By: Sarah Lancaster, Anita Dille, Peter Tomlinson and DeAnn Presley
Now is the time to start considering how to finish winter cover crops in preparation for summer crops. Given the drought status in many parts of Kansas, some producers may not have much cover crop biomass. However, keep an eye out for it as temperatures start to soar this spring. Some cover crop species, such as oilseed radish or fall-planted oats, are likely to be killed by freezing during the winter. But many cover crops will need to be terminated mechanically or chemically in the spring. Once the cover crop has been planted, there are two factors you can control in ceasing the cover crop: method and timing, and your choices regarding these factors interact. It’s also important to remember that NRCS guidelines for cessation times have implications for program compliance (Figure 1).
Figure 1. USDA map illustrating termination time guidelines.
Methods of mechanical termination of cover crops, such as rolling or rolling (Figure 2), tillage and mowing have the potential advantage of reducing selective pressure on herbicide-tolerant weeds by deferring herbicide use to crop applications. Termination with a roller or roller bender may be more effective for monoculture plantings than a winter grain, such as rye or triticale. The growth stage of cover crops is a key factor in achieving a successful kill. For example, cereal rye and other winter cereal grains are killed more consistently when using a roller folder in the milk or dough stage, while legumes are best controlled in full bloom. Tillage is also a mechanical termination option in some cropping systems. Some species, especially clovers, may not be effectively killed by tillage. Multiple tillage passes may be required, which can negate the health and soil conservation benefits of the practice. Mowing as a termination method is more suitable for smaller areas.
Figure 2. Roll folder used to finish a summer cover crop of sorghum and Sudanese grass. Photo by Peter Tomlinson, K-State Research and Extension.
Herbicides are an effective cover crop removal method that can be used in a variety of cropping systems. Selecting the most effective herbicide varies according to cover crop species and stage of growth. In general, more mature cover crops are more difficult to control with herbicides, especially once the plants have begun reproductive development. Selective herbicides, such as SelectMax (clethodim) for grasses or 2,4-D for broadleaves, can be used to control single species plantings, but mixing group 1 herbicides such as clethodim with group 4 herbicides such as 2,4 -D will probably reduce the weed check. Non-selective herbicides such as glyphosate, glufosinate or paraquat are recommended for the control of mixed species plantings. Combinations of glyphosate and 2,4-D can increase the mortality of broadleaf crops. While glyphosate is the most effective herbicide for controlling most cover crop plantings, 2023 price and availability issues could make alternative herbicides more attractive to some growers. Different herbicide options and their effectiveness are listed in Table 1.
Residual herbicides such as Fierce (pyroxasulfone + flumioxazin), Authority Maxx (sulfentrazone + chlorimuron), or Prefix (fomesafen + S-metolachlor) can also be included in sprays for termination of cover crops. A recent research project conducted in Kansas and other soybean-producing states found that including Fierce in the spray mix when quitting grain rye may increase swinegrass control (Figure 1). Also in this study, greater weed suppression was observed when corn rye was stopped at planting time rather than two weeks before planting. However, producers considering delaying cessation should also consider whether the additional weed control outweighs the soil profile used by the cover crop and the potential yield penalty for the cash crop.
Figure 1. Influence of Fierce (YES PRE or NO PRE) on Palmer amaranth density (top) and growth rate measured in days until POST-emergence application (bottom) when soybeans were planted on two dates (early or late) with three rye-based cover crop strategies (No-till = no cover, Early = finished two weeks before sowing, Plant green = finished at sowing). The study was conducted in Rossville, KS. Funding was provided by the United Soybean Board. Charts by K-State Research and Extension.
If choosing to include a herbicide with residual activity, it is especially important to consider the risk of injury or rotation restrictions of the crop that will be planted later (Table 1).
Additional cover crop cessation resources can be found at
Table 1. Herbicide considerations for chemical cessation of cover crops.
Herbicide1
Effectiveness of termination2
Potential rotation restriction3
Cereal rye
Austrian Winter Pea
Roundup PowerMax (glyphosate)
G/E
F/G
Nobody
SelectMax (clethodim)
G/E
No
corn
Roundup + atrazine
g
G/E
soy
Roundup + Clarity (dicamba)
G/E
G/E
soy
Roundup + Canopy (chlorimuron + metribuzin)
F/G
G/E
corn
Roundup + Sharpen (saflufenacil)
G/E
g
soy
Rounding + SelectMax (clethodim)
G/E
g
corn
Rounding + 2.4-D
G/E
G/E
corn, soy
Gramoxone SL (paraquat)
F/P
F/G
Nobody
Gramoxone + atrazine
f
AND
soy
Gramoxone + metribuzine
f
P
corn
Gramoxone + 2,4-D
F/G
AND
corn, soy
Gramoxone + dicamba
g
AND
soy
Gramoxone + metribuzin+2,4-D
G/E
AND
corn, soy
Gramoxone + metribuzin + 2,4-D + Classic (chlorimuron)
G/E
AND
corn, soy
1Use of trade names does not indicate endorsement of any product. 2E=excellent, G=good, F=fair, P=poor, N=none; The growth stage of the cover crops and the environmental conditions will affect the effectiveness. Herbicide labels replace this information. 3Rotation restrictions are influenced by the application rate and herbicide resistance in the crop. Herbicide labels replace this information.
