The majority of the corn in Kansas is currently in the reproductive period. From this point until harvest, farmers and consultants can begin to make reasonable estimates of corn yield potential. If no ear was formed within a week or two after pollination, then that specific corn plant will remain “barren” until the end of the season. In that unfortunate situation, you need to choose whether to harvest it for silage or leave it in place for grazing the residue.
The number of potential kernels per ear can be adversely affected either before silking time (Figures 1 and 2), if no potential ovule develops, or after silking. After silking, kernel numbers are reduced under any or all of the following conditions:
Figures 1 and 2. Determination of the potential kernel number in corn ears as seen under a microscope (left) and magnifying glass (right). The tip kernels are the first one to start the abortion process under any environmental (abiotic) stress. Photos by Ignacio Ciampitti, K-State Research and Extension.
If ears are present a week or two after silking, producers can get a reasonable yield estimate by the time the corn plants are at the milk or dough stages. Before the milk stage, it is difficult to tell which kernels will develop and which ones will be aborted (Figure 3). The milk stage takes place about 15 to 25 days after flowering, depending on the environmental conditions. We can easily recognize this stage by opening the husk. In the milk stage, a milky white fluid will be evident when the kernels are punctured with a thumbnail (kernel moisture is ~80%).
Figure 3. Grain abortion at the top of the ear (early abortion) and in the mid-section of the ear (late abortion, brown coloration in right corner picture). Photos by Ignacio Ciampitti, K-State Research and Extension.
Figure 4. Corn at the milk stage (R3 growth stage), a milky white fluid is evident when the kernels are punctured with a thumbnail (kernel moisture is approximately 80%). Photo by Ignacio Ciampitti, K-State Research and Extension.
Farmers can get some estimate of the failure or success of the pollination process by examining several corn ear silks. Pollination is successful when silks turn brown (R2 stage, kernel blister stage) and when they can be easily detached from the ear structure as husks are removed. If the silks remain green and are still attached to the ear after growing several inches in length, pollination has failed (Figure 5). In this situation, the ovules will not be fertilized, and kernels will not develop.
Before estimating corn yields, few points are noteworthy. Yield estimates are more accurate as the corn is approaching maturity. Also, yield estimates can be accurate as long as the sample areas reflect the “real” variation of corn yield within the field. The precision of the method increases as the number of sample areas increases, properly reflecting the variability within the field. Yield estimations before harvest can: (1) facilitate the decision of harvest timing, (2) estimate the need for additional inputs before maturity, and (3) serve as a scouting tool since the method of yield estimation involves examining diverse areas of the field.
Figure 5. Unpollinated silks have grown in several inches in length. Photos by Ignacio Ciampitti, K-State Research and Extension.
Estimating yields using “yield component method”
The concept of estimating yields using the “yield component method” has advantages and disadvantages. The primary advantage is that it can be used early in the growing season (milk stage, R3). It involves the assumption that the kernel weight is constant. The method only estimates the “potential” yield because the kernel weight component is still unknown until the crop reaches final maturity (R6 stage).
Estimating potential corn yield with this method uses the following elements:
1) Total number of ears (ears per acre): This is determined by counting the number of ears in a known area (Figure 6). With 30-inch rows, 17.4 feet of row = one-thousandth 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. Make ear counts in 10 to 15 representative parts of the field or management zone to get a good average estimate which fairly represents the field variation. The more ear counts you make, if they are representative of the rest of the field, the more confidence you have in your yield estimate.
Figure 6. Total number of corn ears per unit area. Photo by Ignacio Ciampitti, K-State Research and Extension.
2) Final kernel number per ear: Count the number of rows within each ear and the number of kernels in each row (Figure 7). The final number of kernels per ear is calculated by multiplying the number of rows by the number of kernels within each row. This is just a quick estimation of the potential yield.
Figure 7. Two different size of ears with similar number of rows (16 rows in total) but different kernel number per row and kernel sizes (left). The photo at right shows the determination of rows per ear from a vertical position (20 rows in total). The final number of rows per ear is defined earlier in the season than the number of kernels per row, and can be a function of the hybrid and growing conditions. Photos by Ignacio Ciampitti, K-State Research and Extension.
The number of kernels within each row is not standard and can vary from row to row, depending in part on the number of kernels aborted (“abnormal ears”). Do not count aborted kernels or the kernels on the tip 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. The more you can count, the more precise will be the estimation. Avoid odd, non-representative ears.
Finally the number of kernels per acre is estimated by multiplying the first and second components.
Kernels per acre = Ears per acre x Kernels per ear
Kernels per bushel: This will be more precisely defined at maturity. For this case, common values range from 75,000 to 80,000 for excellent grain filling conditions, 85,000 to 90,000 for average, and 95,000 to 105,000 for poor 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.
For corn in 30-inch rows with an average total number of ears in 12 areas of the field (17.4-foot lengths of row) of:
Number of ears = [(25 + 24 + 22 + 21 + 24 + 26 + 20 + 21 + 22 + 20 + 25 + 26)]/12 = 23 (a)
An average of 23 ears were counted within the 17.4-foot lengths. This can be scaling up to an acre basis by multiplying the number of ears by 1,000 (constant factor if the counts were taken in a 17.4-foot length).
Ears per acre = 23 x 1000 = 23,000 (b)
From those 23 ears, we will take between 2 and 5 ears to calculate the rows per ear and the kernels per row. The average number of rows was 14 with 27 kernels per row.
Kernel number per ear = 14 rows per ear x 27 kernels per row = 378 (c)
The final number of kernels per acre is the outcome of the multiplication of (b) ears per acre and (c) kernel number per ear.
Kernels per acre = 23,000 ears per acre x 378 kernels per ear = 8,694,000 (d)
Kernels per bushel
Under hot, dry conditions, grain filling duration and biomass translocation from the whole plant to the ear (kernels) can be severely affected. Otherwise, a reasonable value to use is about 105,000 kernels per bushel (e).
The final number of kernels per bushel is affected by diverse factors such as genotype, management practices (for example, plant density), and the environment. Plant density can strongly affect the kernel weight and the number of kernels per bushel. Lower plant densities (if growing conditions are optimum) will result in lower values for kernel number per bushel. Also, expect a lower kernel number per bushel as N is more deficient. More information regarding the influence of these management practices on the kernel weight and the number of kernels per bushel is available from an article titled “Corn Grain Yield Estimation: The Kernel Weight Factor” from Dr. Tony Vyn, Purdue University, at: http://extension.entm.purdue.edu/pestcrop/2010/issue22/index.html#corn
Final yield: Calculation of bushels per acre
The final calculation of the potential yield to be obtained at the end of the season is simply the outcome of dividing the component (d) by (e).
Bushels per acre = 8,694,000 kernels per acre ÷ 105,000 kernels per bushel = ~83
In this example, if projected conditions prove to be accurate, the corn should obviously be kept and harvested for grain. From previous experiences, the yield component method of estimating yields often seems to provide optimistic outcomes (slightly overestimation). If the conditions during the reproductive period are predicted to worsen (severe heat stress and lack of precipitation); then the kernel weight can be reduced, and the number estimated for component (e), kernels per bushel, should be higher. That will reduce the yield expectations.
New technologies for estimating corn yields: App
If you have smartphone or tablet devices, there is a “free” app that can provide assistance in estimating corn yield at on-farm scale. The app, developed by the University of Wisconsin, is named “Crop Calculators for Corn” and can be downloaded at:
The Crop Calculators app has a section for estimating yields: “Grain Yield Estimator.” In that section only four inputs are needed for predicting the final yield: (1) plants per 1000th acre (17.4-feet length of row); (2) rows per ear; (3) kernels per row; and (4) kernel weight, or mass. The last factor refers to the individual kernel weight for corn and it is expressed in mg per kernel. This factor normally varies from 150 to 400 mg per kernel. If conditions will be favorable until harvest, then the “kernel mass” should be higher (e.g., 300 mg per kernel). On the opposite end, unfavorable conditions with a short-grain filling period will produce a lower value (e.g., 180 mg per kernel). This factor will ultimately be defined at maturity, but a projection can be used based on forecasted weather conditions for the remainder of the season.
Links with further discussions on the yield estimation can be found at:
The Ohio State University
University of Kentucky
University of Wisconsin
Further details on corn growth and development can be found at:
Ignacio Ciampitti, Cropping Systems and Crop Production Specialist