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.
