Now is the time to set your yield potential for next year
Palle Pedersen, Soybean Extension Agronomist
Originally published in the Iowa Soybean Review, October 2006

The biggest topic of interest this year has been the weather and shortage of rainfall in many areas. Many were surprised that soybean were able to “hang in there”, but now looking back it all makes some kind of sense. Soybean has a taproot system and just like alfalfa, canola, and other plants with taproots, they prefer dry conditions. For that reason, soybean was able to develop a larger root system than we normally find in Iowa.

Any reduction in root development in a dry year will have a detrimental impact on yield. Soybean cyst nematodes (SCN) are the issue of interest. It is the most damaging pathogen that we have in our soybeans, and on average it costs us nearly 60 million bushels a year in Iowa alone. Despite this, many still ignore it. A good indicator for SCN is a reduction in canopy development. Remember that high yielding soybeans often have to be managed as a “biomass generator”.  We want as much leaf area as possible during flowering and pod set. Any stress on the plant, such as SCN, that interfere with water and nutrient uptake will influence canopy development and yield significantly. Even if you feel that your SCN populations are low, it is still worth managing. There is not a yield drag between SCN-resistant and susceptible varieties anymore. That is why we need to start managing SCN as soon as we find it.

Fields should be sampled for SCN this fall if they are going to be planted into soybeans next spring. Samples need to be collected before the soil freeze and not when the soil is very wet since nematodes are difficult to extract from soil with the consistency of mud. Keep the samples away from sunlight in a cool area until shipped to either a commercial laboratory or to the Plant Disease Clinic, Iowa State University, 323 Bessey Hall, Ames, IA 50011.

As soon as you get your results back it is important that you take those into consideration when you select your varieties. The difficult part when selecting SCN-resistant varieties is this: SCN-resistant varieties that yield comparably do not necessarily control the nematode equally. SCN-resistant varieties can vary considerably in how well they control nematode population densities. Selecting SCN-resistant varieties based solely on yield data is short-sided and risky, because some relatively high-yielding, SCN resistant soybean varieties can still build up the nematode population densities. Yes, the long-term consequences may be much larger than you really think. The best way today to select SCN-resistant varieties is using the ISU SCN variety trial conducted by ISU Extension Nematologist Dr. Greg Tylka. Information about the trial can be found at www.isuscnvarietytrials.info.

Last, you need to remember that it is worth waiting to get the information from the variety trial. Your discounts for ordering early or being an extremely large farmer will never be more than $15 per acre or 3 bushels. You can easily pick up 12, 15 or even 20 bushels per acre by selecting the right variety!