Issue No. 2, Article 8/April 6, 2007
Controlling Existing Vegetation Prior to Planting
The recent stint of warm weather has spurred the growth of many winter annual (and even some summer annual) weed species. Henbit and purple deadnettle are in flower across many regions, and several mustard species will soon add splashes of white and yellow to the landscape. Chickweed growth in no-till, wheat, and seedling alfalfa fields has accelerated during the past two weeks. Existing weed vegetation present in no-till fields is frequently denser today compared with 10 years ago, which could be attributable to one or more factors. Whatever your plans for cropping practices in 2007, it is best to control existing weed vegetation before planting corn or soybean. A few thoughts related to preplant weed control tactics follow:
- As previously mentioned, some winter annual weed species are already in flower. These weeds should be controlled soon to prevent seed production and addition to the soil seed bank. Successful seed production as a result of delayed burndown applications or tillage operations is one factor that has contributed to the increased prevalence of certain winter annual weed species.
- Growth regulator herbicides are frequently included in burndown applications. 2,4-D is the most common growth regulator used, but some use of dicamba also occurs. Both product labels specify an interval that must elapse between application and planting. The Clarity label specifies that 14 days must elapse and a minimum of
1 inch precipitation be received between application (up to 8 fluid ounces per acre) and soybean planting. The labels of many 2,4-D ester formulations (3.8 lb acid equivalent per gallon) allow application of up to 1 pint per acre at least 7 days prior to soybean planting; increasing the rate over 1 pint increases the interval to 30 days. Pay careful attention to label statements, as some 2,4-D labels also specify a waiting interval between application and corn planting.
- Many Illinois farmers are aware that populations of horseweed (Conyza canadensis, also referred to as marestail) in several states have been confirmed resistant to glyphosate. We previously reported that populations from several areas of Illinois have been identified that are not effectively controlled by glyphosate under field or greenhouse research conditions. Resistant plants are frequently stunted and display some yellowing in the meristem area following treatment. In many instances, the top of the plant may die back, but this is usually followed by profuse branching along the lower stem of the resistant plant. 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 from Illinois that are not adequately controlled by glyphosate necessitates that farmers consider what options (i.e., other than sole reliance on glyphosate) are available to control these populations. Ideally, all horseweed should be controlled prior to planting, especially prior to soybean planting. However, because horseweed may emerge in the spring as well as during the fall it is likely that some horseweed will emerge following crop planting. Thus, farmers will need to consider how to manage glyphosate-resistant horseweed both before planting and after crop emergence. Tankmix partners are needed to provide adequate burndown control of glyphosate-resistant horseweed. Suggested tankmix partners include 2,4-D ester, FirstRate, Gangster, Valor XLT, Canopy, and Canopy EX. Control of horseweed with Gramoxone is often improved when tankmixed 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 First or Sonic, and FirstRate. Utilizing tankmixtures and soil-residual products can be considered as a proactive approach to slowing the spread of resistant populations. Preplant tillage and interrow cultivation are additional options for horseweed control.
- Soil-residual herbicides can be tankmixed with most burndown herbicides, and many soil-residual herbicides themselves possess activity against emerged weeds, especially when applied with spray additives such as crop oil concentrate or UAN solution. Utilizing soil-residual herbicides in corn and soybean production provides many advantages, including reducing the potential for early-season weed interference that can lead to loss of crop yield potential, a reduced density of weeds and a more uniform weed size range when postemergence herbicides are applied, and reducing the intensity of selection for herbicide-resistant weeds by exposing the weed spectrum to multiple sites of herbicide action.
- 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 with a translocated herbicide.