How Much Yield Have You Lost?

June 16, 2010

by Bob Hartzler, Department of Agronomy 

A combination of low glyphosate prices deterring preemergence applications and a wet June preventing timely postemergence applications has resulted in weeds reaching levels capable of interfering with crop growth and yield potential in fields across the state. While many fields will still be able to be 'cleaned up' when they dry, the lost yield cannot be recovered.

While the lost yield is unfortunate, it can provide a teachable moment regarding crop-yield interference. WeedSOFT 8X  is a web-based competition model that can be used to estimate early-season yield losses. All that is required to determine potential yield loss from weed competition is crop stage, estimated weed-free yield potential, weed density and weed size. The model is based on research conducted in the 1990s at land-grant universities across the Corn Belt.

To demonstrate WeedSOFT, weed infestations in two soybean fields outside of Ames were sampled on June 15 by counting weeds in 10 one-square-foot quadrats. The soybean, planted in 30 inch rows, were at the V4 growth stage and had an estimated yield potential of 65 Bu/A.  The yield already lost at the time of sampling and if control is delayed until the V5 stage as predicted by WeedSOFT is presented in the following table. 

 

Field 1 had a heavy infestation of giant foxtail, and WeedSOFT estimated that the foxtail and other weeds had already reduced yields by 6.7 bu/A (10 percent). The second field had fewer total weeds, but the weed species present were more competitive than foxtail and also were significantly larger than the weeds in Field 1, resulting in a loss estimate of 7.4 bu/A. It is important to recognize that the specific yield loss relationship between weeds and crops is highly variable depending upon the environment and cultural practices. The estimates made by WeedSOFT are conservative, thus the model is more likely to overestimate than underestimate the yield loss. However, the estimates are based on the best research available and are realistic. 

The use of WeedSOFT can demonstrate the risks associated with total postemergence programs and the value of early-season weed control. The focus for this year should be to select a postemergence program that will effectively control the weeds present in individual fields. For next year, adjust plans accordingly to minimize the risk of losing yield potential to early-season competition.

 

Bob Hartzler is a professor of agronomy with extension, teaching and research responsibilities.

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 June 16, 2010. The information contained within may not be the most current and accurate depending on when it is accessed.

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Bob Hartzler Professor of Agronomy

Dr. Bob Hartzler is a professor of agronomy and an extension weed specialist. He conducts research on weed biology and how it impacts the efficacy of weed management programs in corn and soybean. Dr. Hartzler also teaches undergraduate classes in weed science and weed identificatio...