Submitted to: Forage Focus
Publication Type: Popular Publication
Publication Acceptance Date: 4/20/2011
Publication Date: 5/2/2011
Citation: Powell, J.M., Bocher, L.W. 2011. Dairy cattle diets, manure chemistry, and soil nutrient cycles: how do they relate? Forage Focus. May 2011. pp 14-18.
Technical Abstract: While most milk leaves the farm as a desirable end product, manure has different fates – both desirable and undesirable. The desirable outcomes are those in which nutrients stay on the farm to help produce more feed and milk; and the undesirable outcomes are those in which nutrients enter the environment where they may have detrimental effects. In this article, based on a review of 6 feeding trials and on additional new research, we look at the nitrogen component of diets commonly fed to lactating dairy cows, manure chemistry, nitrogen cycles, and environmental impacts. High nitrogen content in the diet increases nitrogen in manure, mostly in the form of urea in urine, which is hydrolyzed then either lost as ammonia gas or mineralized readily by soil microorganisms into plant-available nitrogen forms. For example, after application to soil, slurry from cows fed a higher (19.4%) crude protein diet emitS approximately four times more ammonia from soil than slurry from cows fed a lower crude protein (13.6%) diet. However, after ammonia volatilization subsides (48 hours after slurry application), plant available nitrogen levels are greater in soils amended with the higher crude protein manure, which leads to higher levels of plant nitrogen uptake and plant yield, than in soils amended with the lower crude protein manure. There is a negative relationship between the fiber contained in manure and mineralization of manure nitrogen in soil; high fiber diets increase the fiber content in manure, so the manure is mineralized very slowly by soil microorganisms. Highest concentrations of fiber are found in feces from cows fed birdsfoot trefoil-based forage diets, an intermediate fiber level is found in feces from cows fed corn silage-based diets, and the lowest concentrations are found in feces from cows fed alfalfa silage-based diets. After application to soil, feces derived from cows fed a greater proportion of corn silage than alfalfa silage significantly reduced plant available nitrogen in the soil, compared to soils amended with feces from diets that contained lower amounts of corn silage. Plant yield and nitrogen uptake are also significantly lower in pots amended with feces from corn silage-based diets than in pots amended with feces from alfalfa silage-based diets. Results from various dairy nutrition trials, and the research reported in this article, lend strong evidence that diets can be formulated to meet the needs of healthy, high producing cows while at the same time producing manure that has more desirable impacts on the environment.