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Successful termination of cover crops

By Staff | Apr 29, 2020

-Farm News file photo It is recommended cover crops be terminated 10-14 days prior to planting corn, but that timeframe is more lenient when planting soybeans.



Cover crops have provided a green landscape over thousands of acres throughout Iowa so far this spring, but there is a lot to consider when it comes to when and if they should be terminated before planting corn and soybeans.

Meaghan Anderson, Iowa State University Extension field agronomist has tips to help with better manage cover crop termination.

“In particular, as we look towards corn planting with soybean planting after that, it is important we are managing these cover crops well so they don’t become weeds,” she said.

The top three tips Anderson recommends for termination of cover crops are:

1. Timely termination

“We want to make sure we terminate cover crops in a timely manner so that they don’t interfere with our row crops,” she said. “But know soybeans are tolerant to heavy biomass than corn is going to be. Termination timing is less important prior to soybeans than corn, I think.”

2. Herbicide choice

“Make sure if we are using a herbicide option, which is typically going to be the easiest and most effective way, we are using the most effective herbicide choice available.

3. Finding the benefits of a cover crop

“Once we have decided we are going to get the cover crop killed in a timely manner consider how you might be able to make a cover crop work for you,” she said. “Weed suppression is a way we can maybe make a cover crop work for us.”


Anderson said when it comes to terminating cover crops, there are a lot of goals producers are trying to balance. One goal is to look for when soil temperatures are above 38 degrees, as that is the temperature cereal rye will be out of dormancy and growing.

“We are trying to balance a lot of things,” she said. “Making sure we are getting an effective, timely kill. We want to make sure when we are choosing our herbicides we are making the appropriate choices, that we’re choosing the best adjuvants available to us for the species and the environment and we are making sure we are managing the spray appropriately so that when we are actually mixing that together, we are doing everything we can to make sure that herbicide is going to work and we want to make sure if we have the opportunity to, we are maximizing efficiency by incorporating multiple passes together.”

Anderson said if it is applicable, farmers may be considering using a Urea Ammonium Nitrate (UAN) as part of the carrier along with the herbicide for a nitrogen application or they may look for the opportunity for the application of a residual herbicide as well.

“We need to keep in mind, sometimes those will add risk to a cover crop termination in particularly in cool, fluctuating, not perfect weather conditions which are pretty common in the spring,” she said.

Through a study conducted at the University of Missouri, Anderson said they looked at cereal rye, as well as a number of other cover crops and their control with using different herbicide mixtures.

“Universally, across the board for their grass cover crop species, they found mixtures with glyphosate were by far the most successful,” Anderson said adding producers may want to try alternatives. “But, time and time again, across the research, nothing seems to perform as well, and as consistently across different environments and glyphosate does.”

If there is a cover crop mix that includes a legume that may be overwintering, Anderson said a producer is definitely going to want to look at including a product like 2 4-D or dicamba.

“We want to make sure we can get the legume portion of that terminated, but as far as grasses go, glyphosate and glyphosate mixes seem to be the way to go for sure,” she said.

As previously mentioned, Anderson feels a cover crop can be left to grow a little bigger, however, that could bring both an opportunity and a challenge.

“The challenge is the residual herbicides we may want to include in that herbicide application may not be reaching the soil surface when we apply those to large cover crops,” she said.

Herbicides that reach the soil surface, Anderson said are inversely related to the amount of biomass accumulated from the terminated cover crop.

“That is definitely a challenge we can see, but one opportunity we can have with those large cover crops is that we may be actually able to grow a cover crop well enough, if we focus on heavy seeding, we want to make sure we have even biomass cover in the fall, and then when we turn around in the spring we may have enough of the cover crop present we can actually get some weed suppression with the cover crop. In particular, of small seeded weeds like palmer amaranth or waterhemp. That is hugely important for us because those are the two target weeds in many of our crop production systems across the United States,” she said.

How much biomass does it take for consistent weed suppression?

Anderson said researchers have found it would take in the realm of 7,500 pounds per acre of dried cover crop biomass. However that much may not be required to help with weed control.

“I do think we can get at least partial weed suppression with lower biomass accumulation,” she said. “That’s an opportunity, so maybe if we are going to hang up that residual herbicide in that cover crop anyway, perhaps, if we grow a cover crop with a focus of getting weed suppression, we can move that herbicide to a different point in our production system maybe in to a post emergence application when our cover crop has been terminated already, or perhaps save some money on that residual herbicide if our cover crop can be acting as basically a short lived residual herbicide by itself. I think that is a really neat opportunity and it’s one way to try to make that cover crop actually work for us when we are looking at terminating it in the spring.”


Anderson said the general guideline they recommend is terminating a cereal grain at least 10 to 14 days prior to planting corn. For soybeans, this can be done at or right before planting.

Why is timing important?

“We want to make sure we are obeying our crop insurance termination guidelines,” said Anderson. “My understanding is they are much more flexible now then they have been in the past, but it’s important to talk to your crop insurance agent so you are aware when that cover crop needs to be terminated so it doesn’t interfere with any crop insurance you might have.”

Timing is also important, Anderson said to ensure you are minimizing the interference the cover crop might have with the cash crop.

“If we are growing a cover crop, we may be growing it for certain benefits, but we want to make sure the cash crop is what seamlessly going to make us money, so we want to make sure we are managing that cover crop so that we are minimizing yield loss, or gaining yields in some cases,” she said.

Nutrient tie up in the spring, Anderson said is a concern.

“The biomass can potentially interfere with planting itself,” she said. “We can have concerns with greenbridge effects with disease pathogens and insects and cover crops can act as a weed if we don’t get it terminated well. So, the 10 to 14 day timeframe before corn is the timeframe that is most important because of all of these potential issues with the cover crop. Most importantly, I think carrying diseases, or being a host for insects that may like to jump over from that dying cover crop onto our corn plants, the 10 to 14 days is really just a guideline, but to manage risk the best, we would advise that cover crop be brown and dead before you plant your corn into the crop field.”

Soybeans, Anderson said seem to tolerate those potential issues a lot better.

That’s why we see that timeframe really disappear as far as getting it killed before planting,” she said. “Probably the most important thing with soybeans is making sure you are able to plant into as much biomass that is there.”

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