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Saturday, February 1, 1997
filed under: Weeds

Much more paper is required to print out the list of registered herbicides for use with crops like corn and soybeans than for a “minor use” crop like sunflower. And that’s to be expected. When chemical companies look at planted acreages in the neighborhood of 79.5 million or 64.2 million (1996 U.S. corn and soybeans, respectively), it’s obvious why a crop such as sunflower, with its 2.6 million planted acres last year, occupies a backseat on R&D priority lists.

DNA-class preplant herbicides trifluralin (e.g., Treflan), ethalfluralin (Sonalan) and pendimethalin (Prowl) are normally applied on well over four-fifths of the nation’s sunflower acreage, and they do an overall good job of controlling annual grasses and some small-seeded broadleaf weeds. However, the DNAs do not provide good control of wild oats, mustard, wild buckwheat, marshelder, cocklebur and certain other problem weeds of sunflower.

Sunflower’s short list of herbicides is especially limited when it comes to post-emergent treatments. Imazamethabenz (Assert) is registered on this crop for wild mustard control, but it can be injurious if applied at high temperatures. Sethoydim (Ultima 160) controls most annual grasses and quackgrass, though results can be variable on wild oats, volunteer cereals and quackgrass, depending on rates, adjuvants and tank-mixed products.

The desire for an expanded post-emergent herbicide arsenal is widespread among sunflower growers — and particularly among those who have trended toward reduced-tillage production regimens. It’s commonly believed, for example, that the number of no-till sunflower acres would be significantly higher were it not for the lack of post-emergent herbicides.

The only new sunflower registration within the past year has been the granting of a federal label for glyphosate (Roundup Ultra, Roundup RT) as a preplant or pre-emergence broad-spectrum treatment. (As of this writing, Monsanto was considering the pursuit of selected state labels for use of Roundup Ultra as an inter-row shielded application postemergent treatment for sunflower in 1997. No such labels are yet in effect, however.)

Richard Zollinger, extension weed scientist at North Dakota State University, confirms that the main sunflower herbicide “hole” at present is in the broadleaf sector. The DNAs provide generally sound preplant protection against grasses, while on the post side for grasses, there’s Ultima 160. “And we’re probably going to have Select or Prism labeled before long,” Zollinger says. Kochia, redroot pigweed, the nightshade species, smartweed, marshelder, Russian thistle, cocklebur and certain perennials (mainly Canada thistle) are the primary broadleaf concerns for his state’s sunflower producers, Zollinger reports.

With the encouragement and financial support of the National Sunflower Association, NDSU weed scientists have been evaluating a number of pre- and post- herbicides for potential use on sunflower. Here’s a recap of several of the more-promising ones. (Note: None of the following herbicides are presently labeled for sunflower. Their use on this crop is not encouraged — and would be illegal — until such time a label would be granted.)

• ACETOCHLOR (Harness / Surpass) — The acetochlors are of the same “family” as Dual/Lasso and Frontier, “but in our environment, they perform better on broadleaves than the Lasso/Dual- or Frontier-type herbicides,” Zollinger reports. Harness (manufactured by Monsanto) and Surpass (a Zeneca product) can be applied either preplant incorporated or pre-emergence under conventional tillage systems. A formulation called TopNotch has been developed for for reduced- and no-till situations. “It’s microencapsulated, so it is able to work through the residue better, and you get more residual,” Zollinger says.

• SULFENTRAZONE (Authority) — Authority is an FMC product expected to be labeled for 1997 use on soybeans. “Sulfentrazone has six to eight weeks’ soil residual activity for control of important broadleaf weeds like common lambs-quarters, nightshade spp., pigweed spp., common cocklebur and annual smartweed spp.,” Zollinger notes.

Authority performed very well in 1996 NDSU testing on sunflower. Also, no injury to the ’flowers was observed. “It’s better on broadleaves than grasses,” Zollinger advises. “So I used it along with Treflan to pick up the grasses.” A research proposal has been submitted to the National Sunflower Association for a uniform study in North Dakota, South Dakota, Nebraska and Kansas to better determine the response of sunflower to sulfentrazone.

One caveat: Zollinger says FMC plans to make Authority available for soybeans only as a premix with Classic or Command. Due to the long carryover (four to five years) of those herbicides, such a situation “would basically take Authority out of sunflower — unless they’d give it a different name and allow us to use it alone in sunflower,” he explains.

• ISOXAFLUTOLE (Balance) — Isoxaflutole is a new pigment synthesis mode-of-action herbicide expected to have a federal label for corn this year under the trade name of Balance. Soil applied, it controls many annual grasses and most broadleaf weeds. According to Zollinger, this herbicide has displayed good weed control under limited rainfall conditions, with residual control for six to eight weeks following application. “Isoxaflutole has considerable foliar activity, and tests will be conducted to determine if this might allow selective use in sunflower,” the NDSU weed scientist observes.

“We are quite excited about Balance, because cocklebur and sunflower are real weaknesses with it,” Zollinger adds. “So by doing some band application, low-rate application or using it in a tank mix with another herbicide, we would get the weed control from Balance — yet reduce the risk of injury.”

• LINURON (Lorox, Linex, et. al) — The active ingredient linuron is a photosynthetic inhibitor herbicide. “Lorox is used in carrots and works marvelously,” Zollinger reports. Linuron handles numerous annual grasses and broadleaves. It does not, however, perform well in fine-textured, high-organic-matter soils. In the 1997 NDSU testing on sunflower, Lorox will be used in a pre-emergence application.

• OXYFLUORFEN (Goal) — This product, a cell membrane disrupter herbicide, is used with onions, ornamentals and tree plantings. It is soil applied and controls many annual grass and broadleaf weeds. “Oxyfluorfen has a low leaching potential and may be selective in sunflower based on this characteristic,” according to Zollinger. However, its higher price could be a limiting factor in any development for use in sunflower, he adds.

As noted earlier, none of these herbicides are presently labeled for use on sunflower. Whether they ever are will, for the most part, depend upon the “Two E’s”: efficacy and economics.

The first E — whether a specific herbicide provides adequate control of problem weeds without injury to the crop — is being addressed by the ongoing research at NDSU, along with some concurrent work in the High Plains (particularly by Kansas State University).

Assuming the results of such research are positive, the question becomes one of whether registration on sunflower should be pursued — and, if so, how?

Given sunflower’s relatively small acreage, chemical companies may not wish to invest the funds needed to achieve registration for one of their products on this crop. That would leave two other potential avenues, Zollinger explains.

One is IR-4, a New Jersey-based program which works with state university scientists in developing the data required by EPA to consider the registration of pesticides for “minor” crops such as sunflower. At the behest of the National Sunflower Association, NDSU is continuing its research into new herbicides for sunflower. “We’re going to develop a good data base of crop response (safety) and also of weed control,” Zollinger notes. The next step would be to enter into the IR-4 process with one or two herbicides that would do sunflower growers the most good.

A second option would be pursuit of a “grower label” for a specific herbicide. In that case, producers (e.g., the NSA) — not the chemical manufacturer — would be the entity applying to EPA for a crop use label. First, though, EPA would have to establish a tolerance level for the herbicide on that particular crop. Typically, growers using the herbicide would be asked to sign a waiver releasing the sponsoring organization from any liability pertaining to its use. — Don Lilleboe
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