The following information about cover crop termination is taken from the 2016 Weed Control Guide for Ohio, Indiana and Illinois. The information was originally written by extension weed scientists at Purdue University.
Cover crops are unique in that most are planted primarily to reduce soil erosion and otherwise enhance soil quality, and are not harvested for their seed, fruit, or forage (although some are grazed or used as forage). Instead, cover crops are terminated before planting of summer annual grain crops such as soybeans and corn. When not effectively terminated, cover crops have the potential to become weeds in the grain crop and can slow soil drying and warming in the spring. Many cover crop species have characteristics that make them both desirable as cover crops, and troublesome as weed species. Weedy cover crop escapes not only affect the current production crop, but also can produce seeds and establish a seed bank that will result in future weed problems.
Cover crops can be terminated by a number of methods, although herbicide application is the most common method. When selecting a herbicide program for termination of a cover crop, consider:
- the cover crop species
- the cover crop growth stage
- other weed species present
- the production crop to be planted
- the weather conditions at application
Cover Crop Species. Cover crop systems that contain only grass species or only broadleaf species can be terminated using selective grass or broadleaf herbicides. However, producers will often grow combinations of grass, legumes, and non-legume broadleaf species together to receive the maximum benefits that each group presents. Successfully terminating a cover crop that contains grasses and broadleaves will require a nonselective herbicide such as glyphosate, glufosinate, or paraquat. It is possible to combine a selective grass herbicide (sethoxydim, clethodim, quizalifop, fluazifop) and selective broadleaf herbicide (2,4-D, dicamba) to terminate a mixed crop, but it is not advisable because many of these combinations can be antagonistic and poor control will result. Combining glyphosate with either 2,4-D or dicamba can ensure more complete termination of broadleaf species than spraying glyphosate, 2,4-D, or dicamba alone. Effective herbicide control of grasses and broadleaves varies by species. Consult a weed control guide or herbicide label to ensure the herbicide will be effective on a particular cover crop species. See species-specific recommendations below for herbicide programs for some common cover crops.
Cover Crop Growth Stage. The growth stage and height of the cover crop at the time of termination is critical in determining what herbicide and rate will be most effective. Crops that are bolting, jointing, or producing reproductive structures can be difficult to control with herbicides and may require other termination methods. Always take cover crop heights into consideration because taller, more mature plants may require higher herbicide rates than smaller, less mature plants.
Other Weed Species Present. Before choosing a herbicide to terminate a cover crop, carefully consider all the plant species that are present — including weeds. Decide on a herbicide plan before planting or seeding the cover crop, and then amend the plan according to any additional weed species that occur.
Cash Crop to Be Planted. When planning a herbicide termination program, use only herbicides that are labeled for burndown or preplant applications with the summer annual crop you will plant. Be sure you also observe crop rotational restrictions. For example, there is a 14-day restriction when planting soybean after using high rates of 2,4-D in a cover crop termination. The rotational restrictions for corn after applications of selective grass herbicides (sethoxydim, clethodim, quizalifop, fluazifop) range from 30 to 120 days.
Weather Conditions at Application. Environmental conditions affect herbicide performance, and unfortunately these are factors that cannot be controlled or predicted. Typically, cover crop terminations take place in the early spring, so while the exact weather may vary, temperatures tend to be cool with variable cloudiness and high soil moisture. Take these typical weather conditions into account when planning an herbicide termination program — cool, cloudy conditions slow the rate at which herbicides kill plants. Wet soil can also keep sprayers out of fields, which delays spray applications and allows cover crops to reach undesirable heights and growth stages.
A wide variety of cover crop species are available and recommended for specific cropping systems, soil types, and regions. The following section provides herbicide termination recommendations for the cover crop species most commonly planted in Indiana, Ohio and Illinois.
Annual ryegrass (Lolium multiflorum), also called Italian ryegrass or common ryegrass, has become a very popular cover crop throughout the Midwest. Do not confuse annual ryegrass with cereal rye (Secal cereal). Annual ryegrass is a good cover crop because of its ability to rapidly germinate in the fall, grow aggressively in the spring, and add substantial root and forage mass to the soil profile. However, this plant’s aggressive and competitive nature can also make it a weed problem in grain crops. The introduction of annual ryegrass as a cover crop in Indiana and the possibility of it escaping as a weed is a concern. Annual ryegrass has established itself as a weed in orchards, vineyards, and grain crops throughout the western and southern United States and is recognized by multiple scientific weed societies as an invasive weed species. Annual ryegrass is also able to quickly adapt to herbicide selection pressure. The International Survey of Herbicide Resistant Weeds (Heap 2015) reports herbicide-resistant annual ryegrass populations in ten states and across six herbicide sites of actions. Follow these guidelines for successful termination of annual ryegrass cover crops:
- Make applications prior to 8″ plant height
- Glyphosate rates of at least 1.25 lb ae/A are required, although 2.5 lb is preferred for annual ryegrass termination
- Ryegrass must be actively growing, and it is recommended that applications occur only following three consecutive days when air temperatures have been above 45 F
- The addition of saflufenacil to glyphosate can improve control of annual ryegrass
- Combinations of paraquat, metribuzin and 2,4-D or dicamba can control small ryegrass (<6″ in height), but are not recommended for control of larger plants
- Avoid using PSII herbicides (atrazine & metribuzin) in mixtures with glyphosate, as they can cause antagonism and poor control of annual ryegrass.
Cereal rye and oats. Glyphosate at a rate of 0.75 lb ae/A will effectively control both species up to 18 inches tall. Mixtures of glyphosate plus 2,4-D, chlorimuron, chloransulam, atrazine, or saflufenacil can also be applied for additional control of other cover crop species (specifically broadleaf species) and residual control of summer annual broadleaf weeds. The nonselective herbicides paraquat and glufosinate are less effective than glyphosate on these species.
Crimson clover and Austrian winter peas are two popular legume species used as cover crops that typically do not winter kill and require a spring termination. Escapes and failed control of crimson clover and Austrian peas have been documented as rare, so they pose less threat as potential weed species in production crops than annual ryegrass. Information on control of these species with herbicides is limited, but cover crop guides advise that glyphosate and 2,4-D easily control crimson clover and winter peas.