The effect of varying crude protein level and the proportions of fibre in diets containing a mixture of meat-and-bone meal and wheat bran as the only protein source on nitrogen balance indices and energy digestibility in the rat.

1. The experiment was designed to investigate the effect of protein level in diets on apparent protein digestibility (AD), true protein digestibility (TD), biological value (BV), net protein utilization (NPU) and energy digestibility (DE) in rats. The design involved fourteen dietary treatments in which a combination of wheat bran and meat-and-bone meal (1:1, w/w dry matter (DM) basis) was the only nitrogen source. Crude protein (N x 6.25; CP) level ranged from 60 to 329 g/kg DM in approximately even increments. All diets contained a constant level of minerals and vitamins. The balance of the diet consisted of a N-free mixture. 2. AD increased from 0.602 at 60 g CP/kg DM to 0.702 at 120 g CP/kg DM, with little change at the higher levels of dietary protein. The fitting of a curve indicated an asymptote of 0.721. 3. TD did not vary significantly between 60 and 225 g CP/kg DM (mean 0.781). At the higher protein levels there was a slight but significant reduction in the mean value (0.758). 4. BV increased from 0.475 at 60 g CP/kg DM to 0.498 at 80 g CP/kg DM where it plateaued, and then from 160 g CP/kg DM it decreased at an increasing rate to 0.216 at 329 g CP/kg DM. 5. The response in NPU closely reflected that obtained with BV, rising from 0.370 to 0.384, then declining to 0.162 at the highest CP concentration. 6. DE decreased linearly from 0.851 at 60 g CP/kg DM to 0.617 at 329 g CP/kg DM. 7. The results confirmed that AD values should only be compared at a defined protein level. TD was essentially independent of dietary CP level, except for a small reduction above 250 g CP/kg, whereas BV and NPU were stable only within the range 80-140 g CP/kg DM. DE was negatively related to dietary CP and fibre level.