The rates of entry of cystine into the extracellular compartment, and the concentrations of cystine in this compartment, were measured during 12-14 hr infusions ofL-[35S]cystine in Peppin Merino ewes_ The ewes were sampled from flocks selectively bred for high or low clean fleece weight per head; and were fed either 500 or 1100 g of chaffed lucerne hay daily during the experiment.The mean entry rate of cystine was similar for sheep in the two flocks, but was greater in those sheep eating the 1100 g of lucerne daily (73 v. 35 mg/hr: P < 0·05). The concentration of free cystine in the plasma was approximately 20% less in those ewes from the flock selected for high clean fleece weight, relative to those selected for low clean fleece weight (P < 0·05). Although the concentration of free cystine in plasma increased with the amount of feed given (P<0'05), the difference between the flocks was independent of feed intake.The proportion of the cystine disulphide bound to plasma proteins relative to total cystine in the plasma decreased as the amount of feed given increased (P < O· 05), but the proportion was similar for the two flocks. The [35S] activity associated with sulphate sulphur increased linearly between 4 and 14 hr of infusion, with no significant differences in the slopes of the regression coefficients between flocks or levels.The conclusion was drawn that genetic differences in efficiency of conversion of food into wool resulting from selective breeding for fleece weight were not associated with differences in the entry rate of cystine.

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.

Ten, 2-year-old Merino ewes from a flock selectively bred for high clean fleece weight (Fleece Plus) and ten from a flock bred for low clean fleece weight (Fleece Minus) were randomly divided between two dietary treatments: 600 or 1100 g/day of pelleted lucerne hay. After 14 weeks, each ewe received an intravenous injection of . Venous blood samples were collected at 15 specified times until 8 h after the injections, and wool fibres were plucked until 65-75 days after the injections.Protein-free filtrates prepared from blood plasma were bulked within sample times for ewes from the same flock and dietary treatment. Equations relating the specific radioactivity of free cystine isolated from the bulked filtrates to time after injection contained three exponential terms. The entry rate and pool size of cystine estimated from these equations were greater in Fleece Minus than in Fleece Plus ewes (by 25 and 44 % respectively for entry rate and pool size). Both traits were also higher in ewes offered 1100 g lucerne/day than in those offered 600 g/day (58·7 v. 33·9 mg/h for entry rate and 19·2 v. 11· 8 mg for pool size). The concentration offree cystine in plasma was greater in ewes offered 1100 g lucerne/day (3·0 v. 2·1 mg/I; P < O· 05), and greater in Fleece Minus ewes (3·0 v. 2·1 mg/I; P < 0·05).The percentage of the injected radioactivity recovered in the wool clipped to day 70 post-injection differed between genotypes and between dietary treatments (P < 0·05), being greater in Fleece Plus than in Fleece Minus ewes, and greater in those offered 1100 g lucerne/day than in those offered 600 g/day.The relationships between 35S incorporated per 1000 fibres (R) and time after injection (t) were best fitted by equations of the formFor all sheep, n = 3. The coefficient of the second term was significantly greater (P < 0·05) in ewes offered 1100 g lucerne/day, whilst the constant of this term was significantly greater in Fleece Minus ewes.The specific radioactivities of cystine incorporated into wool fibres (SR,) during various intervals of time after injection were derived from these equations and from the measured rates of output of cystine in wool. The equations computed to relate SR, to time after injection (t) were of the form 1=1Again there were three components. The coefficient of the third component was significantly greater

The effects of daily abomasal infusions of L-cystine (2 g) or DL-methionine (2, 5 g) on the production of wool and its content of sulphur were compared in Peppin Merino ewes from paired flocks selected for high (Fleece Plus) or low (Fleece Minus) fleece weight.The six ewes of the Fleece Plus flock produced 16 % more wool than those of the Fleece Minus flock, when the ewes of both flocks were offered a basal diet of 800 g of ground and pelleted lucerne hay daily. Abomasal infusions of L-cystine or DLmethionine increased the rate of wool growth in the two flocks, but an interaction between treatment and genotype (P < 0·05) resulted from a greater response to the amino acids in sheep from the Fleece Plus flock (55 v. 15%). The infusion of DL-methionine caused a greater increase (P < 0·05) in wool production than that due to the infusion of an equivalent quantity of L-cystine (41 v. 32%).Both amino acids increased the sulphur content of the wool (P < O· 05) with no difference in the response due to either amino acid. The wool grown by sheep in the Fleece Plus flock contained 10% less sulphur, and this difference between the flock means was independent of treatments.

Twelve mature ewes from a flock selected for high clean fleece weight (Fleece Plus) and twelve from a flock selected for low clean fleece weight (Fleece Minus) were randomly divided between two dietary treatments: 500 or 1100 g per day of chaffed lucerne hay. After the sheep experienced these dietary regimes for 40 days, each ewe was infused intravenously with L-[35Sjcystine for 12-14 hr. Incorporation of 35S activity into fibres during the infusions, and into wool clipped subsequent to the infusions, was measured.During the final 7-9 hr of infusion, the 35S activity in plucked fibres increased linearly with time. The linear regression coefficients relating 35S activity to time did not differ in the genetic or the dietary comparisons, regardless of whether the rate of incorporation of 35S was expressed relative to a 1000 fibres, or to the number of fibres growing from unit area of skin. The mean specific radioactivity of cystine incorporated into wool proteins during the infusions was less in ewes consuming 1100 g per day of lucerne (23 v. 35 nCi/mg : P < 0'05). This difference was less than the difference in the "plateau" specific radioactivity of cystine in the plasma, indicating that the 35S was differentially diluted with "cold" cystine during its transfer from plasma into the wool fibre; dilution being greater in ewes consuming the lesser quantity of feed (4, 0 v. 2·9 :A similar effect of feeding level was evident on the specific radioactivity of cystine in wool clipped from the ewes subsequent to the infusions. In addition, the specific radioactivity of cystine in wool from Fleece Plus ewes was less than that from Fleece Minus ewes (3, 2 v. 3· 6 nCi/mg : P < O· 05), indicating that greater dilution of 35S with "cold" cystine was occurring in these ewes. As judged by the wool production per unit area of skin, the efficiency of conversion of food to wool would have been greater in Fleece Plus ewes and in those consuming 500 g per day of lucerne. Thus, mechanisms controlling the dilution 0[35S between plasma and fibre, presumably arising