For the fifth consecutive 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. While harvest is already well underway, I wanted to share the results of our tour.
Patrick and I visited 16 corn fields in Polk County on September 8, 2025 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 year was one of the most interesting due to the sufficient (or excessive) summer moisture and the uncertainty regarding what southern rust and other stressors would do to the crop. While other methods exist, the most traditional method of making yield estimates is the “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.
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). In 2024, we estimated 222.6 bu/ac as the county average and waited until June 2025 to learn that USDA-RMA reported the actual average as 226.5 bu/ac.
While yield estimates varied greatly from field to field, the average yield across our 16 estimates was 202.5 bu/ac. The low yield estimate was 174 and the high was 254, but the majority of yields were below 220 bu/ac. This is the lowest yield estimate we’ve made in three years. Our estimate differs substantially from some other Polk County estimates I’ve heard, but we will know what the final RMA numbers are next summer.
According to a Des Moines weather station, the growing season started out similar to 2024, but we had an overall cooler June that slowed our GDD accumulation compared to the last two years. In mid-July, the rainfall far surpassed other recent years, though lack of rainfall in August stalled accumulation. By early September, total GDD accumulation was slightly behind the past two years and total rainfall was similar to 2024 but ahead of 2023. The most notable difference I can see is that we appear to have had a cooler early summer and a warmer late summer than in 2024, combined with more frequent rainfalls. While yields are rolling in, the full image of what the weather and southern rust pressure did is yet to be determined.
See the 2021 article here, the 2022 article here, my 2023 X (Twitter) posts, the 2024 article here, and contact your local Iowa State University Extension field agronomist to let them know about your corn yields!
Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Integrated Crop Management News, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on September 30, 2025. The information contained within may not be the most current and accurate depending on when it is accessed.
- Field Agronomist in Central Iowa