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Could green manuring fit into growers’ cropping systems?

The terms “cover crop” and “green manure” are sometimes used interchangeably, but usually a cover crop refers to any crop type grown specifically to provide ground cover, and a green manure is a legume cover crop grown to fix nitrogen and increase soil fertility. Cover crops, including green manures, are used to reduce soil erosion, improve soil characteristics, and suppress weeds, insect pests and plant diseases. 

Typically in Western Canada, green manuring involves underseeding a biennial or perennial legume in a main crop like wheat. The main crop is harvested in the first year. Then the legume overwinters, resumes growth in the second year, and is terminated in the middle of the growing season.

Fitting green manures into conventional systems
“In Western Canada, the greatest use of green manure crops is by organic farmers, presently. Because organic farmers are restricted from using fertilizers or pesticides, they have to rely on other methods to increase soil fertility or to manage weeds and other pests, so their use of green manures is very high. But for conventional farmers, green manuring is a bit of a trade-off in terms of the extra time and costs required. It’s certainly easier to use fertilizers and herbicides, and it can be just as cost effective to use those products,” says Dr. Bob Blackshaw, a researcher at Agriculture and Agri-Food Canada’s Lethbridge Research Centre. He has led a number of studies to investigate the effects of different cover crop and green manuring options.

He adds, “In most systems that include a green manure crop, there is no cash flow in the year when it’s a green manure crop. That’s probably the number one reason why most conventional farmers don’t use it.”

But green manuring can help meet a conventional farmer’s objectives in some cases. Blackshaw explains, “There probably has to be more than one objective that a producer is trying to fulfil to really encourage them to use green manures. Maybe they want to reduce erosion on an erosion-prone area of their farm or to increase organic matter levels in a very sandy soil or in previously eroded areas. Then they might want to grow a cover crop or a green manure crop to build up soil health while having other benefits like increasing soil nitrogen and suppressing weeds, insects and diseases.”

Perhaps the most obvious situation when a conventional producer might want a green manure is when nitrogen fertilizer prices are high. “With some of our sweet clover plots, we were getting 90 to 180 lbs per acre of nitrogen. If nitrogen is 25¢ a pound, then producers aren’t that interested. But if nitrogen were to reach $1 a pound, then they’re saving $90 to $180 per acre. So when nitrogen fertilizer prices spiked two or three years ago (2007-08), there was incredible interest in having annual legumes and green manure crops in the rotation,” says Blackshaw.

He has also investigated ways to provide some income in the green manuring year. For example, “in one study we planted alfalfa or red clover into winter wheat, harvested the wheat in early August, and let the legume crop continue to grow until mid-October when we terminated its growth. Then we spring-planted a crop in the following year. In that situation we don’t have a year without cash flow.”

Blackshaw notes, “Once the winter wheat is harvested, the legume has a full amount of sunlight and if there’s sufficient soil moisture, then it will continue to grow quite nicely through the rest of August, September and at least into the middle of October. That’s when we think a lot of the nitrogen-fixing is occurring.”

Underseeding the legume to a fall-seeded crop is unusual. The normal recommendation is to establish perennial legumes in the spring to give them time to develop larger root systems so they can overwinter successfully. “When we tried it, we didn’t know whether any of the legume crops would survive,” he says.

Alfalfa worked quite well, while red clover was more problematic. “We’ve been through three cycles and alfalfa has survived every year. But we had some winterkill with red clover. It had 100 percent survival one year, about 50 percent survival next year and then very little survival in another year.”

The alfalfa option had other benefits, too. “It didn’t reduce our winter wheat yields at all. And it provided about 18 to 36 lbs of N per acre, some weed suppression, and ground cover for erosion control,” relates Blackshaw.

Green manuring methods
“In Western Canada, the two main green manure crops right now are sweet clover, which is a biennial, and in the wetter areas, red clover, which is a perennial. These crops are often established with a main crop, so they might be underseeded in wheat, for instance. Hopefully they remain relatively small as an understory crop in that first year and don’t reduce the yield of the main crop. Then they overwinter and resume growth in the second year,” says Blackshaw.

He notes, “Both sweet clover and red clover compete very well with weeds in the second year because they have an established root system and start growth earlier. They are quite productive and fix lots of nitrogen. As long as we terminate them in the middle of the growing season, then we have a reasonable amount of time to get the soil moisture levels back to where we want them. For sweet clover, termination is around the first of July. For red clover, the timing is not quite so critical; you could terminate it then or let it grow into August.”

Annual legumes seeded in the spring and terminated in late summer or early fall do not usually work very well in Western Canada. “They either don’t compete well with weeds or they don’t produce enough growth in a short enough time and fix enough nitrogen to do a good job,” explains Blackshaw.

Like any legume, the green manure seed should be inoculated with rhizobia. The recommended seeding rate is usually the rate for growing the legume as a regular crop. Blackshaw adds, “If the seeding rate is too high, then its growth in the first year will be excessive and reduce the yield of the other crop.”

Various options can be used to terminate the green manure. “The most common method in Western Canada is using a double disk. We’ve done some research on mowing the crop and leaving the residues on the soil surface, using shallow tillage with a cultivator or the Noble blade cultivator, and using herbicides,” notes Blackshaw.

All of these methods can work reasonably well, but there are some differences in the levels of soil nitrogen, soil protection and weed suppression provided.

In terms of nitrogen differences, he says, “Half of the legume’s nitrogen is in the roots, so that remains the same no matter how you terminate the crop. But some of the nitrogen is in the shoots, and more of that may be lost when the residues remain on the surface. However sometimes the differences are pretty small. We had one experiment where the effects of mowing sweet clover and leaving the residues on the soil surface were very similar to disking. Of course, the advantage of leaving the residues on the surface is reduced soil erosion potential.” 

Weed suppression is better when the residues remain on the surface. “Many weeds require light to germinate and those residues on the surface are shading them. Also, tillage is known to stimulate weed seed germination. So every time we till, we actually encourage weeds to germinate,” he explains.

Growers in dry areas may worry about green manures using up soil moisture. Blackshaw says, “It’s a question of whether there is sufficient time to recharge the soil moisture levels before the next crop. If you terminate a green manure crop in early to mid-July, then you have the rest of that growing season and the following winter to trap snow and get the soil moisture levels pretty close to where they would be otherwise, if you’re planting a spring-seeded crop. However, if you were thinking about planting winter wheat right after the green manure, you may very well have a little lower soil moisture levels.”

He adds, “We’ve found that surface moisture levels are often higher with a green manure crop. That’s because there is more residue on the surface and greater snow trap and greater infiltration. So it’s never a question of getting good crop germination and emergence in the following crop. But at deeper layers in the soil, say 50 centimetres (20 inches) deep, we could have a little less moisture. Then if we went into a dry season or a drought condition, that could be a negative.”

New studies
Blackshaw is currently working on a couple of cover crop studies with the University of Manitoba and the University of Saskatchewan. One project is evaluating termination methods with less soil disturbance, and the other is assessing various cover crop mixtures.

The project on termination methods includes mowing, very low intensive tillage like a Noble blade, and using a roller-crimper, which bends the stalks and rolls down the crop. The researchers are assessing crop termination effectiveness and soil nitrogen benefits.

Some of these termination methods are more appropriate for biennials than perennials. Blackshaw explains, “The nice thing with a biennial like sweet clover is when it flowers, we do our killing treatments around 80 percent bloom, the plant is already starting to end its life cycle and doesn’t really want to regrow. If we tried to do a killing treatment before it starts to flower then it’s in a vegetative stage and it will try to regrow. A disking or herbicide might be effective at that stage, but the less intensive tillage or the mowing treatments would not be effective.”

The other project involves evaluating various mixes of cereals, legumes and oilseeds in terms of biomass productivity, erosion control, soil organic matter and residue decomposition rates.

He explains, “Legume crops are high in nitrogen so the soil microbes use it as a food source and decay those residues very quickly. That can be a good thing if we want to plant the next crop relatively quickly. But if we want to maintain cover to reduce soil erosion, then we might like those residues to last longer. If we mix an oilseed or a cereal with the legume, then the residues would decay more slowly. So we’re looking at whether we can establish them together, will they grow together, and what’s their productivity. Then we are measuring decomposition rates over about 12 to
18 months.”

The long view
Looking ahead, the importance of green manures in conventional systems will likely be strongly affected by fertilizer prices. If prices rise, then more information on the nitrogen benefits of green manures could be especially helpful. Blackshaw says, “We know organic nitrogen is released more slowly and more consistently throughout the growing season (compared to nitrogen from a commercial fertilizer). That may or may not affect yield, but it provides more nitrogen at grain-filling time so we almost always have higher protein content in cereals (after a green manure). There could even be some benefit for oil quality in a crop like canola.”

Another factor that could affect adoption of green manuring is the possibility of incentives to grow them because of their societal benefits, like net reduction of greenhouse gas emissions.

Blackshaw concludes, “Cover crops and green manure crops have been used for thousands of years, and they are still very widely used in less developed countries. We’ve replaced their use with pesticides and fertilizer, and that can work quite well. But perhaps there’s a chance to integrate their use back into our conventional farming systems on a limited scale. In certain situations when fertilizer prices are quite high, or in particular portions of a farm or a field where you have a need to reduce erosion or build up organic matter, there may be sort of niche situations that these crops may be used to start with. And then if producers become more familiar with them and more comfortable with them, they may expand their use on a greater scale.”