Kernel development in many cornfields is well ahead of what we’ve usually observed in early August in recent years (especially 2009). Much of the April and early May planted corn  is in the dent stage with milk lines visible. Given current daily heat unit accumulation, much of this corn 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. Farmers may want to start estimating grain yields in their fields prior to harvest in order to help with marketing and harvest plans.
While examining ears to determine potential grain yield, growers may encounter various ear development problems  that may impact yield at harvest (http://agcrops.osu.edu/specialists/corn/resources/of-interest/AbnormalCornEarsPoster_000.pdf ). Troubleshooting these corn ear disorders now rather than at harvest may give growers more time to diagnose likely causes of these problems.
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./acre 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 cornfields 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.
Step 1. Count the number of harvestable ears in a length of row equivalent to 1/1000 acre. For 30‑in. rows this would be 17 ft. 5 in.
Step 2. On every fifth ear, count the number of kernel rows per ear and determine the average.
Step 3. On each of these ears count the number of kernels per row and determine the average. (Do not count kernels on either the butt or tip of the ear that are less than half the size of normal size kernels.)
Step 4. Yield (bushels per acre) equals (ear #) x (avg. row #) x (avg. kernel #) divided by 90.
Step 5. Repeat the procedure for at least four additional sites across the field.
Example: You are evaluating a field with 30‑inch rows. You counted 24 ears (per 17 ft. 5 in. row section). Sampling every fifth ear resulted in an average row number of 16 and an average number of kernels per row of 30. The estimated yield for that site in the field would be (24 x 16 x 30) divided by 90, which equals 128 bu./acre.
The ear weight method can only be used after the grain is physiologically mature (black layer), which occurs at about 30‑35% grain moisture. Since this method is based on actual ear weight, it should be somewhat more accurate than the yield component method above. However, there still is a fudge factor in the formula to account for average shellout percentage.
Sample several sites in the field. At each site, measure off a length of row equal to 1/1000 acre. Count the number of harvestable ears in the 1/1000th acre.
Weigh every fifth ear and calculate the average ear weight (pounds) for the site. Hand shell the same ears, mix the grain well, and determine an average percent grain moisture with a portable moisture tester.
Calculate estimated grain yield as follows:
Step 1. Multiply ear number by average ear weight.
Step 2. Multiply average grain moisture by 1.411.
Step 3. Add 46.2 to the result from step B.
Step 4. Divide the result from step A by the result from step C.
Step 5. Multiply the result from step D by 1,000.
Example: You are evaluating a field with 30‑in. rows. You counted 24 ears (per 17 ft. 5 in. section). Sampling every fifth ear resulted in an average ear weight of 1/2 lb. The average grain moisture was 30%. Estimated yield would be [(24 x 0.5) / ((1.411 x 30) + 46.2)] x 1,000, which equals 135 bu./acre.
Because it can be used at a relatively early stage of kernel development, the yield component method may be of greater assistance to farmers trying to make a decision about whether to harvest their corn for grain or silage. Since drought stress conditions in some fields may result in poorly filled small ears, there may be mechanical difficulties with sheller or picker efficiency that need to be considered. When droughts occur, it’s often cheaper to buy corn for grain than to buy hay for roughage (because of likely forage deficits). Therefore, there may be greater benefit in harvesting fields with marginal corn grain yield potential for silage.