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Don't let less-than-desirable control effect your cash crop yield.
Terminating Cover Crops Before They Become Weeds
  • With the arrival of spring and warmer temperatures, cover crop growth has resumed and soon we will be implementing termination programs.
  • Chemical products and rates needed to kill cover crops should be dependent on cover crop species and growth stage.
  • Complete termination is the key to starting off a successful cropping season.

While cover crops provide a solid agronomic benefit regarding soil erosion, nutrient scavenging, winter annual weed suppression and residue management, THEY ARE NOW WEEDS! If complete control isn’t achieved, the rate of soil warming and drying can slow, leading to a later-planted cash crop, thus hindering yield potential. Less than desirable control can also lead to the cover crop producing seed and establishing a seed bank that could result in future weed issues.  Complete control of cover crops during the early spring is critical for cash crop yield potential. There are two main types of cover crops; broadleaves (ex. Crimson Clover) and grasses (ex. Cereal Rye). Below you will find some guidance on how to find the best programs to successfully kill these two types of cover crops.

Broadleaf Cover Crops:  While most of the broadleaf cover crops do winterkill, do not assume that this has happened this year.  Cold, harsh winters typically result in winter kill of the cover crop, but do not assume complete kill.  Now is the time to scout and asses the winter survival of the crop species we planted last fall.  There are some species, such as Crimson clover and Austrian winter peas, which do not winterkill.  The key to success in managing these broadleaf cover crops is to not rely solely on glyphosate as your chemical means of control.  The addition of a growth regulator such as 2,4-D or dicamba may provide more consistent control and another effective site of action for resistance management.

Grass Cover Crops:   We can further split the grasses into two groups—annual ryegrass and not annual ryegrass.  While glyphosate is probably the most widely used herbicide to manage all these grass species, herbicide rate and timing may vary depending on the grass species present. 

If annual ryegrass has any significant growth, it can become very difficult to manage. The standard recommendation for annual ryegrass is to apply glyphosate at 1.13 lb ae/acre (Roundup PowerMAX @ 32 fl oz) with the appropriate adjuvants to plants less than 6” in height, and actively growing. Research suggests that the air temperature must be above 45°F for 3 consecutive days prior to the application for more consistent control. If annual ryegrass is taller than 6”, increasing the rate of glyphosate is necessary to get control. In terms of growth stage, it is best to control annual ryegrass prior to jointing. We have a little more flexibility with other grass species, cereal rye included.  The standard glyphosate recommendation would be to apply at a rate of 1.13 lb ae/acre with the appropriate adjuvants to stands less than 18” tall.  Again, if the cover crop is taller, we may need to increase the glyphosate rate. Regardless of grass cover crop species, we have additional herbicides that can be used besides glyphosate. The use of paraquat containing products will also provide control of grass cover crops. One thing to keep in mind is to always have either COC or MSO in the tank, if left out, reduced control may be observed.

Are there other methods of terminating cover crops, you may ask? Absolutely. Cultural methods such as mowing, tillage, and crimping can all aid in cover crop termination. Mowing is only successful if we mow at the right timing, when the growing point is high enough that the mower can cut it off. This can lead to more above ground biomass that may be harder to plant into, however, it may be beneficial for weed control. Tillage may be effective if the cover crop is still small, but keep in mind that tillage may defeat the purpose of what a cover crop was intended for in the first place (ie. erosion control). Lastly, crimping is a new method of termination that allows the above ground biomass to be used as a thick weed barrier. The goal of crimping is to terminate without chemical measures. These all come with benefits, but also with downfalls, such as more trips needed across the field, additional equipment, and a potential harbor for certain insect pests. These methods are all additional options that could be considered for terminating cover crops.

The key to successfully terminate cover crops is to manage them with the right herbicide while they are still small.  Complete termination is the key to starting off a successful cropping season.

Reference:  Purdue University and University of Illinois

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