Sources of variance in the bioassay of protein value.

A factorial design was used to simultaneously evaluate the relative effect of 8 experimental factors and their interactions on the weanling rat bioassay of protein value: (1) source of protein: ANRC casein, lactalbumin, high-protein wheat flour; (2) protein level of the diet: 5 and 10%; (3) dietary fat level: 10 and 20% corn oil; (4) animal: ARS-Sprague-Dawley, from Taconic Farms; (5) age of animal: 21 and 28 days; (6) acclimation time: 2 and 4 days; (7) replication: 2 complete replications in time; and (8) duration of the test: food consumption and body weights were measured at 3, 7, 10, 14, 17, 21, 24, and 28 days after starting the test diet and converted to the ratio of grams of weight gained per gram of protein consumed for each weigh day. The official AOAC method for determining the protein efficiency ratio was followed with minor modifications. All 8 factors and many of their possible interactions appeared to significantly influence the measured ratios. When the ratios were adjusted by reference to the corresponding value for casein as a control, fat level, rat source, rat age, and acclimation time were no longer significant sources of variation. Plots of measured ratios and their coefficients of variation against time suggest that the optimum assay time varies with the protein but that the assay time should not be less than 21 days. The generally used 28-day assay time seems to offer no increase in precision over 21 days.