Issue No. 14, Article 2/July 8, 2011
Is Wheat Straw Worth Anything?
As I traveled through the southern part of the state this last weekend, I saw a number of harvested wheat fields, some with bales of straw waiting to be picked up and others with baling yet to begin. In the last three or four years, baling wheat straw seems to have become a more frequent practice. While the uses for wheat straw are varied, a common denominator exists in terms of nutrient removal. Removing straw means exporting nutrients from the field.
I want to explain here how to determine how much nutrient is removed when straw is baled. Knowing the amount of nutrient in straw is important to both determining the straw's value and knowing how much fertilizer is being removed and thus how much to apply back to the field to maintain fertility levels.
The most precise way to determine the amount of fertilizer being removed is to weigh the bales and extract a representative sample of the straw. Using that sample, two values must be obtained from a testing lab: moisture content, to determine the dry weight of the bale; and nutrient content, which is reported on a dry basis.
Of course, this approach might not be practical or possible for all producers, in which case one can estimate the value of straw by following a few simple steps. But keep in mind that there can be substantial variability in the actual value, depending on how the straw is processed.
Step 1 of the calculation is to determine how much straw is being produced. The best estimates are obtained as a function of plant height and grain yield. These estimates vary somewhat, but a typical value is about 1 pound of straw per pound of grain. In this step, remember that although a bushel of wheat is 60 pounds, that includes 8.1 pounds of moisture (13.5%). So a bushel of wheat is 51.9 pounds of dry grain. The dry-grain yield in bushels per acre is multiplied by 51.9 and then divided by 2,000 to obtain the number of dry tons of straw produced.
Step 2 is to determine how much straw is actually removed. Again, this is not an absolute value because it will depend on cutting height and how much of the cut straw is actually baled. In general, a good wheat crop yields around 2.5 to 2.8 tons of straw per acre on a dry matter basis.
Step 3 is to know the nutrient content of the straw. A ton of wheat straw normally contains 9 to 12 pounds of nitrogen (N), 3 to 4 pounds of phosphorous (P) in the form of P2O5, and 25 to 45 pounds of potassium (K) in the form of K2O. K content varies so much because K is not incorporated into organic compounds and so can be easily leached from the residue. The time elapsed, the amount and frequency of precipitation received after the crop reached maturity, and when the straw is removed from the field all impact K content. Typically, K leaching occurs rapidly; it is not unusual to see a decrease of as much as 80% after the straw has been rained on a few times. On the other hand, N and P content in the straw are more or less stable because they are present in organic forms and require microbial decomposition of the straw before they can be leached out.
The fourth and final step in the calculation is to determine the fertilizer value. Use the information determined through the procedures described and the current price of fertilizer to arrive at a value of dollars per acre.
Though I have focused on removal of major nutrients (N, P, K), I want to be clear that there are other costs associated with straw removal; they might not be easily observed in the short term, but they can have important impacts on a field's future productivity.
In addition to the major nutrients, wheat straw contains calcium (Ca), magnesium (Mg), sulfur (S), and micronutrients. In Illinois, the soil's native supply of these nutrients is typically high, so there is no need to supply them through fertilization. But greater removal of these secondary nutrients by harvesting straw can accelerate depletion rates and result in future deficiencies. Removal of basic cations, like K, Ca, and Mg, can increase the need to lime soils to maintain adequate pH levels. Depletion of organic matter and N reserves can lead to less crop availability of N through the process of mineralization (conversion of organic N to inorganic N). Diminishing organic carbon contents can also result in negative impacts on soil conservation and on soil physical, chemical, and biological properties. For all these reasons, carefully consider all relevant factors when you estimate the true cost of straw removal.--Fabián G. Fernández