The aim of the present study was to compare the amount of microbial N entering the duodenum and the efficiency of N utilisation for microbial protein synthesis in normal (NP, 17.4 g N/d) and low protein (LP, 7.5 g N/d) fed Corriedale sheep. Renal functional tests for urea handling studies, and determination of urinary allantoin as an indirect method to estimate the microbial protein production ...