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Cover crops in second-year soybeans


November 30, 1999
By Carolyn King

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Cover crops can provide a lot of benefits. They protect and improve the soil; increase yields of the main crop; and reduce weed, disease and insect problems. So they could help decrease many of the problems that plague cropping systems with continuous or frequent soybeans. But would they help enough to be a practical option for Ontario growers?

Two projects are working on the answer to that question. One is a recently completed three-year study led by Adam Hayes from the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA). The other is a project that is underway, led by Dr. Bill Deen and Dr. Dave Hooker, both from the University of Guelph. “In Ontario, we have a significant number of acres with either continuous soybeans or frequent soybeans in the rotation,” explains Deen. “There are issues with these soybean-intensive rotations related to a lack of surface protection, making the soil susceptible to wind and water erosion, and this leads to issues related to offsite movement of sediment and nutrients. Soybean-intensive rotations are also associated with reductions in soybean yield, poorer soil structure, lower soil organic matter, lower water-holding capacities and higher incidence of soybean cyst nematode (SCN).”

He adds, “The other issue is that soybean-soybean-soybean may undermine the success of no-till production systems. No-till doesn’t work as well if it is combined with a poor or no rotation. If you add the compounding effects of poorer soil structure, lower organic matter and other problems of soybean-intensive rotations, the chances of no-till working well are reduced even further.”

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These sorts of issues prompted Hayes to set up his study in 2006 to assess the value of cover crops in second-year soybeans.

Funded by the St. Clair Region Soil and Crop Improvement Association, the study had sites in Essex, Kent and Lambton counties. Hayes says, “In this area, with a lot of poorly drained clay soils, many growers don’t want to grow corn, mainly because they don’t believe it will yield well enough to cover input costs. That leaves them with soybeans and wheat. Often they feel they can make more money on soybeans than wheat, or they plant wheat but it may not make it through the winter, so they end up growing quite a few years of soybeans in a row. That is not good from several perspectives, including disease, soybean cyst nematode, soybean yield and soil health. So a number of the farmers in the area were interested to see if something could be done to try to help improve yields of soybeans after soybeans and reduce some of the other problems.”

Hayes chose rye and winter wheat as the cover crops. “Rye and winter wheat will continue to grow until winter sets in, and then in spring they grow again until they are terminated.” That means they are able to protect the soil and outcompete weeds. “Also they have fibrous root systems, so they provide some benefit to the soil structure and soil organic matter. And some research has shown that rye could help reduce soybean cyst nematode populations,” he explains.

The three-year study
Cover crops were seeded in the fall after soybean harvest. Because the sites were located in southwestern Ontario, the growing season was long enough for the cover crops to become well established before winter, despite some difficult fall weather. The cover crops were terminated with glyphosate in the spring. Then, the co-operators followed their normal soybean seeding practices, either using a no-till drill to seed into the cover crop residue or, in a few cases, tilling and then seeding. Some of the trials included a corn residue strip to compare a corn-soybean rotation with continuous soybeans.

Hayes summarizes some of the key results: “The study showed cover crops could be grown at minimal added expense to the grower. But they didn’t provide as much of a yield benefit for the second-year soybean as I would have liked on the clay soils. Although the cover crop provided quite a significant advantage at the sandy loam site, unfortunately we didn’t have enough other sites like that to verify that.”

The study’s results also reinforced that soybean yields are higher in soybeans after corn than in soybean after soybean. He says, “A good rotation, which can include corn, provides more benefits. You get that break in diseases and a 10 to 15 percent yield boost.”

Although most of the sites had either no SCN or only moderate infestations, the project did provide some verification of a rye cover crop’s ability to control SCN. “In one of our fields, the soybean cyst nematode population was very high, and where we had the rye cover crop, we saw a reduction in the SCN populations,”
notes Hayes.
He also found that a rye cover crop in combination with an SCN-resistant soybean variety further reduced SCN populations.

Overall, rye tended to provide greater cover crop benefits than winter wheat. He says, “When we did see a yield advantage, it tended to be with the rye, and we certainly got a lot more growth and better weed suppression with the rye.”

The effects on the soil were difficult to assess because the cover crop was only in place for a short time each year and because the project lasted for only three years. Hayes adds, “But I think if the grower got more comfortable with using a cover crop, he could potentially look at letting the crop grow a bit longer and having more organic material on the surface and more root growth providing more organic matter to the soil, Over time there might be more soil benefits.”

Current study
Deen and Hooker’s project could provide an opportunity to assess such longer-term effects. Their project, which started in 2008, is comparing three main systems to determine their effects on factors such as soybean yield, soil characteristics and SCN: continuous soybeans with no cover crop, continuous soybeans with a cover crop, and a corn-soybean rotation.

The cover crops in the study are annual ryegrass and cereal rye. The researchers chose cereal rye because it is a common cover crop in Ontario, it tends to be very hardy and establishes well, and it may reduce SCN. They chose annual ryegrass in part because it is a cover crop of interest in many parts of North America. “We want to test some of the claims being made about annual ryegrass in the US. Proponents of annual ryegrass claim a long list of benefits including improved soil structure and reduced nitrogen losses, and improved success of no-till,” says Deen.

Fall-seeded annual ryegrass tends to have fairly good winter tolerance. In the spring, it has to be terminated before it elongates; after elongation, it becomes very difficult to control. It does not produce a great deal of biomass; proponents in the US believe its benefits are due mainly to the plant’s extensive, fibrous root system.

The researchers are also evaluating two approaches to seeding the cover crop: drilling it in after soybean harvest and broadcasting it into the soybean crop at the pre-leaf-drop stage. They are terminating the cover crops with a glyphosate burndown.

All treatments are split into a no-till versus conventional till comparison.

The plots are located at the University’s Elora Research Station and its Huron Research Station at Centralia, north of London, Ontario. Deen notes, “You get the biggest problems with continuous soybean and the biggest benefit from cover crops on more marginal land with poorly drained, heavier textured soils. However, at Elora we have a pretty good soil type, and so the effects of continuous soybean and the mitigating benefit of cover crops may be lessened. But if we see benefits there, we would expect to see greater benefits on some of our heavier textured, more poorly drained ground.”

In the project’s first two years, both locations had a very cold fall, resulting in a late soybean harvest and a limited growing opportunity for the cover crop. In 2009, there was no yield benefit for soybeans with a cover crop compared to soybeans without a cover crop. The results for 2010 are not available yet.

Given the cold fall conditions, it is not surprising that results were better when the cover crop was seeded into the soybean crop at pre-leaf drop. Deen adds, “In a lot of our soybean production region in Ontario, I think we are looking at pre-leaf drop seeding for the cover crop to extend the growing season.”

The researchers are monitoring for SCN, but have not detected this pest in any of the plots so far.

More study needed
The first two years of Deen and Hooker’s project were funded by OMAFRA’s Great Lakes Program. The researchers hope to obtain more funding for the study by 2013 when they expect to be ready to do some additional measurements, such as testing of soil properties. Deen notes, “I think we’ll find within just a few years that continuous soybean is starting to degrade the soil relative to corn-soybean. But the question is, to what extent will the introduction of the cover crop reduce or eliminate that effect?”

It is too early to say whether cover crops make economic sense in a soybean-intensive rotation. Deen says, “We may find there is a substantial yield improvement that justifies all this effort, but we may find the yield benefit alone does not justify using cover crops.” Also, it is also not clear yet whether the effect of cover crops in reducing insect, disease and weed problems would sufficiently reduce chemical pest control input costs to affect the overall economics of the system.

If the direct economic benefits to the producer do not support using cover crops, then Deen suggests, “Perhaps some sort of cost-shared initiative would need to be put in place to encourage farmers to use cover crops. That cost-share is justified on the basis that there may be a yield benefit for the farmer and there are benefits for society. For instance, cover crops sequester carbon, keep soil in place and out of our waterways, and perhaps keep nutrients from moving off site.”