in W to the other (control, C), have been measured under restricted and ad libitum feeding of lucerne chaff in pens. Differences in favour of the S group were observed for W and W/I under both regimes, the respective relative values of S/C being under restricted feeding 110/100 and 112/100, and under ad libitum feeding 113/100 and 112/100. Of these between-group differences, only W/I on restricted feeding was significant (P < 0.05). Differences between groups for B and I were small and not significant. On restricted feeding the S/C values were 98/100 for both; on ad libitum feeding they were 99/100 for B and 101/100 for I. When the data from the two feeding regimes were combined, the between-group differences in both W and W/I were significant (P < 0.05), but no significant group x nutritional level interaction was observed in any character. The two levels of nutrition in the pens yielded wool production rates similar to those observed contemporaneously on natural pastures. There were large positive phenotypic correlations between net efficiency (W/I under restricted feeding) and gross efficiency (W/I under ad libitum feeding), and between net efficiency and W under both regimes. There were also positive phenotypic correlations between I and both W (moderate) and B (large) under ad libitum feeding. W/I accounted for 92% of the between-group (genetic) and 70% of the within-group (phenotypic) variation in W. There was no significant difference in plasma protein-bound iodine (PBI) between S and C ewes, while correlations between W and PBI were not significant within either group. The energetic efficiency of the wool production process is discussed; in this experiment no more than 1.5% of the energy consumed appeared as energy in the form of wool fibre.