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Preharvest Glyphosate: The ’07 Experience

Saturday, December 1, 2007
filed under: Harvest/Storage

Tim DeKrey was motivated by curiosity when he hired Helm Flying Services of Harvey, N.D., to spray a glyphosate — Monsanto’s RT3™ — on about 85 acres of sunflower on September 12. When he harvested those ’flowers three weeks later, on October 3, his curiosity was pleasantly rewarded.

DeKrey was among a number of Upper Midwest producers who applied a preharvest treatment of RT3, Roundup WeatherMAX®, Roundup PowerMAX™ or Roundup Original Max® to sunflower this fall. They were able to do so because of new supplemental labeling for those Monsanto products for preharvest weed control in sunflower and safflower.

For sunflower, the labels advise treatment “when the backsides of sunflower heads are yellow and bracts are turning brown and seed moisture content is less than 35%.” The maximum labeled rate for sunflower is 22 fluid ounces. A minimum of seven days should transpire between treatment and either harvest or livestock feeding.

The product labels clearly state that the preharvest application is intended to provide weed control in physiologically mature sunflower. Knowing how late in the year this crop typically is harvested, Monsanto understood that postharvest applications of glyphosate products were not viable for many producers — especially in the Northern Plains states. Hence the motivation to register preharvest treatment options.

And they do indeed provide weed control when applied according to the label. DeKrey, who farms near Steele, N.D., says that while it took longer than normal for the glyphosate to work (due to the cool seasonal temperatures), he was generally pleased with the level of perennial weed control. Canada thistle and quackgrass were two key targets. “We could see the weeds when harvesting,” he says. “Weed kill wasn’t the best on those below the canopy; but overall I was satisfied.”

One unexpected benefit for DeKrey came to light during harvest. Like other sunflower growers, he’s careful to wash down his combine on a regular basis to avoid the threat of fires from the accumulation of “fines” on the engine manifold from the threshed heads. This year, he noticed a sharp decline in the volume of fines on his glyphosate-treated acreage. He didn’t need to clean his combine nearly as often; nor did he have any fire issues.

Equally as important as weed control — and more so for some sunflower producers — was a parallel benefit of the preharvest glyphosate treatment: plant desiccation.

While there is no mention of sunflower desiccation on the product labels, spraying glyphosate on this non-Roundup Ready crop will obviously have an effect similar to that on weeds. And that’s a good thing — as long as the sunflower plants are physiologically mature.

Truth be told, desiccation was the primary motivation for a number of growers who had their sunflower fields treated with a preharvest glyphosate this fall. Bruce Due, Breckenridge, Minn.-based agronomist with Mycogen Seeds, says the practice conveyed several benefits along with weed control:

• Cutting losses to blackbirds — Though it commonly took from two to three weeks for the glyphosate to bring seed moistures down to a satisfactory harvest level, treated fields often were harvested a week to two earlier than nearby non-treated ones. For growers in areas plagued by blackbird depredation, that’s a big advantage.

• Reducing drying costs — Much of North Dakota and northwestern Minnesota experienced a cool, wet fall. Given the high price of propane, having seed moistures drop steadily out in the field translated into big savings on prospective drying costs.

• Avoiding late-season inclement weather — Blackbirds aside, the longer ’flowers stand out in the field, being rained upon, jostled against each other by the wind and subject to lodging, the higher the potential for seed loss.

Glyphosate uptake by the sunflower plants would have transpired faster had latter-September and October temperatures in the region been warmer, Due points out. He believes the length of time it took for the Roundup products to dry down treated sunflower fields — commonly two and a half to three weeks — was the main concern growers had with this management practice in 2007.

Still, the seed company agronomist also believes it paid off for the majority who tried it. Due is familiar with one northeastern North Dakota sunflower grower who, more than two weeks after treating with Roundup, saw little visible impact. He’d been combining in 20%-moisture sunflower that had not yet received a killing frost. But within three or four days of the frost’s arrival, the ’flowers fell to 12%. “He was thinking, ‘I’m not sure [the glyphosate] did anything,’ “ Due recounts. “But then they moved to a field they hadn’t sprayed — and those ’flowers were still at 18 or 19% moisture. So there was a six- to eight-point moisture difference — though it still took a frost to really get them to the point where they were harvestable.”

The region’s 2007 fall environment had a lot to do with the rate at which the glyphosate was absorbed and translocated within sunflower plants, Due reiterates. “We had a lot of overcast skies; and while temperatures were ‘OK,’ it wasn’t real warm.” While understanding the labels list 35% as the maximum seed moisture for preharvest application, Due expects that as growers become more experienced with the practice, “they’ll figure out they need to be spraying a little earlier” than was the case this year. “A week’s time, with warmer temperatures, will make quite a difference in how those plants react to the herbicide.”

Due is aware of some northeastern North Dakota growers who believe they may have hurt their sunflower yields by using a preharvest Roundup treatment. Not so, in his opinion. Excessive rainfall from late August through September — seven to 12 inches, in some instances — triggered serious disease pressure in numerous fields and subsequent poor yields. “Bottom line: if the sunflower plants are physiologically mature, you can’t hurt them (in terms of seed yield and oil content) by spraying glyphosate,” Due emphasizes.

Tanya Hawkins is an agronomist with Helm Flying Service. This fall, the Harvey, N.D., aerial applicator treated approximately 4,000 acres of sunflower preharvest with RT3.

Hawkins estimates overall grower satisfaction with the new management practice at more than 80%. The main concern she heard from growers “was the amount of time it took for the sunflower to dry down. But my reaction was, regardless of what crop it was this year, the glyphosate took a long time to work.” Cool weather was the reason. “We had a lot of 40-degree mornings,” Hawkins points out. While she would normally expect the glyphosate to achieve its full effect within 10 to 14 days, this fall it typically took about three weeks “just because of the (cool) temperatures.”

Hawkins and her colleagues at Helm encouraged growers to go with the maximum labeled rate of RT3: 22 ounces. “And we tried to keep water volumes up there at three to five gallons” for optimum coverage.

The biggest challenge with preharvest glyphosate programs this year, says Hawkins, “was to make sure we had the timing right with the growers — and to fully explain to them that it would take longer to dry down (compared to a product like paraquat).” Looking toward 2008, she urges growers to plan well ahead in order to get the product applied as soon as possible once the seeds hit 35% moisture.

For his part, Bruce Due would like to see the timing eventually edge closer to 40% seed moisture — though he knows the current labels list 35% as the earliest level. He points to recent North Dakota State University research showing that even when applied with seed moistures above 40%, glyphosate did not hurt seed yield or oil content. Due certainly doesn’t recommend treating at those higher moisture levels. “But if we’re talking 40% moisture versus 35, that’s splitting hairs,” he states. “If they’re physiologically mature, you can’t hurt them.”

To view research reports on three NDSU glyphosate/desiccation studies — including one on confection sunflower — go to the Research Forum Papers page on the National Sunflower Association web site. Enter the keyword ‘Desiccation.’ These three studies also were discussed in an article in the August/September issue of The Sunflower, which is available on the NSA website's archive. Additionally, the magazine’s March/April issue addressed the Monsanto glyphosate products’ new preharvest labels. — Don Lilleboe

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