Nitrogen study looks at two cover crops’ potential to reduce nitrate leaching

A new Extension handout details research results from a study led by Extension specialist Fabian Fernandez at the Rosholt Research Farm in Westport, Minnesota. The farm is in the Bonanza Valley and has an irrigation system. The handout shows data from 2016 to 2019 for two cover crop systems: winter rye and Kura clover living mulch. 

Started in 2011, this research project is a long-term partnership between the University of Minnesota, the Pope County Soil and Water Conservation District and the Minnesota Department of Agriculture, funded by the Minnesota Clean Water Land & Legacy Amendment. Since its inception, this project has provided research projects for four graduate students.

The following is a Q&A between Extension communications specialist Paul McDivitt and Fabian Fernandez, lead researcher of the study:

What was your motivation for this study?

We actually started this project before the new groundwater protection rule, but we knew that these areas are highly susceptible to nitrate leaching and groundwater contamination. The reason we started this study was to investigate in depth these cover crops. In essence, doing things to see what works and what doesn't so that the farmers don't have to be spending money, time, or resources on practices that may have little benefit. One thing I would mention to farmers in the field when we started this project and we had no corn or soybean planted where we were establishing Kura clover plots is that if it really comes down to not being able to farm these fields at all because they're vulnerable to groundwater contamination, while one year of no crop is really not a good situation, it may be a better alternative than not being able to farm at all. So we wanted to look at questions such as: can we establish this living mulch and other annual cover crops, and be able to continue to grow crops in this vulnerable region?

Why did you choose rye and Kura clover for your cover crops?

Rye was selected because of its winter hardiness. It survives the winter. It's pretty good at taking up nitrogen, it grows well early in the spring with cooler temperatures, and it is easy to terminate before planting the cash crops. The first year of the study we planted rye during the summer, but nothing survived except at the very edges of the plot. There was simply too much shade from the cash crops for it to survive. Now we wait to seed the rye cover until the corn and soybean crops start to die off so there is more sunlight reaching the soil. Doing this before actual harvest is important to give rye as much time as possible to grow before it gets too cold. If we wait until harvest then it's too late.

The Kura clover was selected because we wanted to have a perennial cover. With rye, we still have a period of time after it is terminated where little or no vegetative growth (or nitrogen uptake) is occurring in the field. Once Kura clover gets established, it's a pretty good bulletproof kind of system. The other potential benefit is that Kura clover is a legume, so it fixes nitrogen. Though this legume uses a lot of nitrogen, it also fixes nitrogen from the atmosphere. The thought is that a well-established Kura clover stand can help us by 1) sequestering (taking out of the soil) nitrate early in the growing season when it is most susceptible to leaching, and 2) providing nitrogen to the cash crops during the summer through symbiotic fixation and release of some of the nitrogen as some plant tissues decompose. Kura clover is a cool-season crop, so it grows vigorously early in the spring and slower in the summer, especially if it is under a corn or soybean canopy that limits the amount of sunlight it gets. 

Why do you think the rye cover crop didn’t do a good job of reducing nitrate leaching in this study?

Well, I think the problem was two-fold. One is establishment. The rye cover crop grew every year, but it was very spotty. We have tried several ways to improve the establishment but it has been inconsistent through the years. By the time the rye is terminated in late April or very early May there is not a lot of biomass or nitrogen taken up by the cover crop.

Timing is the other issue. We have measured between ½ and ¾ of the total annual drainage and nitrate leaching load happening during late April and mid-to-late June. At that time, since rye is already terminated and the cash crops are starting to grow, there are not very many active roots taking up large amounts of water or nitrate. Unless the decomposing rye biomass is immobilizing nitrogen out of the soil, the benefit of rye in reducing nitrate leaching would be limited, which is what we observed in our measurements.

Anticipating that we might have limited success with rye because of what I just mentioned, I decided that it would be important to have a contrasting treatment with the Kura clover that is growing vigorously during the April-June timeframe when we need the most help in reducing drainage and nitrate leaching.

Why did the Kura clover living mulch hurt the corn and soybean yields so much?

As I said earlier, Kura clover does need nitrogen. We plant the cash crops after strip-till, but we noticed that Kura clover quickly invades the tilled strips. We tried to suppress the Kura clover with a reduced dose of glyphosate, and some years also mowing it before the corn crop could be damaged by the operation, but that was not sufficient. This year, we actually used a full dose of glyphosate on the rows and a reduced dose between rows. That kept the rows pretty clean for a good portion of the growing season, but still, just looking at the crop, there was still some competition. Hopefully it was less than previous years. We will be able to check that out soon once we harvest the crops.

Have you read other research on these cover crop systems? How do your results compare to other studies?

We actually have some people here at the University of Minnesota that have Kura clover studies. Dr. John Baker has some long-term Kura plots where they have actually seen a benefit where they need to apply less nitrogen than without the Kura clover. So that's where I'm encouraged that maybe as our system gets better established (it takes at least three years for the Kura to be at full capacity), we will be able to start seeing some of that cycling where the Kura clover actually becomes a contributor of nitrogen rather than a competitor for the crops during the summer months.

I've seen some research, such as Jeff Vetsch's study in Waseca, where the corn crop required more nitrogen fertilizer the first year after a rye cover crop. Why is that?

There are a few variables to consider. One is obviously that the plants do take some nitrogen out of the system that becomes immobilized because it's in the organic form in the plant. Another is that once the rye cover is terminated, for that nitrogen to cycle back into the soil as inorganic nitrogen, the residue has to be broken down by microbes. If the carbon-to-nitrogen ratio of the residue is too high, the microbes use inorganic nitrogen from the soil to help break down that residue and this nitrogen is temporarily unavailable for the corn crop. Normally, the longer we wait to terminate a rye cover crop, the higher the carbon-to-nitrogen ratio becomes and the longer it takes for the nitrogen to become plant-available again. This is one of the reasons why it is important to try to terminate the cover crop early in the spring. Lastly, it is possible that we lose some nitrogen in the rye residue through ammonia volatilization during the breakdown process. This is something I would like to secure funding to investigate.

Looking at your data, it seems like there's a pretty clear relationship between how good a cover crop is at reducing nitrate leaching and how much it negatively impacts the crop in terms of yield. Do you think that’s true?

For the Kura clover in our study, for sure. It's pretty clear. The nitrate reduction is huge. We get less nitrate leaching out of that system with nitrogen applied than we do in a check plot with no nitrogen. Unfortunately, at present we are also seeing a substantial yield reduction for corn and soybean. 

With the rye cover crop there is very little if any difference in nitrate leaching reductions. Across different studies the results have been inconsistent. In some situations, rye reduced nitrate leaching, and in others, like this study, there was no benefit. There are many complex interactions impacting results, but I believe rye cover crop establishment in the fall and growth in the spring are two of the major factors. 

In our study, comparing rye cover and no-rye treatments showed no difference for corn-after-soybean in the EONR (economic optimum N rate) or yield at the EONR, but in continuous corn, the crop needed 24 lbs N/acre more to achieve the EONR (and similar yield) than the crop grown without a rye cover crop. So, at this time we have seen little overall benefit wth the rye cover crop. We are trying to find ways to get better establishment and growth and consistency across years. Doing so, we might see better performance at least in nitrate leaching reduction, and hopefully with little or no negative impact on the cash crops. Continuing this research to fine-tune the establishment and growth of rye will be very important. 

Do you have any takeaways from this study for farmers who are considering cover crops?

I think there are some important environmental benefits of having cover crops. We have not done much in this study yet at looking at other potential benefits related to soil conservation and soil health, which can be highly relevant to improving sustainable agriculture production. That said, the grain yield reductions we have seen with Kura clover are real and there is a real cost associated with it. Another important thing to keep in mind if you are planning to plant cover crops is that it is a lot more involved than a traditional cropping system. Planting, terminating, and managing the cover crop requires time and money, and the results can vary drastically from one year to the next.

I find that it takes time to fine-tune the variables that allow for a successful establishment of the annual rye cover crop. A living mulch is also not easy to establish. In our case it took the whole first year with no cash crop planted. The second year we delayed planting of both corn and soybean to allow the Kura clover to get a head start. It was only in the third year when we started to focus more on the cash crops. Often, people that plant Kura clover wait two years before planting corn or soybean. In our case, we planted after only one year of establishment. I think because we were able to irrigate, the crop looked good in the spring of the second year, but we were pretty concerned about the survival of the Kura clover during that second growing season. Luckily, it survived. People that work with Kura clover typically say that the first year it sleeps, the second year it creeps, and the third year it leaps. That was certainly our experience. Overall, I think cover crops can produce important benefits but there is work involved. No pain, no gain.

What’s next for this project?

This is a very fruitful partnership with the Minnesota Department of Agriculture and the Pope County Soil and Water Conservation District. The plan is to continue this project for a while. The long-term characteristics of this study are very important to preserve. As we transition from this initial phase of establishment, we are interested in beginning to understand how the measurements we are taking will be impacted now that the Kura clover system is working at its peak. Other questions we want to address in the future involve nutrient cycling and soil health. These questions can only be answered with a long-term study in place. So, we are very fortunate to have this opportunity. Another aspect that I would be interested in exploring in the future is the partial removal of crop residue to study its impact on cover crop establishment, nitrate leaching, nitrogen availability for the crops, and impacts to soil health. 

The handout was created by Sarah Jameson-Jones.

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This article was first published in October 2020.