Matt Liebman from Iowa State (left) visits with an audience member about weed control.

January 9, 2004: Want to learn how to manage weeds naturally on your diversified farm? Here’s what the experts at the 2003 National Biodynamic Conference in Ames, Iowa, had to say about effective methods to help you “just say no” to herbicides.

Combination of strategies helps control weeds

Weeds are often the biggest problem that farmers face with biodynamic and organic farming systems. Unfortunately, heavy reliance on cultivation can be harmful to soil and crops, and hand weeding is labor intensive. So what’s the answer?

“To manage weeds, you have to understand the life cycle of a weed species,” said Matt Liebman, an agronomist at Iowa State University. “Focusing on soil-crop-weed interactions will aid the development of ecologically sound farming systems.”

Crop rotations play a big role in weed management, he said. “Some crops are much more effective in suppressing weeds, so you need to tune in to where weed seeds are being added in a rotation. In a rotation that includes corn, soybeans, triticale and alfalfa, soybeans are the weak link when it comes to weed control.” For vegetable producers, crops such as carrots and onions are often less competitive against weeds. These differences and characteristics illustrate the point that the starting phase in your crop rotation can be just as important as the crops you include in the rotation, Liebman said.

“In the first example, if you start the rotation with soybeans, you’ll have more weed pressure later on. But if you start the same rotation with triticale, you’ll have less weed pressure.”

Because weeds can’t get up and move, their fate depends on the soil conditions surrounding them, he said. “Research has compared soil amended with composted manure and green manure versus a rotation with herbicides. Where the soil has been built up with organic matter, weeds are less of a problem.” The amended soils also show less variance in weed pressure compared to unamended soils, Liebman said. “Not only do you get better weed control in fields with healthier soils, but you get more consistent control, too.”

Green manures can provide another weed control benefit, he said. “Researchers have studied the effects of synthetic nitrogen and clover green manure on bean crops and weed performance. The results showed that the biomass of weeds was a lot less with green manure than synthetic nitrogen. This means thousands fewer weed seeds were produced per meter.”

Understanding seed predation

Fewer weed seeds can also be achieved by encouraging seed predators like ground beetles, field crickets, field mice and birds, Liebman said. “Research has shown that these predators are taking very large quantities of seeds from fields at certain times of the year.”

Averaged over several sampling dates during the growing season, 18 percent and 22 percent of velvetleaf and giant foxtail seed, respectively, were removed per day by predators, according to the research cited by Liebman. “This can strongly regulate weed populations over the course of 20 years,” he said.
Perennials such as alfalfa and red clover tend to have higher levels of seed predation than wide-row crops like corn, he said. “Habitat is the key to increasing the number of seed predators. Rodents remove about six times as much weed seed as insects, and they like the canopies that form in alfalfa and small grains.”

Allelopathic weed suppression

In weed seed tests involving red clover extract, the clover’s toxicity effects led to fewer weed seeds germinating, Liebman said, and of the ones that did germinate, their radicles were much smaller. Researchers have also learned that larger seeds have a relatively strong susceptibility to the allelopathy from red clover, he said. This means you can get good weed suppression for three to four weeks in large-seeded crops that follow red clover, but, Leibman warned, “there can be damage to small-seeded crops, unless you wait at least three weeks before planting.”

Other plants offer similar results, he said. “Sorghum is very potent. If you incorporate it green, it will virtually obliterate weeds in your fields. Cereal rye also works, but be careful with this, because it can sequester a lot of nitrogen.”

Little things mean a lot

Another important weed strategy, Liebman said, is to delay fertilizing your fields until your crops are ready to take up the nutrients. “Weeds will thrive on a big dose of fertilizer,” he said. “That’s why a slow release of nitrogen, for example, is much more effective than a big pulse of fertilizer.”

The underlying message, Liebman said, is that ecologically-based agriculture uses a lot of little things that add up to success or failure with weed management. “While these tools are individually weak, they are powerful together and can provide weed control that’s as good as what you get from synthetic herbicides.”