ISSUE 6   June 18, 2009


In numerous no-till soybean fields, omission of 2,4-D ester in burndown treatments resulted in a failure to control marestail, and soybeans have now emerged. As many growers know from previous experience, it can be almost impossible to adequately control marestail plants that have recovered from earlier treatments with herbicide. The best case here is that the marestail plants are not herbicide-resistant, and the soybeans are Roundup Ready, which allows use of a high rate of glyphosate or a combination of glyphosate plus FirstRate or Classic. Resistance to glyphosate and/or ALS inhibitors will limit the effectiveness of these treatments. Control is more difficult in non-GMO soybeans, since FirstRate and Classic are the only options. These two herbicides are not that good on large marestail anyway, and populations in many non-GMO fields are ALS-resistant. All (yes - all) other postemergence soybean herbicides have essentially no activity on marestail, and usually cannot be counted on to even slow the growth of marestail.



The answer to this is generally "yes", and you can expect some activity from herbicide once rain finally moves it down into the soil. Herbicide can be lost from the soil surface via degradation, but dry soil conditions will limit the rate at which this occurs. Minor losses from photodecomposition or volatility are possible with certain herbicides, but most are not especially susceptible to this. The bigger issue here is the emergence of weeds between application and rainfall, which will generally require some type of postemergence herbicide treatment even if the preemergence herbicides are finally "activated".



This question obviously applies only to certain areas within the state, since other areas have received an abundance of rain. The second part to this question was whether postemergence herbicide applications should be delayed in corn to wait for later-emerging weeds, even if it meant switching herbicides. One way to look at this is that in any spring, a certain portion of the weed seedbank has broken dormancy and is "primed" to germinate once environmental conditions (primarily moisture and temperature) are suitable. So, it is certainly possible that forth-coming rain will result in substantial weed emergence in areas that have been dry up to now.

Where preemergence herbicides were applied at planting and are providing substantial control still, it may be possible to delay postemergence treatments to allow time for any late weed emergence. However, this delay can result in the need to switch to a product that is safer on corn that is more advanced in growth stage. Where preemergence herbicides were not applied, or where more than a few weeds are escaping preemergence herbicides, the postemergence treatment should be applied when weeds are small to maximize effectiveness and reduce weed-crop competition. Keep in mind also that weeds emerging after the corn reaches a height of about 20 inches have little potential to reduce yield anyway, even if they are present at crop harvest. Our general recommendation is to apply postemergence herbicides by the time corn reaches this height, and let the crop do the work of suppressing later-emerging weeds. One exception to this is burcucumber, which can require application of herbicides when the corn is upwards of 30 inches tall.



Unharvested corn from the 2008 crop and corn seed going through the back of the combine will contribute to corn becoming the "Weed of 2009". High populations will surely occur this year. What impact the volunteer corn will have on this yearís crop yield and the viable management options available will depend upon in which crop the volunteer corn is present. Making the assumptions that the majority of the volunteer corn present is glyphosate resistant and that glyphosate resistant crops were planted in the field this year, your only management option in corn at this time is cultivation. In soybean you have the herbicide options of the ACCase inhibiting herbicides such as: clethodim, Fusilade DX (fluazifop-P), and Assure II (quizalofop); note Poast Plus (sethoxydim) is not as active as the other POST grass herbicides on volunteer corn. The ACCase inhibiting herbicides are generally targeted on 12- to 24-inch tall volunteer corn. The ALS herbicide, Raptor can also effectively control smaller (2 to 8 inch) volunteer corn.

In 2007, researchers at South Dakota State University indicated that volunteer corn is much less competitive in corn than soybean. The South Dakota study (Alms et al. 2007) evaluated the full season effect of a range of volunteer corn densities (800 to 14,000 plants per acre) on both corn and soybean and reported yield losses that ranged from 0-13% in corn and 0-54% in soybean. A 2007 University of Minnesota study reported yield loss potential in corn that was very similar to the South Dakota study. Iowa State reported one volunteer corn plant per 10 foot of row reduced corn yield 1.3%. This lower impact on corn is likely due to a volunteer cornís reduced demand for resources and the competitive vigor of the planted F1 hybrid. Volunteer corn has a lower yield potential than the planted F1 hybrid resulting from delayed emergence. In the University of Minnesota study, volunteer corn plants lagged from one to six leaf stages behind the crop and few plants produced an ear by harvest.

If volunteer corn populations are high and conditions remain dry, inter-row cultivation is a cost-effective option in corn unless the herbicide resistance traits of volunteer corn and the planted corn differ, then herbicide control is possible. Volunteer conventional corn can be controlled with glyphosate or Ignite; Volunteer Liberty Link (LL) corn can be controlled with glyphosate; Volunteer Roundup Ready (RR) corn can be controlled with Ignite. Unfortunately, stacked LL/RR corn must be cultivated. Ignite can be less effective than glyphosate due to its contact activity, but it will still significantly reduce the competitiveness of the volunteer corn.

Volunteer corn can significantly reduce soybean yield. A study conducted in Minnesota in 1979 and 1980 by Andersen et al.1982 evaluated the effect of clumps of volunteer corn per row of soybean (approx. 8 plants/clump) on soybean yield. When averaged over six experiments, soybean yield was reduced 1% for every 75 clumps per acre. The authors also indicated that at a density of 75 clumps per acre or greater, delays in herbicide application of approximately 3 weeks (from mid-June into July) resulted in a reduction in soybean yield.

Like any weed, early emerging volunteer corn that competes longer into the growing season with soybean will have a greater impact on yield, especially if drought conditions persist. However, volunteer corn emergence is often delayed and extends throughout the growing season making the timing and economics of the time of weed removal more difficult to determine. To save a trip across the field the ACCase inhibiting herbicides mentioned previously can be tank mixed with glyphosate, but adjuvant requirements may need to be adjusted depending upon the glyphosate and ACCase herbicide formulations used (see specific labels for details). Also, different ACCase rates are associated with different volunteer growth stages. Often growers would like to wait later into the growing season before treating for volunteer corn, to allow for the extended emergence period. This response is understandable but must be tempered by the fact that the early-emerging volunteer corn plants will cause the greatest yield loss and extending the herbicide application too late into the growing season can diminish herbicide effectiveness, potentially resulting in reduction in soybean yield.

U of MN entomologists Bruce Potter and Ken Ostlie are proposing additional reasons to remove volunteer corn in soybean earlier rather than later. First, allowing volunteer corn plants to grow into July allows time for any hatching corn rootworm larvae to feed and possibly survive on volunteer corn within soybean fields. Survival rates depend on whether or not volunteer corn expresses the Bt-RW gene. An additional concern is an increase of corn rootworm exposure to sub-lethal levels of the Bt proteins insecticidal properties and potentially accelerating development of resistance. Second, volunteer corn plants can serve as sites for corn rootworm egg laying, potentially increasing rootworm populations in the next corn crop. Therefore, the presence of volunteer corn reduces the crop rotation effect. While much is still unknown in this area of research, removing volunteer corn plants in mid-June rather than July would help to mitigate these potential problems.

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.

Andersen, R.N., J.H. Ford, and W.E. Lueschen. 1982. Controlling volunteer corn (Zea mays) in soybean (Glycine max) with diclofop and glyphosate. Weed Sci. 30:132-136.

Rich Zollinger
Extension Weed Specialist

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