Planting Soybean After Soybean (Part 2): In-Season Management Considerations

By Jenny Rees - Extension Educator | Roger Elmore - Extension Cropping Systems Agronomist | Loren Giesler | Thomas Hunt - Extension Entomologist | Suat Irmak - UNL Extension Water Resources Engineer | Amit Jhala - Extension Weed Management Specialist | Robert Klein - Western Nebraska Crops Specialist | Nathan Mueller - Extension Educator | Charles Shapiro - Extension Soil Scientist—Crop Nutrition | James Specht - Emeritus Professor of Agronomy | Allan Vyhnalek - Extension Educator | Robert Wright - Extension Entomologist

 

 

In Part 1 of this article, we look at considerations for planting soybean after soybean. In this article, Part 2, we share considerations for in-season management.

 

 

Weed control can be extra challenging when soybeans follow soybeans versus being rotated with other crops, particularly if herbicide-resistant weeds are present. Six weeds have been confirmed resistant to glyphosate in Nebraska and they are widespread. Remember, post-emergence herbicide options for weed control in Roundup Ready soybean are far less than corn. Therefore, pre-emergence herbicides should be applied within two to three days of planting soybean.

 

Several new herbicides were recently registered for pre-emergence weed control in soybean. A weed control plan including multiple effective modes of action should be in place to ensure effective weed control. Design the weed control program to target specific weeds to obtain effective weed control and reduce cost of herbicides and application. For example, if glyphosate resistant marestail is present in the field, consider applying herbicides in early spring (at least three weeks before planting soybean). This is important because the majority of marestail emerges in early fall and starts bolting in early spring.

 

Post-emergence herbicides are very limited for effective control of glyphosate-resistant marestail in Roundup Ready soybean. You may also need to consider an additional post-application depending on how quickly canopy closes, especially when you have common waterhemp and Palmer amaranth in the field. If the infestation of these weeds are heavy in the field, consider tank-mixing a residual herbicide with post-emergence herbicide. This combination will provide foliar as well as residual control of common waterhemp and Palmer amaranth to reduce the seed bank. Herbicide programs like these are important for effective weed control now and in the future. Please refer to Guide for Weed, Disease, and Insect Management in Nebraska.

 

 

In general, insect pest considerations for planting soybean after soybeans would be the same as for planting soybean after corn or other crops, with the possible exception of bean leaf beetle and Dectes stem borer.

 

If late season bean leaf beetle populations were high the previous year, the winter was relatively mild, and wooded areas and/or alfalfa fields are nearby, there may be a greater chance of economically damaging levels of overwintered beetles colonizing the first-planted soybean. This is of increased concern if Bean Pod Mottle Virus is a problem in the area. In this case, an insecticidal seed treatment and scouting of emerging and seedling soybean fields is advisable.

 

In the case of Dectes stem borer, if the field was infested with Dectes the year before, they can re-infest it upon emergence. This may result in higher populations than when they have to disperse a distance to find soybean fields and spread out over more than one nearby field.  In rotated soybean fields, often Dectes are initially more abundant on the field borders as they move into the field. This would not be the case in soybeans after soybeans, as larvae overwinter in infested stems at the base of the plant.

 

 

In general white mold would not be expected to be more severe in continuous soybean than in rotated soybean. The main concern for other diseases would be foliar diseases.

 

Frogeye leaf spot is becoming more common in Nebraska and fields with a history of this disease will need to have varieties with resistance or the field will need to be scouted for possible foliar fungicide application. In addition, brown spot will be more severe in continuous soybean and will result in some amount of yield loss without fungicide application. Typically this loss is low and does not warrant an application unless frogeye is also present. (Timing of the application should be R3 to early R4.)

 

 

Soybeans remove more nitrogen than they produce so there will be a net decrease in total nitrogen in the soil. Since they fix about half of what they need, the net removal is about 2 lb N/bu of yield. Most likely the next time corn is grown, the traditional soybean credit can be used, just be aware this is most likely due to lack of high carbon residue and not additional nitrogen.

 

While most Nebraska soils are well supplied with potassium, corn removes only 0.25 lb K2O per bushel compared to soybeans that remove 1.2 lb K2O per bushel.

 

The most important impact will be on soil organic matter. In order to maintain soil structure and other beneficial physical properties, plant residue needs to be left on the field. The high carbon content of corn is an excellent carbon source. Soybeans produce less total residue, have a greater nitrogen content allowing for quicker degradation, and do not help with soil organic matter maintenance.

 

With soil testing and nutrient management, the removal issues can be resolved. Unless an alternate source of carbon is found, such as a fast-growing grass cover crop or cattle manure applications, the carbon/soil organic matter issue will remain.

 

Some have suggested applying nitrogen to soybeans. In many Nebraska studies on rotated soybeans there is about a one-in-three chance of increasing yield by about one bushel, usually at nitrogen (N) rates that are not economic. In a three-year study on a sandy loam soil near Brunswick, continuous soybeans yielded 64.1 bu/ac without nitrogen and 64.2 with 100 lb N/acre. 

 

Alternatively, if manure was spread last fall in anticipation of growing corn in 2017 and the plan changes, the cost of the manure application becomes greater since the nitrogen credit will mostly disappear; this could mean that the spreading went from saving money to costing money, depending on many details.

 

 

In a soybean irrigation study at UNL’s South Central Ag Lab near Clay Center the first irrigation event was delayed until R3 (beginning pod) in two treatments. These plots then were irrigated at a rate of either 100% or 75% actual evapotranspiration (ETa) replacement. Delaying irrigation events to R3 resulted in the greatest yield and water use efficiency (Irmak et al., 2014).

 

The research demonstrated that delaying the onset of irrigation until the R3 stage and practicing full irrigation thereafter for soybean grown on silt loam soils resulted in yields (and crop water productivity) that were similar to full-season irrigation scheduling strategies. This result may be applicable in other regions with soil and climatic characteristics similar to those in south-central Nebraska. Delaying irrigations to the R3 stage, but irrigating at 100% ETa replacement, resulted in 1 inch less water application with greater yields as comparted to the fully irrigated treatment. Delaying irrigations up to the R3 stage and irrigating at 75% ETa replacement thereafter resulted in up to 1.7 inch less irrigation water withdrawal with greater yields compared with fully irrigated treatment. This difference can result in substantial reduction in fuel input cost.

 

Nitrogen timing was also studied in this research. These irrigation treatments also were used to evaluate soybean response to nitrogen (N) application methods (i.e., a pre-plant method versus N injection using the SDI system). Nitrogen injection through the SDI system resulted in slightly greater, but non-significant (p > 0.05), yield than pre-plant applied N in the first year, but slightly less yield in the second year. Thus, the primary advantage of N injection would be mainly operational convenience, as well as less fuel use. When averaged over years, the combined mean yield of the two N application treatments was greater (by about 3%) than the combined mean yield of the two non-N controls. That yield difference may not be economically worth the cost of the N fertilizer, unless the producer can contract the sale of the soybean seed at a commensurate price per ton.

 

Another consideration is to avoid irrigation during bloom whenever possible. This will reduce the development of white mold in parts of the state (northern areas) prone to this disease.

 

 

It will be important to keep in mind the in-season management considerations provided in this article including: