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Volunteer Corn - An Issue in Corn and Soybean

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By Liz Stahl and Jeff Coulter

Growers are finding high populations of volunteer corn in their fields this spring.  Factors likely contributing to this include lodging in many fields last fall due to poor stalk quality and drought conditions, and higher harvest losses due to low grain moisture at harvest.  Other factors that can lead to high populations of volunteer corn the following year include storm damage and ear droppage.  The question arises:  When are populations of volunteer corn high enough to warrant control?  


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Volunteer Corn in Corn - U of MN Research Results: 

To help determine the effect of volunteer corn on corn yield and at what point a control tactic should be implemented, research trials were conducted by the University of Minnesota in 2007 and 2008 at the Research and Outreach Centers in Lamberton and Waseca.  Extension Educators Liz Stahl and Ryan Miller along with Scientists Jodie Getting and Tom Hoverstad conducted the trials.

Treatments included seven populations of volunteer corn plants (target populations of 4,000, 8000, 12,000, 18,000, 24,000, 30,000, and 36,000 plants/acre), two populations of volunteer ears corresponding to 2 and 5% ear droppage (650 and 1,600 ear clumps/acre), three combination treatments of volunteer ears and plants (650 ears/acre plus 12,000, 24,000, or 36,000 plants/acre), and a control which contained no volunteer corn.  Carry-over kernels and ears from a glyphosate-resistant hybrid planted the previous year were used for the volunteer corn treatments.  Plots were hand-seeded with the carryover seed, with kernels being planted at two to three times the target population.  Plots were field cultivated and then planted to a glyphosate-resistant corn hybrid at 32,000 to 33,000 seeds/acre in 30-inch rows.  Acetochlor was applied preemergence for weed control followed by glyphosate postemergence.  Within two weeks after the glyphosate application, volunteer plant and ear populations were assessed and plots were thinned to target volunteer populations where needed.  Corn grain yield and moisture content were measured at harvest.       

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Effect of Volunteer Corn Plants on Corn Yield:  Corn yield was significantly affected by volunteer corn in 3 of 4 site-years (Figure 1), although results were variable.  In 2007, low germination of the carryover seed used for volunteer corn treatments contributed to variability among the plots, and average volunteer corn populations were 39 to 62% of the target population.  In 2008, germination of the volunteer seed was much higher and plots were thinned to reach target populations of volunteer corn after the postemergence glyphosate application.

In 2007 at both locations, the lowest level of volunteer corn to impact yield (yield losses averaged 8% compared to the control) was at the 18,000 plants/acre target treatment.  This corresponded to an average volunteer population of 8,000 and 11,000 volunteer plants/acre that year at Waseca and Lamberton, respectively.  In 2008, corn yield was reduced at Lamberton by only the three highest volunteer populations evaluated (yield losses ranged from 23 to 26% compared to the control), or once volunteer populations reached at least 24,000 plants/acre.  Yield was not significantly affected by volunteer corn populations at Waseca in 2008. 

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Effect of Volunteer Corn Ear Clumps on Yield:  Except at Waseca in 2007, yield was not affected by the low (650 ear clumps/acre) or high (1600 ear clumps/acre) population of volunteer ear clumps (Figure 2).  At Waseca in 2007, yield was reduced 9 to 11% by volunteer ear clumps compared to the control, although yields were similar between the two volunteer ear clump treatments.  In 8 of 9 comparisons, yield was not reduced by the addition of 650 volunteer ear clumps to a low, middle or high population of volunteer plants (target populations of 12,000, 24,000, or 36,000 plants/acre).  The only time yields were reduced in a combination treatment compared to the corresponding volunteer plant population was when ear clumps were added to the mid-level volunteer plant population at Waseca in 2007. 

Other observations:  In this trial, contribution to yield by volunteer plants appeared to be minimal.  Emergence was typically delayed in volunteer plants compared to the planted hybrid, and volunteer plants were typically one to six leaf collar stages behind the hybrid.  Most volunteer corn plants, particularly at higher volunteer populations, did not produce an ear and if ears were produced, they were small with poor seed set.  Stalk lodging and harvest moisture were also evaluated, but no differences were detected among treatments. 

What Does this Mean for Control of Volunteer Corn in Corn?  This research demonstrates that volunteer corn can reduce corn yield, although the effects varied by growing environment.  In this study, low levels of volunteer corn did not impact yield.  In years where volunteer corn affected corn yield, volunteer corn populations had to reach at least 8,000 plants/acre before yield was reduced an average of 8 %.  For a 200 bu/acre corn crop, that would be the equivalent of 16 bushels/acre.  Using a corn price of $5.00/ bushel, this calculates to a break-even cost for a control tactic of $80/acre.  At the populations evaluated, volunteer plants also had a greater impact on yield than ear clumps.  These results demonstrate that if volunteer corn populations are high enough, control of volunteer corn in corn may be justified.

Volunteer Corn Control Options in Corn: 

Control options for volunteer corn in corn are limited and complicated by the fact that most hybrids contain at least one herbicide resistant trait if not more.  If a Roundup Ready (RR) hybrid was planted the previous year, glyphosate will not control volunteer corn the following year.  Likewise, if a Liberty Link (LL) hybrid was planted the previous year, glufosinate will not control volunteer corn the following year.  Glufosinate is an option to control volunteer RR corn, and glyphosate is an option to control volunteer LL corn.  However, many hybrids are now stacked with both of these traits, making row cultivation the only option.

Volunteer Corn in Soybean:

Now that most growers in a corn/soybean rotation rotate RR corn with RR soybeans, volunteer corn has become one of the major weed issues in soybean fields today.   Volunteer corn can be a major yield robber in soybean:  Research conducted by Bill Johnson at Purdue University found that soybean yields can be reduced by populations of 8,000 to 16,000 volunteer corn plants/acre (Johnson, et. al., 2011).   Although this population is similar to the population of volunteer corn that impacted corn yield in the U of MN trials described above, previous work by South Dakota State University demonstrates that volunteer corn is much more competitive in soybean than corn (Alms et al., 2007).  Similar populations of volunteer corn (800 to 14,000 plants/acre) resulted in yield losses ranging from 0 to 13% in corn while losses in soybean ranged from 0 to 54%.

Yield impacts are not the only concern with volunteer corn in soybean.  Volunteer corn can act as a bridge between corn crops for corn rootworm, and potentially encourage the development of resistance to Bt-corn rootworm hybrids (Potter, 2012).  For long-term management of corn rootworm, control of volunteer corn in soybean is recommended.       

Volunteer Corn Control Options in Soybean:

There are several effective herbicides that can be used to control volunteer corn in soybean.  According to Jeff Gunsolus, U of MN Extension Weed Specialist, herbicide options include products with the ACCase mode of action such as:  Assure II (quizalofop), Fusilade DX (fluazifop-P), Select Max (clethodim), and Fusion (fluazifop-P and fenoxaprop) (Gunsolus, 2009).  Poast Plus (clethodim) is not as active on volunteer corn as the other products in this mode of action.  The ACCase products are generally targeted on 12- to 24-inch tall corn.  Another option is the ALS inhibitor Raptor, which is targeted to smaller volunteer corn (2 to 8 inch).  For further details, check out the Minnesota Crop News article on volunteer corn management at http://blog.lib.umn.edu/efans/cropnews/2009/06/volunteer-corn-management-in-c-1.html

Always check the herbicide label for the appropriate application rate and timing, information about tank-mixing with other products, and adjuvants guidelines.  Row cultivation is also a viable option for control of volunteer corn in soybean.           

Summary:

--Corn yield can be reduced by populations of volunteer corn.  Volunteer corn populations of at least 8,000 plants/acre were needed before yield losses were observed in U of MN research trials. 

--Volunteer ear clump populations simulating 2 and 5% ear drop contributed less to yield losses than the volunteer corn plant populations evaluated.

--Options to control volunteer herbicide resistant (HR) corn in HR corn are limited.  Depending on the HR traits used, cultivation may be the only option for control when volunteer corn populations in corn are high enough.  

--Volunteer corn is more competitive with soybean than corn, and soybean yield can be affected by populations of volunteer corn at 8,000 to 16,000 plants/acre.

--There are several effective tank-mix options with glyphosate for control of volunteer corn in soybean.

--Volunteer corn control in soybean should be considered not only for potential impacts on yield, but also for potential issues with long-term management of corn rootworm.

Acknowledgements: 

Funding for the University of MN Volunteer Corn in Corn trial was provided by Monsanto Company. 

References:

Alms, J., M. Moechnig, D. Deneke, and D. Vos.  2007.  Competitive ability of volunteer corn in corn and soybean.  Proc.  North Central Weed Sci. Soc.  62:14.

Gunsolus, J.  2009.  Volunteer Corn Management in Corn and Soybean.  Minnesota Crop News.    http://blog.lib.umn.edu/efans/cropnews/2009/06/volunteer-corn-management-in-c-1.html

Johnson, B., C. Krupke, and P. Marquardt.  2011.  Volunteer corn:  A pain in our Roundup Ready crops.  Online Webinar, Plant Management Network.  http://www.plantmanagementnetwork.org/edcenter/seminars/VolunteerCorn/player.html

Potter, B. 2012. Southwest Minnesota IPM stuff. Issue 7. http://swroc.cfans.umn.edu/prod/groups/cfans/@pub/@cfans/@swroc/documents/asset/cfans_asset_390134.pdf

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