Planting Soybeans on Soybeans May Not Be a Good Idea.
Lower input costs, perceived simplicity of the Roundup Ready system and lower machinery requirements at harvest are reasons farmers site for growing soybeans after soybeans. However, planting soybeans after soybeans does result in yield losses compared with planting soybeans after corn or some other crop. Planting soybeans after soybeans also promotes certain pests that can rob even more yield potential.
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. 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. 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 whether 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. Weed species will try to fill any bare soil. Soybeans planted at the same time each year and sprayed with the same herbicides each year will favor weeds that are less sensitive to the herbicide or weeds that emerge and reproduce at periods when the herbicides are not being used. Continuous soybean production can lead to more weed problems.
Suggestions for producers in northern states who have no choice other than planting soybeans on soybeans where a higher risk of white mold disease exists. 1. Maximize the disease defensive characteristics in the varieties planted. Varieties with lower susceptibility to white mold or other diseases should be selected. 2. Go to wide rows (30 inches) because this planting pattern is less susceptible to white mold problems. 3. Planting shorter-season varieties and delaying planting may in some years help reduce white mold pressure. 4. Use a different weed control program each year soybeans are planted if certain weeds are starting to become a major problem. 5. Rotate chemical families/modes of action. 6. Condition and test any saved seed that will be planted. If the seed is possibly infected, a seed treatment may be appropriate, depending on the disease. It should be noted however to growers that Roundup Ready soybeans cannot be saved and replanted because it’s against the law. 7. 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. Heavy tillage can increase the potential for soil erosion. 8. Do not use nitrogen fertilizer. In regions where DAP (diammonium phosphate) is the most reasonable source of phosphorus, then the amount of nitrogen used will not help or hurt the soybeans. 9. Producers also do not need to inoculate their seed unless there was poor nodule formation in last year’s crop. Phosphorous (P) and potassium (K) fertilization maybe required, depending on soil tests and crop yield removed last year.
Research results show a 5-10% yield loss in second year soybeans and beyond. This yield loss is often due the “rotation affect”. Yield loss can be considerably greater due to increased insect and disease problems as well as decreased organic matter and reduced soil quality.
In a soybean rotation experiment at Carrington, ND, yield was reduced by over 20% with 4 years of continuously-grown soybean compared to soybean grown on the previous year's wheat ground. The major factor contributing to the yield reduction was root disease. Soybean cyst nematode is not present at the experiment location.
For more information on this or other topics related to soybean production, contact your state extension specialist or your local extension educator/agent.
The following is the link to the soybean extension specialists: state extension soybean specialist