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Start with a Clean Slate

Thursday, April 15, 2004
filed under: Weeds

Weeds that get started prior to crop emergence pose the greatest competitive threat to sunflower – or any other crop, for that matter. Weeds that emerge in the weeks after crop emergence may not compete as well, but still steal precious moisture away from the crop.

The best scenario at the beginning of the growing season is a clean slate, so to speak, to give sunflower the chance to compete with weeds, not try to catch up to them.

Your weed control program in sunflower begins with field selection, says Ron Meyer, Colorado State University extension agronomist. The herbicides used in last year’s crop can help clear the way for that clean slate, so long as residual carryover isn’t a concern. If you’re unsure about herbicide carryover in a field you’d like to plant to ‘flowers, Meyer suggests scooping up some topsoil from the field in question, preferably from a few locations. Plant some sunflower seeds (or tomato seeds) in a container with the soil and put it in a sunny window. If the plants emerge fine, herbicide carryover shouldn’t be a concern.

Leon Wrage, extension weed specialist at South Dakota State University, views an early glyphosate burndown of weeds as a virtual prerequisite to growing sunflower, to provide the crop with an equal footing with weed competition.

Optimum would be a glyphosate burndown several weeks before planting to control the season’s first flush of weeds, including small wild buckwheat at the two to three-leaf stage, followed up with another glyphosate application at planting to get additional weeds.

Limited weed growth (or a busy planting season) may prevent a grower from making two applications. If making only one glyphosate burndown, don’t let weeds get too large. When weeds get too big, it can be difficult to get all of the axillary buds along the stem killed, and you may get regrowth or an incomplete kill, Wrage points out.

Also, use the right rate for the right weeds. “Always remember that perennials and annuals are two different animals. With annuals, the smaller they are the easier they are to kill. With perennials, it’s just the opposite. The more leaf surface there is, the more Roundup you land on the plant that’s taken into the root system,” Monsanto/Dekalb representative Troy Sayler points out. “So perennial grasses are harder to hit early in the spring because you’re working with smaller targets. Same with Canada thistle. You have that small bud that comes through in the spring and a massive root system below ground. It can take a hit of chemical and bounce back. The larger it becomes, the better we can kill it.”

Kochia is often a problem in sunflower, especially in the Dakotas. Thus, consider including Spartan™ (Sulfentrazone) with a preplant burndown or as a separate treatment plenty of time before planting to allow moisture for activation.

Sunflower has 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 7.5. Don’t use the product on coarse textured soils with less than 1% organic matter.

Wrage points out that the sunflower grower’s toolbox for weed control is better than it’s ever been before – it’s a matter of using the right tool, at the right rate, at the right time. – Tracy Sayler

Weed Info Online

Tips in Applying Beyond to Clearfield™ sunflower

The Clearfield system consists of sunflower hybrids bred specifically for use with Beyond™ herbicide, delivering broad-spectrum postemergence grass and broadleaf weed control in all tillage systems. A few tips to keep in mind:

1) Clearfield is not the same chemistry as Roundup-Ready (glyphosate). Beyond herbicide is a member of the imidazolinone chemical family.

2) 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.

3) Clearfield sunflower hybrids are not cross-tolerant to the sulfonylurea (SU) family of herbicides, which are also ALS or AHAS inhibitors.

4) 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.

5) Clearfield is not a complete weed control program for sunflower. It helps growers manage pigeongrass/foxtail, wild oats, wild mustard, cocklebur 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.

6) Beyond will not control ALS-resistant kochia and other ALS-resistant weeds.

7) Beyond performs best as an early post-emergent herbicide in Clearfield sunflower at early stages of growth (2-4 leaf). The herbicide shouldn’t be viewed as a rescue treatment for large weeds that have gotten out of hand.

8) 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.

9) 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.

10) 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.

You’ll find a more complete primer on Clearfield sunflower at Click on the link “Sunflower Magazine,” then “view archives” and “Hybrid Selection/Planting.” – Tracy Sayler

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