Cover Crop Termination

By James Hoorman, Hoorman Soil Health Services, adapted from Green Covers

This spring has been warmer than normal, but Ohio’s subsoil moisture has been dry due to last year’s drought.  Recent rains may have helped depending upon how much rain actually soaked in.  Last year, adequate subsoil moisture allowed farmers to get decent yields, however; what about this year?  According to the National Weather Service, there is a 83% chance for a transition from El Niño to La Niña during April-June and a 62% chance for La Niña to develop by June-August.  Typically, El Niño years are drier while La Niña years tend to be wetter in the Midwest. 

For Ohio, the 60-day weather forecast is for temperatures to be above normal in our area but perhaps drier than normal conditions around the Great Lakes. April may be wetter, but May is expected to turn dry. Farmer’s may be planting earlier than normal depending upon the weather. What about terminating cover crops?  That depends upon soil moisture.   

When dry conditions are expected, terminate cover crops early when they are still in a vegetative stage.  If you are trying to build soil organic matter (SOM), most of that comes from the roots.  Many grass cover crops (especially cereal rye) have their roots grown, so terminating early may be beneficial to conserve moisture if soil is dry. A plant uses half of its total water usage during the reproductive stage so terminating before that stage will help conserve water for the next crop. Additionally, producers who are planting corn or another high nitrogen use crop should consider early termination to save soil nitrogen for the following crop. Early termination is not ideal if there are legumes, such as hairy vetch or winter peas in the cover crop mix.

 Late termination is the termination of cover crops at reproductive stage. Farmers targeting a high carbon cover crop for maximum weed suppression will want to choose a late termination, as well as anybody who has a legume in their cover crop mix and is targeting nitrogen production. If the soil is wet, letting the cover crop grow is beneficial to get the soil to dry out. 

There are several ways to terminate cover crops.  A roller crimper can mechanically terminate a mature cover crop.  The blades destroy the stems on a plant and they tend to die if they are close to being mature. Timing is important.  For cereal rye, crimp when it is in the boot stage or heads out.  For legumes and clovers, when they are at 10-20% bloom stage to maximize nitrogen formation. No-till and organic producers often use crimping.  Corn can be crimped up to the vegetative stage V4 (four true leaves) while on soybeans, not before they emerge past the cotyledon (first two leaf stage) up until they are 4-5 inches tall.  Crimping works best for late termination and not very well for early termination if trying to conserve moisture.

Many farmers use herbicides to chemically terminate cover crops early.  The herbicides work best when crops are young and tender before they become lignified. Conventional farmers and some no-tillers often use herbicides. There are many herbicides, but glyphosate (Roundup) is often used to burn down cover crops although paraquat is another burn down herbicide.

Some farmers use tillage to terminate the cover crop to disrupt the root system.  High speed discs and even undercutters can be used to kill cover crops, however, they tend to ruin the soil structure and promote soil erosion.  If chemical termination or the crimper roller fails, tillage is often used as a last resort.  Tillage can be done anytime to terminate a cover crop, but tillage also dries the soil, losing 0.5 to 1.0 inch of soil moisture per pass. Livestock farmers have the option of grazing, making hay, or haylage from the cover crop.  Dairies often chop cereal rye or other forages for feed.  Cutting the cover crop just as it starts its reproductive phase (boot stage in grasses, 10-20% bloom in legumes/clovers) can effectively terminate the cover.  This does not work on alfalfa, red clover, or annual ryegrass.  Due to our fickle weather, do not wait too long to harvest the cover crop.  Cereal rye can go from a few inches tall to several feet tall quickly.  Harvest as soon and as quick as possible.  Usually, it is best to maximize growing conditions for the next growing cash crop.  If you are planting corn into a harvested cover crop and planting corn for corn silage, it may not be as critical compared to growing corn for grain.  With lower crop prices expected, farmers in our area will need some good yields to make ends meet this year

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