Inspecting fields and troubleshooting yield-limiting factors is not an easy task. Yield estimations are a function of factors that develop throughout the growing season. The yield component method is a popular method to estimate crop yields ahead of harvest; it is recommended as early as the milk stage of kernel development (R3). Earlier estimates can be too optimistic since stress issues can still occur, and they can have a larger negative footprint.
The main components of corn grain yield include ear number per unit of area (item #1 in the list below), kernel number per ear (items #2 & 3), and kernel weight (item #4). These components are determined at different times during the growing season (Figure 1); when it comes to the formation yield, all season-long conditions are critical for reaching good outcomes.
1. Number of ears per area: the number of plants per acre and the number of ears per plant primarily determine the total number of ears per area. The success of germination, emergence, and stand establishment early in the growing season define this yield component. In most hybrids grown in the U.S. Corn Belt, only the upper one or two ear shoots result in ears with harvestable grain, but hybrids have been bred to develop a single ear on the corn plant. Atypical conditions later in the season can reduce the number of ears per area; think of a hailstorm or green snap later in the season.
2. Number of kernel rows per ear and the potential number of kernels per row: ovule formation (potential kernel number) occurs during the mid-to-late vegetative stages, approximately V7 through V14 (seven and fourteen collared leaves, respectively). Potential kernels are arranged in an even number of rows (versus odd), usually 16 or 18. Row number is always an even number because initial rows divide laterally, forming two rows each. Each row can have up to 50 to 55 viable ovules (potential kernels). Plants with a low number of kernel rows per ear would reflect conditions during the mid-vegetative stages, approximately V6 to V14.
3. The number of actual kernels per ear: the success of pollination, kernel fertilization, and kernel retention determines the actual number of harvestable kernels through about R3 (milk stage). Normal ears have the potential to produce about 800 to 900 kernels in each ear. However, due to pollination issues or kernel abortion during grain formation, the number of harvested kernels per ear is generally lower. By the R3 stage, the kernels that will continue to fill will increase in depth and those that were pollinated but aborted will appear yellow and will begin to shrivel. Large issues with unpollinated ovules or aborted kernels suggest stress during the VT/R1 to R2 growth stage.
4. Kernel weight (or so-called test weight): kernel weight is determined during the latter half of the season from about R2 (blister stage) through to R6 (right before physiological maturity or black layer). Ears with low kernel weight would reflect conditions during the second half of the reproductive stages (R3 to R6). At this time of the crop cycle, moisture in the grain is going down, while dry matter accumulation is going up. Approximately 40-45% of grain weight is gained during the first half of the dent stage (R5) alone. By R6 (physiological maturity), kernels have no longer milk line and have reached maximum dry matter. Following physiological maturity (R6), black layer formation takes place. Common values used for yield estimates can be 80,000-85,000 kernels per 56-lb bushel. Smaller kernels would increase this value (approaching 90,000 kernels/bu), whereas strong filling conditions and greater kernel depth may reduce this value to 70,000-75,000 kernels/bu. Special attention and adjustment of kernel weight assumptions are necessary to improve accuracy in yield estimations.
For yield estimations, the first components can be easily measured in the field (ear number, number of kernel rows per ear, and number of kernels per row), but be aware that the final kernel weight uses assumptions. Its final weight is not achieved until physiological maturity and back layer. Due to this and other unknowns, these are just estimations. It is expected to have results within +/-20 bushels per acre of actual yield. An article describing two yield estimation methods can be accessed here.
All this is to say that the crop’s exposure to unfavorable conditions during the growing season can negatively impact ear formation and yield. Adverse conditions can expand from flooding, drought, nutrient deficiencies, low solar radiation, storms, pests, disease, and much more. Understanding the timing at which each yield component develops, and good records of any unfavorable conditions can better inform our diagnostics.
Some resources that can help to sort out the management or stress and timing that can impact corn yield are available here 1) troubleshooting abnormal ears, 2) incomplete kernel set and tipped-back: how do they differ?, 3) arrested ears: how to avoid them?, and 4) other ear abnormalities: when and why they develop?.
Abendroth, L. J., Elmore, R. W., Boyer, M. J., & Marlay, S. K. 2011. Corn growth and development. PMR 1009. Iowa State University Extension. https://store.extension.iastate.edu/product/Corn-Growth-and-Development
Elmore, R. 2017. Estimating Potential Corn Yield. UNL CropWatch, University of Nebraska. https://cropwatch.unl.edu/2017/estimating-corn-yield
Licht, M. 2017. Estimating Corn Yields Using Yield Components. ICM Newsletter, Iowa State University. https://crops.extension.iastate.edu/cropnews/2017/08/estimating-corn-yields-using-yield-components
Ortez, O. A., McMechan, A. J., Hoegemeyer, T., Ciampitti, I. A., Nielsen, R. L., Thomison, P., Abendroth, L. J., & Elmore, R. W. 2022. Conditions potentially affecting corn ear formation, yield, and abnormal ears: A review. Crop, Forage & Turfgrass Management, 8, e20173. https://doi.org/10.1002/cft2.20173
Thomison, P. 2015. Estimating Corn Yields. C.O.R.N. Newsletter, Ohio State University. http://agcrops.osu.edu/newsletter/corn-newsletter/2015-25/estimating-corn-yields
Nielsen, R. L. 2021. Estimating Corn Grain Yield Prior to Harvest. Corny News Network, Purdue University. https://www.agry.purdue.edu/ext/corn/news/timeless/YldEstMethod.html