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large seeded broadleaf weeds

Triazine (5). Atrazine (5) is a common component of many preplant and preemergence herbicide premixes for corn. Where weed pressure is light, a March application of atrazine with crop-oil concentrate and 2,4-D (4) or dicamba (4) can control winter annual weeds such as mustards and marestail and provide control of most germinating weeds up to planting. If kochia is the key target, 0.5 to 1.0 lb/acre atrazine (5) with a pint of dicamba (4) applied in late February to early March can provide excellent control of germinating kochia. It is essential to add glyphosate (9) to the mix if winter annual grasses are present. In a premix with other herbicides, atrazine adds burndown control of newly emerged grasses and broadleaf weeds present near planting time, as well as some residual control of small-seeded broadleaf weeds such as pigweeds and kochia (except for triazine-resistant populations). Unless your situation prohibits atrazine use, always apply atrazine (5) with HPPD-inhibitor (27) and acetamide (15) herbicides.

N-Phenylphthalimide family (14) – Encompass, Resource, Valor

For burndown applications in a no-till system on emerged grass and broadleaf weeds, an application of glyphosate (9) and a product containing dicamba (4) or 2,4-D (4) may be critical. The choice between 2,4-D and dicamba will depend on weed species present. Dicamba products will be more effective on kochia and marestail. 2,4-D is more effective on winter annual mustards. The use of preemergence herbicides, applied just before or following planting, often provides control of weeds for several weeks. This can greatly improve the effectiveness of a postemergence herbicide application, and gives the producer more leeway on post application timing.

Dinitroanaline family (3) – Treflan, Trust, Prowl, Acumen, Sonalan, Balan

An additional ALS-inhibiting (2) herbicide from DuPont is called Resolve (rimsulfuron, 2). Rimsulfuron (2) is also a component in Prequel (2, 27), Instigate (2, 27), Basis (2), and Basis Blend (2), which was previously mentioned. Additional products containing rimsulfuron include Harrow (2) and Crusher (2). Resolve will provide short residual control of grass and broadleaf weeds and should be used as a setup herbicide with a good postemergence weed control program. If ALS-resistant broadleaf weeds are present, these ALS-containing herbicides often will be less effective.

Scepter O.T. Scepter O.T. is a packaged mixture of imazaquin and acifluorfen (the active ingredients in Scepter and Blazer, respectively). Applying 1 pint of Scepter O.T. per acre is equivalent to applying 1/3 pint of Scepter plus 1 pint of Blazer. Scepter O.T. gives excellent control of cocklebur and pigweed and will control other weeds normally controlled by 1 pint of Blazer.

Soybean tolerance of Pursuit is good. See Table 14 for rotational restrictions. Do not plant cotton or most vegetable crops the year following a Pursuit application.

Pursuit kills weeds very slowly, and it may take as long as 3 weeks to kill morningglory. In some cases, the weeds do not die completely but rather stop growing and the soybean canopy fills in above them. In addition to controlling certain broadleaf weeds, a postemergence application of Pursuit usually gives adequate control of broadleaf signalgrass, foxtails, seedling johnsongrass, and shattercane, and it sometimes gives adequate control of rhizome johnsongrass. Pursuit does not control common ragweed, lambsquarter, prickly sida, sicklepod, and certain other weeds.

In addition to proper herbicide selection, timeliness of application and proper application methods are essential. The most common cause of poor results with postemergence herbicides is application when the weeds have grown beyond the optimum size for treatment. As a general rule, postemergence broadleaf herbicides should be applied when weeds are in the two- to four-leaf stage or 2 to 3 inches tall. This normally occurs 2 to 3 weeks after planting.

Although you should strive for good early-season weed control, situations arise where a salvage treatment is necessary. Remember that salvage treatments are not a substitute for good early-season control.

Canopy. Canopy is a packaged mixture containing 6 parts metribuzin to 1 part chlorimuron (Classic). Because it contains a high percentage of metribuzin, the use restrictions discussed previously for Lexone, Salute, Sencor, and Turbo also apply to Canopy. Canopy can be applied in various preplant-incorporated or preemergence tank mixes.

Many of the postemergence broadleaf herbicides act through contact, meaning that there is little to no translocation or movement of the herbicide within the plant. Consequently, thorough spray coverage is necessary for good results. See Table 9 for recommended application pressures and volumes. Postemergence herbicides should be applied using either flat fan nozzles or hollow cone nozzles. Flood nozzles give poor spray coverage and should never be used to apply postemergence herbicides. Most postemergence herbicide labels also specify not to use CDA (controlled droplet applicator) sprayers. See specific labels for application directions.

All photos are either the property of Syngenta or are used with permission.

The best approach to large-seeded broadleaf weed control in corn is to start clean with a burndown application or a burndown application followed by an application of Acuron® or Acuron Flexi corn herbicide. Both Acuron brands contain multiple, effective modes of actions including bicyclopyrone (group 27), which was developed to complement Callisto® herbicide (group 27) and provides improved control of large-seeded broadleaves.

Cocklebur, giant ragweed and morningglory are often referred to as “large-seeded” broadleaf weeds because they produce larger seeds than their small-seeded counterparts: lambsquarters, marestail and waterhemp. While large-seeded broadleaf weeds tend to produce fewer seeds, the seeds are heartier and often remain viable in the soil for decades. Due to their larger size, the seeds often emerge from deep within the soil profile and present a larger plant mass when they appear, making them more established and difficult to control. To complicate matters, they often appear in flushes, which makes choosing a herbicide with strong residual control a must.

Since large-seeded broadleaves often come in flushes and residual is important to maintain season-long control, we recommend applying Acuron or Acuron Flexi in a 2-pass system: a foundation rate of Acuron or Acuron Flexi followed later by the remaining rate. This approach incorporates multiple, effective modes of action and helps ensure long-lasting residual control.

Caption: Urbana, Ohio: Acuron applied pre-emergence at 2.5 qt/A with 1 qt/A atrazine in a field with a history of giant ragweed pressure. Application made on May 28, 2016. Photo taken 88 days after treatment.

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