Diurnal and dietary impacts on estimating microbial protein flow from urinary purine derivative excretion in beef cattle1

Two experiments were conducted to determine the effects of diet composition and time of urine spot sampling on estimates of urinary purine derivative (PD) excretion. In Exp. 1, 116 individually fed crossbred heifers (407 ± 32 kg) were arranged in a randomized block design (82 d). Treatments were arranged in a 2 × 3 factorial design, with two urine spot sample collection times (0700 and 1700 hours; AM and PM) and three diets: 85% steam-flaked corn (SFC); 85% SFC + 1.5% urea (UREA); or 25% SFC, 30% wet corn gluten feed, and 30% corn bran (BYPROD). In Exp. 2, six ruminally and duodenally fistulated steers (474 ± 37 kg) were arranged in a replicated 3 × 3 Latin square design, with dietary treatments identical to Exp. 1 (63 d). Treatment diets were selected to result in varied amounts of microbial crude protein (MCP) in order to evaluate the accuracy of using estimates of urinary PD excretion to predict MCP. Urine spot samples were collected at 0700, 1200, 1700, and 2200 hours. No urine collection time × diet interactions occurred (P > 0.20) for any variable in either experiment. In Exp. 1, dry matter intake (DMI) was greatest with BYRPOD (10.40 kg/d) and lowest with SFC (7.90 kg/d; P < 0.05). Feed efficiencies were greatest for UREA (0.182) and least for SFC (0.141; P < 0.05). Urinary PD:creatinine (PD:C) ratio was greatest for BYPROD (1.25) and least for SFC (0.94; P < 0.05). Urine spot sampling time had a significant (P < 0.05) impact on PD:C, 1.03 for AM and 1.22 for PM samples. In Exp. 2, DMI was greater (P < 0.05) with BYPROD than with SFC and tended (P = 0.07) to be greater with BYPROD than with UREA. Ruminal pH was greatest for BYPROD (5.94; P < 0.05). Flow of MCP was 636, 829, and 1,056 g/d for SFC, UREA, and BYPROD, with BYPROD being greater (P < 0.05) than SFC and tending (P = 0.06) to be greater than UREA. Urinary PD:C was greater (P < 0.05) for BYPROD than SFC and tended (P = 0.09) to be greater for UREA than SFC. Urinary PD:C increased linearly (P < 0.05) with sampling time. Diets formulated to affect DMI and MCP flow resulted in differences in urinary PD excretion, and these results related well with MCP flow estimated from duodenal purines. Collecting spot samples of urine later in the day resulted in greater estimates of urinary PD excretion; purine and PD flows appear to increase with time after one morning feeding per day. This method is well suited to evaluating relative differences between treatments but should not be extrapolated to assume absolute values.