Evaluating corn yield potential prior to harvest can be helpful for harvest scheduling, developing market strategies, and estimating feed requirements. However, yield evaluations can be misleading if care is not given to the evaluation process.
Crop uniformity has a large influence on the accuracy of any yield potential evaluation method. Samples should be taken randomly throughout a field to calculate the best estimation of yield potential. One sample for every 10 to 15 acres should be sufficient unless conditions are variable. More samples may be needed to represent a non-uniform field and improve the accuracy of the estimate.
Two common methods for developing yield potential estimates are the Corn-Yield Component and the Corn-Ear Weight methods. The Corn-Yield Component method is the most widely used.
This method, which can be used as early as R3 (milk stage), is based on the assumption that grain yield potential can be estimated by using the 1) number of ears per acre, 2) number of kernel rows per ear, 3) number of kernels per row, and 4) kernel weight. The first three components can be determined from field samples; however, kernel weight is an unknown factor until physiological maturity (black layer) occurs and must be represented by a calculated factor. The average factor for kernel weight (90) is derived by using the long established estimate of 85,000 kernels per 56 pound bushel. However, a factor of 80 to 85 could be used because average kernel size has increased since the formula was developed and environmental conditions can affect weight and size (Table 2).1
The calculation includes 4 steps:
Step 1: At each sampling site, measure 1/1000th of an acre for the appropriate row width (Table 1) and count the number of harvestable ears (don’t count abnormal ears, nubbins, those on severely lodged plants, or those on the ground).
Step 2: On every fifth ear at the sampling site, count the number of kernel rows and kernels per row and determine the averages. Count kernels that are in complete rings around the cob and avoid counting kernels on the extreme ear end (Figure 1).1
Step 3: Determine an estimate of yield potential per acre at each sampling site by multiplying the number of ears times the average number of rows times the average number of kernels and divide by 90 or the factor that best represents the growing conditions (Table 2).
Step 4: Repeat the process at a representative number of sample sites within the field. Calculate the average yield potential for all the sites to determine a potential yield for the entire field. Example: Harvestable ear count = 26; average number of kernels/ear = 600; growing conditions were average (85). Estimated average yield potential (bu/acre) = 26 x 600 ÷ 85 or an average of 183.5 bu/acre.
Poor growing conditions during grain fill can cause kernel weights to be lower and result in the average estimated yield potential being under estimated. Conversely, very good growing conditions can increase kernel weights and result in an over estimation of potential yield.3
This method, which should only be used after corn has reached R6 (physiological maturity or black layer), may be more accurate than the yield component method because it is based on actual kernel weight. It does have a factor to account for average shell out percentage.3
Steps to determine average yield potential for this method should include:
Counting the number of harvestable ears in 1/1000th of an acre (Table 1) at several random sites throughout the field. At each site, 1) weigh every fifth ear and calculate the average ear weight per site 2) hand shell each of the ears, and 3) with a moisture tester, determine the average grain moisture content of the shelled grain.
Calculation Steps for each site:
Step 1: Multiply the number of ears by the average ear weight.
Step 2: Multiply the average moisture content by the factor 1.411.
Step 3: Add 46.2 to the average grain moisture content × 1.411 (Step 2).
Step 4. Divide the result from Step 1 by the result from Step 3.
Step 5. Multiply the result from Step 4 by 1000.
Example: Harvestable ear count = 26; average ear weight of every fifth ear is 0.5 pound; average grain moisture content is 30%. Estimated average yield potential (bu/acre) = [(26 x 0.5) ÷ ((1.411 x 30) + 46.2)] x 1,000, or 147 bu/acre.2
1 Nielsen, R.L. 2011. Effects of stress during grain filling in corn. Corny News Network, Purdue University.
2 Thomison, P. 2010. “Predicting” corn yields prior to harvest. C.O.R.N. newsletter. Ohio State University.
3 Nielsen, R.L. 2011. Estimating corn grain yield prior to harvest. Corny News Network, Purdue University.
4 Bergland, D.R. and Endres, G.J. 1999. Corn growth and management quick guide. North Dakota State University.
5 Hall, R.G. Corn growth stages with estimated calendar days and growing-degree units. South Dakota State University.
6 Lauer, J.L. 2002. Methods for calculating corn yield. Agronomy Advice. University of Wisconsin.
7 Nielsen, R.L. 2008. Grain fill stages in corn. Corny News Network, Purdue University.
Web sources verified 08/31/2015. 130816033001