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The Preharvest Glyphosate Option

Monday, September 1, 2008
filed under: Optimizing Plant Development/Yields

‘We just want to get more of what you grew in the field into the bin.’

In a nutshell, says Kirk Howatt, that’s the primary reason for using a crop desiccant on sunflower fields. It’s also the main reason why a number of sunflower producers applied (or had an aerial applicator apply) a preharvest treatment of a broadspectrum herbicide — glyphosate — in 2007 and why many more are expected to do so in 2008.

Supplemental labeling for certain Monsanto glyphosate products (RT3™, Roundup WeatherMAX®, Roundup PowerMAX™ and Roundup Original Max®) paved the way for growers to use those products last year for preharvest weed control in mature sunflower and safflower. While the label states “weed control,” the physiological fact of the matter is that glyphosate works the same way on a sunflower plant as it does on a weed. So the expected result is a faster plant drydown and earlier harvest.

When treating sunflower, the labels advise product application “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, with a minimum interval of seven days between treatment and either harvest or livestock feeding.

While sunflower growers in any state can apply a preharvest glyphosate treatment, climate dictates that most of this practice actually occurs in the Dakotas and Minnesota, as opposed to the High Plains. “Cooler fall temperatures and wet conditions often slow the natural desiccation process” in northern areas, notes Howatt, who is a North Dakota State University weed scientist. “Poor weather can also cause a decline of crop yield and quality. Last year, in some of our research studies, we got to a point where drydown should have been happening quickly; but we entered a prolonged period of misty weather.” The moisture total was not substantial, “but the crop was not drying at all during that time.”

The point is that the longer a sunflower crop stands unharvested, the more exposed it is to blackbirds, disease, lodging and other yield- and/or quality-impacting threats.

Late-season weed growth is not an idle threat, either. Canada thistle provides a prime example. “They’re setting seed late in the season, and they’re still just as green as the crop,” Howatt notes. “If we come in with desiccation treatments — especially glyphosate, which is registered for weed management — we kill that undergrowth.

“You’re not going to completely prevent seed production and germination,” the NDSU weed scientist continues. “But if you can cut off the plant’s ability to continue producing seed into the fall, you limit the amount of seed going back into the soil — seed that you’ll have to deal with in the future.” Its usefulness in helping manage weed populations in succeeding years “is certainly one of the reasons why preharvest glyphosate has become popular” in general, Howatt states.

As noted, the Monsanto products’ supplemental labels clearly indicate that sunflower seed moisture content should be “less than 35%” when the products are applied preharvest. In the opinion of some sunflower agronomists, the products could be safely applied at 40% without hurting seed yield or quality. (However, should that occur, the grower assumes the risk, not the chemical company.)

In 2007 Howatt and other university weed scientists conducted a four-site study to look at the effect of desiccant application timing on seed yield and quality. The sites were located at Fargo and Minot, N.D., Brookings, S.D., and Hays, Kan. The evaluated herbicide treatments were paraquat (6 oz ai/ac), Kixor (safluenacil, 0.71 oz ai/ac), glyphosate (12 oz ai/ac) and Kixor + glyphosate. Kixor is a very promising BASF product that is not yet registered for preharvest use in sunflower.

The researchers applied these products at 30, 40 and 50% seed moisture, with their desiccation effect then evaluated across four seven-day intervals. Seed yield and quality were measured at 28 days after application.

They found that seed yield was not affected by treatment at even 50% moisture, though that higher level did result in reduced seed size and oil content. There were no significant differences in seed yield or quality between the 30 and 40% moisture treatments.

While he cannot recommend treating with glyphosate at moisture levels higher than 35%, Howatt does agree that the rest of the label’s timing instruction — i.e., “when the backsides of sunflower heads are yellow and bracts are turning brown” — leaves some room for interpretation. “How yellow is yellow? How much brown do you need? How hard do you have to brush those disk flower parts so that they come off?” he asks. “It’s kind of a moving target — especially as we get more of the ‘stay-green’ trait in sunflower hybrids.”

The NDSU weed scientist says he has noticed in his studies that “if we have brown tissue showing up on the shoulder of the bracts, that correlates with about 30-35% seed moisture — regardless of how green the rest of the plant appears.” While he hasn’t conducted any studies to document this observation, he suggests that those brown bract shoulders provide a “good cue” that the moisture percentage is low enough for spraying.

One issue that some northern growers encountered in 2007 was an excessively long sunflower drydown period following the glyphosate treatment. It was not uncommon to hear reports of two and a half to three weeks being the time it took for fields to dry down sufficiently following application.

Weather conditions obviously played a big role in at least some of those situations, i.e., the cooler the temperature and the more rain that falls, the slower the herbicide is absorbed and translocated within the sunflower plant. In Howatt’s studies, however, the distinction between treated and untreated plots has been pretty obvious. “In our situations, by the time we got to a week and a half after application, the field has always looked brown — except for the untreated controls,” he states.

Not surprisingly, the benefits of desiccation typically are greater in the northern growing areas than in states like Kansas and Colorado. “As you move south, you have more warm, dry temperatures later in the season; so ‘natural desiccation’ works just fine,” Howatt points out. But autumn across North Dakota and northwestern Minnesota often provides a different scenario. “In our studies at Fargo and Minot, we definitely had a benefit from applying that desiccation treatment,” he states. “Even at 30% moisture in Minot, we were able to harvest a week to 10 days earlier [in the treated plots compared to untreated].” — Don Lilleboe

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