Two studies were conducted using rams from the Massey University fleeceweight-selected (FW) and control (C) lines of Romney sheep. In the first study, conducted from April to July, nine ram hoggets from each line (mean liveweight 42.1 kg) were fed 1000 g/day protein-extracted lucerne meal for 6 weeks before being blood-sampled by jugular venipuncture. They were then assigned to one of three groups receiving 0, 30, or 60 g/day formaldehyde- protected casein in addition to the basal diet. Blood sampling was repeated after 4 weeks of casein supplementation. During the pre-casein period, FW rams exhibited lower concentrations of urea (6.1 versus 7.2 mM, SE=0.3,P 0.05) and thyroxine (3.3 versus 4.3 #x3BC;g/dl, SE = 0.5,P 0.10) than Crams. During the period of casein supplementation, intakes of nitrogen (N) in the form oflucerne meal increased with dose of casein (from 24.8 to 26.1 g/day) but were not different between the lines. Casein supplementation also increased plasma urea concentration in a dose-responsive manner with no evidence of a casein level x selection line interaction. Plasma concentrations of the enzymes alanine aminotransferase and aspartate aminotransferase were not different between the lines at any time. In the second study, five rams from each line (mean 60.9 kg liveweight) were fed 110 maintenance as lucerne chaff for 30 days. Mean intakes ofN as lucerne chaff were 32.3 g/day and were not influenced by line. The rams then received intravenous infusions of saline or methionine (25.7 mg/kg liveweight per day) in a switchback design during July-August. FW rams maintained lower plasma urea concentrations (by 1 mM) than C rams during the saline infusions and this difference was halved during methionine infusion. In addition, FW rams had lower concentrations of creatinine and thyroxine in plasma (P 0.01) but neither difference was influenced by methionine treatment. Erythrocyte reduced glutathione concentrations were not different between the lines but were elevated by methionine infusion. It is concluded that selection for greasy fleeceweight in these lines has been accompanied by changes in plasma concentrations of urea, creatinine, and thyroxine. Differences in plasma urea concentration between the lines appear to reflect differences in the efficiency with which amino acids are used for production, and differences in kidney function. The physiological basis of differences in circulating thyroxine is not known. Although these differences are unlikely to be useful as predictors of genetic merit in the field (as they are influenced by non-genetic factors such as nutrition), an understanding of their physiology may help in the development of genetic markers.
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