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Weed seed control technology will be on full display at this year's Farm Progress Show Aug. 30-Sept. 1. Learn the latest strategies from Iowa State University Extension and Outreach. When you hear weed specialists toss around the terms large-seeded weeds and small-seeded weeds, you might wonder which

Weed Seed Destructor and Other Control Methods to Be Showcased

AMES, Iowa – Controlling weeds in farm fields is an annual challenge – especially with more weeds becoming resistant to herbicides.

Fortunately, producers have a wide range of options to counter weeds, including some creative ways that may not have been employed in the past.

At this year’s Farm Progress Show, Aug. 30-Sept. 1 in Boone, Iowa State University Extension and Outreach will showcase one of the more innovative, and practical methods of controlling weeds: a weed seed destructor.

Fitted to a combine, the weed seed destructor does what it’s name implies. It pulverizes and destroys seeds so that they cannot germinate.

The weed seed destructor (by Redekop) will be attached to the back of a John Deere 680 combine and will be available for viewing outside the ISU Extension and Outreach tent. While the machine will not be operating during the show, visitors can see it in operation on a computer screen, and they can ask questions of weed science experts.

“We want to give the public a chance to see and ask about this innovative form of weed control technology,” said Prashant Jha, professor and extension weed specialist at Iowa State. “Farmers in central Iowa and in Harrison County are already using this technology and we expect more will do so in the coming years.”

Other methods of weed control will also be featured, including videos of chaff lining, a method that guides the harvested chaff into narrow bands as it flows out the back of the combine at harvest, which reduces the spread of weed seeds by more than 95% across the fields and contains weed seeds in smaller spaces.

The harvester or combine is modified with a baffle that separates the chaff (containing the majority of weed seeds) from the straw. The chaff is directed into narrow central bands using a chute at the rear of the combine.

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Weed seeds in the chaff are subjected to decay, and burial of small-seeded weed species such as waterhemp in the chaff will potentially result in reduced emergence in the subsequent growing season. High application rates of herbicides or shielded sprayers can be used to selectively control emerged weeds in those narrow bands in the field.

The weed control display will also allow visitors the chance to test their knowledge of weed specimens found in the Midwest. Sixteen different species will be available for visitors to identify.

Visitors will also have the chance to learn more about waterhemp, and how it can be suppressed using cereal rye as a cover crop. Photos and sample trays will show the results of using no rye, rye terminated at 4-6 inches tall, and rye terminated close to heading.

“We’re going to be showing the potential for biomass (cover crops) to suppress weeds like waterhemp, and how the results vary based on the height of the cover crop,” said Jha.

Cereal rye has the best potential to suppress weeds because it accumulates more biomass than other cover crop species. A study that was done for the Farm Progress Show shows an incremental decrease in waterhemp based on the density of rye.

Field studies indicate cereal rye biomass of 4,500 to 5,000 pounds per acre at termination can significantly suppress waterhemp emergence in soybeans, and reduce the size and density of waterhemp at the time of exposure to postemergence herbicides.

Additionally, producers can view a map of where herbicide resistance has been documented in Iowa based on the recent survey, and ask questions to Jha and other specialists about their own experience with herbicide-resistant weeds.

Jha will be joined at the show by ISU Extension and Outreach field agronomists Angie Rieck-Hintz, Meaghan Anderson, Gentry Sorenson and Mike Witt and several weed science graduate students.

Heed Weed Seed Size

When you hear weed specialists toss around the terms “large-seeded weeds” and “small-seeded weeds,” you might wonder which are which and what difference it makes? Good questions.

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“Weed seed size comes into play in particular with no-till and strip-till,” points out University of Wisconsin weed scientist Chris Boerboom. “With those systems, large-size weed seeds don’t get incorporated and therefore don’t germinate well. For example, velvetleaf, a medium- to larger-seeded weed, becomes much less of a problem in no-till farming.”

In contrast, Boerboom notes, small-seeded weeds and annual grasses, if not managed, can become more prevalent in no-till and strip-till. That’s because they’re able to germinate near the surface.

“When the seeds of large-seeded weeds lie on the surface, as they do with no-till, they make only one flush and are easier to control with a single herbicide application,” explains Jeff Stachler, weed scientist at Ohio State University. “Also, since they are on the surface, they are more likely to be eaten by rodents, insects and birds. Over time, because of predation, good weed control, and lack of incorporation, the number of large-seeded weeds declines in no-till fields.”

But small-size weed seeds in no-till fields usually get incorporated by rainfall, tire tracks and other natural soil disturbances just enough to germinate, Stachler explains. Thus, they become dominant weeds.

When fields are tilled, large-seeded weed seed gets incorpo-rated. That allows them to germinate better and over a longer period of time, says Stachler. But small-seeded weeds, when incorporated by tillage, become less competitive because they’re pushed below their optimum germination level, he points out.

Germination and growth patterns of medium-seeded types, says Stachler, fall between the large and small.

In general, he adds, large-seeded weeds are more competitive on a per-plant basis than small- or medium-seeded weeds. “The larger seeds tend to germinate faster because of more energy, and they tend to be larger weeds,” he notes. “It may take four to eight times as many small-seeded weeds to be as competitive as a single large-seeded weed.

“On the other hand, small-seeded weeds like pigweeds and lambsquarters tend to produce more seeds per plant and can spread faster,” Stachler says. “That’s the reason they can take over so quickly in no-till if not properly controlled.”

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Herbicide-wise, soil-applied products such as dinitroanilines (Prowl, trifluralins), acetamides (Harness, Dual II Magnum, Lasso, etc.) and pigment inhibitors (Command, Balance, Callisto) are the most effective for small-seeded weeds, says Stachler. Except for the few products that need incorpo-ration, all can be used with no-till.

Pre-emergence PPO inhibitors (Authority, Valor, etc.) control mostly small- and medium-seeded weeds. “To control large-seeded weeds with pre-emergence products, go with triazines or ALS-inhibitors, assuming there is not a resistance problem,” Stachler advises. “They also get medium- and small-seeded weeds.”

As a rule, large-seeded weeds are more difficult to control with pre-emergence herbicides than are small-seeded weeds, notes Wisconsin’s Boerboom. “That’s partly because they emerge from a greater depth and don’t take up as much herbicide as do those weeds that germinate near the surface. We can usually control large-seeded weeds more effectively with post products.”

Boerboom points out that most pre-emergence grass killers, especially for corn, also are touted to help control small-seeded broadleaf weeds. But, he says it’s important to keep in mind that their control can vary depending on the broad-leaf. When those grass herbicides are premixed with atrazine, control is good and consistent for most small-seeded broadleaves.

Weeds And Their Seed Sizes

The following list of broadleaf and grass weeds, found in the Midwest, comes from Ohio State University:

Large-seeded broadleaf weeds: common cocklebur, giant ragweed, the morningglories, common sunflower and burcucumber.

Medium-seeded broadleaf weeds: common ragweed, velvetleaf, jimsonweed, smartweed, Canada thistle, kochia and common dandelion.

Small-seeded broadleaf weeds: pigweeds (including waterhemp), lambsquarters, eastern black nightshade, marestail, field pennycress, chickweed, purple deadnettle, wild mustard and shepherd’s-purse.

Large-seeded grass weeds: shattercane, johnsongrass, field sandbur, woolly cupgrass, downy brome and wild oats.

Medium-seeded grass weeds: foxtails, barnyardgrass and wild proso millet.

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