A study of the characteristics of wool follicle and fibre in Merino sheep genetically different in wool production

Abstract

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.