Allelopathic Cover Crops, Living Mulches, and Weed Thresholds
for Managing Weeds in Vegetables

As the weed science editor for MBCN, I would like to tell you about some of my research being conducted with Drs. Harry Bottenberg, Catherine Eastman, Darin Eastburn, Vasey Mwaja, and Leslie Weston here in Illinois. As you can probably tell by the long list of names these have been multidisciplinary and long term studies. Our goal has been to reduce pesticides, especially herbicide use, and to determine the effects of cropping system changes on vegetable crop pests.

One area we have been studying is the use of reduced tillage-rye cover crop systems for vegetable production. Reduced tillage reduces soil erosion and avoids bringing new weed seed to the upper soil where they can germinate. This system also uses fall-planted cereal rye. Cereal rye releases allelochemicals which inhibit weed seed germination and growth. Rye also produces substantial biomass which can also inhibit weeds. The rye is killed in the spring either with glyphosate or by mowing. We found the rye mulch controlled most weed species similar to the control resulting from conventional tillage along with preemergent herbicides. The most severe weed problem in the rye mulch was early-season dandelion infestation from wind-blown seed. Supplemental cultivation or a post-emergence herbicide could control the dandelion and was necessary for acceptable late-season weed control.

Rye had a variable impact on insect and plant pathogen populations. In cabbage populations of diamondback moth, imported cabbageworm, cabbage looper, and aphids were lower on cabbage grown in rye mulch than grown in bare ground. Part of the explanation may be due to the smaller size of cabbage plants in the rye. In snapbean, populations of potato leafhoppers and thrips were sometimes reduced. Levels of white mold in snapbeans were lower in the rye mulched plots.

The biggest problem with the rye mulch system is that growth and yield of pumpkins, cabbage, and snapbean are variable, often less than the conventional tillage. The results are likely due to soil compaction and/or nitrogen tie-up during the rye degradation. We are currently determining if strip tillage in the crop row with rye mulch between rows could overcome problems with crop growth and yield.

Another study evaluated the use of living mulches, including perennial ryegrass, red clover, white clover, and canola on growth and yield of okra and peppers. These living mulches were planted between rows of the vegetable crops and a clean-tilled strip maintained near the crop. Living mulches provided excellent control of weeds (velvetleaf, pigweed, purslane, foxtails, and crabgrass) both between and within crop rows. Interestingly the living mulch planted just prior to the vegetables provided better weed suppression that a perennial (1 year old living mulch). We were also surprised that the living mulch suppressed weeds within the clean tilled crop rows. Yields of both the peppers and okra were reduced by the living mulch. Mowing the living mulch did not completely eliminate the yield reduction. White clover caused the least yield reduction. We feel the yield reductions were primarily caused by competition for moisture. The use of drip irrigation may overcome the yield reduction.

The most recent project we have started is research to determine threshold levels for the management of redroot pigweed and large crabgrass in snapbeans. We are also interested on the impact of sub-threshold levels of weeds on insect pests. Last summer we conducted preliminary research which suggested grasses were more competitive that redroot pigweed and grasses also affected leafhoppers in snapbean. We will be conducting this study over the next two summers and I hope to report the results in future issues of the MBCN.

- John Masiunas, University of Illinois - Urbana

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