The removal of wheat straw or corn and soybean stalks from fields following a grain harvest is a common practice on many farms. This crop residue is often used as a bedding source or as a high fiber livestock feed. In both cases, the practice of using or selling crop residue off the field adds “value” to the grain crop. However, there are also associated costs with residue removal. These are:

• The cost of harvesting, handling, and storing the crop residue.

• Reduced soil cover over winter and early spring, perhaps increasing the risk for erosion.

• Reduced organic matter that is returned to the field and eventually broken down by soil microbes.

• Reduced nutrients that will be incorporated back into the soil. 

          Some of these costs are easier to put firm numbers on than others. Certainly harvesting costs can be calculated or applied from current custom harvest rates. Putting a firm dollar value on the contribution of a single crop’s worth of organic matter (crop residue) is difficult, if not impossible. The nutrient value of the residue has become a much bigger economic factor as commercial fertilizer prices have increased in recent years. The data in Table 1 is derived from a combination of recent straw/stalk bale chemical analysis and research results. Given current fertilizer prices, let’s take a close look at the nutrient value of harvested crop residue for wheat, corn, and soybean. Note that each nutrient is unique in contributing to the overall value.

Nitrogen:

          From a value perspective, nitrogen (N) is the most difficult primary nutrient to deal with. Although N is removed with the straw or stover, in no case does it impact the applied N recommendation for the subsequent crop. Nevertheless, 13-15 lbs. N per dry ton is usually removed in the straw or stover depending on the crop harvested (Table 1). Perhaps there is some justification of using all or a portion of the removed N value as a charge for the loss of organic matter in the form of crop stover from the field. 

Phosphorus:

          Phosphorus (P) is found in relatively small amounts in crop stover (Table 1). Further, because it is incorporated in plant organic matter, the concentration remains more stable over time after grain harvest than is the case for potassium. Across crop species, only about 4 to 6 lbs. of equivalent P₂O₅ per dry matter ton are removed in the crop stover. 

Potassium

          When it comes to harvesting crop stover and accounting for removed nutrients, potassium (K) is the “tail that wags the dog.” A large percentage of K in the crop is found in the plant tissue (especially stalks). Further, K is largely found floating around in solutions within and between plant cells. This makes it much more prone to loss from dying or dead plant tissue as it weathers in the field. For example, a recent Iowa State University research study showed that K concentration in soybean straw left in the field declined from about 1.3 percent at the time of grain harvest to 0.6 percent two months later. Hence, the elapsed time between grain and straw harvest has a big impact on removed K. Based on analysis of harvested stover bales for the three crops, an average of 23 to 30 lbs. of equivalent K₂O per dry matter ton are removed when crop stover is harvested. 

Determining value:

          Several factors must be taken into account when valuing crop stover, but equivalent fertilizer nutrient value is an important starting place. The value will vary over time with changes in fertilizer price and be dependent on harvest timeliness and method.