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Western Integrated Pest Management Center

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Targeting Weed Seeds at Harvest

As herbicide-resistant weeds become more common across the country, researchers and growers are looking for other ways to control weeds.

In Colorado, they’re looking to techniques and technology developed in Australia, which has significant issues with herbicide-resistant weeds.

Known as harvest weed-seed control, these IPM-friendly methods are designed to destroy or remove weed seeds during harvest to prevent them from raining down onto the soil and replenishing the weed seed bank. In Colorado wheat, weed species of concern are winter annual grasses that share the grain’s growing cycle, like jointed goatgrass, feral rye and downy brome.

“In harvest weed-seed control, the objective is to prevent those seed-bank increases,” explained Colorado State University doctoral candidate Neeta Soni. “There are a number of ways to do it, and we’re investigating to see if they could be adopted in Colorado.”

One way to destroy the weeds seeds is by directing chaff during harvest into a cage mill – imagine a giant coffee grinder – and pulverizing the chaff and weed seeds into powder. That’s the idea behind an Australian innovation known as the Harrington Seed Destructor (and a new competitor called the Seed Terminator).

Another option is to use a piece of equipment called a chaff deck to gather chaff into mounded strips behind the harvester, capturing the weed seed in those mounds of chaff. In some places those chaff strips can be burned, and in others they’re left alone to allow the weed seeds to decay without entering the soil.

A third option is to use chaff carts and collect all the chaff and captured weed seeds for off-site destruction.

“Our research is focused on finding out if there is potential to use these methods in Colorado,” explained Soni, a graduate student of assistant professor Todd Gaines. “So what we needed to know is whether, at harvest, the majority of the winter annual grass seeds are retained in the upper wheat canopy, where they would be vulnerable to the seed destructor or other methods.”

If the weed seeds have already shattered and fallen to the soil, or if the weed seeds are below the cutting height of the combine, the methods would not be as effective.

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So the Weed Research Lab team measured and counted a lot of weeds.

“What we found is that the majority of seeds are still retained at harvest,” Soni said. “Downy brome is the same height as wheat, rye is taller and jointed goatgrass a little shorter, but growers could adjust their cut height to manage it.”

Soni then counted out 1,000 seeds of each weed species into a specified amount of chaff and drove to the University of Arkansas where they have a seed destructor set up on a test platform. She ran each bundle through the destructor. The pulverized material was dusted across beds of soil to see if any weed seeds germinated. Virtually none did.

“The seed destructor was 98 percent effective on downy brome and jointed goatgrass, and 99 percent effective on feral rye,” Soni said.

The Gaines lab hopes to conduct field trials with the equipment. They will also study if the strips of mounded chaff are effective in Colorado, or if the state’s dry and windy conditions enable weed seeds to survive and spread.

The seed destructor isn’t commercially available in the United States yet, but a number of researchers are testing versions in different regions and in different crops. The initial model was a tow-behind trailer, but both Australian manufacturers now offer the technology integrated into a combine harvester that retails between $120,000 and $160,000 Australian dollars.

Not every grower would need to buy one.

“It is very common that growers here have their harvesting done by a contractor,” Soni said, “so this could be an extra service they provide.”

But not at every harvest. Because whatever specific iteration of harvest weed-seed control Colorado growers may eventually adopt, it should be just one element of an integrated management strategy, Soni cautioned.

“Repeated use could lead to the selection of earlier-shattering weed seeds, or shorter weeds,” she said. “It has to be used in rotation with other integrated measures, including herbicides and crop rotation.”

In short, it should be part of an IPM program.

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Removal and Burial of Weed Seeds by Ants (Hymenoptera: Formicidae) From the Soil Surface of a Cropped Area in Western Australia

Although granivorous ants are known to collect weed seeds from cropping areas in Australia, the fate of these seeds has not been adequately investigated. Seeds of annual ryegrass (Lolium rigidum Gaud.) and wild radish (Raphanus raphanistrum L.) were placed around the nests of five native ant species (Iridomyrmex greensladei Shattuck, Rhytidoponera metallica Smith, Melophorus turneri Forel, Monomorium rothsteini Forel, and Pheidole hartmeyeri Forel) and tracked continuously over a 24-h period. Removal rates and seed preference of the ant species were evaluated. Ant nests were then excavated to determine the placement of seeds that were taken into each nest. Seed preference, seed removal efficiencies, activity, and seed storage all varied between the ant species. Annual ryegrass seed was collected by three species of ants and was removed from the soil surface more efficiently than wild radish seed. Most ant species stored seed below ground at a depth that is inhibitory to emergence, thereby potentially removing that portion of seed from the seed bank, but some seed was placed at germinable depths. Pheidole hartmeyeri was identified as a likely biological control agent for annual ryegrass seeds and wild radish, while Me. turneri and Mo. rothsteini have potential as biocontrol agents for annual ryegrass, but further research is needed.

Keywords: Lolium rigidum Gaud.; Raphanus raphanistrum L.; granivore; seed bank; weed seed predation.

National Summit on Strategies to Manage Herbicide-Resistant Weeds: Proceedings of a Workshop (2012)

Chapter:The Australian Experience of Managing Herbicide Resistance and Its Contrasts with the United States–Michael Walsh

Michael Walsh
University of Western Australia

Since the 1990s, the grain-growing regions of Australia have experienced a major problem with herbicide-resistant weeds. This occurred because of a unique combination of events. First, the highly productive pasture species annual ryegrass (Lolium rigidum) was present at high densities across vast areas devoted to livestock production. Then, from the 1970s, much of this pasture land dominated by Lolium was converted to wheat fields. With this dramatic shift from livestock to crop production, Lolium instantly was a problematic crop weed. The newly available acetyl coenzyme-A carboxylase (ACCase) herbicides were used widely and persistently. Herbicide use without diversity on huge populations of genetically variable Lolium across vast areas resulted in widespread resistance evolution. Cross pollination among resistant survivors ensured multiple herbicide resistance and the loss of efficacy of many herbicides. This shattered the illusion of herbicide invincibility and drove the search for integrated control strategies. Multiple resistance forced diversity in weed-control practices and the use of both herbicide and nonherbicide tools for sustainable weed control. For example, Lolium, like many crop weeds, relies on annual seed production and seed maturity is synchronized with crop maturity. Importantly, Lolium seed remains attached to the plant at the same height as the crop seed heads at grain harvest. Modern grain harvesters are efficient at sorting weed seed from crop grain, thus some 95 percent of Lolium seeds pass intact through the grain harvester to be returned to the crop field in the chaff fraction, perpetuating the ongoing weed problem. Therefore, grain harvest represents an excellent opportunity to target Lolium seed production. Toward this, Australian farmers have developed and adopted several “harvest weed seed control” (HWSC) systems that effectively target annual ryegrass and other weed seeds during the harvest operation. The HWSC systems currently used in Australia include chaff carts, direct harvest residue baling, narrow windrow burning, and the recently introduced Harrington Seed Destructor. The use of HWSC in addition to herbicidal weed control now

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has been proven to reduce Lolium infestations dramatically, clear evidence of the value of new weed-control tools in prolonging the life of herbicides.

When ryegrass pastures were converted to wheat production in Australia, ryegrass (Lolium) became the main weed.

Australian farmers have developed and adopted several “harvest weed seed control” (HWSC) systems that effectively target annual ryegrass and other weed seeds during the harvest operation. The use of HWSC in addition to herbicidal weed control now has been proven to reduce Lolium infestations dramatically.