Estimating yield losses in drought-damaged corn
Given current daily heat unit accumulation, much of the corn crop is likely to achieve black layer (physiological maturity) before the end of August. A continuation of high temperatures will promote rapid grain drydown and the potential for an early harvest. In upcoming weeks, corn growers with drought damaged fields may want to estimate grain yields prior to harvest in order to help with marketing and harvest plans.
Two procedures that are widely used for estimating corn grain yields prior to harvest are the yield component method (also referred to as the "slide rule" or corn yield calculator) and the ear weight method. Each method will often produce yield estimates that are within 20 bu/ac of actual yield. Such estimates can be helpful for general planning purposes.
The yield component method was developed by the Agricultural Engineering Department at the University of Illinois. The principle advantage to this method is that it can be used as early as the milk stage of kernel development, a stage many Ohio corn fields have probably achieved. The yield component method involves use of a numerical constant for kernel weight which is figured into an equation in order to calculate grain yield. This numerical constant is sometimes referred to as a "fudge‑factor" since it is based on a predetermined average kernel weight. Since weight per kernel will vary depending on hybrid and environment, the yield component method should be used only to estimate relative grain yields, i.e. "ballpark" grain yields. When below normal rainfall occurs during grain fill (resulting in low kernel weights), the yield component method will overestimate yields. In a year with good grain fill conditions (resulting in high kernel weights) the method will underestimate grain yields.
In the past, the yield component method equation used a "fudge factor" of 90 (as the average value for kernel weight, expressed as 90,000 kernels per 56 lb bushel), but kernel size has increased as hybrids have improved over the years. Dr. Bob Nielsen at Purdue University suggests that a "fudge factor" of 80 to 85 (85,000 kernels per 56 lb bushel) is a more realistic value to use in the yield estimation equation today http://www.agry.purdue.edu/ext/corn/news/timeless/YldEstMethod.html.
Emerson Nafziger, Ph.D., the University of Illinois notes that under current drought stress “….If there's a fair amount of green leaf area and kernels have already reached dough stage, using 90 [as the “fudge-factor “] might be reasonable. It typically doesn't help much to try to estimate depth of kernels at dough stage, when kernel depth is typically rather shallow anyway, especially if there are 16 or more kernel rows on the ear. If green leaf area is mostly gone, however, and kernels look like they may be starting to shrink a little, kernels may end up very light, and using 120 or even 140 [as the “fudge-factor”] might be more accurate”. http://bulletin.ipm.illinois.edu/article.php?id=1695.