Changes in amino acid and glucose metabolism in response to increments of rumen escape protein (REP) were studied in groups of lambs of three differing dietary histories and consequent weights, but similar ages. Crossbred wether lambs (Merino × (Border Leicester × Merino)) were fed to obtain three distinct growth patterns. The LW group (n = 15) were offered a low quality roughage diet throughout the experiment. The MW group (n = 19) were offered a high quality mixed diet followed by the same low quality diet as LW lambs. The HW group (n = 8) were offered a high quality mixed diet throughout. All diets were offered once daily ad libitum. The LW, MW and HW groups had liveweights of 18, 32 and 41 kg respectively at the commencement of supplementation, and were 33±0·1 weeks of age. REP supplements (formaldehyde-treated casein) were offered at 0, 20, 40, 60 or 80 g/day to MW and LW lambs and at 0 or 40 g/day to HW lambs.

REP increased basal digestible organic matter intake (DOMI), liveweight gain (LWG) and urinary N excretion and tended to increase N balance in LW and MW lambs. DOMI, N intake, N balance and LWG were all higher (P < 0·05) in HW compared to MW and LW lambs. REP tended (P < 0·10) to increase LWG in each dietary history group.

Blood glucose concentration was higher (P < 0·01) in HW than in other lambs but was not significantly altered by REP supplementation. Irreversible loss of glucose was greater (P < 0·01) in HW lambs and increased (P < 0·001) with REP for LW and MW lambs. REP increased (P < 0·05) phenylalanine (Phe) concentration in blood, Phe flux and oxidation and whole body rates of protein synthesis and degradation. HW lambs had higher (P < 0·05) values for all these parameters than did MW and LW lambs.

REP increased (P < 0·05) plasma concentrations of insulin-like growth factor-1, and plasma insulin increased (P < 0·05) in MW but not in LW or HW lambs. REP had no effect on plasma growth hormone (GH) concentration. Plasma concentration of insulin was higher (P < 0·05) in HW than in MW or LW lambs, while GH was not significantly affected by dietary history.

The results show that supplementation of ruminant diets with REP increases the rate of flux and oxidation of amino acids, and the rate of glucose utilization. Amino acid supply appears to influence glucose utilization more through oxidation rate than supply, and this relationship is affected by previous dietary history (weight for age) and energy availability, either from the diet or from body stores.