Incorporating turn-over in whole body protein retention efficiency in cattle and sheep

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Abstract
In pigs the quantification of breakdown and synthesis by powers of body protein led to the estimation of turn-over related protein retention efficiency by the equation kP= {1 + [1 − (P/α)(2/9)Q]−1/6}−1, with α the limit value of whole body protein (P) maturity, so that 0 ≤(P/α)≤1. The factor 2/9 is derived from diffusion attributes indicated by cell and nucleus geometries α and Q represents a scaled transformation of intake, 0 ≤ Q ≤ 1, such that a value of Q = 1 may represent ad libitum intake and Q = 0 the intake at the maintenance requirement. Published observations on finishing steers provide estimates of whole body protein synthesis and breakdown at pre-determined levels of intake in confirmation of the theoretical (2/9)Q power associated with (P/α) inkP. Further confirmation of the (2/9)Q power in cattle follows from satisfactory agreement between an estimate of conventional multiple regression retention efficiency and the turn-over related retention efficiency calculated at the given level of intake, for the mid point of the body mass interval covered by the regression estimate. In addition, a simulation experiment on cattle from the literature gives power estimates of protein breakdown and synthesis in general agreement with those accepted for pigs. Examples on both fine and coarse diets are employed to suggest a general rule for prediction on diets causing submaximal efficiency due to suboptimal intakes.In sheep, evidence derived from estimates of conventional multiple regression efficiencies suggests that the rule (a-b) = (2/9) Q for the calculation ofkPshould be reserved for the description of compensatory growth. Protein retention efficiency for ordinary growth should be described by an adaptation of the rule derived for suboptimal intakes.