Animals are 15N-enriched compared to the diet they consume and the extent of this isotopic enrichment ( 15N) has been shown to vary according to different animal and environmental factors. We investigated the relationship between 15N, the efficiency of N utilization (ENU) and N excretion in young finishing beef cattle. Young bulls (n=35) were individually fed after weaning one of the 4 experimental diets based on either corn silage or pre-wilted grass silage offered at two different intake levels (high and low) according to a complete block design experiment. Plasma was sampled from all animals the day before the first slaughter and diet ingredients were sampled throughout the experiment. The natural abundance of 15N in plasma protein and diets was analyzed and 15N was calculated. The N balance of 8 extra animals fed the same diets was conducted according to a replicated 4 × 4 Latin square design to evaluate the fecal and urinary N excretion. Animals on the corn silage diets had higher ENU and lower total N excretion and 15N compared to grass silage fed animals. The 15N had a good correlation with ENU and could be used to evaluate the environmental N excretion of beef cattle in different feeding conditions.