Making Corn Yield Estimates - 2024 Edition

Again this year, I joined my colleague Patrick Hatting, Farm Management Specialist for central Iowa, and checked several Polk County corn fields as a part of our “Polk County Yield Tour.” After R3 (milk stage) is a great time to venture into corn fields to make yield estimates as kernel abortion is less likely and plant stress will result in reduce kernel size or fill rather than kernel loss. In addition to performing yield checks, walking late reproductive stage corn provides a good opportunity to check the field for other things like stalk rots or other standability issues and foliar disease, especially if you left any test strips of untreated vs. fungicide-treated areas of fields. These late-season checks can be invaluable to plan for harvest and future growing seasons.

Patrick and I visited 12 corn fields in Polk County on September 3 to do yield estimates. In each field, we arbitrarily chose a location sufficiently far from the field edge or any confounding areas (waterways, demonstration plots, etc.). This was especially challenging this year due to the variability in corn fields from early-season water stress. While other methods exist, the most common method of making yield estimates is the traditional “yield component method.” If you’re interested in the full details on estimating yield using this method, check out this ICM Encyclopedia article on the topic.

Steps to estimating yield using the “yield component method”:

1. Measure 1/1000^th of an acre and count number of plants with harvestable ears. In 30-inch row corn, 1/1000^th of an acre is 17 feet, 5 inches.
2. Choose several ears in the 1/1000^th of an acre to count kernels on. We chose 6 per measurement and selected them by choosing every 5^th harvestable ear. You can choose any method, but we suggest using a method to remove some subjectivity from selection and try to reduce the chances of biasing your sample.
3. Count the number of kernel rows around each ear and determine the average number of kernel rows around.
4. Count the approximate number of harvestable kernels on the length of each ear and determine the average kernels per row.
5. Use the following math equation to determine an approximate yield for the field based on your sample:
   
6. Collecting more ears per sample and more samples from an individual field should lead to a more accurate representation of yield. A more thorough yield estimation might use 10 ears per sample and collect samples from 5 different areas of the field.

One hotly debated topic is the appropriate denominator number to use for the kernels per bushel in the yield component equation. 90,000 has always been a standard used in this equation, but more updated sources suggest that modern hybrids may have fewer kernels per bushel. The key factor is that this number is simply assumed and is likely to be variable based on corn hybrid and environmental conditions during grain fill. In years with significant stress during grain fill, kernels are likely to be smaller and that denominator of kernels per bushel may need to be higher – perhaps 100-120,000 kernels per bushel. In other years, the number could be lower than 90,000. This can be especially challenging when trying to determine a yield estimation but is an important note since so much of yield depends on kernel size and kernel fill during the later reproductive stages.

Remember that yield estimates are just that – yield estimates. I failed to post last year’s estimates to this blog (sorry!), but you can see them in this thread on X (Twitter). Last year, the average yield of our 12 estimates using the yield component method was 206 bu/ac in Polk County and USDA-NASS reported the average yield in Polk County to be 206.4 bu/ac.

This year, the average yield across our 12 estimates was 222.6 bu/ac. This may be the least confident I’ve ever been in this estimate due to the variability across fields this year, but we usually end up ‘in the ball park.' Unfortunately, I'm aware we may not receive county-level yield estimates in February 2025, so we may not get a good idea of how close we were this fall until later next year. Despite what felt like a remarkably wet start to the season, the corn benefitted from relatively normal rainfall early, greater July and August rainfall during the grain fill period, and fewer stress degree days (SDDs). Even though it was warmer and growing degree days (GDDs) were ahead of 2022 and 2023, the crop had plenty of moisture available to support grain fill.

Be sure to note areas where yield estimates are made in order to make comparisons after combines have run; you may be able to approximate the number of kernels per bushel and see how close you were to the correct number (for this year, and this hybrid).

See the 2021 article here, the 2022 article here, my 2023 X (Twitter) posts, and contact your local Iowa State University Extension field agronomist to let them know about your corn yield estimates!