Bill Johnson
Paul Marquardt and
Glenn Nice

Volunteer Corn Competition and Control in Soybeans

Created: 6/23/2010

We have observed a lot of soybean fields with volunteer corn over the last several years. In fact a recent survey of weed control talks in the North Central Weed Science Society proceedings shows that volunteer corn is the 5th most common weed found in titles of talks given at the annual conference. This indicates that volunteer corn has reemerged as a major weed in Midwest soybean and corn cropping systems.

Table 1. Most common weeds mentioned in North Central Weed Science Society titles from 1948 to 2007.

Rank 1948 1955 1967 1977 1987 1997 2007

1 Canada thistle Canada thistle Field bindweed Common milkweed Canada thistle Common sunflower Lambs-qurters
2 Field bindweed Field bindweed Western ironweed Field bindweed Downy brome Common waterhemp Common waterhemp
3 Leafy spurge Large crabgrass Wild buckwheat Johnson-grass Musk thistle Giant foxtail Giant ragweed
4 Perennial sow thistle Leafy spurge Wild oat Wild buckwheat Johnson-grass Palmer amaranth Horseweed
5 Russian knapweed Quackgrass Yellow nutsedge Yellow nutsedge Velvetleaf Velvetleaf Vol Corn
Grasses 0 2 2 2 2 1 1
Broadleaves 5 3 3 3 3 4 4
Annuals 0 1 2 1 2 5 5
Biennials or Perennials 5 4 3 4 3 0 0

The purpose of this article is to review our data on the competitiveness of volunteer corn in soybean and go over the control options. To evaluate the competitiveness of volunteer corn with soybean, field trials were conducted just south of Lafayette, Indiana in 2008 and 2009. Soybean was planted in 7.5 inch rows and volunteer corn (from ears collected the previous fall) was seeded on the same day as corn was planted. See figure 1 for an idea of what the different densities looked like in the middle part of the growing season.

Figure 2. Volunteer corn densities within the study.

In 2008 densities of 2 or more plants per square meter caused significant yield reductions.  In 2009, 4 or more plants per square meter caused significant yield reductions. Densities as high as 16 plants per square meter reduced yields 40 to 45% The results indicate that soybean can tolerate some competition from volunteer corn, but if densities exceed 2-4 plants per square meter, treatment measures should be implemented.

It is important to note that volunteer corn emerges at the same time soybeans do, yet we typically do not treat until the second treatment of glyphosate is applied to the soybean field. In the past we have typically thought that volunteer corn is a late emerging weed. However, in this study, we attempted to establish volunteer corn densities at the V3 stage of soybean growth and found that plants that emerged by V4 soybean or later, did not survive until the end of the growing season.  So, by the time the second postemergence treatment of glyphosate is applied to soybean we are likely 6-8 weeks into the growing season and significant amount of competition has occurred since we are treating plants that emerged at the same time soybean emerged.

Volunteer corn can be controlled in soybean with postemergence grass herbicides that were used on a widespread basis until we had Roundup Ready soybean (Table 2).  In Roundup Ready or non-GMO soybeans, any type of volunteer corn can be controlled by inclusion of Assure II/Targa, Select Max, Fusion, Fusilade or clethodim in postemergence herbicide programs. Activity of these herbicides is maximized by use of COC or MSO, plus AMS. Control can be more variable when applied in a mixture with a glyphosate product that contains surfactant, in the absence of COC or MSO. Generic clethodim is often less effective than the other products listed here in mixtures with glyphosate, unless COC or MSO is included in the mixture.

Table 2. Postemergence herbicides for control of volunteer corn in soybeans.

Product Volunteer Corn size Rate /acre

Select Max Less than 12 inches
12 to 24 inches
6 oz
9 oz
Clethodim (Arrow/Section) 4 to 12 inches
12 to 18 inches
4 oz
6 oz
Assure II / Targa 10 to 12 inches
12 to 18 inches
18 to 30 inches
4 oz
5 oz
8 oz
Fusilade DX Less than 12 inches
12 to 18 inches
4 oz
6 oz



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