Donnie K. Miller and Bill J. Williams
Many row crops in Louisiana are produced using some type of conservation tillage system. In these systems fields are allowed to remain undisturbed through the winter and spring until planting. In the absence of spring tillage, herbicide programs are required before planting to successfully manage the weeds that emerged over the winter and early spring.
Control of weeds in the spring before planting has most often relied on programs including glyphosate or paraquat as the primary herbicide with tank mix partners such as 2,4-D, Goal or Harmony Extra. The tank mixes expand the spectrum of weeds controlled and prevent future weed germination. In addition, paraquat applications at planting are often made following a glyphosate tank mix treatment to ensure weed-free conditions for the emerging crop.
Although not always directly related to herbicide effects or competition, winter weeds present in the spring before planting may negatively affect crops because they may serve as hosts for insect and disease organisms.
The LSU AgCenter recommends that herbicides be applied six to eight weeks before planting to remove winter vegetation and eliminate problems with insects migrating from weeds to emerging crops. However, rain and wind often make it difficult to achieve this interval treatment. Delays in spring herbicide applications not only lead to increased insect problems but to reduced herbicide efficacy. As weeds grow larger, they are less susceptible to herbicides.
Overreliance on widely used herbicides, such as glyphosate, has led to an increased incidence of weed resistance in other states. In Louisiana, weeds such as marestail, henbit and ryegrass often require a tank mixture of as many as three different herbicides, once optimum herbicide application timing is missed. Even then these weeds are not always adequately controlled, and cost is definitely increased. Furthermore, previous research by LSU AgCenter weed scientists has shown winter weeds to be competitive in corn if not adequately controlled before planting.
Off-target drift is another problem with spring herbicide applications. Herbicides applied later in the spring on cotton and soybean fields can lead to situations of off-target drift that can negatively affect emerged crops that are planted earlier, such as corn and wheat, or home gardens or flower beds. A possible alternative to spring application of herbicides is a late fall/early winter application to eliminate or reduce the amount of weeds that germinate in winter and are present in the spring before planting. In addition, there is an opportunity to use herbicides with different modes of action than glyphosate or paraquat, which will aid in resistance management.
Research conducted with late fall/early winter herbicide applications by LSU AgCenter weed scientists has primarily focused on weed control efficacy with the herbicides Valor, Goal, Reflex, Dual Magnum, Envoke, Resolve, Python, Firstrate and Grasp. Herbicide applications have been evaluated in late November to late December in most trials, but earlier applications in September or early November have been evaluated on a limited basis.
In general, soil activity during winter months following herbicide application has been good (85 percent to 100 percent control) on some of the more common winter weeds including henbit, chickweed, shepherds’ purse, annual bluegrass, swinecress and cutleaf eveningprimrose. Research conducted in neighboring states has also observed excellent control of the glyphosate-resistant weed marestail with Valor and Envoke.
On some occasions, research plots have remained weed-free until planting, with no additional spring herbicide application necessary. On other occasions, a follow-up spring herbicide application is necessary, however, for smaller weeds at lower populations that are more easily controlled in spring.
Weeds have emerged in research plots approximately 120 days after herbicide application. Limited research in 2008 indicated that herbicide applications in September or early November are less effective than applications made in late November or December. The reason may be that soil microorganisms remain highly active before the onset of lower temperatures and break down the effectiveness of the herbicide.
The majority of the herbicides being studied either have strictly soil activity or exhibit primarily broadleaf activity when applied to emerged weeds, with limited control of emerged grass species. In cases where winter grasses have emerged before application, the addition of paraquat or glyphosate has enhanced control.
One concern regarding fall/winter herbicide applications is that with native winter vegetation removed, soil is left bare to be exposed to rainfall during winter months, which may affect row integrity for planting in spring. In addition, conservation programs may require a certain amount of vegetation to avoid erosion on certain soils. Because a majority of the herbicides evaluated have little or no activity on winter grass species once emerged, future research will concentrate on identifying programs that remove common broadleaf winter weeds but maintain some winter vegetation, primarily annual bluegrass, which is easier to control in the spring.
Conclusions to this research are that to obtain maximum effectiveness of fall/winter herbicide programs, applications should be timed to emergence of winter weeds preferably once temperatures have cooled in late November or December. Producers are cautioned that although effective through winter months, these programs may not eliminate the need for a follow-up treatment before or at planting. Therefore, the cost of these programs must be factored in to management decisions. In addition, producers should always confirm whether the use of fall/winter programs and removal of winter vegetation would affect their compliance with participating conservation programs.
(This article was published in the summer 2008 issue of Louisiana Agriculture.)