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Agronomy Update

Cover Crops: Termination Timing and Weed Suppression

Published on Monday, February 06, 2017

Cover crop acres have been steadily on the rise for the last few years. According to a recent survey by the Sustainable Agriculture Research and Education Department, farmers in the U.S. increased their cover crop acres by 147 percent from 2014 to 2016. But, this rapid adoption did not come without growing pains. Many farmers have struggled with terminating their cover crops on time and, in many cases, the cover crop persisted into the growing season and actually became detrimental to yield. How and when you should kill your cover crop will be dependent on the cash crop you’re planting as well as the species and growth stage of your cover crop.

Many farmers are using cereal crops (cereal rye, wheat, etc.) as a part of their mixture because of their relatively low cost and ability to produce high amounts of biomass above and below ground. Soybeans have a greater ability to overcome cereal competition early in the year so termination can be delayed up to 7 to 14 days after planting. Corn lacks the early season “grit” that soybeans have and thus, the cereal cover should be terminated at least 14 days ahead of planting. University of Tennessee Weed Scientists Dr. Garret Montgomery and Dr. Larry Steckel have seen a negative impact on corn stands and early season vigor when a standalone cereal cover crop was used. However, when a legume (vetch) was introduced to the mix, a significant difference in vigor was seen (Figure 1).
 


In 2016, the local growing conditions were very conducive for growing corn and no yield loss was observed. However, I believe that in a normal year where some stress is observed, the lack of early season vigor could translate into yield loss. 

Another consideration when using a cereal cover crop is the use of a roller crimper. Roller crimpers are an effective way to kill cereal cover crops if your crop has already moved to the reproductive stage. Although, you may still have to use a herbicide to terminate your cover crop, a roller crimper can still provide added benefits. When utilized in front of the planter it lays the cover crop over and creates an organic mat that can suppress weeds that would normally compete with the cash crop for nutrients, water, and sunlight.
 


In the South, we deal with a fun little weed called Palmer amaranth (Palmer). This weed can be extremely competitive and very hard to control, especially in soybean fields. Palmer amaranth has shown resistance to six different modes of action; most recently expressing resistance to PPO’s (Flexstar/Reflex, Cobra, etc.) Cover crops could potentially be another weapon in the fight against Palmer amaranth. Another portion of Dr. Montgomery’s research is focused on evaluating how long Palmer amaranth can be suppressed with the use of cover crops (Figure 2). 

 

As you can see in Figure 2, terminating the vetch + cereal rye cover crop 0 to 14 days after planting provides 32 to 38 days of Palmer amaranth suppression. When utilized with a soybean crop, this could potentially reduce the need for one of your herbicide applications. However, I would still suggest overlaying a residual herbicide soon after planting to ensure a weed-free field.

In summary, when planting soybeans into a cereal rye + vetch, cover crop termination can be delayed up to and even after planting without adversely affecting crop vigor while still prolonging weed control. When planting corn, the aforementioned cover should be terminated a minimum of 14 days ahead of planting. 

When planting into green cover crops, there is the potential for a “green bridge” which could lead to increased insect pressure. That being said, the addition of an insecticide may be warranted with your burndown application. In the study from the University of Tennessee, Montgomery and Steckel observed increased cutworm pressure on the corn and a higher number of three-cornered alfalfa hoppers in the soybean crop.

There are many factors to consider when using cover crops in your crop rotation, which can make it seem overwhelming. Consult your local Beck’s representative for the best management practices when using cover crops.
 

 

 

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