Partitioning of limiting protein and energy in the growing pig: testing quantitative rules against experimental data.

Literature solutions to the problem of protein and energy partitioning in the growing pig are quantitatively examined. Possible effects of live weight, genotype and food composition on the marginal response in protein retention to protein and energy intakes, on protein and energy-limiting foods are quantified. No evidence was found that the marginal response in protein retention to ideal protein supply, when protein intake is limiting, is affected by live weight, genotype or environmental temperature. There was good evidence that live weight does not affect the marginal response in protein retention to energy intake when protein intake is not limiting. Limited data for different genotypes suggested no effects on this response. A general quantitative partitioning rule is proposed that has two key parameters; e(p)* (the maximum marginal efficiency for retaining the first limiting amino acid) and R* (the maximum value of R, the energy to protein ratio of the food, MJ metabolisable energy (ME)/kg digestible crude protein (DCP), when e(p)* is just achieved). When R<R* the material efficiency of using ideal protein is (e(p)*/R*) x R. The value of e(p)* was determined to be 0.763 (SE 0.0130). There was no good experimental evidence that e(p)* is different for different amino acids. The best estimate of R* was 67.9 (SE 1.65) MJ ME/kg DCP. Live weight, genotype and temperature did not affect the values of either parameter. A more general understanding of partitioning, including the effects of 'stressors' such as disease, may be achieved by using the preferred rule as a starting point.