Effect of Different Amounts of Protein and Varying Proportions of Corn Silage and Alfalfa hay on Milk Production and Nitrogen Excretion of Dairy Holstein Cows

Four treatments were assigned to determine the effects of dietary potential digestible fiber (pdF) and crude protein (CP) levels in mid lactating Holstein cows on milk production and nitrogen efficiency. Sixteen primiparous (n=8) and multiparous (n=8) Holstein cows (body weight (BW)=546±72 kg; days in milk (DIM)=118±50 day) were allocated to one of four diets in balanced randomized complete design in two periods (each period including 28 day). Diets consisted of 50% forage and 50% concentrate. The experimental diets were as follows: 1) 19% CP and low pdF (HPLDF); 2) 17% CP and low pdF (MPLDF); 3) 15% CP and high pdF (LPHDF) and 4) 17% CP and high pdF (MPHDF). Milk yield was similar among dietary protein levels in each treatment, when data were combined across treatments. Both milk urea nitrogen and blood urea nitrogen concentrations increased as the protein content of diet was increased. As die-tary protein was increased from the lowest to the highest concentrations, mean fecal N concentration increased from 2.8 to 3.0% and urinary N from 5.8 to 7.3 g/L (P 0.05). Overall, based on N utilization as well as milk production, 17% protein in diets utilizing various proportions of alfalfa hay and corn silage as the forage source appeared sufficient for cows producing 27 kg/d of milk in this study. Reducing protein to this amount can reduce N excretion, especially environmentally labile urinary N, without affecting milk production.