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Ohio State University Extension


C.O.R.N. Newsletter 2011-26

Dates Covered: 
August 9, 2011 - August 16, 2011
Rob Leeds

Yellow soybeans in response to glyphosate applications

Chorosis of soybean leaves seems to be more prevalent than usual this year, and some of this can be attributed to POST glyphosate application.  The following resources cover this subject, and may be helpful. 

Presentation by Dr. Mark Bernards, University of Nebraska,

Monsanto Agronomic Spotlight on leaf yellowing – on OSU weed science website,

Prevent annual weed seed production in fallow fields

There is still time to control weeds in fields that were not planted to crops this year due to wet weather.  A major goal of any control implemented at this time should be prevention of seed production by summer annual weeds.  It’s not always necessary or practical to kill weeds, especially large ones, to prevent seed production.  Summer annual weeds that are mowed now, or substantially affected by herbicides, should produce few seed even if they are able to still reach maturity.  The choices for control are probably limited to mowing or herbicides at this point.  Tillage is certainly an option, but control of the large weeds in many fields would require more than a shallow tillage pass.

As much as we would like for growers to avoid glyphosate applications in order to minimize further selection for glyphosate-resistant weeds, a glyphosate-based herbicide program may still make the most sense here.  Large weeds are most effectively controlled by systemic herbicides and the low cost allows glyphosate to be used at 1.5 to 3 lbs ae/A in these fields.  It’s important not to apply glyphosate alone, however.  A mixture of glyphosate and 2,4-D may be the most cost-effective approach.  This should control most grass and broadleaf weeds, and injure the glyphosate-resistant marestail enough to prevent or greatly reduce seed production.  Use 2,4-D amine and avoid applying near areas that could be damaged by 2,4-D movement.  It’s also possible to apply a mixture of glyphosate and dicamba, or a premix of 2,4-D and dicamba, but the potential volatility of dicamba at this time of year is a major concern.  Where it’s not possible to use a mixture of glyphosate and 2,4-D, use of either Ignite or a mixture of glyphosate and Sharpen can achieve the goal of minimizing weed seed production, especially for marestail.  It may be possible to accomplish this with Gramoxone also, but only at the higher labeled rates and in combination with 2,4-D or a high rate of metribuzin.  Use a spray volume of 15 to 20 gpa when applying Sharpen, Ignite, or Gramozone, and avoid use of nozzles that produce primarily large droplets.

Screening common and giant ragweed populations for herbicide resistance

We will be screening ragweed populations for response to glyphosate and other herbicides this fall/winter in the greenhouse to get a better idea of the current herbicide resistance situation in Ohio.  We will be collecting seed and soliciting seed collections from fields where the response to glyphosate has decreased over time, or where resistance is suspected.  Let us know if you have a field or fields where this is the case, or collect mature seed and send it to us.  The important caveat here is that fields must have been treated with an appropriate glyphosate program to begin with.  This essentially means that the field was weed free at the time of planting, and glyphosate was then applied postemergence one or more times at a reasonable weed size.  Fields where ragweed survived burndown, or where glyphosate was used as the only burndown herbicide this year do not generally qualify, since weeds were large and old by the time burndown herbicides could be applied due to wet conditions.  Fields with a several-year history of glyphosate response problems also qualify.  For more information, contact Mark Loux, 614-292-9081,

Soybean Aphids and Two spotted Spider Mites on Soybeans

As we are getting into August, we are beginning to hear of soybean aphids and twospotted spider mites showing up on soybeans, not only in Ohio but in neighboring states.   While aphids are not yet being reported in high numbers in our state, they are reaching economic levels elsewhere farther to our north.  And then in the driest areas of Ohio, twospotted spider mites are reaching levels requiring spot treatments. 

Growers should remember that with the overall late plantings this summer, most soybeans have a lot of growth remaining, and will remain green into September.  Whether either pest reaches greater levels over the state remains to be seen.  Because of the possibility, we do recommend that growers and scouts maintain a watch on their fields for aphids and in areas that are still dry, twospotted spider mites, to ensure neither catches them off-guard.  See our field crop insects’ web page,, for fact sheets, treatment thresholds, and available insecticides and miticides.

Bt Resistance Found in Western Corn Rootworm in Iowa

A team of Iowa State University researchers led by Dr. Aaron Gassmann has documented the first case of resistance to Bt by rootworms, published last week in the journal PLoS:One  (see the article for free at  Their study observed increased survival of western corn rootworms from 4 populations in eastern Iowa when reared on Bt corn compared to non-Bt corn.  More specifically, resistance was found in Cry3Bb1, which is contained in varieties such as VT Triple Pro and Smartstax.  Resistance was not found in the other rootworm-Bt trait, Cry34/35Ab1.  Rootworms from these 4 populations were all collected from continuous corn fields, where varieties containing Cry3Bb1 were grown for 3-5 years.  It may be no surprise, then, that resistance occurred under this strong selection pressure.

What does this mean for Ohio growers?  If this trend and selection pressure continues in Iowa, populations could spread.  Think of the western corn rootworm variant which spread from Illinois into Western Ohio.  Or, if this same pattern of continuous corn with the same trait occurs in Ohio, Bt resistance can arise. (We should also point out that we have NOT had any indication of failure of this or any Bt-trait in Ohio). This research should serve as an example and reminder of good, responsible IPM (such as switching traits, rotating with soybean, ensuring refuge compliance, etc.) is still needed even with Bt technology.

ECO Farming Field Day

ECO Farming stands for Eternal No-till, Continuous Living Cover, and Other best management practices. Eternal no-till means keeping the soil undisturbed as much as possible to improve soil quality and soil health. Continuous living cover is growing a crop on the soil 100% of the calendar year and may include grain crops followed by cover crops, pasture, hay, or perennial crops. The goal is to have live roots to feed the soil microbes, recycle soil nutrients, and to protect the soil from soil erosion. Other BMP’s may include controlled traffic, water table management, integrated pest management, and manure management. This system of farming closely mimics and enhances natural cycles in the soil that conventional tillage destroys. For more information, attend our ECO Farming Field day, August 19, 9:30 AM to 2:30 PM at the Jeff Rasawehr Farm, 8820 Kuck Road, Celina, Ohio 45822. Lunch is free but you must register by August 17th by calling 419-586-2179. See this website for more details:

Southwest Ohio Corn Growers and Fayette County Agronomy Field Day

The Southwest Ohio Corn Growers Association and Fayette County Agronomy Committee will hold their annual field day on August 16th at the Fayette County Demonstration Farm. Registration begins at 9:30 AM, and the educational program starts at 10:00 AM. This year's topics include: Understanding corn maturity ratings, refuge requirements for insect resistant corn, fungicide usage in corn, Crop scouting 101, and soil compaction/2011 crop issues. In addition to the educational programming we will have our agribusiness displays and health screenings.

This year's featured speaker will be the National Corn Growers Association, Policy Committee Chairman, Anthony Bush. Dwayne Seikman, OCGA Executive Director, will provide an update on the State organization. We will also hear from Rocky Black, Deputy Director of the Ohio Department of Agriculture on the activities of the Ohio Livestock Care Standards Board.

Cargill AgHorizons has joined as our event sponsor. Aside from their financial support, they have volunteered to host our afternoon activities at the Bloomingburg elevator. At 2:00 PM, the field day participants are encouraged to attend our Grain bin safety demonstrations, and tour the Valero Ethanol facility.

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Crop Observation and Recommendation Network

C.O.R.N. Newsletter is a summary of crop observations, related information, and appropriate recommendations for Ohio crop producers and industry. C.O.R.N. Newsletter is produced by the Ohio State University Extension Agronomy Team, state specialists at The Ohio State University and the Ohio Agricultural Research and Development Center (OARDC). C.O.R.N. Newsletter questions are directed to Extension and OARDC state specialists and associates at Ohio State.