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An individual feeding experiment was conducted to estimate diurnal and dietary impacts on microbial CP (MCP) production estimated from urinary purine derivative (PD) and creatinine excretion. Heifers were fed one of three diets formulated to produce differences in MCP production: an 85% steam-flaked corn-based diet (SFC); the SFC diet with 1.5% urea (UREA); or a corn milling byproduct-based diet (BYPROD). Spot samples of urine were collected at 0700 and 1700 hours. No urine collection time x dietary treatment interactions were present for any variable. Dry matter intake, ADG, and F:G were poorest with the SFC treatment. Urinary PD:creatinine (PD:C) ratio was greatest with the BYPROD treatment and lowest with the SFC treatment, measuring 0.94, 1.18, and 1.25 for the SFC, UREA, and BYPROD treatments, respectively. Regardless of diet, PD:C was greater with samples collected later in the day, and differences in PD:C due to diet can be observed regardless of collection time.