LETHBRIDGE – Cover crops have been part of nutrient management in North America for more than two centuries.
However, low-cost nitrogen fertilizer and changing agronomics prompted conventional agriculture to largely abandon the practice.
The few exceptions included its use to control soil erosion and to grow crops organically.
Now, cover crops are poised to make a comeback.
Chris Procyk of the Southern Applied Research Association (SARA) in Lethbridge says conventional farmers are looking for alternative sources for at least some of their fertilizer needs.
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“Last year’s (input) prices were a real kick in the pocketbook,” he said.
“It caused a lot of questions about other ways to get the yields with as small a fertilizer purchase as they could get away with.”
Bob Blackshaw of Agriculture Canada’s Lethbridge research centre said the merits of green manure are overlooked on the modern prairie farm.
“Beyond depositing nutrients, like nitrogen, green manure crops play valuable roles in increasing organic matter, improving soil structure and creating more diverse and biologically active microbial communities in the soil,” he said.
“But the history goes way back. Thomas Jefferson recommended that legumes be grown to improve the soils of colonial America. Hairy vetch was planted during World War Two when we couldn’t get nitrogen fertilizers.”
Where moisture levels will support two crops, cover crops are planted into existing crops for pest control or after an early crop is harvested to reduce weeds and prevent soil erosion through fall and winter.
Farmers either use herbicides to kill the cover crops or let winter do the job for them.
Winter annuals such as hairy vetch are tough enough to survive most western Canadian winters and will fix substantial amounts of nitrogen for a spring seeded crop.
However, Blackshaw said in some cases these crops are also used as a living mulch.
Research in Manitoba and Ontario has found that legume cover crops such as hairy vetch and red clover can act as a living mulch when planted directly into winter wheat “on the frost” or in early spring.
Nearly two decades ago, Michigan researchers found that red clover planted in this way could deliver as much as 100 pounds of nitrogen per acre in one growing season.
Now retired from Agriculture Canada’s research centre in Swift Current, Sask., Bix Biederbeck conducted soil research in the late 1990s that showed green manure-spring wheat rotations could successfully replace chemical or tillage fallow.
Using chickling vetch, black lentils, field peas and Tangier flat peas, he found these green manures could double or triple the amount of nitrogen available in the soil compared to summerfallow.
Blackshaw’s research has found that nitrogen fixation can exceed 100 lb. per acre when sweet clover is established with a companion crop such as wheat and then left as a summerfallow cover crop the following year.
Nitrogen isn’t the only nutrient that cover crops can make available. Deep rooted covers can draw up other nutrients into the shallow root zone. Crops such as lupin, buckwheat and sweet clover secrete acids that make phosphorus plant-available.
“Sweet clover can use P and (potassium) from rock phosphate and feldspar, making these nutrients available for the subsequent crop … and you can get vesicular-arbuscular mycorrhizal fungi living in the roots of some crops. They also assist in P uptake and make it available to crops later on,” Blackshaw said.
“Any of these strategies, to be of use, has to deliver a net economic benefit for farmers. The cost to the producer and to the grain crop can’t be higher than a replacement with a commercial fertilizer source.”
Blackshaw found that sweet clover as a companion crop with wheat and a second season cover crop had several advantages over traditional systems.
Yields in the first season were comparable to those of a traditional cereal crop. During fall and winter, the clover provided additional soil cover and weed control. As well, a hay crop of clover could be taken in late June and sold, improving the practice’s economic return and sequestering about half of the nitrogen needed for a spring wheat crop.
“Farmers have shown concern that this cover would result in a shortage of moisture the following season, but that wasn’t the case. The soil recharged and stubble from the clover was adequate cover for the fall and winter,” Blackshaw said.
“It would be possible to follow that same year with a winter wheat crop, but we didn’t look at that. We did find that sweet clover did a better job of fixing N in than did annual legumes.”
Studying results
Agriculture Canada’s research centre in Lethbridge has inter-seeded alfalfa, red clover and Austrian winter pea with winter wheat, killing the alfalfa two months after harvest.
“This would result in being able to take a grain crop each year without a fallow season,” he said.
Researchers are still analyzing the data from the three-year study that ended this fall, but it appears the alfalfa performed as expected without hurting winter wheat yields. Subsequent crops of canola appeared to fare better than those without alfalfa grown first.
Austrian winter peas proved too aggressive in some cases, reducing winter wheat yields by 25 percent.
Red clover was subject to winter- kill.
Alberta Agriculture, Agriculture Canada and SARA are growing new cultivars of winter lentils and winter peas developed by American researchers.
They also have the potential to be fall planted and either taken as a grain crop the following summer or terminated as a green manure.
“These strategies are more common in other parts of the world, but for conventional producers here in Canada, these old ideas are new again,” Blackshaw said.
