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Herbicides step aside

Interest in biological-based weed management strategies has never been stronger. Certainly the booming North American organic farming sector has much to do with this interest, but it is also due to concerns in conventional farming circles about herbicide resistance and its environmental impact.

September 17, 2009
By Treena Hein

 For years, combines have been credited as being most efficient at spreading weed seed, but newer designs and modifications are attempting now to gather weed seed before it is spread.  

Interest in biological-based weed management strategies has never been stronger. Certainly the booming North American organic farming sector has much to do with this interest, but it is also due to concerns in conventional farming circles about herbicide resistance and its environmental impact.

Instead of focusing on the use of herbicides to wipe out weeds “in one blow,” crop scientists are exploring the use of several ways of controlling weeds without the use of chemicals. “It’s a strategy of using a multitude of small hammers instead of one big one,” says Dr. Adam Davis, a weed ecologist with the United States Department of Agriculture’s Agricultural Research Service in Urbana, Illinois. 

 Dr. Clarence Swanton, known for his work on the critical weed free period in crops, defends leaving behind weed seeds as part of the food chain in healthy soils.



Biological control
Several of these “small hammers” involve enhancing the activity of soil microbes and invertebrates. Dr. Clarence Swanton of the Plant Agriculture Department at University of Guelph, says “Predation of seeds by soil microbes and invertebrates is the most significant method of biological control of weeds.”

Populations of invertebrates that feed on seeds can be effectively augmented with management strategies, says Swanton. “This is something a farmer can manipulate. It’s a matter of healthy soils. Leaving residue and using a good crop rotation are very important.”

While Swanton acknowledges that there is value in the argument that leaving residue on fields might boost incidence of some fungal diseases, he says “The benefits of leaving residue on the soil to support invertebrate and microbe populations outweigh the disadvantages.”

In addition, Swanton champions the idea that weed seeds serve an important ecological role. “Some of my current research aims to support the idea that weed seeds are an important source of energy for the food web,” he says.

Predation of weed seed can also be enhanced through choice of tillage method, says Swanton. “Shallow till or no-till leaves seed on or near the soil surface,” he says. “In this scenario, predation is much higher from microbes and invertebrates. In addition, the weed seeds are exposed to increased environmental stress compared to seeds that are deeper in the soil. They are left to face changes in temperature and moisture.”

Swanton says some studies have pointed to night tillage as having value in reducing weed seed germination, “But it didn’t demonstrate a large enough impact to move forward in terms of supporting it as standard practice.”

Davis and his colleagues have been studying soil microbes that initiate seed decay. “It’s the ones that initiate decay that are most important,” he says. “Once decay has started, there are many species of microbes breaking down the seed. We are making good progress in determining which ones initiate decay, and then we can work on recommending actions which maximize their activity. This might involve applying them to the soil and/or figuring out which types of soil best support their growth.”

The other main biological strategy to control weed seed production is to reduce weed growth by making competition with crops much more difficult. This method is all about the crop canopy, which Swanton says is the second most important biological weed seed control factor after predation. “It depends on the soil type and crop you’re working with,” notes Swanton, “but anything you can do to increase the rate and degree of canopy development will decrease weed seed production. This might include increasing seeding density and decreasing row width.
“The more uniform your cop stand,” adds Swanton, “the less reliance you’ll have on chemistry.”
Davis’ colleague Marty Williams, also of USDA-ARS, has demonstrated in field tests that sweet corn cultivars that grow tight canopies, that is, those that produce wide thick leaves and have other attributes that block sunlight to weeds, resulted in significantly reduced weed seed production.

Davis says “There is a lot of interest among commercial sweet corn companies in which of their hybrids are most weed-suppressive. This will help them make recommendations to low-external-input or organic farmers.”

Mechanical control
Another “small hammer” being studied as a way to control weeds without herbicides involves the use of machinery that removes or destroys weed seed.

“Early combine harvesters gave farmers the option of collecting and destroying weed seed, but the modern ones just spit them out along with the chaff,” notes Davis. “The combine is the most efficient weed seed dispersal device that’s ever been invented, and so what we need is a harvesting device that can destroy rather than disperse weed seed.” He adds that some organic growers in the US have already created modified combines that collect weed seeds.

Among others, Davis’s team has established that physical injury is an effective way to destroy persistent weed seeds. “They are vulnerable to physical injury (abrasion, cutting),” Davis says. “We have discovered this is because their chemical defences aren’t very strong, not strong enough to protect them from opportunistic pathogens like fungi and bacteria that enter the damaged seed coat.”  

Davis is doing preliminary work with colleagues in the University of Illinois’ mechanical engineering department to eventually create a combine unit that would cause damage to weed seed in soybeans and corn. “Right now, we are in the pre-design stage,” Davis says. “We are looking at what types of damage we can inflict on weed seeds, how much energy such a unit would require, how much it would weigh, etc.”