The ruminant is less efficient at utilising dietary nitrogen (N) for growth than the non-ruminant. Some of this inefficiency may be due to differences in the timing of energy and N absorption following a meal. In the non-ruminant, both energy and amino acids are absorbed together, relatively rapidly following a meal. By contrast, the ruminant absorbs its nutrients asynchronously and this may lead to reduced post-prandial anabolic stimulus. A series of experiments were conducted in which the effects of different temporal patterns of posts ruminal nutrient supply on whole body and tissue metabolism were assessed. When energy and protein were supplied twice daily in three hour pulses, thus mimicking the non-ruminant, urea production was markedly reduced whilst tissue insulin exposure over the twelve hour infusion cycle was increased. This combination most likely indicates that net amino acid retention was increased by the synchronous infusion pattern. Infusion pattern does not influence protein synthesis in muscle, skin, gut or liver, however, and therefore the changes in whole body urea metabolism are most likely mediated through modulation of proteolytic systems. Maximising the capture of N in the post-prandial period may therefore be a means of increasing the efficiency of ruminant production systems.

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The aim of the present work was to study the protein utilization during energy undernutrition in sheep. In four different trials, the effects of varying levels of protein with submaintenance amounts of energy on the energy &'38 N balances of sheep were studied using the intragastric infusion technique and the respiration chamber. The effects of change in energy and protein supply on plasma metabolites and hormonal concentrations were also studied. When animals were given protein well in excess of their maintenance requirement with little or no non-protein energy, they attained positive N balance although they were in negative energy balance, apparently by efficient utilization of endogenous energy (presumably body fat). As body fat was used to fuel the energy needed for protein retention, fat and protein deposition were negatively correlated. However, at very high level of casein infusion, the oxidized component of the supplied protein can contribute up to 36&'37 of the total ME requirement. About 16 kJ of endogenous energy was used for each g of protein accretion. The efficiency of endogenous energy utilization ranged between 0.56 to 0.60. There was no clear evidence, that there is any minimum level of body fatness which is necessary before body fat can be utilized to support protein retention during exogeneous non-protein energy restriction. Protein utilization during exogenous energy restriction was found to be more affected by the growth potential than the adiposity of the animal. Both fasting-heat production &'38 N excretion were reduced when the glucogenic needs of animals were met. Similarly plasma glucose, -hydroxybutyrate and free fatty acids concentrations were not affected by the energy status of the animals, when glucose requirements of the animals were met.