Davies, K. L., McKinnon, J. J. and Mutsvangwa, T. 2013. Effects of dietary ruminally degradable starch and ruminally degradable protein levels on urea recycling, microbial protein production, nitrogen balance, and duodenal nutrient flow in beef heifers fed low crude protein diets. Can. J. Anim. Sci. 93: 123-136. The objective was to determine the effects of ruminally degradable starch (RDS; 28.6 and 69.2% of total starch) and ruminally degradable protein [RDP; 48.0 and 55.0% of crude protein (CP)] content on urea recycling, nitrogen (N) balance, duodenal nutrient flow, and microbial protein production in beef heifers fed low CP (10%) diets. Four ruminally and duodenally cannulated beef heifers (723±57 kg body weight) were used in a 4×4 Latin square design with a 2×2 factorial arrangement of dietary treatments with 23-d periods. Jugular infusions of [¹⁵N¹⁵N]-urea (220 mg d^-1; 98 atom percent) were conducted for 4 d (days 18-22) to estimate urea kinetics, with total collection of faeces and urine. Proportions of [¹⁵N¹⁵N]- and [¹⁴N¹⁵N]-urea in urinary urea, and ¹⁵N enrichment in faeces were used to calculate urea kinetics. Ruminal microbial N production was estimated using ¹⁵N as a marker. Ruminal ammonia-N concentration was greater (P=0.01) in heifers fed high RDP as compared with those fed low RDP, and it was also greater (P=0.01) in heifers fed low RDS as compared with those fed high RDS. Microbial N flow to the duodenum increased as RDP level increased on the high RDS diet, but was not affected by RDP level on the low RDS diet (interaction; P=0.04). Urea-N entry rate and urea-N transfer to the gastrointestinal tract were similar (P>0.05) across diets. The amount of recycled urea-N incorporated into microbial N increased as RDP level increased on the high RDS diet, but the opposite was observed on the low RDS diet (interaction; P=0.008). These results indicate that at a low CP level (10%), increasing both RDS and RDP levels can increase microbial N flow to the duodenum and improve the efficiency of use of recycled urea-N for microbial N synthesis.