Natural cover crop termination

By James Hoorman, Hoorman Soil Health Services

As the planting season progresses, many cover crops and weeds continue to grow. Letting cover crops grow may reduce soil moisture, improve soil structure, reduce dust storms, and add soil carbon.  Crop rollers naturally kill cover crops by mechanically terminating (crimping) them.  Crimpers are used to kill grass cover crops (cereal rye, oats, barley, whet, millets, sorghum species), vetches (hairy, common), annual clovers (crimson and balansa), brassicas (kale, rape), buckwheat, sunflowers, and multi-species cover crops. Crimpers do not work well with perennial cover crops like red clover, alfalfa, or annual ryegrass.  Best results when the heads or flowers are in the “boot” or head stage, when mechanically crushing cover crop stems kills them.

Crimping advantages include suppression of weeds by forming a natural mulch, reduced summer soil temperatures, it conserves soil moisture, decreases soil erosion, adds organic matter, and reduces blowing soil.  Crimping cover crops works well on corn and soybeans but not on small seeded crops like hay. A disadvantage is that the cover crop has to be crimped at the right stage (boot or head stage) to be effective. Several local Soil & Water Conservation Districts (SWCD’s) now have crop rollers to rent out.

Crimpers are 16-inch rolling steel drums with blunt steel blades either tractor pulled or front mounted.  As the crimper rolls through a cover crop, the blunt blade “crimps” or injures plant stems every 7 inches, crushing the xylem and phloem so the plant dies. The blades are usually positioned at a 7-100 angle to reduce bouncing, soil movement, and to increase maximum plant stem crimping.   If done properly, 90-100% natural kill is possible. Organic farmers use crimpers without any herbicides while conventional farmers may use it in combination with other herbicides to reduce herbicide resistance, especially to glyphosate (Roundup).  Farmers often have to spray once after crimping but it may eliminate or reduce one chemical spray pass per year.

Crimpers or crop rollers are used several ways. A crimper can be front mounted on the tractor, placed in front of the planter, trailing the planter, or as a separate operation.  The advantage of one-pass crimpers (front mounted, in front of planter, or trailing) is less time, labor, and fuel however; farmers may have to wait to plant either corn or soybeans to terminate the cover crop properly.  Also, the planter and the crimper should plant and roll in the same direction.  This system works well for small farmers but not larger farmers. Usually, it pays to get your crops planted earlier, so waiting to plant when crimping conditions are ideal may be a disadvantage.

Planting green into the cover crop improves crop yields but a separate operation is needed to roll or crimp the cover crop.  The crimper size does not have to match the planter width and the crop can be rolled in any direction.  Crimpers can be operated at 8-10 mph using a mid-sized tractor.  Relatively little horsepower is needed to roll the crimper, but a larger tractor allows for the extra hydraulic pressure needed to raise and lower the crimper to prevent digging and soil disturbance on end rows. At this speed, every foot of crimper can terminate 1 acre per hour (15 foot =15 acres, 30 foot =30 acres).   

Under dry hard soil conditions, adding water or oil increases crimper weight and reduces bouncing to improve cover crop termination.  A crimper with water puts 75 pounds per square inch (psi) down pressure on the plant stem.  Crimper cylinder drums should be drained before winter (even if using oil) to prevent freezing and cylinder drum cracking. Sometimes farmers crimp at a slight angle to improve crimping performance or crimp it twice in different directions.

Corn can be crimped or rolled up to V4 (possibly V5) because the growing point is below the soil surface until V6 and the corn plant is still young and quite flexible.  V4 is when a corn seedling has 4 true leaves.  Crimping soybeans 3-4 inches tall (5 inches maximum) increases pod set (decreases internode spacing) resulting in slightly higher crop yields. Never crimp soybeans until at least 1-2 tri-foliates are visible.  If a cover crop is drying the soil out too much, use herbicides first to kill the cover crop, then crimp or roll the crop later to form a mulch to prevent soil drying and reduce soil temperatures.

Vegetables farmers (pumpkins, tomatoes, squash, cucumber, muskmelon, watermelon, green beans, peas, sweet corn) often plant seed or transplants into covers, resulting in cleaner produce and less weeds.  Crimping cover crops reduces herbicide use and improves the soil, so many farmers are starting to use and learn this new technique.        

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