Milk production and nitrogen excretion of dairy cows fed different amounts of protein and varying proportions of alfalfa and corn silage.

Four trials were conducted to determine the effect of dietary protein amount on lactation performance and N utilization. Each trial used one of the following alfalfa-to-corn-silage ratios for the forage part of the diet: 100:0, 75:25, 50:50, and 25:75. All trials utilized 16 mid-lactation Holstein cows (days in milk averages ranging from 80 to 140 among trials) in a replicated 4 x 4 Latin square design with 3-wk periods, including 2 wk for adaptation and 1 wk for data collection. Diets consisted of 50% forage and 50% concentrate (dry matter basis) and were formulated to contain 15.00, 16.25, 17.50, or 18.75% protein in each trial. The analyzed protein content of the diets was 15.7, 16.9, 18.0, and 19.2% when averaged across trials. Milk yield was similar among dietary protein levels in each trial, ranging from 35.2 to 36.1 kg/d when data were combined across trials. Changes in milk fat and protein due to the protein content of the diet were small and inconsistent. Both milk urea nitrogen and blood urea nitrogen concentrations increased linearly as the protein content of the diet was increased, ranging from 9.9 to 13.1 and from 9.9 to 13.8 mg/dL, respectively, across trials. As dietary protein was increased from the lowest to the highest concentrations when data were combined and analyzed, mean fecal N concentration increased from 2.8 to 3.0%, and urinary N from 5.8 to 7.3 g/L. At the same time, mean total N excretion increased from 484 to 571 g/d, and conversion of intake N to milk N decreased from 0.27 to 0.22, resulting in an average change of 18%. Of the N excreted, urinary N accounted for an increasing proportion, ranging from 41 to 48%, as dietary protein was increased. Overall, based on N utilization as well as milk production, 17% protein in diets utilizing various proportions of alfalfa and corn silage as the forage source appeared sufficient for cows producing 38 kg/d of milk in this study.