Fourteen Merino ewes from each of two flocks, selectively bred for either high (Fleece Plus) or low (Fleece Minus) clean fleece weight per head, were randomly allotted to two feeding levels (17 v. 34 g kg-1 liveweight) of a good quality diet and offered these levels for 100 days. During the final 42 days, the rate of wool growth and its components were measured in each sheep. The Fleece Plus sheep had a greater density of wool follicles and produced more wool per unit area of skin. The latter trait interacted with the nutritional level; Fleece Plus sheep were more responsive to the higher nutritional level (45 v. 13%). Both the rate of growth of fibre length and the diameters of fibres produced in secondary follicles were the components associated with this interaction. The sulfur content of the wool was 12% less in wool from Fleece Plus ewes, but the difference between the two flocks was unrelated to the nutritional level. The average diameter (c. 83 8m) of the bulbs of the follicles did not differ between flocks or dietary levels, but follicles in the skin of Fleece Plus sheep were deeper (P < 0.05) below the skin surface (1.52 v. 1.38 mm). Despite the similar maximum bulb diameters, random longitudinal sections of follicles from Fleece Plus sheep had a 10% greater area of mitotically active tissue (P < 0.05). The difference was independent of nutritional level. When colchicine (10 8g) was injected intradermally, the number of cells revealing metaphase arrest within the mitotically active area of the follicle increased linearly with time to 6 h after the injection. The rate of increase in these cells expressed per unit volume of bulb tissue was not however influenced by either genotype or nutritional level. The average volume of cortical cells isolated from the fibres did not differ between genotypes or nutritional levels, but as with fibre volume, the volume of the cortical cells of the Fleece Plus sheep was more sensitive to a change in nutritional level (interaction: P < 0.05). Calculation of the rate of incorporation of cortical cells into fibre from both rate of production of fibre and the size of the cortical cells indicated that 20% more cells were incorporated by the Fleece Plus sheep and 17% more cells were incorporated by those sheep offered the greater dietary allowance. There was no significant interaction between genotypes and nutritional level for this trait.
The efficiency of conversion of feed to wool was measured in three jocks of Peppin Merino sheep, namely : Fleece Plus-Selected for nine years on high clean fleece weight ; Fleece Minus-Selected for nine years on low clean fleece weight ; Random-An unselected, control flock. Ten 18 month old ewes from each of these flocks were individually housed and fed ad lib on hammer-milled lucerne hay for 54 weeks. Highly significant differences between flocks were observed in feed intake, wool growth, and efficiency of conversion of feed to wool. The relative productive levels for the Fleece Plus, Random and Fleece Minus flocks were-for feed intake, 108 : 100 : 88-for clean fleece weight, 118 : 100 : 70-and for efficiency 109 : 100 : 79. The correlation between efficiency and daily feed intake was very highly significant and negative within each flock, (-0.918, -0.929 and -0.924, for Fleece Plus, Random and Fleece Minus flocks respectively), whereas the correlation between jocks was significantly positive, r = 0.997. The relative importance of the factors determining the 18 per cent increase in clean fleece weight of the Fleece Plus over the Random flock was : efficiency, 55 per cent and feed intake, 45 per cent-and comparing the Random to the Fleece Minus flock : efficiency, 60 per cent and feed intake, 40 per cent. Approximately half the difference in feed intake was independent of body weight.
The efficiency of conversion of feed to wool was measured with adult ewes at three levels of nutrition. During the period at which each level was fed, changes in body weight and feed intake were minimized. These ewes were chosen as representative of the total drops of three flocks, one of which had been selected for high clean fleece weight (Fleece Plus), one for low clean fleece weight (Fleece Minus) and a control flock selected at random (Random). The intermediate and high levels of nutrition were intended to maintain 20 per cent and 44 per cent higher body weights than at the low level of nutrition. The average intakes of lucerne pellets at the low, .intermediate, and high levels of nutrition were approximately 500, 800, and 950 grams a day respectively. The average body weights at these intakes were 30, 36, and 45 kg and the average gains in body weight were -11, +7, and +16 grams a day during the 18 weeks of measurement at each level. There were no differences between the flocks in these measurements. The relative wool production of the Fleece Plus, Random, and Fleece Minus flocks changed between levels of nutrition, being 101 : 100 : 96 at the low level, 120 : 100 : 88 at the intermediate and 121 : 100 : 96 at the high level of nutrition. Part of this differential response could be attributed to different feed intakes between the flocks and part to a smaller decrease in the efficiency of the Fleece Plus flock with increasing level of nutrition. The relative net efficiencies measured at the intermediate level were 116 : 100 : 86.5 for the Fleece Plus, Random, and Fleece Minus flocks respectively.
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