Testing the cover crop hypothesis

Agronomy researchers are catching up with what farmers are doing in their fields

It’s an exciting time for cover crop research. Last summer, many large-scale cover crop trials were underway across the Prairies looking at everything from cover crop combinations, rotations and planting methods to pollinator strips.

One of the biggest ongoing projects, funded by Western Grains Research Foundation, Manitoba Wheat and Barley Growers and Manitoba Pulse and Soybean Growers is called “Testing the Cover Crop Hypothesis across Prairie Canada.” This study compares rotations with and without cover crops.

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Closeup of a plowed field, fertile, black soil.

Trials are taking place at five locations: the University of Manitoba’s Carman and Glenlea research farms, the University of Saskatchewan at Saskatoon, South East Research Farm at Redvers, Sask., and Farming Smarter at Lethbridge, Alta.

The four-year trials are comparing a four-year crop rotation grown with and without fall shoulder season cover crops. These two treatments will be compared to a soil-building perennial crop and a standard, two-year wheat/canola rotation. The outcomes will be assessed in terms of soil health and crop performance. Each of the four-year rotations includes wheat and canola along with a pulse crop (peas or soybeans) and a second grass crop, depending on the location. The Saskatoon location is unique because it includes irrigation and potatoes in place of the second grass crop in the rotation. The experiment is fully phased so that each crop is present in each of the four years of the study.

“We have got a range of locations, one of them drier, one of them more northern and the idea is to look at how cover crops fit into grain cropping systems in each of those areas,” says Dr. Yvonne Lawley, who is leading the overall project along with trials at the University of Manitoba’s Glenlea and Carman research farms. “The crops in the four-year sequence depend on what’s representative for each area, and we tried to pick cover crops that complement those crops. We tried to do that as best we can to make the cover crops successful at each phase of the rotation and compare different cover crop types and planting methods between the four different crops in the four-year rotation.”

Some of the cover crops are overwintering, with winter rye included in most locations. Others are cover crops that winterkill. Some cover crops are inter-seeded into the crop, while others are drilled in after harvest.

“We’ll have a sense at each of those locations if drilling after harvest is a good strategy versus broadcasting into a standing crop for the windows that we have at each of the locations,” says Lawley.

Data from sub-projects

The project is in its second year. In 2020 several related sub-projects will get underway looking at detailed nitrogen cycling, microbial communities and comparing greenhouse gas emissions of overwintering and winterkill cover crops against a no-cover crop system.

“The hypothesis is that having living cover in the springtime reduces the nitrous oxide flux we tend to see in the springtime,” says Lawley, who also has a graduate student looking at the soil carbon and soil physical property changes such as infiltration and aggregate stability in the soil.

A living laboratory

Most of our current knowledge of Prairie cover crops is based on full-season cover crops grown by organic producers for green manure crops or mixed livestock operations growing annual feed. Grain farmers looking to grow shoulder season cover crops after harvest will mostly find current research information from the U.S. and Eastern Canada. Local knowledge comes from farmers who have been innovating ahead of researchers; the science is trying to catch up.

Field-scale cover crop trials in Manitoba are also looking at how plants and soils respond to cover crops under commercial field conditions.
photo: File

“What I’m hoping this project can do is put some numbers to what those innovators are experiencing and observing right now,” says Lawley. Growers who are watching the increase in cover crops may be more decisive about making their own investments when more data is available. “I’m hoping it becomes a platform for other detailed studies as what we are learning about the microbial community and nutrient cycling increases. We’ll have this living laboratory where we can look at these new questions in a range of environments and ultimately address the questions we have now about the fit for cover crops in Prairie cropping systems,” Lawley says.

Paired with small-plot trials, field-scale cover crop trials in Manitoba are also looking at how plants and soils respond to cover crops under commercial field conditions. With funding from General Mills and Manitoba Agriculture, Lawley is establishing on-farm experiments with six farmers in Manitoba over the next three years.

“We’re working within farmers’ existing rotations and adding replicated strips of cover crops every year — or as often as we can — for the next three years,” says Lawley. “I think farmers right now are looking for some research, but also for people to talk with and hear about the experiences of others — both their successes and problems to avoid.”

Planting strategies

Other studies at the University of Manitoba are also looking at cover crop agronomy in Manitoba. “We’re doing a fall planting date study where we take seven different species in a mixture and plant them August 15, September 1 and September 16 and look at which species perform well in each of those windows and get at the question of how late is too late?” says Lawley. “How much productivity can we get with those later planting dates?”

Another study is looking at the benefits from an overwintering cover crop like rye. “Providing soil cover in the early spring is a really important objective for using cover crops,” says Lawley. “That’s when we’ve seen soil erosion, so we are looking at different termination dates for our rye and then planting low-residue crops like soybeans and edible beans into the rye.”

The trial includes terminating two weeks before planting or green seeding directly into the living cover crop and either terminating the cover crop just before seeding, at seeding, or looking at what the penalty of terminating the crop two weeks after seeding.

The same study is also combining strip tillage and green seeding cover crops into soybean and corn residue.

Intensifying the rotation

Old ideas become new again for cover crops, says Lawley. The last time Prairie researchers were focused on cover crops was in the late ’80s and ’90s when no till was still developing and there was still a significant amount of fallow in Prairie crop rotations. “These green fallow crops were being looked at to reduce soil degradation during fallow without using too much moisture for the next crop,” says Lawley. There is no fallow in current rotations, so new cover crop research is focusing on intensifying rotations.

Later research on the Prairies focused on full-season cover crops for organic systems. Now, says Lawley, “we’re looking at intensifying the small fallow periods at the beginning and the end of our annual crop production cycle. Rather than taking a year out of production, how do we intensify our systems on those two shoulder seasons?”

Many livestock producers have been realizing the value of cover crops (often referred to as polycrops) because they can use them as a source of annual forage. The research needs that remain are about the potential for long-term benefits for grain farmers.

“The current interest I’m hearing right now from some of the grain farmers I talk with is that maybe they’re not going to do that across the whole field, but rather focus their efforts on areas of their field with problem soils that are excessively wet or have salinity, those are areas where I see some grain farmers wondering if cover crops can have a fit,” says Lawley.

Another project will begin next year planting cover crops in wet areas of fields to see if they can mitigate risks of managing excessive moisture.

Farmers have different goals and cover crops may offer solutions for many of them. “I like to think about using plants as a soil management tool,” says Lawley. “So often, we think about how we manage soils for crop plants but the exciting thing to me about cover crops is we can then start thinking about how we use plants themselves as a soil management tool.”

About the author

Contributor

Angela Lovell

Angela Lovell is a freelance writer based in Manitou, Manitoba. Visit her website at http://alovell.ca or follow her on Twitter @angelalovell10.

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