Cover Crop Termination Timing

For optimum weed suppression, the cover crop termination timing needs to be based on the crop species and its growth stage. Termination of the cover crop should be delayed so that the highest possible biomass is achieved to aid in weed suppression, but it should be done prior to the development of mature/viable seeds. In southern states (Florida, Alabama, Georgia, Mississippi, etc.), termination at 2–3 weeks before planting the main crop also allows enough time for soil moisture recharge by rainfall. However, to maximize nitrogen contributions from legume cover crops to a subsequent crop, planting of the subsequent crop should occur soon after termination. Table 1 provides the appropriate growth stages for terminating various cover crops.

Cover Crop Termination Methods

Herbicide Application

Herbicide application is the most common cover crop termination practice for conventional systems. It is important to consider several aspects when terminating cover crops with herbicide application.

1. Cover crop family: There are several cover crop families, such as small grains (grass species), legumes, and mustards. Herbicides need to be selected carefully based on the specific cover crop because herbicide response varies among these crop families and species. Herbicide selection is especially important in fields planted with mix of two or more cover crop types—a common practice in many areas. Additionally, cover crop growth stage is a critical factor to consider. To achieve proper termination, herbicides need to be applied so that the cover crop does not mature past a specified growth stage (as listed in Table 1). For a cover crop mix, multiple herbicide products might need to be tank-mixed to achieve an effective kill of all the cover crop species.

2. Main crop after cover crop: If residual herbicide products will be tank-mixed for cover crop termination, then effects on the subsequent crop (main crop) should be considered. Residual herbicide products usually have longer plant-back restrictions that might limit the crop choices for the subsequent planting. Therefore, using herbicide products labeled for burndown or preplant application for the subsequent crop is important.

3. Selecting and tank-mixing herbicide products: Herbicide products can be contact or systemic types. Contact herbicides such as paraquat (Gramoxone, etc.), glufosinate (Liberty), or saflufenacil (Sharpen) do not move in the plant; therefore, they only affect the plant parts where droplets deposit. Poor coverage can limit contact herbicide performance and compromise cover crop termination. Enhance spray coverage by using nozzles that produce medium droplets, spraying at a higher volume (greater than 15 gallons per acre), applying herbicide at a higher rate (i.e., higher end of the labeled rate range), and adding surfactants. Systemic herbicides such as glyphosate (Roundup and various others), 2,4-D or dicamba (various products), or grass herbicides (Select Max, etc.) are taken up and translocated to the growing points of the plant; as a result, complete coverage is not as important for their effectiveness. However, cold weather, cloudy conditions, or generally poor growth (due to drought, deficiencies, etc.) may delay herbicide performance, and termination of the cover crops might take longer.

It is a good practice to tank-mix herbicide products with preemergence (PRE) and postemergence (POST) activity, but be cautious about potential negative effects. If herbicide products used in the tank-mix are not compatible, reduced herbicide activity may result. For instance, tank-mixing of herbicides such as atrazine, metribuzin, diuron, or chlorimuron reduces glyphosate activity on cereal rye, but tank-mixing 2,4-D, dicamba, or saflufenacil does not affect glyphosate performance. While tank-mixing PRE or residual herbicides, be mindful of the subsequent crop and plant-back restrictions. Various POST herbicides could be tank-mixed (Table 2) to kill cover crops effectively and to broaden the weed control spectrum.

Roller-Crimper Termination

A roller-crimper is an implement that can be mounted on the front or back of a tractor. The implement is usually a hollow cylinder (filled with water to increase weight) with welded blades on the outside to crimp stems. This implement crushes and creases stems to facilitate cover crop termination. Cover crop species such as hairy vetch, barley, triticale, cereal rye, and oats can be terminated effectively using the roller-crimper method (Table 3). The following considerations are important when using a roller-crimper to terminate cover crops.

1. Cover crop stage: Effective termination with a roller-crimper is highly dependent upon the appropriate growth stage of a cover crop species. For cereal rye, roller-crimper termination should be implemented between 50% anthesis and early milk stages (Padro et al. 2019). If cereal rye is rolled too early (before 50% anthesis), it can still produce new tillers and seeds. Likewise, if rolled too late (after the milk stage), seed heads can continue to develop and mature. For hairy vetch, appropriate timing for roller-crimper termination is from full bloom to early pod set (Padro et al. 2019). If a cover crop mix (e.g., legume and cereal rye) is planted, roller-crimper termination could be challenging due to differences in growth habit between the two species.

2. Mechanical considerations: Roller drum angle relative to the center beam (or travel direction) can impact the effectiveness of roller-crimper termination. For example, when termination was performed with roller drums perpendicular to travel direction (to rye rows), the rye rebounded after rolling. However, when the roller drum was operated at a 75-degree angle to the center beam, the rye was tossed diagonally, and fewer plants rebounded (Little 2019). In addition to better termination, the latter technique enhanced soil surface coverage with residue after cover crop termination. Some growers roll the cover crop parallel to rows, and others commonly roll at angles up to 45 degrees. Regardless, planting the main crop in the direction of rolling will limit residue accumulating and dragging through the planting equipment.

Tillage

Tillage is an option for cover crop termination, but it is not preferred. Multiple tillage passes are required to effectively terminate cover crop species that have deep root systems (such as clovers). Moreover, tillage termination is impractical for no-till or minimum-till systems where cover crop residues are maintained on the soil surface to prevent soil erosion and enhance soil health benefits. Even in strip-till systems, it is better to first terminate the cover crop with an herbicide application and follow up with the strip tillage.

Conclusion

Regardless of method, it is important to effectively terminate cover crops to prevent them from spreading as a weed during the subsequent crop season. The best practice for cover crop termination is to select an appropriate termination method and to implement it at crop-specific growth stages. It is also important to conduct follow-up scouting to ensure that cover crops are fully terminated. Follow-up scouting will help to identify skipped cover crop patches, escapes, or regrowth that might need spot application for complete termination. Overall, a properly managed cover crop can suppress many annual weed species and contribute to an effective weed management program.

References

Bosak, E., and V. Davis. 2014. "Cover Crop Termination." University of Wisconsin Extension. Wisconsin Weed Crop Science 204. Accessed on September 15, 2020. https://fyi.extension.wisc.edu/covercrop/terminating-cover-crops/

Little, R. 2019. "A Roller-Crimper for Cover Crop Termination and Weed Suppression." Accessed on September 20, 2020. https://cropwatch.unl.edu/2019/roller-crimper-cover-crop-termination-and-weed-suppression

Padro, B. B., J. Wallace, M. Barbercheck, R. Hoover, C. Keene, and W. Curan. 2019. "Terminating Cover Crops with a Roller-Crimper in Organic Grain Rotations." Accessed on September 28, 2020. https://extension.psu.edu/terminating-cover-crops-with-a-roller-crimper-in-organic-grain-rotations

Palhano, M. G., J. K. Norsworthy, and T. Barber. 2018. "Evaluation of Chemical Termination Options for Cover Crops." Weed Technology 32:227–235. https://doi.org/10.1017/wet.2017.113

Whalen, D. M., M. D. Bish, B. G. Young, S. P. Conley, and D. B. Reynolds. 2020. "Herbicide Programs for the Termination of Grass and Broadleaf Cover Crop Species." Weed Technology 34:1–10. https://doi.org/10.1017/wet.2019.73

Wright, D. L., C. Mackowiak, and A. Blount. 2017. Cover Crops. SS-AGR-66. Gainesville: University of Florida Institute of Food and Agricultural Sciences. https://edis.ifas.ufl.edu/aa217

Tables