Keys to cover crop termination and burndown timing

Optimal planting times and ambient temperatures are looking favorable to trigger an onslaught of soybean and corn planting very soon. As larger planters and more efficient equipment dot the landscape, planting can progress very quickly when soil conditions and weather forecasts align favorably. As of April 22, about 10% of corn and 7% of soybeans had been planted in Illinois. Soil conditions are mostly favorable for planting, so fieldwork will be controlled by rainfall amounts, drying rates, and soil fitness. In Illinois, 64% of crop acreage has adequate moisture in both the surface soil and subsoil. In Iowa, which has been through more drought conditions, 46% of surface soil and 36% of subsoil had adequate moisture, with more than 50% of the soils below optimal moisture. With dry soils as a backdrop to the timing decision, it is time for most of the vegetation, either weeds or intentional cover crops, to be terminated.

📸: BASF Contributing Writer/Featuring a comparison of cover crops. Henbit covers this flat timber soil field (L) and may have already produced seed for next year's seedbank. A closed canopy of wheat, or cereal rye (R), has done its job and should be terminated to save soil moisture under most conditions.

Do weeds and cover crops slow or speed soil drying?

A living cover crop with a closed canopy pumps water out of the soil via transpiration. Soils undergo slow but steady drying down to the depth of the root zone. Surface drying is most rapid on bare soil that is exposed to wind and solar radiation (which heats the soil and drives evaporation), but it primarily affects the upper few inches of soil. The worst combination for soil drying is a dead or desiccated cover crop that will protect the soil surface from wind and heat but not promote transpiration. Slow-working herbicides (glyphosate in cool weather) may leave the stomata open and allow water to move through the weeds or cover crop even as they are dying. Contact herbicides that cause cell rupture to shut this process down more quickly. In the photo below, the grass on the right is likely still conducting water into the atmosphere, whereas the ponding on the left makes drying dependent on evaporation and will proceed more quickly in the open water zones than where there is a dead cover crop.

📸: BASF Contributing Writer/Featuring Glyphosate leading to a slow death of plants, especially in cool weather (L). Low spots in a killed cover crop field (R) depend on evaporation or movement into the subsoil for drying.

Adding growth regulator- or PPO-herbicides to burndown

Glyphosate remains an excellent herbicide for grass-cover crops but will need help for broadleaf cover crops or glyphosate-resistant weeds. Additions of Sharpen^® Herbicide or either 2,4-D or dicamba will help in either situation. Pay attention to plant-back intervals as they relate to the seed platform and crop you are using.

📸: BASF Contributing Writer/Featuring PPO herbicides like Sharpen^® Herbicide, which rapidly desiccate plant cells (L), while growth regulator herbicides (R) like 2,4-D or dicamba stop growth but retain some ability for the plant to transpire.

Wounded but not terminated cover crops and weeds

A famous "line" uttered in the highly awarded movie Apocalypse Now (1979), starring Marlin Brando and Martin Sheen, applies here. Captain Willard (Martin Sheen) is sent on a mission to terminate the command of a wayward Colonel with the haunting "order" "terminate with extreme prejudice." When terminating a cover crop or weed infestation ahead of your target crop, it is best to do it right. In the pictures below, an attempt was made to disk a field that contained grasses and clover. I watched this process with the knowledge that rain was coming that evening. Had it turned windy, hot, and dry for a few days, there might have been more success, but as the pictures show, the clover clumps were simply transported and transplanted, aided by the evening rainfall. Herbicides would have been a more efficient option. Even worse, the use of a double precision disk gang on marginally wet soils has likely caused soil compaction.

📸: BASF Contributing Writer/Featuring a double precision disk gang moving across a weed, grass, and clover mix (L). A rain that evening revived clover plants that had been just slightly "moved" by tillage (R).

Reviewing cover crop purpose

In my last article on Cover crop scenarios, I discussed the various purposes of a cover crop. It is useful to review how many of these potential objectives have been achieved and how that relates to termination timing.

• Erosion control: If a decent stand exists, the job has been done. Terminate the cover crop.
• Nitrogen fixation: N fixation by leguminous cover crops will continue to increase, but planting date yield penalties should be carefully evaluated. Terminate now or soon.
• Weed suppression: A good stand of a grass cover crop will provide surface soil shading and some allelopathic control of germinating weed seeds. The release of those chemicals requires the cover crop to be dead, desiccated, and leached by rainfall. Terminate now.
• Compaction alleviation: Tillage radishes may have been winter-killed, and where they haven't, they need to be terminated as the difficulty of control increases as they begin to flower. Terminate now.
• Soil health improvement: The spring growth of cover crops should supply the rhizosphere with enough goodies to improve soil health. Terminate now.

Coming next: How soil-applied herbicides work.

📸: @mvburling via Canva/Featuring a close up shot of cover crops.

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