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Issue No. 7, Article 3/May 8, 2009

Large Weeds + Wet Fields = Challenging Conditions

The wet field conditions that have delayed planting have conversely been a boon to the growth of both winter and summer annual weed species. The purple coloration present in fields two to three weeks ago has been largely replaced by yellow from the flowering of certain mustard species and butterweed. White-flowered mustard species, such as field pennycress and shepherd's purse, are close to reaching maturity. Several species of summer annual weeds have emerged and are making significant growth. Many fields where no herbicide or tillage operation has occurred currently can be described as "woolly."

Preplant tillage operations can effectively control existing vegetation while a seedbed is being prepared. However, as weeds become larger the effectiveness of tillage to control weeds before planting can be reduced. Dense stands of certain winter annual weeds, such as common chickweed, can "ball up" in a field cultivator. Stems of larger common lambsquarters plants bent over but not completely severed from the roots during tillage may spring back upright in a C-shaped or S-shaped configuration. While the winter annual weeds not completely controlled by preplant tillage eventually will complete their life cycles, summer annual weeds that survive preplant tillage are often much more difficult to control with herbicides applied after crop emergence.

Reduced weed control may also occur when fields are a bit wet during the preplant tillage operation. Soil disturbance may not be as extensive when soils are retaining moisture, and clods are more likely to form. Weeds are also more likely to take root again after tillage when soil disturbance is inadequate but soil moisture is abundant.

Would applying a burndown herbicide before preplant tillage improve control of larger weeds? This would likely provide improved control in many situations, but several points should be considered:

1. If you are applying a translocated herbicide such as glyphosate, it would be advisable to wait 24 to 48 hours between application and tillage to provide adequate time for the herbicide to translocate in the target vegetation. Generally, the longer the interval, the more complete the control of existing vegetation ultimately will be.

2. Growth regulator herbicides may not be the best choice if tillage will be done soon after application. Tillage soon after application may incorporate some of the growth regulator herbicide into the upper soil profile, placing it in the crop seeding zone and subsequently increasing the likelihood of crop injury.

3. Contact herbicides, such as paraquat, may not be as effective as translocated herbicides against larger weeds, but they can begin to desiccate existing vegetation much more quickly than translocated herbicides.

4. Soil-residual herbicides can be included with the burndown herbicide, but keep in mind that the subsequent tillage operation may not provide the most desirable distribution of the residual herbicide in the soil profile. Issues of physical incompatibility and antagonism should also be considered when combining one or more soil-residual herbicides with glyphosate.--Aaron Hager

Aaron Hager

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