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The Risk of Beans on Beans (on Beans)

Friday, September 15, 2006
filed under: Rotation

It might come as a surprise to learn just how full of beans – literally – the Northern Plains have become.

Take North Dakota, for example. Cass County in N.D. has led all U.S. counties in soybean acreage for ten consecutive years, and has been at or near the top during that time for highest total production. Last year, the southeastern N.D. counties Cass (17 mil bu), Barnes (11) Richland (10.7) Stutsman (10.6), and LaMoure (9.3) each produced more soybeans than the statewide production of Georgia (4.5 mil bu) Alabama (4.8) and Texas (5.9).

South Dakota leads the nation in the adoption of biotech soybeans. Last year,

only Illinois and Iowa produced more soybeans than Minnesota, the first state in the U.S. to require the use of biodiesel.

However, with all those soybean acres – and a good number of continuous beans on beans – agronomists warn that the soybean bubble could burst like the sunflower bubble did in the early 1980s, when growers began to run rotations too short, resulting in an increased risk in pest problems.

North Dakota State University extension agronomist Duane Berglund advises no more than two years of continuous beans. “It’s the third year of continuous beans and on where we risk running into all kinds of potential problems,” says Berglund.

Research indicates yield potential drops further in three or more years of continuous beans, compared to two years of continuous beans. The decline is about 4 to 5 bu/ac, a loss of about $20 to $25 per acre. Berglund speculates that poor nodulation and weaker root systems in continuous beans are a key contributor to lower yields. The yield effect can be hidden, if there is no check field to compare continuous beans to beans in a proper rotation with a grass crop like wheat or corn.

Berglund says the production risks of continuous beans are many:

• Reduced root structure and health

• High potential for root rot

• Increased potential for foliar and stem diseases

• Sclerotinia risk

• Risk of weed species shifts in Roundup-Ready soybeans

• Reduced soil structure/tilth

Soybean aphids were a problem this year in many soybean fields, a function not of crop rotation but wide availability of the host crop – soybean aphids are exclusive to soybeans – together with weather favorable for development of not only soybean aphids but other types of aphids and insects as well. Since weather every year is different, there’s no way of predicting at this point whether soybean aphids will be a problem again next season.

SCN Found in Cass, Clay Counties

Soybean Cyst Nematode is a ‘different animal,’ so to speak, than the soybean aphid; it is affected by crop rotation (or lack thereof) and is recognized as one of the most serious yield threats among soybean pests in the U.S.

The plant-parasitic nematode is a microscopic roundworm, which leaves eggs in the soil that can remain viable for years even in the absence of a suitable host. SCN feeding on the soybean root system may reduce N-fixing nodules, and result in increased susceptibility to a number of soybean diseases, including sudden death syndrome.

Yield losses up to 30% have been measured from SCN. Hot, dry weather may be conducive for plants to show the above-ground symptoms of SCN infection such as stunting, yellowing, and premature death.

SCN was first reported in North America in 1954, in North Carolina, and has since spread to 28 soybean-producing states and Canada. In Minnesota, SCN was first detected in 1978 near Frost in Faribault County. By 2000, its presence had been detected in 52 counties in the state, according to the U of M.

A soybean field in Cass County near Argusville was confirmed by NDSU plant pathologist Berlin Nelson this summer as being infested with SCN. This is the first confirmation of SCN in Cass County. The field apparently had a fairly high population of SCN, and most likely has been infested for a few years.

Richland County was the first and only county in North Dakota with confirmed SCN prior to this find. Across the river in Minnesota, SCN was confirmed in Clay County for the first time this year as well. A map that shows SCN confirmations in other states is available at the North Central Soybean Research Program Plant Health Initiative website:

While southern Minnesota is largely a corn-bean rotation, some growers are looking for other crops to work into their rotation. In Yellow Medicine County of Minnesota, for example, southeast of Watertown, S.D., some growers are finding that sunflower is a good option to work in with soybeans in rotation with corn, and some wheat.

“It’s one less year of beans,” says Dave Dybsetter, Helena Chemical, Canby, Minn. “Certainly rotation helps with SCN, but it’s not the first and foremost factor. Iron chlorosis is the bigger issue around here, and sunflowers will grow in soils that soybeans will not.”

Once established, SCN cannot be eradicated; it can only be managed, says Carl Bradley, extension plant pathologist at North Dakota State University. “The keys to management are good crop rotation practices – no more soybeans on soybeans – and the use of adapted resistant varieties. There are currently a little more than a handful of SCN resistant varieties that are adapted to our growing conditions and maturity, but more are on the way.”

The Plant Health Initiative has an excellent SCN Management Guide online at that includes scouting, management, and sampling information. – Tracy Sayler

Following Soybeans with Sunflower

Most agronomists say it is preferable to follow a broadleaf crop like sunflower or soybean with a grass crop like wheat or corn, but that sunflower can follow soybeans, keeping field history in mind.

A change in federal crop insurance rules several years ago allows sunflower (both oil and non-oil) to be grown on fields planted to soybeans, dry peas and lentils the previous year, without affecting federal crop insurance coverage.

USDA’s Risk Management Agency made the policy change since Sclerotinia head rot is caused primarily by windborne ascospores – the biggest risk of head rot in sunflower is when weather is conducive for the development and spread of these ascospores, which may be blown in and infect sunflower and other at-risk crops regardless of crop rotation. North Dakota State University still recommends a minimal crop rotation interval of three or four years from highly susceptible crops such as canola and dry beans, where Sclerotinia is called white mold.

In the absence of weather conditions favorable for disease, there is no risk in sunflower yield or quality planted on the previous year’s soybean ground, says NDSU extension agronomist Duane Berglund. In fact, there may be an agronomic benefit. Weed pressure should be greatly reduced following Roundup-Ready soybeans (or Roundup-Ready corn) and Berglund says sunflower would also take advantage of a 40-lb/ac nitrogen credit following soybeans.

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