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Issue No. 4, Article 6/April 21, 2006

Control Existing Vegetation Before Planting

Decreased use of soil residual herbicides in soybean, several recent "mild" winters, and a general reduction in fall tillage all have contributed to changes in the weed spectrum across much of Illinois. One change that is particularly noticeable this time of year is the robust amount of weed vegetation present in many fields where no fall tillage or herbicide applications occurred. Compared with 10 years ago, the amount of existing vegetation to be dealt with prior to planting is often much greater and composed of species not familiar to everyone.

Much of this existing vegetation consists of winter annual weed species, such as chickweed, henbit, field pennycress, butterweed, and purple deadnettle. These species generally emerge in the fall and overwinter, but sometimes (depending on weather and soil moisture conditions) they may emerge during the early months of the year. Some early-emerging summer annual species, such as prostrate knotweed, kochia, common lambsquarters, and giant ragweed, already have made their presence known. This "mat" of vegetation can cause significant problems with planting operations and crop establishment if not adequately controlled. In most situations, farmers should control existing vegetation prior to planting no-till corn or soybean.

2,4-D can be used as a burndown treatment alone or tank-mixed with several other herbicides. The ester formulation is usually preferred over the amine formulation since the waiting period between application and planting is generally less for an ester formulation. The labels of many 2,4-D ester formulations (3.8 lb acid equivalent per gallon) allow applications of up to 1 pint per acre 7 days prior to soybean planting; increasing the rate to more than 1 pint increases the waiting interval to 30 days. Keep in mind that some 2,4-D ester formulation labels also specify a waiting interval between application and corn planting (for example, 7 days for up to 1 pint, 14 days for rates between 1 and 2 pints). In addition to waiting intervals, labels sometimes 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 coming in direct contact with the crop seed increase the probability of severe crop injury.

Injury to soybean from a preemergence application of 2,4-D less than 7 days before planting.

Several soil-applied herbicides used in corn and soybean have both soil and foliar activity. This foliar activity can provide some control of small annual weeds. In corn, products such as atrazine and Balance PRO or premixes containing these herbicides can provide control of small weeds. In soybean, products such as metribuzin (Sencor, in Boundary, Canopy, and Domain), Valor, Gangster, and Extreme have foliar activity and can be applied prior to planting.

Keep in mind that most of these herbicides work best on small annual weeds, especially when applied with a crop oil concentrate or liquid nitrogen solution (consult the respective product label for additive recommendations). If existing vegetation is larger than 2 to 3 inches, adding another herbicide to the tank can often improve burndown activity. Gramoxone Inteon and glyphosate have foliar activity but lack any soil residual activity. These herbicides are often tank-mixed with corn or soybean preplant herbicides to improve control of existing vegetation.

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 compared with a translocated herbicide.

Last season, we reported the identification of horseweed (aka marestail) populations in Illinois that were not effectively controlled by glyphosate. Since this initial report, the greenhouse experiments have been repeated, and the results from the second experiment were nearly identical to the first. Thus, it is important to be aware that exclusive reliance on a glyphosate-containing product for horseweed control may not provide adequate control. Because glyphosate use is pervasive both prior to planting (i.e., burndown) and following crop emergence (i.e., postemergence in glyphosate-resistant varieties/hybrids), identification of horseweed populations in Illinois that are not adequately controlled by glyphosate necessitates that farmers consider alternative options to control these populations.

Ideally, horseweed should be controlled prior to planting. However, because horseweed can emerge in the spring as well as during the fall, it is possible that some horseweed may emerge following crop planting. Thus, farmers will need to consider how to manage glyphosate-resistant horseweed both before planting and after crop emergence. Since a higher percentage of soybean acres are planted no-till compared with corn, the following discussion will focus on options for horseweed control in soybean systems.

Control of existing horseweed before planting: In situations where tillage is not an option, existing horseweed plants should be controlled while still in the rosette stage, or before they exceed 6 inches in height. While glyphosate remains effective on many other weed species, tank-mix partners are needed to provide adequate control of glyphosate-resistant horseweed. Suggested tank-mix partners include 2,4-D ester, FirstRate, Gangster, Canopy, and Canopy EX. Control of horseweed with Gramoxone is often improved when tank-mixed with Sencor and/or 2,4-D. Valor and Python are not very effective at controlling emerged horseweed but can provide soil residual control. Other herbicides that provide soil residual control include Sencor, Classic, and FirstRate. Utilizing tank mixtures and soil residual products can be considered as a proactive approach to slowing the spread of resistant populations.

Control of horseweed after soybean emergence: Tank-mix partners with glyphosate include FirstRate and Classic. Few other options, if any, are available for postemergence control of glyphosate-resistant horseweed in soybean.

Our colleagues in Ohio have dealt with glyphosate-resistant horseweed populations longer, and on a larger scale, than we have here in Illinois. A recent article in the C.O.R.N. newsletter, written by Mr. Jeff Stachler and Dr. Mark Loux from Ohio State University, provides a snapshot of the horseweed problems faced by Ohio farmers. Their article, "A Marestail Update," can be viewed at corn.osu.edu/.

Table 2 is reproduced from the 2006 Illinois Agricultural Pest Management Handbook. The table includes weed control ratings for several corn and soybean herbicides used to burndown existing vegetation prior to planting.--Aaron Hager and Dawn Nordby

Aaron Hager
Dawn Refsell

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