This is the time during the growing season when crop tours and seed companies start posting yield predictions for corn. Most of the corn crop in Ohio is probably at the dough stage (R4). Given the tremendous variability in crop quality across the state and between and within fields, it will be particularly interesting this year see how close yield estimates come to matching what's harvested this fall. Moreover, although there may be little or no yield from many fields damaged by excessive rainfall and saturated soil conditions (and related problems, e.g. N deficiency, poorly developed root systems), the fate of other corn fields has yet to be determined. Other factors could cut yields further. Many fields that were excessively wet several weeks ago could now benefit from rain. Shallow, limited root systems attributable to excessive soil moisture may predispose corn to late season soil moisture deficits. Several foliar diseases, esp. northern corn leaf blight and gray leaf spot, are widespread. Not surprisingly, the predictions I've received thus far indicate a wide range in corn yields.
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. For more on this check http://www.agry.purdue.edu/ext/corn/news/timeless/YldEstMethod.html.
Step 1. Count the number of harvestable ears in a length of row equivalent to 1/1000th acre. For 30‑inch 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 85.
Step 5. Repeat the procedure for at least four additional sites across the field. Keep in mind that uniformity of plant development affects the accuracy of the estimation technique.
The more variable crop development is across a field, the greater the number of samples that should be taken to estimate yield for the field.
Example: You are evaluating a field with 30‑inch rows. You counted 29 ears (per 17' 5" = row section). Sampling every fifth ear resulted in an average row number of 16 and an average number of kernels per row of 33. The estimated yield for that site in the field would be (29 x 16 x 33) divided by 85, which equals 180 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 1/1000th 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 A) Multiply ear number by average ear weight.
Step B) Multiply average grain moisture by 1.411.
Step C) Add 46.2 to the result from step B.
Step D) Divide the result from step A by the result from step C.
Step E) Multiply the result from step D by 1,000.
Example: You are evaluating a field with 30‑inch rows. You counted 24 ears (per 17 ft. 5 in. section). Sampling every fifth ear resulted in an average ear weight of 1/2 pound. The average grain moisture was 30 percent. 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. This will be an important consideration this year given the limited ear development present in many fields exhibiting highly variable plant growth.
Reference: Nielsen, RL. 2014. Estimating Corn Grain Yield Prior to Harvest. Corny News Network, Purdue University.http://www.agry.purdue.edu/ext/corn/news/timeless/YldEstMethod.html. (URL checked Aug 2015).