CORN
Crop Observation and Recommendation Network

March 17-31, 2003
C.O.R.N. 2003-6

In This Issue:

A) Can Dandelions Be Controlled In The Spring?
B) Weed Control Consequences Based Upon Prior Years' Practices
C) Currently Available Insecticide For Corn, Soybeans, Wheat And Alfalfa

A) Can Dandelions Be Controlled In The Spring? - Loux CORN Questions

Dandelions are extremely problematic in many corn and soybean fields in the eastern corn belt. Herbicide treatments in fall are the most effective strategy to control established dandelions, and we have a good handle on which treatments are most effective in fall. Conversely, dandelions do not always respond well to spring herbicide treatments, and we still lack data on how to maximize control with herbicides in the spring. One of the problems in the spring is the inability of systemic herbicides to readily move into the roots until dandelion is advanced in growth stage in late spring, which results in much variability in control, especially for dandelions with large taproots.

Research in turfgrass at Purdue University showed a relationship between the effectiveness of 2,4-D and degree day accumulation. Control generally improved as degree days accumulated, but 2,4-D was not consistently effective until late spring. We expect that dandelions in no-till fields are more difficult to control than those in turf. Available research indicates that the activity of glyphosate on dandelions in spring may be even more variable than that of 2,4-D ester, and less likely to be predicted by degree day status (which reinforces the utility of adding 2,4-D to glyphosate in spring burndown treatments).

A second problem is the emergence of dandelions from seed, which occurs primarily from mid April through late May. Herbicide programs that lack preplant herbicides with residual activity create an environment perfect for the emergence of dandelion seedlings, which are often than not well controlled with postemergence herbicides either. The result of seedling emergence - a new dandelion infestation even where a herbicide treatment the previous fall controlled established plants. Effective dandelion control programs are therefore a combination of herbicides applied at the proper time for control of established plants, and use of preplant herbicides with residual to control spring-emerging dandelion seedlings.

Under last season's dry conditions, dandelions were very competitive with corn and soybeans, with severe yield loss in areas where dandelions were not adequately controlled. The competitiveness of dandelion will vary with environmental conditions, but the combination of established and newly emerging dandelions in a field can result in interference with both crop establishment and early to mid season growth. For this reason, dense stands of dandelions should probably be controlled well before crop planting, rather than at the time of planting. The question is - how to accomplish this given the variable effectiveness of spring herbicide treatments? We hope to be able to provide a better answer to this question after another season of research, but we can offer the following suggestions at this time:

• Tillage can effectively control dandelions, but it must be aggressive enough to completely disrupt them. This is unlikely to occur with most spring tillage tools.
• Use a preplant burndown treatment, and include 2,4-D ester at the highest rate labeled for the time of application. Increasing the 2,4-D ester rate from 0.5 to 1.0 lb/A can improve control of dandelion, marestail, and other tough weeds. The 0.5 lb rate (1 pint of a 4 lb/gallon formulation) of all 2,4-D ester products must be applied at least 7 days before soybean planting. Most 2,4-D ester labels state that the 1.0 lb rate should be applied at least 30 days before planting. An exception is the label for Weedone 650 (Nufarm), which allows application of 1.0 lb/A at least 15 days before planting (1 1/3 pints of a 6 lb/gallon formulation).
• Do not skimp on the glyphosate rate in burndown treatments. Minimum rate for dandelion control is 0.75 lb/A (22 oz of Weathermax or 32 oz of other glyphosate products). Consider increasing the rate in dense populations of well-established dandelions. Adding low rates of Canopy XL to a glyphosate/2,4-D burndown may improve control and help prevent regrowth (and provide residual control of seedling dandelions). The addition of Valor to a glyphosate/2,4-D treatment can also speed the rate of burndown, but dandelions are more likely to regrow compared to Canopy XL.
• - Gramoxone does have a fit in the spring for burndown of dandelions, but only in specific combinations. In OSU research, the mixture of CanopySP plus 2,4-D ester plus Gramoxone has been a very effective treatment for control of dandelions prior to soybean planting. Canopy XL is slightly less effective than Canopy SP in the mixture, but is still an effective choice where rapid burndown of dandelion is needed. Adding a few ounces of Sencor to a will improve control with mixtures of 2,4-D plus Gramoxone plus CanopyXL. Somewhat similar results can be achieved with a mixture of Sencor (8 oz) plus 2,4-D ester plus Gramoxone, but the dandelions are more likely to regrow with Sencor versus either Canopy product.
• In corn, combinations of Gramoxone plus atrazine plus 2,4-D can be extremely effective for dandelion burndown, with more rapid activity than glyphosate treatments, especially in early to mid spring. Callisto and Balance have activity on dandelions also, but university data are lacking to make more specific recommendations on these products. We suggest applying either product in combination with atrazine and 2,4-D for most effective control.
• Include herbicides with residual activity to control seedling dandelions that emerge throughout the spring (Canopy XL or SP, Sencor, Valor, Scepter, etc).
• Use appropriate herbicides and rates in postemergence treatments if dandelion are present. Classic, Synchrony, and glyphosate are the most effective postemergence soybean herbicides. In Roundup Ready soybeans, mixtures of Classic plus glyphosate can improve postemergence dandelion control. A number of postemergence corn herbicides have activity on dandelion, including Hornet, any dicamba product, 2,4-D, and other combinations that include any of these.

B) Weed Control Consequences Based Upon Prior Years' Practices - Stachler and Loux CORN Questions

Weed populations change over time, and the production practices implemented by producers have a major influence in determining the nature of these changes. In effect, the weed control and crop production practices of prior years have caused some of our current weed problems. Below are some examples of the weed population shifts or weed control problems that have occurred over the past 55 years due to management practices:

• Annual grasses became a problem in corn after the introduction of 2,4-D in 1947, because of the heavy reliance upon 2,4-D to control broadleaf weeds and the subsequent reduction in row cultivation;
• Fall panicum became a new weed problem in corn after the introduction of atrazine, because of the heavy reliance upon atrazine, which did not control fall panicum, continuous corn, and a reduction in row cultivation;
• Atrazine-resistant weeds began to occur in 1968, 11 years after the introduction of atrazine, because of the continued heavy reliance upon atrazine and continuous corn;
• Perennial weeds became more of a problem in corn and soybean fields after the introduction of no-tillage practices. By the 1990's, trees became a component of the weed population in long-term no-tillage fields;
• ALS-resistant weeds began to occur in Ohio in the mid-1990's, due to the heavy reliance upon Pursuit, Preview/Canopy, and other ALS-inhibiting herbicides and continuous soybeans;
• Annual grasses become a problem in no-tillage and reduced-tillage corn in the 1990's due to heavy reliance upon Extrazine, surface-germinating grasses that were not adequately controlled by chloroacetamide herbicides, the reduction of atrazine rates, and the elimination of row cultivation;
• Winter annual weeds increased during the late-1990's due to warmer winters, the practice of delaying burndown herbicide applications until planting (that allowed winter annual weeds to set seed), increased use of postemergence herbicides, reduced usage of preemergence herbicides, earlier harvesting (that allowed more time for winter annual weeds to establish), and increased no-tillage acreage;
• The increase in dandelion populations currently being experienced is due to the reliance upon only glyphosate in the burndown and in-crop applications in Roundup Ready soybeans, a decrease in the use of residual herbicides in soybeans and to some degree corn, the large number of no-tillage acreage, continuous soybeans, and the failure to use 2,4-D in burndown treatments;
• Glyphosate-resistant marestail is now present in Ohio due mostly to continuous no-tillage soybeans and the usage of glyphosate only for burndown and in-crop applications in Roundup Ready soybeans since 1997;
• One positive change has been the near extinction of jimsonweed. This has occurred due to early planting, no-tillage, and use of ALS-inhibiting herbicides.

These examples illustrate the consequences or weed population shifts that can and do occur based upon the usage of a limited number of management practices. This is especially true when herbicides are relied upon as the main weed control tool. Diversity in the sites of actions of herbicides is important to minimizing the number of herbicide resistant species. To achieve this diversity, preemergence herbicides should be used more often in soybean production, especially no-tillage soybeans.

Despite the many negative examples cited above, there are currently more methods and tools available to control weeds than at any time in the history of weed control!

C) Currently Available Insecticide For Corn, Soybeans, Wheat And Alfalfa - Eisley and Hammond CORN Questions

There have been some additional insecticides labeled during the past several months and we have tried to mention those in previous newsletters. Tables that summarized current chemicals labeled on each crop with some of the insects controlled and restrictions listed on the labels can be found at: http://corn.osu.edu/library/articles/03insectupdate.html. Tables include Corn-Soil Insecticides and Seed Treatments, Corn-Foliar Insecticides, Soybeans-Foliar Insecticides, Alfalfa and Small Grains. There may be additional insects or label restrictions that are not listed in the tables. Check the current labels for the most up-to-date information before using any insecticides.

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Past versions of C.O.R.N. can be found on the World Wide Web at: http:/www.ag.ohio-state.edu/~corn/archive/

C.O.R.N. is a summary of crop observations, related information, and appropriate recommendations for Ohio Crop Producers and Industry. C.O.R.N. is produced by the Ohio State University Extension Agronomy Team, State Specialists at The Ohio State University and Ohio Agricultural Research and Development Center. C.O.R.N. Questions are directed to State Specialists, Extension Associates, and Agents associated with Ohio State University Extension and the Ohio Agricultural Research and Development Center at The Ohio State University.

Editor: Greg LaBarge        Web Editor: Nathan Watermeier

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