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The objectives of this experiment were to determine the effects of feeding a combination of modified wet distillers grains with solubles (WDGS) and wet corn gluten feed (WCGF) on nutrient digestion, purine derivative excretion, and N utilization. Multiparous (n = 20) and primiparous (n = 20) cows were arranged in a replicated 5 × 5 Latin square with 21-d periods. Animals were fed one of 5 treatment diets during each period: 1) 0% co-products (control); 2) 15% WDGS (15WDGS); 3) 15% WCGF (15WCGF); 4) 7.5% WDGS and 7.5% WCGF (15MIX); and 5) 15% WDGS and 15% WCGF (30MIX; dry matter basis). A portion of forages, corn, and soy-based protein was replaced with WDGS, or WCGF, or both. Dry matter intake was greater for 15WDGS (25.1 kg/d) and 30MIX (25.5 kg/d) than for control (22.4 kg/d), 15WCGF (23.2 kg/d), or 15MIX (23.5 kg/d). Dry matter digestibility was greatest for 15WCGF and 30MIX (63.6 and 64.1%, respectively) and least for 15WDGS (59.8%), and neutral detergent fiber and N digestibility were greatest for 30MIX (50.7 and 68.6%, respectively) and lowest for 15WDGS (41.3 and 61.5%, respectively). Excretion of purine derivatives in urine was greater for co-product treatment diets than for control. Fecal N was greatest for 15WDGS compared with other treatment diets (311.0 vs. 263.3 g/d), whereas urinary N was greatest for 30MIX (330.0 g/d), intermediate for 15WCGF and 15MIX (319.3 and 320.5 g/d, respectively), and lowest for control and 15WDGS (308.5 and 312.2 g/d, respectively). Manure N (fecal + urinary N) was greatest for 15WDGS, intermediate for 15MIX and 30MIX, and lowest for control and 15WCGF. Treatment diets did not differ in 4% fat-corrected milk production. Compared with the ration containing WDGS, the ration with a 30% mixture of WDGS and WCGF improved nutrient digestibility and N utilization with reduced manure N excretion and increased N retention. Thus, it appears feeding WDGS and WCGF in combination reduces some of the negative effects of feeding WDGS alone.