Cover crop weed control

By James Hoorman, Hoorman Soil Health Services

A new study by Purdue University, Dr. Bill Johnson shows the benefits of cover crops for weed control.  Cover crops are known to control grasses and marestail, but Dr. Johnson wanted to document other weed control benefits.  For example, can you control weeds with out residual herbicides?  What other weeds do cover crops control? 

Dr. Johnson found residual herbicides may not be needed if crop residue is high enough to suppress marestail and annual grasses weeds.  For other broadleaf weeds, the cover crop residue was not enough to suppress broadleaf weeds. The cover crops plus residual herbicides were 100% effective at controlling weeds in his trials.  Cocklebur was a problem weed which required the full rate of herbicide plus the cover crop residue to control it.

Dr. Johnson discovered several other important weed facts about cover crops.  Planting green or planting soybeans into cereal rye later had a much higher success rate than terminating the cover crop early.  The Purdue research team found that cover crops terminated 2-3 weeks before planting had higher water hemp, giant ragweed, then grasses concentrations than cover crops terminated at planting or later. 

Cereal rye has an allelopathic effect or antagonistic effect on small germinating weed seed.  Most of the chemical responsible for this effect resides in the stems and leaves.  As the plant is terminated and starts to decompose, this allelopathic effect can last for several weeks.  Naturally, the bigger the cover crop when it is terminated, the more natural herbicide there is to suppress weeds.  Soybeans are not affected by this allelopathic effect and once soybeans canopy, they outcompete weeds for sunlight and nutrients.

Johnson noted that cereal rye was harder to terminate with glyphosate plus a residual like Acuron when the weather was cool.   Herbicide antagonism results when glyphosate and Acuron were used together, resulting in a slower rate of cereal rye termination, but Johnson felt the Acuron was needed for late residual weed control.  Johnson, a weed scientist and herbicide expert, however did not try terminating the cereal rye mechanically with a crimper crop roller.  The crimper crop roller might have reduced the need for the burndown (glyphosate) and allowed the residual herbicide (Acuron) by itself to reduce late season weeds escapes. 

Johnson also investigated whether the higher soil biological activity associated with the cover crops might lead to a reduction in herbicide efficiency due to faster herbicides break down.  Johnson used cereal rye for three years and documented a 23% rise in beta-glucosidase and 76% rise in dehydrogenase enzyme activity, verifying increased soil microbial activity.  The increased microbial activity did increase common corn herbicide breakdown of atrazine (41%) and mesotrione (Callisto, 36%), but not significantly compared to bare or follow fields.   For Johnson, the bottom line was that cover crops help with weed control but residual herbicides may be needed to get 100% weed control.

While cereal rye is a great cover crop to control weeds after soybeans, Johnson did not investigate the benefits of other cover crops.  The best weed fighting cover crops are Sorghum Sudan grass, radish, cowpea, buckwheat, cereal rye, annual ryegrass, and oats; which out compete weeds for space, water, sunlight and nutrients and/or have natural herbicides (glucosinolates) that bio-fumigate the soil. Farmers should avoid tillage which preserves and replants weed seeds.

While cover crops may suppress and outcompete weeds, they also help reduce crop diseases. Most corn and soybean diseases (Phytophthora, Fusarium, Pythium, and Rhizoctonia) thrive under wet soils. Cover crop roots dry the soil profile to reduce disease pressure. Using glyphosate (Roundup) to kill your cover crops decreases manganese availability and may promote Fusarium. Cereal rye before soybeans reduces Phytophthora and Rhizoctonia while oats reduces Fusarium and makes manganese available, counteracting glyphosate. Annual ryegrass and cereal rye also reduce soybean cyst nematodes 80-90% if planted early in the fall when soil temperatures are above 500F. Some no-till farmers plant green and use crimper crop rollers to terminate cover crops or use alternative herbicides to minimize the nutrient tie up problems associated with glyphosate (Roundup). 

One item not mentioned in the Purdue University (Johnson research) is the amount of cover crop residue needed to suppress weeds.  Organic producers often seed cereal rye at 2-3 bushel per acre to suppress weed seed.  The higher the amount of cover crop residue that is degrading, the greater the weed suppression is a common theme in most research.  However, higher and thicker cover crop stands can also create problems with planting and promote other pests like slugs, and voles (field mice). Farming sustainability requires a balance of practices, more, knowledge, and more patience. 

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