Special Edition   April 24, 2009

WEED CONTROL ISSUES

The excessive moisture and flooding this spring may impact weed management this spring. If planting is completed before mid-May, few adjustments in weed management will be needed, with a few exceptions. As planting is delayed beyond mid-May weed management programs should be critically evaluated and changes may be necessary. Flood waters will have some impact upon weed populations. The delayed harvest of crops last fall may increase the likelihood of volunteer corn and soybeans. Special management of volunteer Roundup Ready crops will be needed. The weed management issues listed below should be reviewed to determine necessary changes in your weed management program.

1. Control of volunteer Roundup-Ready crops. Volunteer crops may be more prevalent this year due to the delayed harvest in 2008. Consult pages 19 and 74 of the 2009 North Dakota Weed Control Guide for the effectiveness of various herbicides to control volunteer Roundup-Ready crops. Select Max controls volunteer Roundup Ready corn more effectively than clethodim 2 EC formulations (Arrow, Clethodim, Intensity, Section, Shadow, Trigger, and Volunteer) when used at equivalent active ingredient rates. However, the Select Max label recommends a lower use rate, which can compromise control.

2. Impact upon weed populations from flooding. Buoyant weed seeds can be picked up by flowing water from one area and be deposited to another area downstream. Buoyant weed seeds are usually deposited along the waters edge where other trash is deposited. Areas lower in topography in which water stood for an extended period of time will allow dense weed seeds to be deposited. This allows for the spread of weed species or biotypes not previously present in a field. Common cocklebur is a great example of a very buoyant seed. Herbicide-resistant seeds may have been moved from one field to another by moving water. Scout fields before planting and before herbicide applications to determine the presence of introduced weed species. Fields should also be scouted after all herbicide applications to determine the effectiveness of the application in case difficult to control weed biotypes entered from another field.

3. Impact upon preemergence herbicides with delayed planting. With the increase usage of preemergence herbicides, especially in corn and soybeans, a few reminders are important. All residual type herbicides require rainfall/water for activation. Some herbicides require less rainfall for activation compared to others. Incorporating preemergence herbicides usually improves weed control. As planting is delayed into late-May and early-June, the chances of rainfall diminish, which reduces the opportunity for herbicide activation. As planting is drastically delayed more weeds will be present at the time of tillage or preplant herbicide applications compared to early planting. A reduction in weed density during the growing season usually occurs when weeds are removed late in the spring before planting. Therefore, the later a crop is planted the greater the risk that a preemergence herbicide will not be activated and fewer weeds should be present during the growing season.

4. Impact upon preplant herbicide applications in no-tillage with delayed planting. Weeds will be larger at the time of a preplant application as planting is delayed. Use the appropriate herbicide and rate for the weed species present and size of weeds at the time of the preplant application. If 2,4-D is removed from the preplant application due to a desire to plant corn and soybeans earlier, increase the rate of the non-selective herbicide being used. This is especially true for those species more effectively controlled when 2,4-D is mixed with the non-selective herbicide. If glyphosate-resistant weeds are known to be present in a field, keep the 2,4-D in the preplant herbicide mixture and delay planting or use a high rate of paraquat or Ignite or some other effective preplant alternative herbicide. Another option may be to apply glyphosate at 2.25 lb ae/A or greater as long as the weeds are less than 4" tall.

5. Impact upon postemergence herbicides if a preemergence herbicide is not applied. If a preemergence herbicide is not applied to control early season weed species, then the postemergence herbicide must be applied to small (less than 2-3") weeds for maximum control. Glyphosate-resistant weed biotypes should be more effectively controlled from a glyphosate application to 1-inch weeds compared to weeds greater than 1 inch. If planting is drastically delayed, fewer postemergence herbicide applications should be required compared to early planting.

6. Herbicide carryover. Please remember to consider what herbicides have been previously applied if a change in crop rotation is necessary.

Jeff Stachler
Agronomist - Weed Science
jeff.stachler@ndsu.edu


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