Nutrient removal with drought-stressed corn
The severe drought conditions that persist in much of the Midwest have caused complete crop failure in some areas or have reduced yield to the point that it would not be economical to harvest for grain. Many are looking at options to deal with the crop material that was produced and are considering baling it or chopping it for silage. One question being asked is how much nutrient will be removed if the stunted corn crop is taken out of the field.
The condition of the drought-affected crop varies so much between fields that it would be difficult to establish a removal rate that represents every condition. In the most severely affected areas, some crops have died while they were still at vegetative stages, others are dying with barren ears, and yet others are still hanging on trying to fill the kernels that started to form. These differences make it difficult to determine not only the amount of biomass produced but also its nutrient content. Both factors are critical in determining total nutrient removal when material is taken out of a field.
How much biomass has been produced? The first step in determining total nutrient removal in stover is to calculate how much stover is produced. In a normal year corn stover is typically estimated from a harvest index (also known as a residue-to-corn grain ratio). The most widely used dry weight ratio is 1:1 residue:grain. Using this 1:1 ratio to calculate the pounds of dry residue produced, then, the grain yield (in bushels per acre) is multiplied by 47.3. (A 56-pound bushel of corn at 15.5% moisture contains 8.7 pounds of water.) The value can then be divided by 2,000 to obtain the number of dry tons produced.
This year, of course, that approach is not going to work very well. As a rough estimate one could expect to remove, on an acre basis, up to about 1 ton of dry biomass per foot of corn height if the stalks are small in diameter. If the crop developed further into the growing season and stalks have a more typical diameter, I would estimate as much as 1.2 to 1.3 tons of dry biomass per foot of corn height. Again, these are just rough estimates, and they may over- or underestimate the actual biomass produced depending on the conditions of the specific field. Of course, the amount of biomass removed will also depend on the cutting height. The higher the height, the less stover that will be removed. For these reasons, the best way to determine total amount produced is by weight of bales or silage loads, adjusted for moisture content.