Use of urinary purine derivatives to estimate microbial protein flow in cattle

Urinary purine derivatives (PD) are the excretion products of purines, and may be used as an alternative to duodenal purines to estimate microbial crude protein (MCP) flow. When urine volume is quantified using urinary creatinine excretion, spot samples of urine can be used to estimate relative differences in MCP flow. Four experiments were conducted to further advance the use of urinary spot samples to estimate MCP flow. In the first experiment, six steers were fed diets with three levels of roughage with and without calcium magnesium carbonate. Increasing roughage level resulted in a linear increase in ruminal pH and a linear decrease in total tract organic matter digestibility, but did not affect urinary PD:creatinine (PD:C) ratio. To determine the relationship between ruminal MCP flow and urinary PD excretion, 116 heifers were individually-fed diets formulated to produce differences in ruminal MCP flow. When urea was added to a steam-flaked corn diet, feed intake and urinary PD:C increased. Further increases were observed when a corn milling byproduct was added to the ration. In a metabolism experiment utilizing the same dietary treatments, feed intake and urinary PD:C ratio again increased when a corn milling byproduct was added to the ration. Microbial CP flow estimated from duodenal purines was related to urinary PD:C ratio, suggesting that spot samples of urine can be used to predict relative differences in MCP flow. In both experiments urinary PD:C increased with time after feeding, suggesting that a diurnal effect may occur. A final experiment concluded that level of feed intake did not affect microbial purine:N ratio, and determined that urinary PD are related to duodenal purine flow. It was also concluded that the diurnal effect on urinary PD:C ratio previously observed was likely due to feeding time, as no diurnal impact was observed when steers were fed diets in six equal portions throughout the day