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April 29, 2004 Planting Soybeans on Soybeans Current high soybean prices and good loan support have some producers considering planting two, three or even more years of continuous soybeans in some fields, according to Duane Berglund, North Dakota State University Extension agronomist. “This may appear to be a good option after the initial pencil pushing but should be examined closely. Although production costs may be lower, there is a good chance that yields may be lower as well. Additionally, producers may be setting themselves up for long term management headaches.” The two major concerns with planting soybeans on soybeans are disease potential and buildup and weed specie shifts. “It is not a
certainty that a second year of soybeans will produce poor yields, although
it does tempt fate,” Berglund says. “Organisms that attack
soybeans are a major concern. Problems that can occur are white mold (Sclerotinia),
certain root rots, brown stem rot and soybean cyst nematode (SCN was identified
in southeastern N.D. in 2003). “It has been documented in other
states that SCN is a major problem in continuously planted soybeans,”
Berglund says. “If we have a dry or very dry summer, disease incidence could be down,” Berglund says. “However, if we have normal or above normal rainfall this summer, disease outbreaks could become a major problem. One only has to remember the early 1980s when we planted sunflower in tight rotations which caused disease and insect problems. That led to sunflower acreage reductions and production losses.” The higher the disease pathogen levels, the greater the potential for yield loss. “Some good news, some root rot causing pathogens (Phytophthora, Fusarium, Pythium, and others) are already prevalent in the soil so a few more probably won't make a major difference,” Berglund says. There are now a number of cultivars that are resistant or tolerant to Phytophthora root rot. It mostly 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 can occur because it escapes by herbicide tolerance or time of emergence. The increase of certain weeds such as nightshades, biennial wormwood, ALS resistant Kochia, horseweed (marestail) and waterhemp are examples. By planting the second or third year of soybeans and applying the same herbicide or using the same cultural practices, you are helping speed the selection process along. From a natural ecological perspective, a field wants to have weeds growing in it. The objective is to prevent a buildup of those weeds that are hard to control. “If you have no other choice, maximize the disease defensive characteristics in the varieties planted,” Berglund says. “Varieties with lower susceptibility to white mold should be selected. Go to wide rows (30 inches) as this planting pattern is less susceptible to white mold problems. Planting shorter season varieties and delayed planting may, in some years, help reduce white mold pressure.” Producers should consider using a different weed control program in 2004 than in 2003 if certain weeds are starting to become a major problem. Rotate chemical families/modes of herbicide action. Condition and test any “conventional” saved seed if that is what you will be planting. If you suspect the seed may be infected, a seed treatment may be appropriate depending on the disease. “Remember that Roundup Ready soybean cannot be saved and replanted,” Berglund says. “It’s against the law!” Moldboard plowing may help control some (but not all) of the fungi and bacteria that survive on residue. Heavy tillage of soybean residue may have other negative impacts so spring moldboard plowing is not advisable. Soil tilth can be destroyed which may increase the potential for erosion. Reduce or eliminate any nitrogen application. Also, you may not need to inoculate seed unless you had poor nodule formation in last year’s crop. Phosphorous fertilization may be required, depending on soil tests and crop yield last year. ### Source:
Duane R. Berglund, (701) 231-8135, duane.berglund@ndsu.nodak.edu |
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North Dakota State University |
