The question that is often asked about drought-stressed corn: “If I sell the standing corn as silage or hay, what is the value of the nutrients that are removed from the field?”
Although nutrient levels in 65 percent silage (Table 1) are similar for stressed and non-stressed corn, there is less plant growth and less uptake with droughty corn.
Nutrient uptake may be 50 percent of normal for fields affected early in the growing season with plant heights perhaps reaching only 4-5 feet. However, nutrient uptake may approach 80 percent to 90 percent of normal for late season drought that limits grain fill and nutrient transfer within the plant.
The values listed are only averages and a more accurate nutrient profile can be determined by sampling the field and analyzing for N, P, K and S. The variability in corn growth during a drought often varies not only by area of the state, but by field and within the field. Available water differences cause most of the growth variability but soil fertility also will influence the nutrients taken up by the plant. For a field sample, 15 to 20 plants should represent the area to be cut or baled.
Although there are other secondary (Ca and Mg) and micronutrients in the silage, these are very abundant in our soils and/or are in very small amounts in the plant.
The value of N, P2O5, K2O, and sulfur in drought-stressed corn can be found by multiplying the above concentrations by the current fertilizer values (Table 2).
Other benefits of leaving corn stover on the field also need to be considered and include: 1) soil cover (less erosion, trapping snow, less evaporation and runoff), and 2) greater soil organic matter levels.
Another question often asked “with little or no grain yield and leaving the stover and those nutrients in the field, how much can I reduce my fertilizer recommendation for next year?” Unfortunately, not much if any. Because of the lower plant growth we also have lower nutrient uptake - from 20 percent to 50 percent less in many cases. In addition, some of those nutrients that are taken up are in the stover even in a good year and are not credited in nutrient calculations. Of those that are “extra” less than ½ of any nitrogen would become available for next year’s crop and perhaps less than ¼ under no-till management.
Potassium is quickly released from the stover, but again there is less K uptake with stressed corn. For 150 bu corn grain removed there is about 45 lbs/a of K2O that is removed with the grain. If drought stressed corn only took up ½ of the normal K, then we have about 22 lbs/a that would be available in a drought year (assuming no grain yield). If corn yielded 30 bu/a, there would be only about 18 lbs “extra” K.
Although these rough calculations are possible, a better solution is to depend on soil tests to measure the unused nutrients. There is often higher carryover nitrate-N in a drought year that can be measured with a resulting decrease in next year’s fertilizer N recommendation for any non-legume crop that is grown the following year.