No. 4 Article 5/April 18, 2008

Multistate Ratings for Burndown Herbicide Efficacy

Elimination of tillage in agronomic cropping systems places heavy reliance on herbicides to control both existing weeds before planting and weeds that emerge after planting. Cool and wet soils have delayed the development of winter annual weed species somewhat (at least relative to recent years), but a return to warm temperatures will quickly spur these weeds to rapid growth. Whatever your plans for cropping practices in 2008, it is best to control existing weed vegetation before planting corn or soybean.

Many Illinois farmers and custom applicators know that some Illinois populations of horseweed (Conyza canadensis, commonly known as marestail) are resistant to glyphosate. Following treatment with glyphosate, resistant plants are frequently stunted and display yellowing in the meristem area. Often the top of the plant dies back, but this is usually followed by profuse branching along the lower stem. Because glyphosate use is pervasive both before planting (burndown) and following crop emergence (in glyphosate-resistant varieties/hybrids), the presence of glyphosate-resistant horseweed in Illinois requires farmers to consider options other than sole reliance on glyphosate for control. Ideally, all horseweed should be controlled before planting, especially before planting soybean. However, because horseweed may emerge in spring (especially in southern Illinois) as well as in fall, it is likely that some horseweed will emerge after crop planting. Farmers thus need to consider how to manage glyphosate-resistant horseweed both before planting and after crop emergence.

Tank-mix partners are needed to provide adequate burndown control of glyphosate-resistant horseweed. Suggested partners include 2,4-D ester, FirstRate, Gangster, Valor XLT, Canopy, and Canopy EX. Control of horseweed with Gramoxone is often improved when it is tank-mixed with Sencor and/or 2,4-D. Valor and Python alone are not very effective at controlling emerged horseweed, but they can provide soil residual control. Other herbicides that provide soil residual control include Sencor, Classic, Authority (sulfentrazone)-containing products, and FirstRate. Preplant tillage and interrow cultivation are additional options for horseweed control.

In addition to glyphosate-resistant horseweed, Illinois farmers should consider the potential challenges introduced by glyphosate-resistant waterhemp. We often consider waterhemp a species most commonly encountered well after crop planting, but emergence can begin as early as late March/early April. In 2007, we noticed several instances of no-till fields where winter annual weeds were essentially absent but adjacent fields were excessively green with winter annual vegetation. We suspect that the fields devoid of winter annuals may have been treated with a fall-applied herbicide that was very effective against the spectrum of winter annuals. So how are these observations related to concerns about glyphosate-resistant waterhemp? By late April or early May, the fields without winter annual weeds contained extremely dense stands of waterhemp. These observations suggest what could be considered a worst-case scenario: if these waterhemp populations were in fact resistant to glyphosate and if soybean was planted without first controlling the waterhemp, it is very unlikely that any (selective) chemical control options could be used to effectively control the waterhemp. Put another way, the end result of this potential scenario might be replanting the field.

Growth regulator herbicides are frequently included in burndown applications to enhance the spectrum of weeds controlled. 2,4-D is the most common growth regulator used, but some use of dicamba also occurs. Both the amine and ester formulations of 2,4-D are labeled for burndown applications prior to soybean planting, but the ester formulation 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 soybean seed. Also, the ability of esters to better penetrate the waxy leaf surfaces of weeds often results in better control of large weeds and during periods of cool air temperatures. 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 before soybean planting; increasing the rate to more than 1 pint increases the waiting interval to 30 days. 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 coming in direct contact with the crop seed increase the probability of severe injury.

Clarity is labeled for application before planting corn or soybean, while Distinct may be used before planting soybean to control existing broadleaf vegetation. If using these products before planting soybean, a minimum of 1 inch of precipitation (rainfall or overhead irrigation) and a waiting interval (30 days for Distinct, 14 or 28 days for Clarity, depending on application rate) are required before planting. These requirements must be satisfied to reduce the potential for soybean injury.

In past years we have included various tables of herbicide effectiveness with articles published in the Bulletin. Many of the tables originated from the Illinois Agricultural Pest Management Handbook. We assign numeric efficacy ratings to various herbicides to describe how well they control agronomically important grass and broadleaf weed species. Our scale ranges from 0 (no control) to 9 (excellent control). Several other states in the north-central region also rate herbicides on similar (and sometimes different) weed species. Recently we summarized burndown herbicide efficacy ratings from several states (Indiana/Ohio [which publish a joint recommendation guide], Kentucky, Michigan, Missouri, Nebraska, and Pennsylvania); the ratings for those states and for Illinois are presented in Tables 1 through 4.

Please be aware that other states' rating scales can vary from the one we use in Illinois. Some states use a letter-based scale in lieu of numbers; each scale has advantages and disadvantages. The state rating scales are summarized in Table 5. A blank cell in a table generally indicates that no rating was given for that particular weed/treatment combination. If Extension specialists from states whose ratings we have summarized note any errors, please let us know.--Aaron Hager

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