Weed management strategies may ease transition to organic farming


LAWRENCE, Kan. — As the demand for organic produce continues to grow, more farmers are converting their cropland to organic production.

Certified organic cropland acres in the U.S. increased from about 850,000 in 1997 to 1.7 million in 2005.


One of the biggest challenges facing organic agriculturists is how to manage weeds without using prohibited fertilizers and pesticides — especially during the initial transition years.

Little research has been done to identify the most effective ways to control weeds after ceasing traditional weed management practices and converting to non chemical methods.

One newly published study examines how tillage systems and field-cover crops affect weed levels and seed densities during the three-year transition period required before cropland and the products grown on it can be certified as organic.


The study’s results are featured in the article Effects of Initial Seed-Bank Density on Weed Seedling Emergence during the Transition to an Organic Feed-Grain Crop Rotation by Richard G. Smith, Randa Jabbour, Andrew G. Hulting, Mary E. Barbercheck and David A. Mortensen in the September-October 2009 issue of Weed Science, published by the Weed Science Society of America.

Because weeds drop seeds into the soil — essentially laying the groundwork for weeds in future years — farmers can gain an advantage by reducing the density of these seed banks through tillage, crop rotation and the use of cover crops.

For the study, researchers planted rye and timothy as cover crops in the first year of transition and examined how they affected the seedling densities of three common annual weeds: lambsquarters, velvetleaf and foxtail.

Weed seeds were applied at the start of the study in low, medium and high densities, with seedling densities assessed in each subsequent year.

Responded differently

The researchers found that the three weed species responded differently to cover-crop management, with densities varying from year to year — possibly, they say, a result of environmental factors.

The researchers also found that crop changes can rapidly affect the composition and abundance of the weed seed bank, noting: “Individual crop rotation phases in some years exerted strong influences on weed seedling densities in this experiment.”

Unexpected result

Perhaps the most unexpected result of the study was the effect of tillage on weed density. Tillage can decrease the quality and organic matter of soil, so few organic farmers prefer it.

And while weeds typically grow more densely when tillage is reduced, this study found the opposite: “That reduced tillage did not result in substantially greater summer annual seedling densities in this study was surprising.”

The most effective weed management method in the study was the full-tillage strategy with timothy as a cover crop.

“Some organic transition management systems are more consistently weed-suppressive than others are,” concluded the researchers.

Little relation

But overall, the study found little consistent relation between the initial weed seed density and the seedling densities at the end of the three-year transition period.

While weeds may be more prevalent during the switch to organic agriculture, the researchers say this does not mean weeds will flourish in the future, giving hope to farmers considering the move to organic agriculture.

“Initial failures in weed management…may not necessarily doom growers to persistent and intractable weed management problems,” they wrote.

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