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Glyphosate Labeled For Late Season Sunflower

Sunday, April 15, 2007
filed under: Harvest/Storage

Farmers have a major new tool in their pre-harvest toolbox, with EPA approval this spring of glyphosate for late season weed control in sunflower.

The EPA this spring approved a supplemental label for Monsanto’s Roundup WeatherMax® and supplemental labels are pending for Original Max® and RT-3®. Monsanto is in the process of registering the products in various states.

Preharvest use instructions on the label of Roundup WeatherMax states that “This product provides weed control when applied as a harvest aid to a physiologically mature crop prior to harvest of sunflower. For sunflower, apply when the backsides of sunflower heads are yellow and bracts are turning brown and seed moisture content is less than 35%.”

The label indicates to allow a minimum of 7 days between treatment and harvest or livestock feeding, and to apply no more than 22 fluid ounces of the product at a preharvest timing to sunflower.

While the labeled use is for late season weed control, there is an obvious parallel benefit of its effect as a desiccant. In addition to managing weeds such as Canada thistle, there can be a number of advantages in harvesting a sunflower crop early:

• Marketing

• Reduce bird damage

• Reduce shattering and lodging especially if stalk strength is compromised by stalk insects

• Use aeration instead of late-season drier

• Spread out harvest

• Reduce or escape disease pressure

The NSA has been supporting glyphosate research for late season weed control in sunflower for the past few years at North Dakota State University. The team of researchers involved with the project includes Robbie Holthusen, Burton Johnson, and Kirk Howatt, all with the NDSU Department of Plant Sciences, as well as Paula Peterson, and the late Robert Henson at the Carrington Research Extension Center.

They’ve conducted experiments at Casselton, Prosper, and Carrington on oil and nonoil sunflower, comparing glyphosate (Roundup Ultra Max II) and paraquat (Gramoxone Max) to the control – natural drydown via Mother Nature. They’ve evaluated sunflower drydown, desiccation and timing, looking at glyphosate applied to sunflower at 70% (Gly-1), 50% (Gly-2), and 30% moisture (Gly-3), and paraquat at 30% moisture.

Table 1 shows the effect of the various drydown treatments on sunflower seed moisture at Prosper in 2005, on oil sunflower (Mycogen 8377NS) with a population at 22,000 plants/A. Seed moisture was evaluated weekly after desiccant treatment. The control naturally dried down to an acceptable harvest moisture when evaluated Oct. 7. Glyphosate applied at approximately 70% seed moisture (Gly-1) on Aug. 27 dried down to 11% moisture 4 weeks later, on Sept. 23. Gly-2 applied at 50% seed moisture on Sept. 9 dried down to 15% seed moisture two weeks later. Gly-3 (30% seed moisture) applied on Sept. 16 dried down to 13% seed moisture when checked at Sept. 30, and paraquat took only a week to dry down to 16% seed moisture.

On average, the Gly-3 (applied at 30% seed moisture) treatment dried down more than 7 days earlier than the control (Mother Nature) in 2005, and paraquat two weeks earlier than the control and an estimated 6 days earlier than the Gly-3 treatment.

Tables 2 and 3 show the effect on yield and oil. As to be expected, applying a desiccant too early is detrimental to both yield and oil, as illustrated by Gly-1 applied at 70% seed moisture, which had the lowest yield and oil. Paraquat and the Gly-2 and Gly-3 treatments produced similar yield and oil content as the control. However, the Gly-2 treatment tended to reduce yield in other sunflower studies, and yield was reduced by about 10% at one location.

Results were similar with experiments on confectionary sunflower. In experiments with stay-green nonoil hybrid Red River Commodities 2215 at Casselton and Prosper in 2005, glyphosate treatments Gly-1, Gly-2, and Gly-3 were targeted at 70, 50, and 35% seed moisture, respectively. The paraquat treatment was targeted at physiological maturity moisture of 35%. There was also a non-desiccated treatment (control) for moisture loss comparisons. Plant samples were harvested at approximately 7-day intervals following desiccant application.

Drydown response to the glyphosate treatments indicated the Gly-1 and Gly-2 treatments reached 13% or less moisture 4 and 3 weeks earlier than the untreated control, respectively. The Gly-3 treatment also indicated effective desiccation compared to the untreated control. During the first week after application, the paraquat treatment showed the greatest seed moisture loss (26%) compared to the glyphosate treatments. Yield was equal to the control (2800 lb/acre) for the Gly-2, Gly-3, and paraquat treatments at Prosper in 2005, but yield was reduced 12% for the Gly-1 treatment compared to the control.

See complete research details online at – click on ‘Research’ then ‘Research Forum Papers.’ Search for papers under ‘Johnson Burton L’ all categories, all years. Desiccant research going back to 2002 will be found.

Here’s what the results indicate: Paraquat will result in faster drydown than glyphosate, with no effect on yield and oil. Glyphosate applied at 30 to 35% seed moisture won’t dry plant material as fast as paraquat, but is faster than natural drydown, about 10 days faster, with no effect on yield or oil content. Glyphosate applied at 50% seed moisture did not statistically influence yield and oil content. Glyphosate applied too early, at 70% seed moisture, clearly will hurt yield and oil.

Paraquat is still a better sunflower desiccant than glyphosate, but then again, glyphosate isn’t labeled as a sunflower desiccant – it’s labeled for late season weed control. However, applied to sunflower at 30 to 35% moisture, glyphosate can be an effective tool for weed control as well as crop desiccation. The positive aspect of glyphosate for this use is that the window of application appears broad before yield reductions occur, says NDSU’s Burton Johnson. “More research will be conducted to determine when application timing begins to affect yield. Another negative with paraquat is that rainfall after paraquat application can result in moisture absorption by the damaged areas of the sunflower head. Rainwater that accumulates on the backside of the head seems to seep into the tissue, creating heavy, waterlogged heads that can cause stem breakage or encourage pathogen infection. This has not occurred with glyphosate.

Keep in mind that the visual appearance of sunflower physiologically maturity may vary by hybrid, particularly those with the stay green characteristic. It might be a good idea to consult with your seed rep about how your hybrid might be expected to appear at 30-35% moisture, particularly if it has the stay green characteristic. For the most accurate reading, have seed moisture tested. – Tracy Sayler

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