Interaction between protein and energy supply on protein utilization in growing cattle: A review

Based on research with growing pigs, most of the nutrient requirement systems for ruminants assume that the efficiency of amino acid (AA) utilization is not affected by energy supply. The objective of this literature review is to examine the information available to support that assumption for growing :ruminants. Because the study of interactions between protein and energy supply is difficult due to ruminal metabolism, different experimental approaches have been used. Some experimental models (i.e. dietary modifications) are simple, but in some instances have led to inconclusive evidence and misinterpretation of the results. The use of totally intragastrically maintained animals allows energy and protein supply to be varied independent of each other. When this technique was used, the results on the effects of energy supply on protein deposition were variable, but most of the evidence seems to suggest that energy supply increases protein deposition, even at limiting levels of protein supply. In addition, fat mobilization may support protein synthesis when energy is limiting, but an adequate AA supply is provided. With preruminant calves, energy supply also was associated with increases in protein deposition, even when protein intake was limiting. Similar results were observed with growing steers provided with supplemental energy and protein through ruminal and abomasal infusions. According to this model, energy intake, independently of the source of energy, improved the efficiency of AA utilization, but the magnitude of improvement may depend on which AA limits protein deposition. Although the results were variable, and direct comparisons among experimental approaches are difficult, most of the experimental evidence indicates that energy supply affects the efficiency of protein utilization. Thus, the assumption of a constant efficiency for a broad range of energy intake is unlikely to be appropriate for growing ruminants. Although more research is needed to understand the mechanisms involved in those effects, modeling of AA requirements in growing cattle may require consideration of the amount of dietary energy supplied. (c) 2007 Elsevier B.V All rights reserved.