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Manage Wild Sunflower in Clearfield™ Sunflower

Thursday, December 15, 2005
filed under: Weeds

Bruce Due was dismayed to hear of an incidence in South Dakota this past summer in which a field of Clearfield™ sunflower was “loaded” with wild sunflower. At first glance this seemed puzzling, since Beyond herbicide should control wild sunflower in a field of Clearfield sunflower.

Turns out that the field wasn’t even sprayed.

“So here you have a very high chance of wild sunflower cross pollinating with the Clearfield sunflower,” says Due, district agronomist with Mycogen Seeds. That is troubling, he explains, because if cross-pollination occurs when wild sunflower in close proximity to Clearfield sunflower is blooming during the same time, the wild sunflower could develop tolerance to Beyond.

The herbicide-tolerant wild sunflower could spread, and eventually diminish the effectiveness of this technology for wild sunflower control.

Clearfield sunflower actually owe their herbicide resistance to a wild sunflower species (Helianthus annuus) accession that Kansas State University weed physiologist Kassim Al-Khatib collected from a soybean field in 1996.

USDA Agricultural Research Service plant geneticist Jerry Miller, with the Red River Valley Agricultural Research Center in Fargo, learned of Al-Khatib's discovery and requested seed specimens. Miller germinated seed and grew out eight-leaf seedlings inside a herbicide-spray chamber, then sprayed the seedlings with imazethapyr (Pursuit) and imazamox (Beyond) at 2 to 15 times the herbicides' label-recommended rates. The goal was to identify the healthiest survivors, and then transfer the plants' resistance to cultivated sunflower, eventually furnishing growers with a crop that could withstand direct spraying, while killing weeds within the crop.

Now, Clearfield sunflower with Beyond herbicide represents the only real post-emerge option for controlling key problem weeds like marshelder and cocklebur in sunflower. Clearfield/Beyond provides contact and residual activity on a number of other grasses and broadleaf weeds as well, including nightshade, pigweed, foxtail species, wild oats, volunteer cereals, puncturevine, and non-Clearfield wild or volunteer sunflower.

Beyond herbicide is a member of the imidazolinone chemical family . Its active

ingredient is imazamox, and it is a member of the herbicide family of AHAS or ALS inhibitors. Members of this family control susceptible weeds by inhibiting the acetohydroxyacid synthase enzyme.

Due urges growers who produce Clearfield sunflower to be astute in managing it, including control of wild sunflower within Clearfield sunflower fields, as well as field perimeters. “This isn’t like Roundup Ready. The herbicide tolerance in is single gene expression in Clearfield sunflower hybrids,” says Due. “We need to be prudent about watching for skips in spraying that might miss wild sunflower, and wilds in the field perimeters. If we don’t take care of this technology, the wilds could blow right through it.”

Due suggests that growers make plans to spray a non IMI herbicide for weed control in fields where cross pollination amongst Clearfield sunflower and wild sunflower may have occurred.

Clearfield Sunflower Stewardship

Vince Ulstad, technical services representative for BASF, which makes Beyond for Clearfield sunflower hybrids, urges growers to follow good stewardship practices to keep the chemistry viable as a weed control option in sunflower. Specific management practices need to be followed, and these practices should span across crops and years to promote sound herbicide resistance management. Other Clearfield crops, such as Clearfield wheat and Clearfield canola, also need to be taken into account within the crop rotation.

Following are key considerations to follow in managing Clearfield sunflower:

Always grow Clearfield sunflower in rotation with other non-Clearfield crops, i.e wheat/corn/sunflower. This breaks the cycle of continuous sunflower production and allows use of alternate mode-of-action herbicides and tillage. It also promotes good agronomics by reducing disease and insect pressure in sunflower.

Use alternate (non-ALS) mode-of-action herbicides with activity on sunflower in the rotational crop, i.e. growth regulator or photosynthesis inhibitor. This reduces the selection pressure from continuous dependence on the ALS-inhibiting herbicide, and provides alternate mode-of-action to control volunteer Clearfield sunflower and other ALS resistant weeds that may be present.

Do not plant Clearfield sunflower on land with a history of a heavy infestation of wild sunflower. This reduces the threat of outcrossing of Clearfield sunflower with wild sunflower.

Control wild sunflower in adjacent areas to Clearfield sunflower fields (road ditches, field borders, fence rows) through the use of non-ALS herbicides and/or mowing prior to seed set. This minimizes the potential of cross-pollination of wild-type sunflowers with Clearfield sunflower.

Control emerged wild sunflower prior to planting Clearfield sunflower with non-ALS burndown herbicides in no-till/min-till fields or tillage with conventional-till. This reduces reliance on ALS herbicide in controlling wild sunflower, and eliminates any emerged naturally occurring biotype that may be resistant to ALS-inhibiting herbicides

Limit the sole reliance on ALS herbicides to no more than two out of four years in the same field. Where applicable, use sequential or tank-mix partner herbicides with multiple modes-of-action on target weed species in the sunflower crop and in rotational crops.

– Tracy Sayler

Key Points about Clearfield Sunflower

•Clearfield is not the same chemistry as Roundup-Ready (glyphosate), nor does it give the same level of weed control.

•While the Clearfield system is available for several different field crops, including corn, wheat, rice, and canola, the “Imi” chemical formulations are not interchangeable. Thus, for example, Lightning is labeled only for use on Clearfield corn hybrids, not Clearfield sunflower hybrids. As well, one would not apply Beyond on Clearfield corn hybrids.

•Clearfield sunflower is not cross-tolerant to the sulfonylurea (SU) family of herbicides.

•Beyond must be applied only to Clearfield hybrids. Application of Beyond in conventional, non-Clearfield sunflower will result in significant crop injury and plant death.

•Clearfield is not a complete weed control program for sunflower. It helps growers manage pigeongrass/foxtail, wild oats, wild mustard, marshelder, cocklebur, devil’s claw (apply before the weed grows more than 4” tall) and other tough grasses and broadleaf weeds that plague sunflower growers, but does not take the place of other herbicide treatments that may still be needed in sunflower, such as preplant treatments.

•Clearfield has some activity against kochia, but should not be expected to give complete control. Growers will need to use Clearfield in combination with other herbicide strategies or another pre-emerge herbicide product, and consider a glyphosate burndown at planting as well.

•Clearfield sunflower should be grown in rotation with other crops, and use alternate mode-of-action herbicides and tillage, to help prevent the formation of herbicide-tolerant weeds.

•Pyrethroids (Warrior, Asana XL, Baythroid, Scout X-Tra) are safe to tank mix with Beyond. Other chemistries, such as organophosphates (Lorsban, parathion) and carbamates (Furadan) may result in plant injury in Clearfield sunflower. Refer to the Beyond label for more details.

•The need for good sprayer cleanout when switching from one chemical to another is already obvious, and is something to watch as well when adding Clearfield sunflower to your crop mix. Sunflower is highly sensitive to herbicides such as 2,4-D, picloram, dicamba, MCPA, Ally, and Amber. And non-Clearfield sunflower is highly sensitive to Beyond. Use caution as well to prevent drift when spraying Clearfield sunflower near non-Clearfield sunflower.

•The earlier Beyond is applied within the recommended spraying window the better. Growers have the legal flexibility to spray up to the 8-leaf stage in cases when application is delayed, but weed control performance is best when Beyond is applied to Clearfield sunflower at the two to four leaf stage.

•Growers should view Clearfield sunflower hybrids as one tool in the toolbox, and use them on fields where they’re needed. Pick your fields; there are fields where other hybrids may be a better choice.

Find more specific information about Clearfield technology online at

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