Sometimes one plus one equals more than two.
A case in point is combining cover crops with manure.
Cover crops are good. Manure is good. At times, however, cover cropping as a practice has been presented as a sort of alternative to manure; some even refer to cover crops as “green manure” because of the nutrient benefits to soil.
But it doesn’t have to be either-or. In fact, combine the two and you get something that is more than the sum of its parts, experts say.
“There are a couple of benefits to me, but the biggest one is when you put manure and cover crops together you get this sort of synergy,” says Christine Brown, a field crop sustainability specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA).
“First, the cover crops are using the manure nutrients during the growing season when you’ve got the maximum amount of microbial activity in the soil and they all congregate around the root systems. What you get is more biomass and bigger root systems. And when you apply manure during the growing season, you’re not getting (leaching) … and you’re potentially reducing greenhouse gas emissions from nitrous oxide. Basically, it gives you that extra organic matter, water holding capacity and root development a little more quickly when you combine manure with cover crops.”
Another major benefit of combining manure with cover crops is that it can prevent soil degradation or erosion, says Matthew Ruark, a professor of soil science at the University of Wisconsin-Madison. That is especially important with a crop such as corn is harvested for silage leaving the soil exposed and a cover crop combined with manure can greatly reduce the chances soil erosion, he explains.
Combining cover crops with manure is hardly a new idea. In fact, it’s been around for generations. However, the idea appears to be gaining traction in some agricultural communities across North America of late.
It’s a trend Melissa Wilson has witnessed firsthand. Wilson is an associate professor and extension specialist in manure nutrient management at the University of Minnesota. Lately, she has been fielding a lot more questions from producers about how to best combine the two practices.
Wilson thinks there’s a couple of reasons for the increased interest. First, more and more farmers have become interested in using cover crops as they learn about the benefits they provide such as boosting soil health and preventing erosion. Secondly, tighter regulations are being introduced to govern the application of manure, especially earlier in the fall. That means growers in some locations must now have a best management practice in place, such as the use of cover crops, in order to apply manure.
So, what does a grower need to know if they are considering combining cover crops with a manure application on their farm?
A close-up view of manure being injected into a cover crop.
Image courtesy of University of Minnesota
Brown says one of the most common questions she gets from growers is whether its better to plant the cover crop first and then apply manure or apply manure first and the cover crop second. The simple answer is it depends on the type of manure and the conditions on an individual farm.
In a case where the cover crop seed is being planted with a device like a drill, she suggests it is often better to seed after manure has been applied and incorporated into the soil. The benefit to applying manure before seeding is it helps reduce the risk of soil or nutrient movement once the cover crop has emerged from the ground. Post-seeding application can work when good moisture conditions are present in the soil, or when liquid manure can be applied below the crop canopy with equipment like dribble bars or drop hoses, she adds.
Another consideration for growers is to be aware of when the conditions are right for applying manure to a cover crop. In the case of a fall application, Wilson says it’s generally recommended that the soil temperature should be 50 F (10 C) or cooler when manure is applied. That’s because the microbes in the soil have slowed down and they’re not converting nitrogen into a form that is more easily lost, meaning it will stay in place in the soil longer and be more effective.
“Soil microbes convert it into nitrate and that is a form of nitrogen than can leach and can move with ground water,” Wilson says. “In wet years, it’s particularly important to not let it get to the point where it’s in nitrate form. Cooler temperatures really help with that.”
Another consideration is the timing of any application in relation to the development of the cover crop. Brown says farmers should avoid applying manure when a cover crop has just started to emerge and is still in a fragile state because any kind of wheel traffic may negatively impact the plant’s stand. It’s also important not to use any form of highly concentrated manure until after a cover crop is well established since it is likely has a high level of salts that can damage the top growth of a plant or even burn its roots, she explains.
Both solid and liquid manure can be used in conjunction with cover crops. The choice of which form to use often comes down to what a farmer has available to them, Brown says. One potential upside to liquid manure is that it generally has a higher ammonia content meaning nitrogen is more readily available for the crop, she adds.
If solid manure is being used, Wilson cautions growers to use a light application and make sure the cover crop is still visible once the manure has been applied.
“If you put it on too thick and you smother the cover crops that’s obviously going to be a problem,” she says.
Ruark says something growers should be conscious of is the fact that they may need to supplement the amount of nitrogen they apply to a field once a cover crop “gets really big.” A recent study his university conducted showed that once a cover crop produces more than 2,000 pounds of biomass it begins to take up a lot more nitrogen, meaning growers will likely need to add more nitrogen to their soil than they originally planned.
“If you were planning on a certain amount of nitrogen being there for the next crop, that’s going to be less and you’re going to have to supplement it if the cover crop gets above 2,000 pounds,” he says.
Ruark’s advice to farmers who are considering combining cover crops and manure for the first time is to be careful about which type of cover crop they choose. He says the safe choices are grasses that will succumb to winter kill and won’t require any chemical termination in the spring.
“Stick with the core ones, maybe some clovers if you have time, or maybe go with the brassicas such as a mustard or a radish. Stick to the ones that are well-known and we know how to manage.”
One of the most popular cover crops to use in combination with manure is winter rye. Ruark says the reason for that is it provides “really nice cover crop growth,” but does require chemical termination or tillage to kill the cover crop in the spring.
Matthew Ruark says growers may need to supplement the amount of nitrogen they apply to a field once a cover crop grows to be very large.
Images courtesy of University of Wisconsin-Madison
Ruark says another popular cover crop choice in northern climates like Wisconsin is spring barley or oats, especially if they can be planted in early September. But unlike winter rye, they don’t require any chemical termination in the spring, as they typically die off in winter.
Wilson agrees that winter rye is generally a safe choice in northern climates like Minnesota, especially in combination with rye grass. She says a recent study conducted by her school showed that rye grass tends to do better in shaded conditions like when it’s planted into a standing crop. Another project conducted by the school showed a winter rye/oat/radish mix did really well with a manure application as long as there was minimal disturbance during the manure application.
Brown is a strong supporter of using oats as a cover crop. In addition to providing a lot of fibrous roots, oats will winter kill by itself and is also a great natural weed suppressant. The only caveat with oats, she says, is that it needs to be planted relatively early in the fall. If you aren’t planning to plant a cover crop until mid-October, you should probably consider another option. She also likes cereal rye as a cover crop since the cost of seed is relatively inexpensive and it grows well in most situations.
She advises growers to think twice about choosing straight tillage radish as a cover crop. One drawback is if it’s planted too early it can end up plugging up drainage tiles. It can also create a noticeable odour when it winter kills. She recommends anyone planting radishes as a cover crop to mix it with oats which can help it take up nitrogen from manure.
A field demonstrating the difference in cover crop growth with and without manure.
Images courtesy of OMAFRA
Brown also urges growers to do their homework when it comes to applying manure and using eight-way cover crop mixes as they might not be the most economical choice in some cases.
“With the eight-way mix, the nutrients generally are picked up by the oats or the oilseed radish or cereal rye and they outgrow some of those other species. You’re generally better off to just stay with a relatively simple cover crop mix.”
One of the trends in cover crops Brown has noted is an increasing number of farmers turning to harvestable options such as cereal rye and oats. Several recent studies, including one conducted by the On-Farm Applied Research and Monitoring (ONFARM) project, have shown that harvestable cover crops are beneficial because they don’t leave as much residue and provide another source of feed or additional revenue.
A recent three-year study conducted by the University of Minnesota looked at the best ways manure and cover crops can be used in combination. The study’s results showed the earlier a cover crop is planted, the more biomass it will produce, and later is better when it comes to the fall placement of manure.
Still, Wilson says growers need to be patient when it comes to using cover crops in combination with manure. In some cases, it can take four or five years for a farmer to see the benefits from it.
“It does seem with cover crops you really need to kind of wait for the new normal to establish,” she says.
“(Cover crops and manure) changes the dynamics of the nutrient cycling in your soil when you have that added cover crop and residue in there. Just be patient. It may take a few years for you to figure things out but don’t be discouraged. Some of the longer-term yield benefits and nutrient cycling benefits will (take time) to shift.” •
