Cropping Systems Influence Biological Weed Control

Boosting organic matter in soil creates a healthy environment for soil-dwelling bacteria that suppress weeds. That’s according to Agricultural Research Service scientists who for the first time have determined which cropping systems provide the best home for these beneficial bacteria.

ARS scientists report that to create ideal soil conditions, farmers should rotate their crops, reduce tillage and keep herbicide applications to a minimum.

The beneficial microbes, called deleterious rhizobacteria (DRB), live on--or within millimeters of--weed roots, and they feed on substances that ooze from those roots. As the name DRB implies, these bacteria are bad for weeds. Although they suppress weed growth, DRB normally don’t interfere with crop plant growth.

Robert J. Kremer, a microbiologist with the ARS Cropping Systems and Water Quality Research Unit in Columbia, Missouri, says many DRB keep weed seeds from germinating and produce toxins and excessive concentrations of plant growth hormones that put the life processes of weed seedlings in “overdrive.” Consequently, root cells may rupture and leak, replenishing the DRB diet. Once weakened by DRB, weeds are less able to compete with other plants, and they become more vulnerable to other control measures.

Kremer and graduate student Jianmei Li researched cultures of DRB associated with the most dominant species of weeds in six different cropping systems. In general, the highest numbers of weed-suppressing DRB came from fields where crops were rotated, chemicals and tillage were minimal, and organic materials like composts were added. DRB fared best in a corn-soybean-wheat-cover crop rotation. An organic strawberry system with compost was a close second.

The researchers believe the research information can be used to modify current cropping practices or design novel ones to promote development of DRB and take advantage of their natural weed-suppressive effects.

Scientific contact: Robert J. Kremer, ARS Cropping Systems and Water Quality Research Unit, University of Missouri, Columbia, MO., phone (573) 882-6408, fax (573) 884-5070, KremerR@missouri.edu.