University of Illinois

No. 12/June 13, 1997

Postemergence Herbicides for Grass Control in Soybeans

Once dominated by soil-applied herbicides, the majority of the grasscontrol market in soybeans is now shared among several postemergence herbicides. These products are very effective against many annual and perennial species and fit well in most tillage systems. The rain-free interval for all these herbicides is one hour, all are translocated throughout the plant, and each can be tank-mixed with broadleaf herbicides. For additional information on these herbicides, refer to Table 5 in the previous issue of this Bulletin.

When tank-mixing postemergence grass and broadleaf herbicides, keep in mind that antagonism can sometimes occur, which results in reduced grass control. Antagonism can be defined as "an interaction of two or more chemicals such that the effect when in combination is less than the predicted effect based on the activity of each chemical applied separately." This definition can be simplified to "less grass control when the grass herbicide is applied with a broadleaf herbicide than when the grass herbicide is applied alone." There have been several investigations into causes of this type of antagonism. Some studies have suggested that the broadleaf herbicide may limit the amount of grass herbicide absorbed and translocated throughout the grass weeds, thus leading to the observed antagonism. What can be done to reduce this antagonism? Additive selection can help to some extent, as can increasing the rate of the grass herbicide; but the most effective method is to apply the grass herbicide separately from the broadleaf herbicide. Most of the postemergence grass herbicide labels indicate that, if separate applications are to be made, you should apply the grass herbicide one day before the broadleaf herbicide, or about 7 to 10 days after the broadleaf herbicide. Why wait so long if the broadleaf herbicide is applied first? Several postemergence broadleaf herbicides also have activity on grass species, activity that can range from partial control to leafburn only. To adequately absorb the herbicide, the grass needs time to recover (about 7 to 10 days) from this stress before the grass herbicide is applied. If application is made prior to recovery of the grass, the amount of grass herbicide needed for control may not be absorbed.

Each of these products may be used to control perennial grass species. Adequate control often can be achieved with one application, but repeat applications are sometimes needed. The second application should be delayed until sufficient regrowth has occurred.

Corn injury caused by drift of postemergence soybean grass herbicides can be severe. The corn crop across much of the state has been growing under stressful conditions for most of the season and thus may be more susceptible to drift of these herbicides.

Select 2EC (clethodim) may be applied at 4 to 8 fluid ounces for control of annual grass species, and 8 to 16 fluid ounces for control of perennial species. The 4-ounce rate may be used when most annual species are less than 3 to 4 inches tall. Always include a crop-oil concentrate (COC) with Select; 1 to 2 quarts of a liquid fertilizer may also be included.

Assure II or Matador 0.88EC (quizalofop) may be applied at 5 to 9 fluid ounces for control of annual grass species, and 8 to 10 fluid ounces for control of perennial grass species. Always include a nonionic surfactant (NIS) or petroleum-oil concentrate (POC) with Assure II; an ammonium nitrogen fertilizer may be added to the spray mix.

Fusilade DX 2EC (fluazifop) may be applied at 5 fluid ounces for control of volunteer corn only, 6 to 12 fluid ounces for control of most annual species, and 8 to 12 for control of perennial species. Applications must include either an NIS or COC; a liquid nitrogen fertilizer may be included.

Fusion 2.66EC (fluazifop + fenoxaprop) may be applied alone at 6 to 8 fluid ounces to control annual grass species. You may need to increase rates when Fusion is tank-mixed with a broadleaf herbicide. Rates of 8 to 12 fluid ounces are needed for control of perennial species. Applications must include either an NIS or COC; a liquid nitrogen fertilizer may be included.

Poast Plus or Prestige 1EC (sethoxydim) may be applied at 0.75 to 2.25 pints per acre; the most common rate is 1.5 pints per acre. A COC or DASH HC should be included.

Roundup Ultra (glyphosate) may be applied to Roundup Ready soybeans from crop emergence through full flower. Roundup Ultra also provides control of certain broadleaf species. Rates range from 24 to 48 fluid ounces, with a maximum in-crop amount of 2 quarts per acre. No additives other than ammonium sulfate may be included.

Aaron Hager, and Marshal McGlamery, Department of Crop Sciences, (217)333-4424