Donnie K. Miller, P. Roy Vidrine, Steve T. Kelly and Rebecca G. Frederick

Conservation tillage systems, whether no-till or stale seedbed, require use of herbicides before crop planting to rid fields of native winter vegetation and planted cover crops. Elimination of competing vegetation, which is called burndown, helps improve soil moisture and assure crop stand establishment, rapid early season growth and efficient fertilizer use. The herbicide 2,4-D is used in burndown weed control because it eliminates a number of problem broadleaf winter weeds at a relatively low cost.

One problem with 2,4-D is that even though applied post-emergence, it can stay in the soil long enough to injure sensitive crops such as cotton. The injury is more likely to occur if cotton is planted shortly after herbicide application. Unfortunately, most 2,4-D herbicide labels indicate no strict plant-back interval and include vague language such as “90 days or until dissipated from the soil.” The LSU AgCenter is participating in a regional research collaboration to assess plant-back intervals to cotton following application of 2,4-D.

As part of this research, studies were conducted in 2001 and 2002 at the Northeast Research Station in St. Joseph, the Dean Lee Research Station in Alexandria and the Macon Ridge Research Station in Winnsboro to assess effects of 2,4-D applied at various preplant intervals on growth and yield of cotton. The 2,4-D was applied in two formulations, amine and ester. The two 2,4-D formulations were applied at the labeled rate and twice the labeled rate to rows undisturbed before application.

In 2001, each herbicide was applied seven, 14 and 21 days before planting. In 2002, application intervals were seven, 14 and 28 days before planting. A nontreated control was included for comparison. Cotton was planted and maintained weed free by hand-hoeing or postemergence herbicide. To determine possible negative effects, these assessments were made: visual injury, plant height, plant population about 30 days after planting and seedcotton yield.

Visual injury as high as 70 percent at St. Joseph in 2001 and 22 percent at Alexandria in 2002 was observed on cotton following a 2,4-D application before planting. For both locations, differences in injury could not be detected between herbicide formulations or rates, or among application intervals. Injury was either not observed or negligible at St. Joseph in 2002, Alexandria in 2001 and Winnsboro in 2001. Differing injury response among experiments could not be explained by rainfall patterns before or after 2,4-D application.

Although significant injury was observed in two of five experiments, plant height and population and seed-cotton yield were not reduced at any location when compared to cotton receiving no preplant application of 2,4-D. At St. Joseph in 2001, extreme variability in the data may have reduced the ability to detect differences on cotton growth and yield. Results indicate that 2,4-D can be applied seven days before cotton planting without having negative effects on growth and yield. Cotton was able to recover from this early season injury and compensate because of a full growing season and intensive scouting and management of weeds and insects. Under different growing conditions, cotton may not have been able to recover fully.

Although results of this study support 2,4-D label changes, such changes must be initiated by the herbicide manufacturer and approved by the U.S. Environmental Protection Agency. Producers are cautioned to follow current label restrictions for planting cotton following 2,4-D application.

Acknowledgment

Donna R. Lee and A. Lawrence Perritt, Research Associates, Northeast Research Station; Derek Scroggs, Research Associate, Dean Lee Research Station; and Barrett McKnight, Extension Associate, Scott Research, Extension and Education Center, for their work and Helena Chemical Company for providing funding.

(This article was published in the 2003 spring issues of Louisiana Agriculture magazine)