Fall 2006

Is there too much P in distillers grains with solubles? A systems approach to answering the question.


Manure sampleBy Wendy Powers, Department of Animal Science, Iowa State University

Ethanol production in Iowa and the U.S. continues to increase. During the ethanol production process, energy is removed from the grain. Therefore the co-product that is produced, in addition to the ethanol, contains nutrients that are more concentrated than in the feedstock grain. The concentrated nutrients include phosphorus (P) and nitrogen (N). Whereas corn contains approximately 10 percent crude protein and 0.30 percent P on a dry matter basis, dried distillers grains with solubles (DDGs) contains approximately 26 percent crude protein and 0.84 percent P. As a result, concerns regarding the concentrated P have been raised as to whether or not feeding DDGs will create more P in manure than can be managed following land application.

To address this concern, one has to think about the farm as a system with boundaries that often extend beyond the property line. A recent publication from the Iowa Beef Center approaches this issue by considering the dietary P content when DDGs is varied in the diet from 0 to 40 percent of the diet dry matter. Please see Use of Distillers Grains in Feedlot Diets: Impact on Phosphorus Excretion (IBC 29) at http://www.extension.iastate.edu/Publications/IBC29.pdf.

In addition to estimating the amount of P that is excreted when steers are fed the different diets, mass of P excreted was estimated as was the acreage requirements for P removal in corn. While the number of acres increased as amount of P in the diet increased, it is important also to consider the number of acres needed to grow the corn that was fed to the steers as either corn or DDGs. When that aspect of the farm system is taken into consideration, using DDGs does not appear to present a P problem. In fact when 40 percent DDGs is fed to the example steer, more P is needed to grow the corn for both corn grain and DDGs than is actually provided in manure. Getting the manure P to the corn fields is the real issue; nutrient distribution rather than nutrient abundance. No question, producers will be reluctant to haul manure as far as may be needed but it is imperative that the real issues be unveiled so that solutions can be developed.

Cows feedingMost of the attention has focused on P content of DDGs with little attention on N content. However, as particulate matter and ammonia emissions become a greater challenge for all livestock producers to contend with, it is important to consider dietary N and manure N, particularly the fraction of manure N that can not be captured and is volatilized to the atmosphere. Using the same diets as our example, the Iowa Beef Center publication demonstrates that when only 50 percent of the N is captured from a feedlot, increasing the DDGs content of the diet from 15 to 40 percent, N lost to the atmosphere increases by 50 percent. However, in the event that all of the excreted N can be trapped, the producer feeding 40 percent DDGs needs much less commercial N on the corn grown to supply the grain and the DDGs when manure is applied based on P removal application rates.

The bottom line is that we need to consider all aspects of the system when determining the environmental impact for what appears obvious may in fact have plausible solutions and less obvious issues may pose greater challenges. For more information on the examples developed to make this point, logon to the Iowa Beef Center website at: http://www.iowabeefcenter.org/


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