Dietary manipulation to reduce nitrogen and phosphorus excretion by dairy cows

The metabolism and utilization of nitrogen (N) in dairy cows can be affected by phosphorus (P), as insufficient P intakes reduce ruminal fermentation, resulting in low microbial protein production and nutrient digestibilities. There has been renewed research in recent years on the requirements of N and P of dairy cows in an effort to reduce their excretion, but most studies were conducted with a nutrient fixed while evaluating the level of the other nutrient in the diet. This experiment was undertaken to study the effect of reducing both dietary N and P on excretion and production of dairy cows. Eight lactating cows were paired in a replicated 4 × 4 Latin square design with 20-d periods, and cows in each pair were randomly assigned to one of the following four dietary treatments: low crude protein (CP), low P (15% CP, 0.34% P), low CP, medium P (15% CP, 0.39% P), low CP, high P (15% CP, 0.44% P), and high CP, high P (17% CP, 0.44% P). Reducing dietary protein from 17 to 15% resulted in increased conversion of intake N to milk N (P = 0.05) and reduced milk urea N (P < 0.01), ruminal ammonia N (P = 0.03), plasma urea N (P = 0.01), and urinary N (P < 0.01). Reducing dietary P from 0.44 to 0.39 and 0.34% decreased (P < 0.01) P excretion. Feed intake, milk yield, milk composition, nutrient digestibility, plasma metabolites, and ruminal pH and volatile fatty acids were not affected by varying dietary CP or P. Reducing the amount of protein and P fed to dairy cows in the current experiment reduced N and P excretion and increased the efficiency of utilization of these nutrients without affecting milk production.