Stressed corn: Formula for estimating corn yield potential
With the ongoing heat and drought conditions in much of Kansas, many corn growers are trying to decide if their corn crop is worth keeping for grain harvest or if it should be harvested for silage or left in place for residue benefits.
Where tassel, silking, and pollination are complete, or nearly complete, producers can begin to get some idea of what the potential yield might be. To get a reasonable yield estimate, corn should be in the milk, dough, or dent stage. Before the milk stage, it is difficult to tell which kernels will develop and which ones have been aborted.
Producers can get some estimate of the success of pollination by examining ear silks. With successful pollination, the exposed silks should be turning brown and should easily separate from the ear when the husks are removed. Silks that have not been successfully pollinated will stay green, possibly growing to several inches in length (Figure 1). Unpollinated silks also will be connected securely to the ovaries (the undeveloped kernels) when the husks are removed.
Figure 1. Long silks primarily reflecting floral asynchrony. Silks that have not been successfully pollinated will stay green. Infographic by Ignacio Ciampitti, K-State Research and Extension.
Estimating corn yield
Yield estimates can be made using the yield component method. This method uses a combination of known and projected yield components of corn to calculate an estimate of the potential yield. It is “potential” yield because one of the critical yield components, kernel size, will not be known until physiological maturity. Before then, one can use only an estimate of predicted yield based on what you think the grain filling period might be like (e.g. favorable, average, or poor). Estimating potential corn yield using yield components uses the following elements:
- Ears per acre: This is determined by counting the number of ears in a known area. With 30-inch rows, 17.4 feet of row = 1,000th of an acre. This is probably the minimum area that should be used. The number of ears in 17.4 feet of row x 1,000 = the number of ears per acre. Counting a longer length of row is fine, just be sure to convert it to the correct portion of an acre when determining the number of ears per acre. Make ear counts in 10 to 15 representative parts of the field or management zone to get a good average estimate. The more ear counts you make (assuming they accurately represent the field or zone of interest), the more confidence you have in your yield estimate.
- Kernels per ear: This is determined by counting the number of ear rows and number of kernels in each row. Multiply those two items to arrive at kernels per ear (number of rows x kernels per row). Do not count aborted kernels or the kernels on the butt of the ear; count only kernels that are in complete rings around the ear. Do this for every 5th or 6th plant in each of your ear count areas. Avoid odd, non-representative ears.
- Kernels per acre = Ears per acre x kernels per ear
- Kernels per bushel: This will have to be estimated until the plants reach physiological maturity. Common values range from 75,000 to 80,000 for excellent, 85,000 to 90,000 for average, and 95,000 to 105,000 for poor grain filling conditions. The best you can do at this point is estimate a range of potential yields depending on expectations for the rest of the season.
Ears per acre: (30-inch rows)
- 10 different 17.4-foot lengths of row provided counts of 25, 24, 22, 21, 24, 26, 20, 21, 22, 20
- average of these counts is (25 + 24 + 22 + 21 + 24 + 26 + 20 + 21 + 22 + 20)/10 = 225/10 = 22.5
- scaling up to an acre gives 22.5 x 1,000 = 22,500 ears per acre
Kernels per ear:
- The 4 or 5 ears from each 17.4-foot area had an average of 14 rows and 27 kernels per row
- 14 x 27 = 378 kernels per ear
Kernels per acre:
- 22,500 ears per acre x 378 kernels per ear = 8,505,000 kernels per acre
Kernels per bushel:
- Given that this field has been exposed to 100° F and above with no significant precipitation for the past couple of weeks and the prediction for the next 7-10 days is for triple digits every day and no rain, it may not hurt to assume below-average fill conditions and use a fairly large number of kernels per bushel (because kernels will be small). Based on the ranges mentioned above, a reasonable value might be 105,000 kernels per bushel.
Bushels per acre:
- 8,505,000 kernels per acre ÷ 105,000 kernels per bushel = about 81 bushels per acre
If these estimates are close to correct, the field in this example is probably worth taking to grain harvest provided it is still living and likely to keep filling grain. Past experience indicates that this method of estimating yield usually provides fairly optimistic estimates. Use a larger number for kernels per bushel if you want the process to be a bit more “pessimistic.”
See the June 25, 2018 Agronomy e-Update Issue 699 , “Management options for stressed corn”, for the range of yields that might lead to a decision to chop the crop for forage or maybe even leave it in place for the value of the residue.
More information on stressed corn and related topics:
- Nitrate toxicity in drought-stressed corn
- Effect of high night temperatures on corn yield
- EP169 “Abnormal corn ears” https://www.bookstore.ksre.ksu.edu/pubs/EP169.pdf
Further details on corn growth and development can be found at: http://www.bookstore.ksre.ksu.edu/pubs/MF3305.pdf
Links with further discussions on the yield estimation:
- http://www.ca.uky.edu/agc/pubs/agr/agr187/agr187.pdf University of Kentucky
- http://www.agry.purdue.edu/ext/corn/news/timeless/yldestmethod.html Purdue University
Ignacio Ciampitti, Crop Production and Cropping Systems Specialist