Nutrient removal with drought-stressed corn

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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.

How much nutrient is present in the biomass produced? Nutrient concentration is the other important piece of information needed to determine nutrient removal amounts. In a growing season with normal conditions, there can be large variability in the amounts of phosphorus (P) and especially potassium (K) present in biomass. Typical values for stover after grain harvest range from 5 to 8 lb of P2O5/ton and from 5 to 40 lb of K20/ton (with 20 to 30 lb K2O/ton being most common). The lower K contents are caused by the leaching of K from the plant by rain after the crop starts dying, which obviously would not apply at this time.

So nutrient removal values can vary even under normal circumstances; the droughty conditions make it even harder to estimate a value that would work across all locations. The amount of P and K present in the plant material is a function of how much is taken up by the crop. Under severe drought conditions the capacity of the crop to take up nutrients is diminished (see "Water Stress and Nutrient Deficiency" in issue 15 on July 13), so amounts lower than normal would be expected in the corn crop's vegetative tissues this year.

Under normal growing-season conditions, by R1-R2 development stage, nearly all of the K (around 170 lb K2O/acre) and about 50% of the P (around 40 lb P2O5/acre) is taken up. Of course, all of the nutrients are present in the vegetative tissues at these stages, as there are no large seeds present yet. As the seed starts to form, part of the P and K in vegetative tissues translocates into the seed, and by physiological maturity (R6), about 20 lb of P2O5/acre and 120 lb K2O/acre are present in vegetative tissues and cobs. Again, I suspect that the amounts present at R1 for the current crop would be lower because of the droughty conditions.

The best approach to accurately determining nutrient content is to send a representative sample of the harvested material to a commercial lab for analysis. If for no other reason, nutrient analysis is important with drought-stressed corn to determine nitrate concentrations for managing livestock feeding. It would not cost much more to have P and K analysis done at the same time. For more information regarding how to manage livestock feeding of drought-stressed corn with high nitrate concentrations, see the University of Illinois Extension website on drought resources.

What's the bottom line? While there are many factors that make it difficult to generalize about how much nutrient will be removed if biomass is harvested out of drought-damaged corn, the following notes offer general guidelines or at least illustrate important points to consider when making P and K fertilizer decisions for this fall.

For P, the removal rate by harvesting biomass will most likely be lower than what is normally removed when only grain is harvested. If a low yield of 4 tons of dry biomass/acre is harvested at 5 lb P2O5/ton, the removal would be 20 lb P2O5/acre. Even if we use a higher value of 40 lb P2O5/acre (the amount accumulated in vegetative tissues by R1 under normal conditions), these removal values are lower than the 77 lb P2O5/acre removed when harvesting 180 bushels of grain per acre.

For K, however, removal rates can be much higher than the normal removals with grain harvest. Using the same low yield of 4 tons of dry biomass/acre, if the material contains 20 lb K2O/ton, the crop would remove 80 lb K2O/acre; if the material contains 30 lb K2O/ton, then 120 lb K2O/acre would be removed. The estimates can be higher still if we use 170 lb K2O/acre, which is the accumulation in vegetative tissues by R1-R2 under normal conditions. Whichever estimate is used, they are all much higher than the normal removal rate of 50 lb K2O/acre for a grain yield of 180 bushels an acre.

On the other hand, if grain yields are low and biomass is not harvested, the removal rates will be lower than normal. Whatever your particular situation, it is very important to correctly account for removal rates in order to appropriately manage fertilizer applications this fall.

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