No. 2 Article 10/April 9, 2010

Controlling Weeds Before Planting

The challenges imposed by unfavorable environmental conditions during and after last fall's harvest continue to plague many farmers across the state. Fall herbicide applications, which have become increasingly popular as a mechanism to control or suppress populations of winter annual weeds, were delayed or precluded after the 2009 harvest in many areas of Illinois. As a result, fall-emerging weeds often became well established before the onset of winter conditions.

During the past week, warm temperatures coupled with adequate soil moisture have accelerated the growth of existing weed vegetation. Several winter annual species are flowering, and growth of early-season summer annual species is robust. It is advisable to control all existing vegetation before planting your 2010 crop.

Before you implement any plan, scouting to determine what weed species are present, and at what densities, can be time very well spent. Accurate identification often becomes easier as the plants become larger, but it can be more challenging to control larger weeds effectively. "Identifying Early-Season Weeds" in this issue of the Bulletin provides descriptions and color photographs of 20 species common across many areas of Illinois. Other excellent resources are also available to help you identify weeds at various growth stages.

In most instances, farmers use either tillage or herbicides to control existing vegetation before planting. Following are some comments and considerations for each method.

Tillage before planting will likely be relatively common in 2010. Wet soil conditions and heavy equipment used during the 2009 harvest formed ruts that will probably need to be "corrected" with tillage before planting. These "rut repair" tillage operations also can control existing weed vegetation, but large clods are often formed if tillage is done when the soil is too wet. These clods are not ideal for achieving good distribution of soil-residual herbicides that might be applied before or after planting. Also, tilling when the soil is too wet frequently does not provide complete control of existing weed vegetation, especially larger weeds. Large weeds that escape tillage ("cultivator avoidance") are often damaged by the physical disturbance and can be very difficult to control with a subsequent herbicide application. This becomes especially important if preplant tillage is used to control herbicide-resistant weeds, such as glyphosate-resistant horseweed (aka marestail).

Preplant tillage can also be used to incorporate soil-applied herbicides. Herbicides applied to soil surfaces (especially dry surfaces) need to be moved into the soil solution or be in a vapor phase to be absorbed by young weed seedlings. The physical movement of the herbicide is most often accomplished by precipitation or tillage. Uniform herbicide distribution is more readily achieved when the field is free of large soil clods and the implement used places the herbicide into about the top inch of the soil profile. Deeper placement might improve control of certain species (for example, large-seeded broadleaf species), but it can dilute the herbicide and reduce its effectiveness. Keep in mind, however, that the labels of some herbicides (for example, some products containing flumioxazin) specify they should not be mechanically incorporated after application due to the possibility of reducing residual weed control.

If you plan to control existing vegetation with herbicides, be sure to tailor them to the weeds present in specific fields. Remember that you cannot achieve adequate control of glyphosate-resistant horseweed by relying exclusively on glyphosate; tank-mix partners or alternative herbicides are needed to provide adequate burndown control. Products containing saflufenacil, 2,4-D, dicamba, glufosinate, or paraquat can be used to control horseweed prior to corn or soybean planting. In situations where tillage is not an option, existing horseweed plants should be controlled before they exceed 6 inches in height. Research has repeatedly demonstrated improved control of emerged horseweed using two- or three-way herbicide tank mixtures compared with single-herbicide burndown treatments.

Growth regulator herbicides are frequently included in burndown applications. The most common is 2,4-D, but dicamba is also used. Both amine and ester formulations of 2,4-D are labeled for burndown applications prior to planting, but the ester is usually preferred. The low water solubility of an ester reduces the potential for it to be moved into the soil by precipitation, where it could cause severe injury to germinating crop seeds. Also, the ability of esters to better penetrate weeds' waxy leaf surfaces often results in improved control of large weeds and better control during periods of cool air temperatures. The labels of some 2,4-D ester formulations (3.8 lb acid equivalent per gallon) allow preplant applications without a specified waiting interval between application and planting, while other formulations require a 7-day waiting interval between application and corn planting. In addition to waiting intervals, labels sometimes also indicate that tillage operations should not be performed for at least 7 days after application and that the seed furrow must be completely closed during the planting operation or severe crop injury may result. Factors that increase the likelihood of the 2,4-D's coming in direct contact with the crop seed increase the probability of severe crop injury. Pay careful attention to label statements of any 2,4-D formulation used before corn planting.

Cool temperatures can slow the activity of many burndown herbicides, and translocated herbicides are sometimes slower-acting than contact herbicides under these conditions. For example, glyphosate is very effective for control of common chickweed, but symptoms of activity may take several days to develop during periods of cool air temperatures. Contact herbicides may not be as slow to act as translocated herbicides under cool conditions. When the forecast calls for several days or nights of cool air temperatures, symptoms of activity on existing vegetation may develop sooner with a contact herbicide than a translocated herbicide.

What if both tillage and herbicides will be used? Would it be advisable to spray first and then till, or vice versa? The "correct" answer depends on the specific situation in question. For example, if weeds are large before any management operation is implemented it might be advisable to spray a burndown herbicide a few days before the tillage operation. If aggressive preplant tillage is planned to alleviate field ruts, it's probably better to till before applying a soil-residual herbicide.--Aaron Hager

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