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Clearfield™ Sunflower Primer

Tuesday, December 9, 2003
filed under: Hybrid Selection/Planting

The Clearfield™ production system promises to be one of the best tools available for weed control in sunflower. Like any new tool, however, it needs to be understood and used appropriately to be effective.

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.

The Clearfield trait for sunflower was first identified by Kansas State University researchers in 1997, when it was found in a population of wild sunflowers. The wild sunflowers had been growing in a soybean field with seven years of continuous ALS-inhibitor herbicide use. Using traditional non-GMO plant breeding procedures, USDA sunflower geneticist Jerry Miller, Fargo, N.D, crossed the resistant wild sunflower to four USDA cultivated sunflower genetic stocks. The tolerance trait was maintained through several backcross generations, and the germplasm lines made available to commercial seed companies.

By incorporating the tolerance trait from these germplasm lines into their most advanced breeding stock, plant breeders developed new, elite Clearfield breeding lines and hybrids. Field studies were established across major sunflower growing areas to test hybrids, herbicide rates, application timing, and necessary additives. Herbicide tolerance trials established with seed companies developing Clearfield sunflower ensured that finished hybrids have complete natural tolerance to Beyond herbicide.

Mycogen Seeds and Seeds 2000 were two companies that offered Clearfield sunflower hybrids in 2003; they will be joined by other seed companies including Interstate Seed and Proseed to offer Clearfield hybrids in 2004. BASF is the commercial developer of the Clearfield system and manufacturer of Beyond herbicide, which received full label approval by EPA in 2003.

Because it is safe to birds, fish, mammals, and other non-target species, Beyond is considered environmentally friendly, and carries the least restrictive word designation for a pesticide (“caution”).

However, there are a number of critical factors that need to be made clear about the Clearfield production system in sunflower:

1) Clearfield is not the same chemistry as Roundup-Ready (glyphosate). 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 sythase enzyme.

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 (see photo).

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 (see table of weeds controlled) but does not take the place of other herbicide treatments that may still be needed in sunflower, such as preplant treatments.

Strategic Weed Control

Leon Wrage, extension weed scientist at South Dakota State University, points out that while the Clearfield system is new to sunflower, much is already known about the “imi” chemistry on which the system is based. “It provides another weed control option that sunflower growers didn’t have before,” he says.

It’s a control option that needs to be used strategically, depending on field history, previous crop, and weed situation, he says. In clean fields, a postemergent Clearfield application might be the only treatment needed. In other cases, a glyphosate burndown might be needed three to four weeks before planting to control the season’s first flush of weeds, including kochia and wild buckwheat. In addition, sunflower growers may still want to consider a pre-plant application of Spartan for kochia control in no-till, or Prowl for preplant control of foxtail.

Richard Zollinger, extension weed specialist at North Dakota State University, says that the earlier Beyond is applied in the recommended spraying window the better. “But you have the legal flexibility to spray up to the 8-leaf stage in case application is delayed.”

Zollinger says Beyond will do an excellent job of cleaning up key weeds that have been problematic in sunflower, including cocklebur and marshelder. “But we’ll still be fighting ALS-resistant kochia, so it’s recommended to apply a pre-emerge product like Spartan down, followed by the post-emerge Beyond.”

Kansas State University extension agronomist Roger Stockton says Beyond should control devil’s claw, if applied before the weed grows more than 4” tall. “We really haven’t a product available to help us with that weed until now,” Stockton says. “But we still have ALS resistant pigweed and kochia that remain a problem down here, and Beyond won’t touch those.”

Ron Meyer, Colorado State University extension agronomist, says Clearfield hybrid and weed control performance have been evaluated in area plot trials for the past two years. “From a weed control standpoint, it works very good. The varieties have performed OK. They are not the top-performing varieties, but not bottom ones either. Overall, it looks like pretty good technology.”

Bruce Due, an agronomist with Mycogen Seeds, says that last year, company dealers meticulously reviewed the Clearfield program with prospective customers before completing sales. “The dealers went over the tech sheets on Beyond, and how best to use the product,” he says. “And customers were asked why they were interested in Clearfield. There are growers who do not need this technology, because they are using other ways to handle weeds. Those with problem weeds that this product controls, that was our customer. This system needs to be targeted to those fields and growers where the system has a good fit.

“We have other hybrids in our lineup that perform better in a non-weed environment,” he continues. “They have better oil, and as we go north, they are earlier in maturity. But if that grower has a weed problem, we’ve seen a 500 to 600 pound yield difference between controlling the weeds and not. Plus, the grower has a cleaner field to work with for the next crop in rotation.”

Due says he did not receive one complaint about how the hybrid and herbicide performed this past season. “That’s a real positive that goes back to the expectation level we set for using the product.” Growers should expect that the first wave of Clearfield hybrids are longer season compared to other earlier maturing non-Clearfield hybrids. That will be addressed in a year or two, when seed companies release subsequent Clearfield hybrids that are earlier maturing.

“One positive that seems to be evident after this growing season is that it seems to have better stress tolerance than expected, both drought and excess water. And agronomically, it has demonstrated a lot of plant health.” Due says he has also been impressed with the tolerance of Clearfield hybrids to Beyond. The hybrids were screened heavily for the herbicide tolerance during the breeding process, and that is clearly evident in the finished product.

Due says that in the last two years of experimenting with Clearfield in plots, he has seen the best performance when Beyond was applied to Clearfield sunflower at the two to four leaf stage. “You don’t have a thick canopy of weeds out there, so you do a better job of getting chemical to the soil, and Beyond does have residual action in the soil. At this stage we also control weeds better since they’re smaller, and spraying conditions are better since it’s usually not as hot or humid.”

In the last two years of trials at Breckenridge, Minn., Due notes that weed control in plots sprayed with Beyond at the 1x rate (4 oz) on Clearfield sunflower in early stages of growth (2-4 leaf) exceeded weed control performance of plots sprayed at an experimental 2x rate (8 oz, twice the labeled rate) on Clearfield sunflower at the 6-8 leaf stage. “So this illustrates the advantage of spraying early,” he says. – Tracy Sayler

Clearfield Sunflower Stewardship

To preserve the efficacy of the Clearfield Sunflower Production System, and to prevent or delay herbicide resistance in the wild sunflower population and in other weed pests, specific management practices need to be followed. These practices should span across crops and years to promote sound herbicide resistance management.

“We are really stressing stewardship, especially with a crop like sunflower to prevent outcrossing,” says Gary Fellows, BASF’s technical marketing manager for Clearfield sunflower. “Rotating crops and herbicides are probably the two biggest things to keep in mind. It’s on the growers behalf to take care of this technology, so we can maintain it as a weed control program.”

Indeed, university researchers have demonstrated that the herbicide tolerance trait in Clearfield sunflower can move (outcross) into a wild sunflower population, transferring the tolerance trait into the resulting offspring which may exhibit resistance to Beyond and other IMI herbicides. Although the chance of this occurring is low, it is not zero and must be managed.

Always grow Clearfield sunflower in rotation with other 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 (no-till/min-till) or tillage (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.

Applying Beyond to Clearfield Sunflower

The application rate is 4.0 oz per acre, limited to one application per season.

Required herbicide additives needed are nonionic surfactant and nitrogen based fertilizer. Treatment may be applied by ground or air. See label for more specifics.

For conventional tillage, it’s recommended to apply Prowl 3.3 EC at 3.0 – 3.6 pts/A preplant incorporated followed by Beyond at 4 oz/A applied early postemergence (see graphic that outlines application timing).

For no-till or minimum-till, a glyphosate burndown plus Prowl 3.3 EC at 3.6 pts/A Preplant or Preemergence followed by Beyond at 4 oz/A applied Early postemergence is recommended.

Tank-mixing with Insecticide

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.

Watch Drift, Clean Sprayer

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.

Proper sprayer cleanout steps are given on many herbicide labels and the procedure on the label should be followed for specific herbicides. Sprayers generally should be cleaned as soon as possible after use to prevent the deposit of dried spray residues. A sprayer should not remain empty overnight without cleaning; fill the tank with water to prevent dried spray deposits from forming. – Tracy Sayler

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