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Desiccation: The 'Sharpen' Option

Tuesday, August 30, 2011
filed under: Optimizing Plant Development/Yields

Sunflower growers had a new desiccation tool available as of 2010: Sharpen® (saflufenacil), part of the Kixor herbicide group from BASF. While Sharpen had received a full Section 3 label from EPA in 2009, the label came through too late for use during that growing season.

Though some Sharpen was used on sunflower in the High Plains (for timely planting of winter wheat on sunflower stubble), most of the applications were conducted up north in North Dakota, northwestern Minnesota and to a lesser extent, in South Dakota. Chris Wharam, BASF technical service representative for eastern North Dakota and western Minnesota, says the company estimates approximately 200,000 acres of sunflower received a Sharpen treatment last fall.

How successful was the product’s first year of usage on sunflower? It depends upon which grower you ask. Some were quite satisfied with its effectiveness; others certainly less so.

North Dakota State University weed scientist Kirk Howatt, who has been conducting research on Sharpen and other sunflower desiccants the past few years, says some producers he’s visited with indicated Sharpen performed below their expectations. But he also has visited with sunflower growers who were quite satisfied with their results.

Why didn’t Sharpen perform up to expectations in some cases? It could be due to any of several reasons — or a combination.

Weather obviously was one factor. “Last fall we didn’t get the typical killing frost across much of our [Northern Plains] geography, with the actual killing frost anywhere from four to five weeks later than the historical average,” Wharam recalls. “That’s probably part of it. Some areas of central and western North Dakota, for instance, received an atypical amount of spring moisture, thus delaying planting and overall crop progress; so desiccants, in turn, went on later than they normally would — and we got into some light frost situations and drizzly-type days” during the normal application period.

Water volumes and adjuvant rates likely also influenced Sharpen’s effectiveness in a number of cases. The product’s initial label allowed for a minimum of three gallons of water per acre, aerially applied. It also permitted the addition of methylated seed oil on a 1% volume/volume basis. Those minimum levels were too low, especially under less-than-ideal environmental conditions, for maximum desiccant efficiency. Wharam says some of the disappointing 2010 treatments also could have been due to the use of lower-than-recommended water and adjuvant rates.

In response, BASF has changed its recommendations for 2011. The water minimum of three gallons per acre with aerial application has been moved up to five gallons minimum. Regarding the adjuvant, “we still allow for the 1% volume/volume,” Wharam explains. “But if you’re going to be using a lower water application — anything under 12.5 gallons per acre — we want a minimum of 1.0 pint MSO per acre. That’s the bottom, and we feel it will provide a lot better efficacy.” It’s critical, he adds, to use a methylated seed oil-based adjuvant, not a non-methylated one.

As to timing of application, the same steadfast rule applies for Sharpen as with any sunflower desiccant: physiological maturity, meaning the seed moisture must be at or below 35%.

While oven drying samples is the most accurate way to determine seed moisture, it’s also time-consuming and inconvenient. So most growers tend to rely more on visual observations in the field. Bract color typically is the most useful visual indicator. When the tip of the bract is brown, seed moisture is between 50-40%, advises Kirk Howatt. When the neck of the bract has turned brown, the seed is at about 40% moisture. And when the bract shoulders are brown or dried, seed moisture is in the 30-35% range. (See diagram below.)

“It’s very critical that we get that timing right,” Chris Wharam states. “First, if it’s past physiological maturity, the plant is shutting down and not actively taking in the herbicide. It could be well on its way to drying down naturally. So you may be making an application that gives you a day or two advantage — but not much more.

“Second, if you’re far too early, number one, it would be off-label; and two, it logically would take much longer for that plant to dry down.”

Howatt believes this year’s changes in water volume and adjuvant rate will aid Sharpen effectiveness under a wider range of conditions. Still, given the generally late, wet season across much of his state this year, “we’re coming into a fall where it could be difficult to get any of these preharvest desiccants to work well — simply because of how cool it’s been and how much prolonged moisture we’ve had.” The key will be the date by which the crop is able to reach physiological maturity, he says.

“If the sunflower is reaching that physiological maturity stage and you’re not expecting a killing frost for two weeks or more, I’d strongly consider using [a desiccant] — especially if there’s plentiful soil moisture,” the NDSU weed scientist advises. “Wet soils are going to push the sunflower to ‘hang on’ longer” before starting to dry down naturally.”

— Don Lilleboe
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