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Target Glyphosate at Seed Moisture Under 35%

Saturday, September 15, 2007
filed under: Harvest/Storage

This is the first growing season that glyphosate has been approved by the EPA for late season weed control in sunflower.

The EPA approved a supplemental label for Monsanto’s WeatherMax® as well as Monsanto’s RT3™ and OriginalMAX® formulations (check various glyphosate formulations for similar supplemental labels). The glyphosate label is a good addition to existing products labeled as preharvest desiccants in sunflower, the restricted use pesticide paraquat (Gramoxone Max/Inteon) and sodium Chlorate (Drexel Defol). All products are labeled for both oil and confection types. All labels indicate application when seed is 35% seed moisture or below.

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. With today’s high values, getting the crop harvested several weeks early can preserve yields. Late season crop damage is well recognized when strong winds can lodge plants or rub seed from heads. Blackbird damage can be reduced and desiccation may also slow down head diseases such as Sclerotinia. Shattering and lodging potential can be reduced, especially if stalk strength is compromised by stalk insects.

Sometimes the market will pay a premium for early delivered seed. And, it is important to harvest sunflower before corn. Early dry down can minimize artificial drying costs. September and first half October harvesting can often allow the use of natural air drying to lower seed moisture in storage, rather than drying with heat. Air temperatures are higher and humidity tends to be lower during this time frame, compared to late October and November harvesting.

Research supported by the NSA indicated that glyphosate applied at 30 to 35% seed

moisture permits harvest about 10 days earlier than natural drydown. See complete research results online at – click on ‘Sunflower Magazine’ then ‘View Archives’ and ‘Harvest/Storage.’

Warm sunny days following a desiccant application are needed to give the best results.

Keep in mind that glyphosate is slow acting (especially in cool weather) and research indicates that there is in most cases a margin of error if application occurs earlier than 35% seed moisture without sacrifice to yield, oil content or test weight.

Still, you don’t want to apply too early.

“You want to make sure the seed is mature, and that you don’t prevent the completion of seed filling,” says Kirk Howatt, weed scientist at North Dakota State University. “Some of the research work indicated that even applying glyphosate at 50% seed moisture did not statistically influence yield and oil content. And visually, 50% isn’t that far from 35%. However, even at 50% seed moisture, research indicated that some environments can get a yield reduction of about 10%. So we want to make sure we’re close to 35% seed moisture when applying glyphosate.”

Visual appearance of physiologically maturity (about R9) may vary by sunflower hybrid, particularly those with the stay green characteristic. However, Howatt says 35% seed moisture correlates closely with when the florets or flower parts on the face of the sunflower head brush off easily. The backsides of sunflowers heads will be yellow, with bracts turning brown. “Even with stay green hybrids that I have worked with, when seed moisture gets to about 35%, there is some brown coming into the bracts, even if stems are still green,” Howatt observes.

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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