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Ruminally and duodenally fistulated Holstein steers were fed at 40% and 85% of ad-libitum intake. Microbial purine:N ratio did not differ between intake levels, which makes estimating microbial protein easier in production or experimental settings. Urinary purine derivatives (PD):creatinine (PD:C) ratio and microbial CP flow estimated from the duodenum increased with the higher feeding level. Urinary PD:C and duodenal purine flow were related, suggesting that urinary PD:C ratio can be used to estimate relative differences in microbial CP flow.