Timing Cover Crop Removal

  • Cover crops can increase soil organic matter, decrease soil erosion, and some species can supply nitrogen for the next cash crop.
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  • Timely removal of a cover crop can help maximize benefits while minimizing the potential negative impacts the cover crop can have on the yield potential of a crop that is to be planted. 
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  • Because different cover crops should be removed at different times, removal timing and method should be considered independently for each cover crop species.  
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Cover crops are mostly planted for adding benefits to a field, and are not harvested for their seed, fruit, or forage. Cover crops are terminated before planting production crops to avoid becoming weeds and/or hinder crop production.

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When to Remove a Cover Crop?

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Timing of removal is specific to each cover crop species and farming conditions.  Cover crop termination four to eight weeks prior to commercial crop planting can allow soil warming, soil water replenishment, and residue drying and decomposition.1  Termination that occurs less than four weeks before commercial crop planting may provide the benefits of increased cover crop biomass, soil and water conservation, and possible nitrogen sources.

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Nitrogen release and commercial crop yields can be affected by the timing of cover crop removal.  Early termination of small grain cover crops can result in more rapid decomposition of the residue due to the narrow carbon to nitrogen ratio of young plant tissue.  Carbon to nitrogen ratios of 30 to 1 and greater are reached at the flowering stage of small grains.2  This wider ratio may be desirable for increasing above-ground biomass production and residue coverage.  

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The crop being planted following a cover crop should be considered when terminating cover crops.  For example, termination of winter-hardy cereals including cereal rye, triticale, and wheat should be timed to allow at least a two-week window prior to corn planting to avoid a yield reduction.3 

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Termination Methods

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Cover crops may include grasses, broadleaves, legumes, or any combination of these.  The four methods used to terminate cover crops are: winterkill, tilling, mowing, and applying herbicides.  A survey of farmers in 2014 found that approximately 48 percent of cover crop growers applied herbicides, 21 percent used tillage, 20 percent depended on winter kill, 10 percent terminated cover crops with mowing, 1 percent reported using a roller-crimper, and 6 percent used some other method to terminate cover crops.4  Winterkill is a method in which the cover crop is left until it is terminated by a hard freeze. This method is only possible in northern climates and for certain cover crops that are susceptible to the first hard frost (temperatures below 25° F) such as turnip and radish.2  While tillage is the second most common method of terminating cover crops, the use of tillage can be expensive and can reduce the benefits of the cover crops.   

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Mechanical. Aside from tillage, other mechanical methods of cover crop termination include use of a roller-crimper or mower.  Roller-crimpers (mechanical rollers) can be used to kill tall-growing cover crops by breaking or crimping the stem.  This method can be effective when used on cover crops at the flowering stage or later.1  Mow-kill is a method of cover crop removal that is effective on some species.  Cover crop regrowth and residue distribution should be evaluated when mowing is used for termination.  

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Herbicide. Effective herbicides come in contact with the target plant, are absorbed at the leaf surface, and move to the site of action at a lethal dose.  The following factors should be considered when selecting herbicides for cover crop termination: 

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  • Herbicide contact with target plants. A cover crop mixture of grasses and broadleaves has different plant heights and structures and requires non-selective herbicides for control.  Large plants that are bolting, jointing, or in reproductive stages may need greater herbicide rates or additional termination methods. 
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  • Herbicide absorption at the leaf surface. Non-selective herbicides may be contact herbicides (Gramoxone® brands) or systemic (glyphosate).  Systemic herbicides are translocated to the site of action, but the rate of translocation is influenced by plant metabolism.  Actively growing crops have metabolisms that will move applied herbicides to the site of action.  Therefore, applications should be made after three to four days of daytime temperatures in the high 50° to low 60° F range with nighttime temperatures greater than 40° F.5
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  • Herbicide at a lethal dose.  Moist soil conditions in the spring can delay field work, which can allow cover crops to reach advanced height and growth stages.  Full rates of herbicide are needed to achieve control of larger plants and avoid selection for herbicide-resistant plants.  
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Spring conditions may affect activity from a single herbicide application more than activity from a tank mixture of herbicides.  A tank mix of Roundup PowerMAX® herbicide with 2,4-D or dicamba can improve broadleaf termination.6  Other herbicides such as a residual herbicide may be included with herbicides for cover crop termination.  Consider the weed species are present and any carryover risks when selecting residual herbicides.  Also avoid tank mixtures of glyphosate with a contact or burn type herbicide, as it can reduce the ability of plants to absorb glyphosate. 
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Herbicides Recommendations 
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Right rate of Roundup PowerMAX herbicide (4.5 lb ae/gal):
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  • Spring application- use at least 44 fl oz/acre
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  • Fall application- use at least 32 fl oz/acre
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Ammonium sulfate (AMS): 
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  • 8.5 lb to 17 lb (or 2.5 to 5 gal liquid AMS) per 100 gal spray solution.
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Tank mixture with additional herbicide: 
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  • 2,4-D (1 to 2 pt/acre) 
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Follow proper mixing order:
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  • Water, AMS, dry product, agitate, drift control agent, liquids, glyphosate.
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Ensure good coverage:
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  • 10 to 15 GPA, medium to coarse droplets.
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Spray at the right time:
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  • Actively growing plant. 
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Annual Ryegrass
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  • Cool-season, upright annual, late-maturing, and widely adaptable to many soils including rocky and heavy clay soils.
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  • Thrives in dark, rich soils in mild climates, but can survive a range of climates.
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  • Does not survive hot, dry weather or severe winters, but tolerates fairly wet soils even with poor drainage.2 
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Figure 1

Figure 1. Control of annual ryegrass in the spring should begin before the first node is just starting to form.

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Tips for Annual Ryegrass Removal
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1.   Roundup® brand glyphosate-only agricultural herbicides should be used at a full rate (44 fl oz/acre).
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2.   Avoid tank mixtures of Roundup brand glyphosate-only agricultural herbicide with atrazine or mesotrione herbicides.  
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3.   Control is most effective prior to the first node/joint formation.  Once the third node/joint appears, control is poor because of limited translocation, as active growth in the plant goes to reproduction.
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4.   Fields should be scouted to confirm complete death of all cover crops and determine further control needs.
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5.   A second herbicide application with alternate site of action, such as a tank mix of Gramoxone® brands + atrazine, should be considered. 
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6.   Herbicide applications late in the day, approximately three to four hours before sunset, should be avoided. 
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Summary

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  • Cover crop species differ in their ability to survive winter conditions.
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  • Herbicides are often the preferred removal method compared to tillage for overwintered cover crop, as tillage can decrease some of the benefits of the cover crop. 
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  • To help manage the seed bank, the cover crop should generally be removed prior to it setting seed.  
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