Three series of nitrogen balance experiments were performed with growing rats to test the effect of type and level of protein, fiber and starch on N excretion patterns. The design involved 8 treatments in a 3 .times. 3 .times. 2 factorial experiment with 5 rats/dietary treatment. The 18 treatments resulted from a combination of 3 protein treatments, 3 fiber treatments and 2 starch treatments. Protein treatments consisted of 1 level (15 g N/kg DM [dry matter]) of casein fortified with methionine, a protein of high digestibility, and 2 levels (15 and 20 g N/kg DM) of autoclaved brown bean (Phaseolus vulgaris), a protein source of low digestibility. The fiber treatments were 2 levels of cellulose powder and 1 level of barley hulls. The 2 starch treatments were autoclaved potato starch and autoclaved and raw potato starch (1:1, wt/wt). The inclusion of raw potato starch reduced true protein digestibility markedly when the protein source was casein but the corresponding biological values were increased significantly with this treatment. This strongly indicated a movement of urea from the blood to the intestines. This assumption was also supported by significantly lower blood urea concentrations in animals given raw starch. The influence of raw starch on true protein digestibility was significantly less when cellulose and barley hulls were included. This is probably due to reduced transit time from fiber inclusion. The nature of the gut contents also supported this hypothesis. The inclusion of raw potato starch when brown beans were the source of protein had much less effect on true protein digestibility and biological value than when casein was the protein source. This was probably due to the low digestibility of DM and protein in this food leaving sufficient energy and protein for maximum microbial growth. The inclusion of fiber also had little effect on N excretion patterns with the brown bean diets. An increase in the level of brown bean inclusion reduced true protein digestibility only on the diets containing raw starch whereas the biological value was consistently reduced regardless of starch treatment. The lower biological values were associated with significantly higher blood urea concentrations. Increasing the level of brown bean inclusion also resulted in higher fresh weights of cecum, colon and contents. Through dietary manipulation, it is possible to affect N excretion patterns in rats.
