Bill J. Williams, Donnie K. Miller and Steven T. Kelly

Conservation tillage systems, including no-till and stale seedbed, require successful control of native winter vegetation or planted cover crops before planting. Some winter vegetation is easy to control, such as annual bluegrass and common chickweed, while others are difficult, including curly dock and ryegrass.

Glyphosate or paraquat form the backbone of most burndown programs in Louisiana. Few residual herbicides can be mixed with glyphosate or paraquat and applied in multiple crops. Goal and Direx are the main residual herbicides used in Louisiana, but they are limited to selected crops.

Valor (flumioxazin), a herbicide marketed by Valent, is one of just a few residual herbicides that can be tank-mixed with glyphosate or paraquat to improve burndown programs for cotton, soybean, corn, grain sorghum, rice, sugarcane and wheat. Valor efficacy and crop tolerance were evaluated in 2001 and 2002 at the Northeast Research Station near St. Joseph, La., on Commerce silt loam and Sharkey clay soils.

Selecting the optimum Valor rate depends on weed size and the level of soil residual needed:

• Valor at 0.5 to 1 ounce per acre applied with glyphosate or paraquat in February improved cutleaf eveningprimrose, henbit, chickweed, curly dock, swinecress and smartweed control 10 percent to 40 percent compared to either applied alone.
• Valor at 0.5 ounce per acre provided limited residual benefit but was adequate for enhancing burndown of small weeds (less than 2 inches), including cutleaf eveningprimrose.
  Primrose 2 to 4 inches was controlled by 1 ounce per acre of Valor; 2 ounces per acre was required to control 4- to 8-inch primrose. Primrose taller than 8 inches has not been consistently controlled with Valor.
• Valor at 2 ounces per acre has resulted in soil residual control of smartweed for up to six weeks. Smartweed is extremely sensitive to Valor, and other weeds may not be controlled for that length of time. Valor at 1 ounce per acre typically provided three to four weeks of residual control.

Overall, Valor in combination with glyphosate or paraquat applied in February had the broadest spectrum and best residual control of winter vegetation when compared to mixtures with Linex, Direx, Aim, Clarity, Harmony Xtra or Goal. Research indicated that Valor is an excellent tool when burning down early and residual control are desired; however, 2,4-D, Clarity or Harmony Xtra may be better options on larger weeds where residual control is not an issue.

Soybeans and rice demonstrated the most Valor tolerance. Valor applied at 1 to 2 ounces per acre 30, 21, 14 and seven days before planting did not injure soybeans or rice.

Corn injury from Valor was strongly influenced by rainfall following emergence. In 2001, rain occurred four weeks after the corn emerged. Valor at 2 ounces per acre caused 10 percent injury when applied 30 days before planting and 35 percent injury when seven days before planting. However, within 10 days most of the visual corn injury had decreased to negligible levels and did not affect yield.

In 2002, visual corn injury was similar to that observed in 2001, but corn yield was reduced 22 and 52 bushels per acre when 2 ounces of Valor per acre was applied 14 and seven days before planting, respectively. Corn injury from Valor was generally higher (up to 40 percent) on the Sharkey clay than on the Commerce silt loam.

Rain occurred in grain sorghum in 2001 right after emergence. Valor at 2 ounces per acre caused 20 percent injury 30 days before planting up to 70 percent injury seven days before planting. As with corn, however, grain sorghum injury from Valor was short-lived.

In 2002, only 2 ounces per acre of Valor applied seven days before planting resulted in visual injury to grain sorghum. Valor did not affect grain sorghum heights, stands or yields in any trial.

In 2001, 2 ounces per acre of Valor injured cotton up to 63 percent when applied seven days before planting. Cotton yield was not influenced by Valor. In 2002, weather prevented the establishment of cotton tolerance trials.

Valor is labeled for preemergence use in soybean at 1 to 3 ounces per acre and can be applied anytime before planting. In cotton, corn, grain sorghum, sugarcane and rice, no more than 2 ounces per acre of Valor can be applied within 30 days of planting. Valor at 1 and 2 ounces per acre tank-mixed with paraquat or glyphosate improved the control of most winter weeds.

Some trials indicated that less than l ounce per acre of Valor could be used for weed control, but this rate did not produce consistent results. Some trials indicated that Valor could be applied as close as 21 days before planting cotton and corn, and possibly maybe even as close as 14 days before planting grain sorghum and rice. The severe injury in corn supports the 30 days before planting restriction on most crops.

Additional, research is under way to refine Valor rates and timings for improved crop tolerance. When applied as recommended, Valor is an excellent tank mix partner for glyphosate and paraquat and can be used safely on most row crops grown in Louisiana.


(This article appeared in the summer 2003 issue of Louisiana Agriculture.)