In corn production, the first weed control applications are termed “burndowns,” meaning almost complete desiccation of the weed species present in a crop field. This early-season weed control before planting offers several benefits including: removal of winter vegetation, proper and rapid crop establishment, efficient fertilizer use during the season and decreased competition from early-season insects such as cutworms.
In Louisiana, it is suggested that burndown herbicide applications be made three to six weeks before planting is anticipated. The reason for this is that it allows winter vegetation to die in addition to reducing the existing insect populations. Herbicide options for burndown applications are numerous, but there are really two compounds, glyphosate and paraquat, that are applied most frequently either alone or in tank-mixes with other compounds.
In a typical two-compound system, glyphosate can be used early to get an initial systemic kill before the contact compound paraquat is applied just prior to planting. Some producers choose to add a broadleaf compound such as 2,4-D with the initial glyphosate application. Several compounds, including Harmony Extra, Goal, Clarity, Valor or Aim in addition to others, can also be used as tank-mix partners. All of these compounds differ in plant-back restrictions, and the compound label needs to be consulted for the exact number of days that planting is restricted after application.
Another suggestion for herbicide selection involves properly identifying the weed species most difficult to control and selecting the appropriate herbicide for its control. More than 20 winter weeds are found in Louisiana cornfields in the spring, with annual ryegrass, geranium species, curly dock, clover, henbit, dandelion and assorted legume cover crops making up most of the weed populations.
After applying the initial burndown application, especially if it is applied several weeks before planting, weeds do have the potential to regrow or new weeds may actually germinate. Another single application of glyphosate or paraquat before planting has been shown to be effective for these situations.
Weeds and insects can jeopardize a good stand of corn if not properly controlled. If fall bed preparation has been successful, a good burndown program should help establish a successful corn crop. I noticed last year that producers were doing a great job with early burndown applications and then rehipping and rolling the rows just before planting. If the weeds are dead, there is no reason to worry about their coming back, and actually planting the crop, instead of reworking the fields, will not only conserve moisture, which is often critical, it will save money through decreased labor, fuel and time.
David Lanclos is the Extension corn and soybean specialist with the LSU AgCenter.