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Winning The Weed War

Sunday, April 1, 2001
filed under: Weeds

Winning the Weed War

Just as important as the limited arsenal of herbicide products

is how and when you use them for sunflower

Canada thistle has replaced leafy spurge as the number one noxious weed problem in North Dakota, according to Richard Zollinger, North Dakota State University extension weed specialist. “Wet weather makes it grow like crazy, and the weed’s massive root system makes it difficult to control, since it keeps sending up shoot after shoot,” says Zollinger.

There is little one can do to control Canada thistle in sunflower—the key is to control it (as well as other problem broadleaf weeds in sunflower) in the previous crop, which often is a small grains crop such as wheat. “Curtail (clopyralid plus either MCPAe or 2,4-D) in small grains will offer suppression, but Canada thistle is an expensive weed to control,” says Zollinger.

Fall applications when Canada thistle is in the rosette stage offers the most effective control. Along with Curtail, dicamba, glyphosate, Stinger (clopyralid), Tordon (picloram), and 2,4-D have the greatest activity on Canada thistle. Tillage in late fall after spraying Canada thistle increases treatment success, according to NDSU.

While Canada thistle may now be North Dakota’s top weed, marshelder is a weed which caused a lot of problems in sunflower fields last year. The pre-emergent herbicide Spartan (sulfentrazone) gives erratic activity on marshelder. “For some reason, 2000 was a banner year for marshelder. Some producers might have reduced rates too much, and if so, marginally controlled weeds can be missed.” In some cases too, precipitation might not have been adequate for product activation, he adds.

Still, Zollinger says Spartan has been beneficial to sunflower producers, particularly those with no-till. “We’ve had two years of Section 18s (emergency label use) with Spartan, and I think everyone understands that it’s best for small seeded, broadleaf weeds, and that control directly correlates with soil pH and rainfall immediately after application.” Indeed, while sunflower has shown good tolerance to Spartan on medium to fine textured soils with organic matter above 3%, crop injury may occur on soils with low organic matter below 1.5%, and soil pH greater than 7.8.

“I tend to advise staying on the lower end of recommended rates, since our soils tend to be lower in organic matter and slightly higher in pH,” says Drew Lyon, agronomist at the University of Nebraska Panhandle Research and Extension Center, Scottsbluff. With limited herbicide options, area sunflower producers use a variety of methods to suppress weeds, including cultivation, using a herbicide burndown before planting, using plant populations and row spacings to limit weed competition, and using shielded or hooded sprayers to apply Gramoxone Extra (paraquat) for post-emergent weed control.

Ron Meyer, Colorado State University extension agronomist, Burlington, says sunflower damage was evident with Spartan used at the higher end of recommended label rates (4-5 oz.), particularly on high pH soils. “Last year we used both 2 and 3 oz. rates in our Spartan treatments, tested 15 sunflower varieties across those two rates and saw zero Spartan damage and had very good weed control,” he says. “In soils with a pH of 7.5 or higher, use the 2 oz. rate, and apply Spartan well ahead of planting, up to two weeks beforehand. That’s worked well, giving good weed control and little risk of injury. Spartan worked well in both no-till and reduced till systems.”

Meyer says one weed control strategy that some conventional till sunflower producers use is to let broadleaf weeds germinate, apply Treflan (trifluralin), Sonalan (ethalfluralin) or Prowl (pendimethalin), incorporate it, then plant sunflower, thus allowing sunflower plants to emerge and “shade out” the next flush of weed competition.

Select (clethodim) or Prism, a post-emergent product with recently approved label registration, offers better grass control than currently available products, says Zollinger. It should be applied with an oil adjuvant when grasses are two to six inches tall.

Leon Wrage, South Dakota State University extension weed specialist, says it’s not so much that weed problems change from year to year, but the weather, which can both facilitate and handicap weed development and control efforts.

An example is weed control products that need moisture for activation—like Spartan—which proved less effective last year in controlling weeds in areas that did not receive sufficient precipitation for product activation. “It’s been an important tool for kochia, pigweed and nightshade, especially for no-till sunflower. It has performed well in many situations on medium and heavy soils,” he says.

Preplant herbicide burndown treatments can be successful in controlling early weeds, but not always, so producers should be prepared to use post-emergent products for weed suppression if need be, says Wrage. “Weed management is more than just the chemicals. A good window of control often depends on consistency, timing, and using products when and where you need them,” he says.

As crop rotations become more integrated, Wrage stresses the importance of sprayer cleanup following treatments, not only to prevent residual chemical from injuring small grain or broadleaf crops in subsequent applications, but also to prevent residue from one particular chemical from antagonizing the performance of another.

He also says that canopy establishment is critical for suppressing weeds in sunflower as the growing season progresses. “It’s absolutely a major factor in sunflower. Even if there are two or three sunflower plants missing from a row, weeds seems to find those open spots. Narrower rows, a full plant population, a uniform stand all help control weed flushes later.”

Wrage says Canada thistle is also problematic in South Dakota, so much that state agriculture officials convened a conference in February to discuss suppression measures. Waterhemp is another weed that growers need to watch closely to keep from spreading. “One plant can produce 100,000 seeds, and it has little competition other than what the crop provides, so if a few plants are missed, it can become an economic problem in just a few years,” he says.

NDSU’s Zollinger says the introduction of Clearfield (Raptor or imazamox-resistant) technology will significantly benefit annual grass and broadleaf weed control in sunflower, including marshelder and cocklebur. Developed through conventional or non-GMO methods, full federal registration for Clearfield sunflower isn’t expected until 2004 or 2005, but Section 18 or emergency label use may be available as early as the 2002 growing season.—Tracy Sayler

Sunflower Herbicide Notes, Tips

Assert (imazamethabenz): Registered through ND supplemental labeling at 0.6 to 0.8 pt/A for POST control of wild mustard in the rosette stage and prior to bloom and sunflower at the 2- to 8-leaf but before 15 inches tall. Severe sunflower injury may occur with high temperatures and humidity at application. Sunflower variety, growth stage, weather conditions, humidity, spray volume, spray additives, and type of application may affect sunflower safety. Risk of injury should be considered when deciding whether treatment is warranted. Do not apply to sunflower under drought or heat stress. Apply Assert only when air temperature plus relative humidity is below 150. Sunflower damage from misapplication may range from plant stunting to head deformation. Do not tank mix Assert with any insecticides or herbicides. After Assert application, wait 4 to 5 days before between-row cultivation. Read and follow the label information.

Eptam (EPTC), Prowl/Pendimax (pendimethalin), Sonalan (ethalfluralin), and trifluralin are preplant incorporated herbicides that can be applied spring or fall. Sonalan at 1.5 to 2 pt/A or 5.5 lb 10G/A and trifluralin at 1 pt/A or 5 lb 10G/A can be applied on sandy soil. Eptam must be applied and incorporated immediately to prevent herbicide loss. Eptam may be applied in late fall before soil freeze-up at 4.5 pt 7E/A or 20 lb 20G/A on coarse textured soil and 5.25 pt 7E/A or 22.55 lb 20G/A on fine and medium textured soil. Eptam controls wild oat better than Prowl, Sonalan or trifluralin. Sonalan may be tank-mixed with Eptam. Rainfall after Prowl applied pre-emergence is needed for activation.

Poast (sethoxydim) at 0.5 to 1.5 pt/A plus oil additive control annual grasses. Observe a 70 days preharvest interval and do not feed treated sunflower forage to livestock.

Select (clethodim) at 6 to 8 fl oz/A or Prism at 12.8 to 17 fl oz/A plus oil additive controls annual grasses and quackgrass. Observe a 70 day preharvest interval and do not feed treated sunflower forage to livestock.

Spartan (sulfentrazone) applied pre-emergence at 2.67 to 5.33 oz water dispersible granules/A controls most annual small-seeded broadleaf weeds, such as kochia, pigweed species, lambsquarters, nightshade, smartweed, and biennial wormwood. Spartan may suppress other weeds like buckwheat, mustard, ragweed, and Russian thistle. Spartan provides little grass and no perennial weed control. Adjust rate for organic matter. Spartan may be applied up to 30 days prior to planting but use the higher rate in the appropriate rate range. Sunflower has shown good tolerance to Spartan on medium to fine textured soils with organic matter above 3%. Crop injury may occur on soils with low organic matter and soil pH greater than 8.0, especially on calcareous outcropping. Do not use on coarse textured soils with less than 1% organic matter. Provide adequate furrow closure and soil covering to avoid crop injury. Poor growing conditions at and following sunflower emergence, cold temperatures, soil compaction, or rate too high based on soil type and organic matter may result in sunflower injury.

NDSU research has shown excellent weed control with Spartan in many different environments throughout the Great Plains region. However, consistent control of sensitive broadleaf weeds and control of grass and marginally controlled broadleaf weeds greatly depends on at least 0.75 inch rainfall shortly after application and before weeds emerge.

Source: North Dakota State University

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