Four experiments were conducted to evaluate different sources of protein as supplements for growing cattle fed corn stover silage;Isonitrogenous and isocaloric supplements containing soybean meal (SBM), dehydrated alfalfa (DEHY ALF), corn gluten meal (CGM), casein (C), urea (U) or a negative control diet (CNL) were compared in a digestion trial. Source of protein did not affect feed intake of the steers. Digestibilites of dry matter (DM) and organic matter (OM) were not affected by source of protein. Crude protein (CP) was most poorly digested by steers fed the DEHY ALF containing supplement. Digestion of CP by steers fed the CGM, SBM and U containing supplements was similar. Neutral detergent fiber (NDF) digestibility was not affected by source of protein. In a feedlot experiment involving 60 heifers and lasting 98 d, protein supplements containing U, SBM, formaldehyde treated SBM (TSBM), TSBM+U, TSBM+C or CNL supplement were studied. Crude protein and metabolizable protein content of the supplements were: 21.7, 8.0; 21.7, 10.5; 14.9, 10.8; 21.7, 10.8; 21.7, 10.9 and 14.9 and 9.0%, respectively. U, SBM, TSBM+U, and TSBM+C supplementation resulted in higher rates of growth and improved feed utilization over CNL and TSBM. Ruminal ammonia concentration and DM digestibility were higher for animals fed the U supplemented diet than animals fed all other supplements;The effects of varying ruminal ammonia (NH(,3)) concentration on utilization of corn stover were also studied in a digestion and a feedlot trial. The protein supplements, SBM and 5 supplements containing TSBM along with 0.0, 1.43, 2.47 or 4.47% urea contained 20.9, -2.5; 14.4, 0.0; 18.4, -1.5; 21.3, -2.5; 24.1, -3.5; and 26.9 and -4.5% CP and urea fermentation potential, respectively. Ruminal NH(,3) concentration increased linearly with increasing level of urea in the diet. Digestion of OM and NDF increased with increased urea in the supplement and approached maximum when urea was 3.47% of the supplement. Microbial N synthesis increased with urea level and tended to plateau at 3.47% urea. Increasing urea level in the diet did not affect microbial N synthesized per kg OM digested in the rumen. Average daily gain and feed efficiency of heifers fed the 6 supplements increased linearly and quadratically with increasing urea in the supplement. It is estimated that ruminal NH(,3) concentration for maximum microbial N synthesis and utilization of corn stover is about 9.32 mg/dl.