University of Illinois

No. 11/June 6, 1997

Postemergence Broadleaf Herbicides for Soybeans

The soybean crop, as well as many annual weed species, has emerged across much of Illinois. With the cool growing conditions encountered thus far, it appears that in many areas the main flush of annual weeds may be yet to emerge. Whenever these weeds make their appearance, several postemergence broadleaf herbicides are available for control.

Summer annual weed species that emerge early in the season (smartweed or lambsquarters, for example) have made significant growth in many fields that did not receive a preemergence broadleaf herbicide. If a postemergence herbicide will be used to control these weeds, keep in mind that some postemergence broadleaf herbicides have a minimum soybeansize requirement, in addition to a maximum-size limit. For example, the Concert label indicates that applications may be made anytime after the first soybean trifoliate has expanded fully. Applications made to very small soybeans and cotyledon-stage broadleaf weeds may result in enhanced injury and poor control, respectively. Before applying any herbicide, consult the label for recommended crop growth stages for application.

Cultivation can often supplement the weed control afforded by postemergence herbicides. Most postemergence herbicide labels recommend that cultivation not occur within 5 to 10 days prior to or following application. If weeds emerge several days or weeks after application or control was not complete, a cultivation may be all that is necessary.

Table 5 contains additional information on spray additives, rainfree intervals, reentry and preharvest intervals, and grazing restrictions for postemergence broadleaf and grass herbicides used in soybeans. For additional information, consult the respective herbicide label.

Table 5. Postemergence broadleaf herbicides for soybeans.
HerbicideAdjuvant and nitrogenRain-free period (hr)Reentry interval (hr) Preharvest interval (days) Feed or graze forage?
Postemergence-grass only
Assure IIPOC or NIS, NH4 optional11280No
Fusilade DXCOC or NIS, NH4 optional112PrebloomNo
FusionCOC or NIS, NH4 optional124PrebloomNo
Poast PlusCOC or Dash HC, NH4 optional11275Hay?
SelectCOC, UAN optional11260No
Postemergence-broadleaf, contact
BasagranCOC, NH4 optional6a12NoneYes
BlazerNIS or UAN6a4850No
CobraCOC or NIS, check humidity0.51245No
Flexstar HLCOC + NH4 124PrebloomNo
GalaxyCOC or/andb NH46a4850No
ReflexNIS or COCc, NH4 optionall24PrebloomNo
ResourceCOC, NH4 optional11260No
StellarCOCc, NH4 optional11260No
StormCOC or NIS or NH46a 4850No
Postemergence-broadleaf, systemic
Butyrac (2,4DB)Noned6­84860Yes
ClassicNIS, POCc, or MSOc + NH411260No
ConcertNIS or POCc + NH411260No
Pinnacle NIS or COCb,c + NH4 11260No
PursuitCOC or NIS + NH411285No
Roundup Ultrae,fAMS optional1­24FlowerNo
ScepterCOC or NIS11290No
Scepter O.T.NIS or COCcc44890No
Synchrony STSPOC or MSO + NH412601No
TouchdownfNIS, AMS optional4460No
Harvest-aid use
Gramoxone ExtraNIS or COC0.512NAYes?
Roundup UltraAMS optional1­21214No

COC = petroleum oil concentrate (POC) or vegetable oil concentrate (VOC), MSO = methylated seed oil (specialized VOC), NIS = nonionic surfactant, NH4 = ammonium fertilizer, adjuvant = UAN or AMS, UAN = urea­ammonium­ nitrate (28-0-0), AMS = ammonium sulfate (spray grade 21-0-0).
aCurrent label: "Rainfall soon after application may decrease effectiveness."
bUse only if drouthy conditions exist at application.
cPenetrant adjuvant allowed but reduces crop tolerance.
dSome tank mixes allow NIS or COC; see the tank-mix partner's label.
eUse as broadcast treatment only with Roundup Ready­designated soybean varieties.
fMay be used as a spot treatment. Use NIS with wiper applications of Touchdown but not with Roundup Ultra.

Postemergence "contact" broadleaf herbicides

Basagran 4S (bentazon) may be applied at 1 to 2 pints per acre. Addition of UAN or AMS generally improves control of velvetleaf. Split applications can result in improved control of certain species, such as giant ragweed, wild sunflower, Canada thistle, and yellow nutsedge.

Blazer or Status 2S (acifluorfen) may be applied at rates of 1/2 to 1.5 pints per acre. Blazer is often applied as a tank mix with Basagran to improve control of pigweed species and nightshade. Soybean injury is often observed following applications of Blazer, but recovery is generally rapid unless adverse growing conditions persist. Premixed combinations of Basagran and Blazer are marketed as Galaxy (central and northern Illinois) and Storm (southern Illinois).

Manifest and Conclude B&G are "co-pack" delivery systems. The "B" component is equivalent to 2 pints of Galaxy (Manifest) or 1.5 pints of Storm (Conclude), while the "G" component is equivalent to 1.5 pints of Poast (not Poast Plus). The higher rate of Poast helps overcome grass antagonism sometimes observed when these broadleaf herbicides are applied with Poast.

Reflex 2S and Flexstar HL 1.88S (fomesafen) contain the same active ingredient, but Flexstar HL is formulated with three proprietary additives. COC (methylated seed oil) is the preferred spray additive, and UAN or AMS may also be included. Maximum allowable use rates vary according to location in Illinois; Reflex or Flexstar may be applied only once every 2 years. Maximum use rates north of Interstate 70 are 1.25 pints of Reflex and 1.3 pints of Flexstar, while maximum rates south of Interstate 70 are 1.5 and 1.6 pints for Reflex and Flexstar, respectively. Soybean injury is usually greater with Flexstar HL than Reflex.

Cobra 2E (lactofen) may be used at 4 to 12.5 fluid ounces per acre when applied alone, or at reduced rates (< 12.5 ounces) when applied as a tank mix with other postemergence broadleaf herbicides. Selection of additives is dependent upon the tank-mix partner or the relative humidity conditions at the time of application. Cobra is effective on many species of pigweed, but among the diphenyl ethers is the weakest on smartweed. Soybean injury is common following Cobra, the amount of injury being dependent upon rates used, additives, and prevailing environmental conditions.

Resource 0.86E (flumiclorac) is applied postemergence at 4 to 8 fluid ounces per acre and is an excellent herbicide for velvetleaf control. Resource is often applied as a tank-mix partner at 4 fluid ounces with another broadleaf herbicide to enhance velvetleaf control.

Stellar 3.1E (lactofen + flumiclorac) is a premix of the active ingredients of Cobra and Resource. Use rates of Stellar are between 5 and 7 fluid ounces per acre.

Postemergence "translocated" broadleaf herbicides

Pursuit (imazethapyr) may be applied at 4 fluid ounces of the 2AS formulation or 1.44 ounces of 70DG per acre. Postemergence applications must include an NIS or COC in addition to a nitrogen fertilizer. Pursuit controls a wide spectrum of annual broadleaf species and can provide some control of certain small, annual grasses. Tank mixes with diphenyl ether herbicides often enhance control of waterhemp but may antagonize grass control.

Scepter 70DG (imazaquin) is applied at 1.4 ounces per acre for control of cocklebur, sunflower, pigweeds, and volunteer corn. The restriction of tank-mixing a postemergence grass herbicide has been removed from the Scepter label, but labels of some postemergence grass herbicides still have restrictions on tank mixes with Scepter. Scepter O.T. 2.5E (imazaquin + acifluorfen) is a premix of Scepter and Status that is applied at 1 pint per acre.

Classic 25DF (chlorimuron) may be applied alone at 1/2 to 3/4 ounce per acre or at 1/4 to 3/4 ounce when tank-mixed with other postemergence herbicides. Split applications can improve control of species such as giant ragweed and burcucumber. Rotational intervals are determined by rate and timing of application(s), so consult the label for specifics.

Pinnacle 25DF (thifensulfuron) is applied alone at 1/4 ounce per acre, and often at reduced rates with other broadleaf herbicides to enhance control of lambsquarters and pigweeds. Velvetleaf and smartweed are other broadleaf species that Pinnacle has good activity against. Soybean injury in the form of shortened plants has sometimes been observed following applications of Pinnacle, especially when the applications were made during hot, dry conditions.

Synchrony STS 42DF and Concert 25DF (chlorimuron + thifensulfuron) are premixes (3:1 and 1:1, respectively) of Classic and Pinnacle. Synchrony STS may be applied only to soybean varieties designated as STS (sulfonylurea-tolerant soybeans), while Concert may be applied to all varieties.

Roundup Ultra (glyphosate) must only be applied to soybean varieties designated as "Roundup Ready." Roundup Ultra controls many grass and broadleaf weed species when applied at the appropriate rate and timing. Applications may be made from emergence through full flower but should be timed according to weed development. The total amount of Roundup Ultra applied in-crop cannot exceed 2 quarts per acre.

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