Agronomist reviews various cover crop options, discusses nitrogen fixationWritten by Natasha Wheeler
“It takes a lot of research and a lot of time to figure out what works where,” notes Dave Robison, Legacy Seed forage and cover crop agronomist.
Between 2009 and 2013, Robison and his fellow researchers placed over 450 cover crop plots in Ontario, Canada, the eastern Corn Belt and the upper Midwest to study how cover crops relate to nitrogen fixation.
“When we look at different cover crops, nitrogen production is one of the benefits we’re going to get from them. Several legumes make good cover crops, and how much nitrogen they produce depends on several factors,” he comments.
Many legumes are sold pre-inoculated, but Robison warns growers that some might not be. Inoculated legumes had better success in their research.
Moving into an overview of different crop species, Robison says that Austrian winter peas are a key product in many situations, especially when planted after wheat or other cereal grains.
“In most areas, it will winterkill. It would be nice to have five or six weeks of good growth, and longer is generally better with peas. We can produce 70 to 135 pounds of nitrogen per acre with Austrian winter peas,” he explains.
Field peas are generally less winter-hardy than Austrian winter peas, but they make a good short-term cover crop, produce excellent forage and work well for weed control, according to Legacy Seed research.
Earthworms love crimson clover, often more than other clovers or peas, and the clover can produce up to 140 pounds of nitrogen per acre within a few months.
“This is my favorite cover crop because it can produce a whole lot of nitrogen in a short period,” Robison remarks. “In 90 days following wheat, we have been able to measure up to 140 pounds per acre of nitrogen.”
“Medium red clover is probably the least costly cover crop. Although it rarely happens, it can get too tall in wheat and affect the harvest. We can produce 75 to 200 pounds per acre of nitrogen. It has a good root system, is a good soil producer, is easily killed and makes excellent forage,” he explains.
Yellow blossom sweet clover is another cover crop option, although it hosts the soybean cyst nematode and is not recommended in soybean fields.
Although, Robison remarks, “It is an excellent soil builder – maybe one of the best soil builders.”
Hairy vetch can produce 100 to 200 pounds of nitrogen per acre, and most of its nitrogen is found in the top growth. It generally has a more shallow root system and older varieties may present a challenge with hard seeds.
“I am learning there may be some different varieties coming out with less hard seed,” he comments.
Chickling vetch is generally more costly, but 50 percent of the nitrogen it produces is reportedly available for the following crop.
“Chickling vetch can produce a lot of nitrogen, 60 to 100 pounds per acre, and it’s a crop that really has a lot of promise and benefit if we can get more seed production and get the seed cost down,” he notes.
Sunn hemp is a cover crop that has gained some recent media attention. The summer legume should be planted nine weeks before a killing frost.
“It can produce up to 120 pounds of nitrogen per acre,” he says, adding that although it has historically been an expensive crop, seed supplies have been increasing in recent years.
“Mung beans are hard to find, so they’re used for sprouting. They're an excellent crop for heat and drought tolerance and a good nitrogen fixer, and the crop can be hayed and grazed,” Robison continues.
Nitrogen scavengers are also important for retaining nitrogen within the soil, and Robison says that radishes and peas are great to use with cover crops and manure.
“We know that turnips are excellent scavengers. There’s not a lot of money spent on advertising turnips, so we don’t usually think about turnips a whole lot when we think about cover crops. But, turnips have very similar scavenging abilities to radishes, and if we’re using the right turnips, we have a lot of soil activity, as well. That can be very beneficial,” he remarks.
Sudan grasses or sorghum-Sudan grasses, milo or other summer and annual grasses can also serve as nitrogen scavengers, with the ability to sequester up to 200 pounds of nitrogen per acre.
“Annual ryegrass is a high risk, high reward cover crop. It can be difficult to kill, and it probably has the deepest roots, but there are millions of acres that have been killed effectively over the years. It probably has the deepest and most fibrous root mass of any of the scavengers that we’re going to be able to find. It’s an excellent scavenger of nitrogen,” he states.
Winter rye can also become a challenge if not monitored carefully, as it can grow and spread quickly in the spring. Yet, it has good rooting depth and winter hardiness and has the greatest opportunity for success planted later in the year with any type of cover crop, according to Robison.
“Even if we have short cover crops, especially when we’re getting into some of our clovers, that doesn’t necessarily mean we aren’t getting a significant amount of nitrogen production,” he comments, adding that the health of scavenger plants can indicate available nitrogen levels in the soil.