Late-fall to early-winter herbicide application is a new approach to managing winter weeds in Louisiana.
But limited research in 2008 by Dr. Donnie Miller and Dr. Bill Williams, associate professors at the LSU AgCenter’s Northeast Research Station in St. Joseph, indicates herbicide applications in September through early November are less effective than applications made in late November or December.
Miller said this possibly results from an interaction between the herbicides and soil microorganisms that are still highly active prior to onset of colder weather.
The majority of the herbicides being researched either have only 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 prior to application, adding paraquat or glyphosate has enhanced control, the researchers said.
One concern regarding fall/winter herbicide applications is that with native winter vegetation removed, soil left bare is exposed to rainfall during winter, which may affect row integrity for spring planting, Miller said.
Producers also are cautioned that while these applications are effective through winter, they may not eliminate the need for a follow-up treatment prior to or at planting.
"Economic consideration should be made when deciding whether to use these programs," Miller said. In addition, producers should confirm whether the use of fall/winter weed control programs and removal of winter vegetation would affect their compliance with conservation programs in which they participate that may require a certain amount of vegetation be present to avoid erosion on certain soils.
Miller said 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.
Weed control in the spring prior to planting has most often relied on programs including glyphosate or paraquat as the primary herbicide, but overuse can lead to increased incidence of weed resistance.
In Louisiana, weeds such as mare’s tail, henbit and ryegrass often require a tank mixture of as many as three different herbicides if the optimum herbicide application timing is missed, Williams said. "Even then, these weeds are not always adequately controlled, and cost is definitely increased," he said.
The LSU AgCenter recommends that herbicides be applied six to eight weeks prior to planting to remove winter vegetation and eliminate problems with insects migrating from weeds to emerging crops, Williams said. But rain and wind can make it difficult to achieve that interval, he added.
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 (such as corn or wheat that was planted earlier) or homeowner gardens or flowerbeds, he added.
Because of such concerns, interest has increased in applying herbicides with soil activity in the late fall and early winter to eliminate or reduce the amount of weeds that germinate in winter and are present in the spring prior to planting, Williams said.
Delays in spring herbicide applications lead to not only increased insect problems but also reduced herbicide efficacy because of larger weeds and/or weed growth stages at which weeds are not as susceptible, Miller said.–Mary Ann Van Osdell
(This article was published in the 2008 edition of the Louisiana Soybean & Grain Research & Promotion Board Report.)