Efficiency of conversion of food to wool. IV. Comparison of sheep selected for high clean wool weight with sheep from a random control group at three levels of dietary protein

Abstract

An experiment is described in which ewes from a flock selected for a number of years for high clean wool weight per head (S) are compared with ewes from a random control flock (C) under ad libitum feeding of three diets differing in crude protein content but not in gross energy. Differences between the S and C groups were significant for clean wool weight (W), efficiency of conversion of gross energy to wool (W/I), fibre number per unit area of skin (N), and mean body weight (B), the S group having higher values for W, W/I, and N but being lighter in B. Differences between the means of ewes fed on the high and medium protein diets (H and M groups) and the means of those fed on the low protein diet (L group) were highly significant for Wand each of its physical and physiological components. The H and M groups had higher values for W, W/I, B, intake per unit body weight (I/B), fibre cross-sectional area (A), staple length (L), and smooth body surface area (S) but lower values for N. The mechanisms of these differences are discussed in relation to other published work. Differences between ewes fed on the H and M diets were significant for Wand W/I, the H groups having the higher value in each case. These responses are contrasted with those reported by other workers and the possible mechanisms are outlined. When differences between the S and C groups on the H and M diets were averaged and contrasted with the differences between them on the L diet, there were significant group x diet interactions in W and W/I and appreciable but not significant interactions in N, A, and L. Taking the value for the C group as 100 in each case, the values for the S group on the L, M, and H diets respectively were: W, 97, 125, 129;W/I, 92,129,132; N,129,110,111; A, 74,114,110; L, 92,109,110. The possible mechanisms for these changes in the differences between the S and C groups are discussed. The group x diet interaction was not significant for any character when the difference between S and C groups on the H diet was contrasted with the difference between them on the M diet, though there were slight increases in the differences in Wand W/I on the H diet.