Soybeans on Soybeans a Bad Idea

FARGO - May 13/05 - SNS -- The availability of Roundup Ready soybeans is encouraging some growers to consider planting two, three or more years of soybeans in the same field. But, this is a bad idea, warns Duane Berglund, North Dakota State University (NDSU) Extension Service agronomist.

"This may appear to be a good option after the initial pencil pushing, but should be examined more closely. This may look good in the short run, but over a longer period could be a disaster.

"Although production costs may be lower, there is a good chance that yields also will be lower. Growers may not know the potential yields they have lost because of planting soybeans on soybeans. Additionally, producers may be setting themselves up for long-term management headaches," Berglund said.

Disease problems and weed species shifts are the two major concerns with planting soybeans on soybeans.

Organisms that attack soybeans, such as certain root rots, white mold, brown stem rot and soybean cyst nematode (SCN), are a major concern. Soybean cyst nematode has been identified in southeastern North Dakota. White mold, certain root rots and SCN persist in the soil. Brown stem rot overwinters and can live as a saphrophyte on soybean residue.

A few other organisms that survive the winter on residue include those causing bacterial blight, stem canker, pod and stem blight and brown spot.

The first year of soybeans allows reproduction and buildup of disease inoculum and nematode populations. The third or fourth year of soybeans planted into elevated disease or nematode levels can have drastic results, such as much lower yields.

"The higher the pathogen levels, the greater the potential for yield loss," Berglund says. "A piece of good news is that some root rot-causing pathogens (phytophthora, fusarium, pythium and others) already are prevalent in the soil and a few more probably won't make a major difference. In addition, there are a number of soybeans cultivars that are resistant or tolerant to phytophthora root rot. It depends on the race and if you have the correct resistant gene in your variety."

Weed species shifts or the increase of a particular weed species in the population of a field are known to occur because the species may be herbicide tolerant. The time of soybean plant emergence also can make a difference. The increase of certain nightshades, such as biennial wormwood, ALS resistant kochia and waterhemp are examples.

"From a natural ecological perspective, your field wants to have weeds growing in it," Berglund says. "The objective is to prevent a buildup of those weeds that are hard to control. By planting the second or third year of soybeans and applying the same herbicide or using the same cultural practices, producers are speeding up the selection process."

Berglund has these suggestions for producers who have no choice other than planting soybeans on soybeans.

* Maximize the disease defensive characteristics in the varieties planted. Varieties with lower susceptibility to white mold should be selected.

* Go to wide rows (30 inches) because this planting pattern is less susceptible to white mold problems.

Planting shorter-season varieties and delaying planting may in some years help reduce white mold pressure.

* Use a different weed control program in 2005 than in 2004 if certain weeds are starting to become a major problem. Rotate chemical families/modes of action.

* Condition and test any saved seed that will be planted. If there are suspicions the seed may be infected, a seed treatment may be appropriate, depending on the disease. Berglund reminds growers that Roundup Ready soybeans cannot be saved and replanted because it's against the law.

Moldboard plowing may help control some, but not all of the fungi and bacteria that survive on residue. "Unfortunately, heavy tillage of soybean residue has other negative impacts, so spring moldboard plowing is not advisable," Berglund says. "Heavy tillage can increase the potential for soil erosion."

Reduce or eliminate any nitrogen application, Berglund advises. Producers also do not need to inoculate their seed unless there was poor nodule formation in last year's crop. Phosphorous fertilization maybe required, depending on soil tests and crop yield removed last year.