The effect of energy supplementation on nitrogen utilization in lactating dairy cows fed grass silage diets.

An experiment was conducted to examine the effect that various isoenergetic diets, containing different quantities of soluble carbohydrate and fiber and different types of starch, have on nitrogen (N) balances. Six lactating dairy cows in early to midlactation consuming grass silage diets with not less than 600 g/kg total DMI as forage were used in the experiment. Four concentrates were prepared that had higher amounts of either fiber, soluble sugars, corn (low degradable starch source), or barley (high degradable starch source). Overall N utilization by the cows was poor, rarely exceeding 0.30 g milk N/g of dietary N intake. Fecal N outputs accounted for more than half of total N excreted in all treatments except for diets supplemented with high degradable starch, in which urinary N excretion was significantly higher compared with the other treatments. Milk yield was unaffected by concentrate type, averaging 19.9 kg/d, but milk protein content decreased from 32.9 for starch-based diets to 30.9 and 30.0 g/kg for the soluble sugar- and fiber-based diets, respectively. The efficiency of N utilization improved in the low degradable starch treatment, which had lower N excretion (65%) and higher protein concentration in milk. Furthermore, feeding cows corn-based concentrates reduced urinary N excretion by almost 30% compared with barley-based concentrates; therefore, feeding corn-based diets is recommended for the reduction of nitrogen pollution in lactating dairy cows.