ST. PAUL -- Controlling weeds can be a battle for corn and soybean farmers, but organic producers face special challenges and higher risks.
Weed management for organic crop production falls into two categories: cultural weed control and mechanical weed control. Cultural methods include diversifying rotations, delayed planting, changing planting rates, timing nutrient applications and using cover crops. These are the first line of defense against weeds.
Diversifying a rotation is the strongest tool against weeds. You can vary crops by different planting dates. For example, wheat is planted and harvested earlier than corn or soybeans, creating excellent weed control options that break up weed life cycles. Growing a perennial crop in rotation with row crops can also prevent weeds from adapting to the planting system. Competitive perennial crops such as alfalfa are especially effective in reducing seed banks of annual grasses and broadleaf weeds, and in suppressing perennial weeds like thistle.
For both organic corn and soybeans, delayed planting will balance yield gains from improved weed control against yield losses from later planting. Rotary hoeing or harrowing and the first row cultivation are the most important operations to reduce losses to weeds.
Some guidelines for mechanical weed control:
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- Go as shallow as possible.
- Till as infrequently as possible. Every tillage pass reduces soil moisture.
- Tailor tillage applications to a specific weed problem.
- Know the weed growth stages most vulnerable to control practices.
- Get weeds when they're small.
Timing is a key point with mechanical weed control. However, too many operations can damage soil structure and cause crop injury and lower yields. You also risk unnecessary time, labor and expense with redundant operations.
Try to strike a balance between controlling weeds and maximizing crop yields. Scouting for weeds is very important. This includes identifying your weeds and determining when weeds emerge. Operations should be timed to coincide with emergence of problem weeds.
For details on weed control in organic corn, soybeans and other organic crops, see a new web-based guide at www.organicriskmanagement.umn.edu . It's titled "Risk Management Guide for Organic Producers," and will be helpful for organic producers and those contemplating a transition to organic. It has 14 chapters by 22 different authors covering a wide range of production topics.
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Jeffrey Gunsolus is a weed scientist with University of Minnesota Extension.
