Do you know your number?
Palle Pedersen, Soybean Extension Agronomist
Originally published in the Iowa Soybean Review, November 2006

In the last issue of the Iowa Soybean Review, I talked about the importance of variety selection and how we very easily can pick up 10 to 15 bu per acre if we are just careful when selecting a variety. That means selecting a variety that is high yielding, consistently high yielding across numerous locations and has the right disease package. For a large part of Iowa, it should have resistance to soybean cyst nematode (SCN).

In 2004, I initiated the “Iowa Yield Potential” project, funded by the checkoff and the Iowa Soybean Association. The first stage of the project will end this fall, and we will continue on the second stage starting in 2007. One thing I would love to share with you from this project is the importance of variety selection. I will give you an example on why you need to know your number; your SCN number. And yes, size does matter. Dr. Greg Tylka has been preaching for the last decade about the importance of sampling for SCN and to start managing SCN when the numbers are low. It is easy to keep a population low, but it takes a lot to drive a high population down.

In figures 1 and 2 you will see an example from the “Iowa Yield Potential” project. This data set was from 2005 and conducted on a farm near Whiting, Iowa, where both in 2004 and 2005 we were able to produce more than 100 bu per acre in plots. This farm is very productive and the SCN population for this specific field we used in 2005 was 151 eggs per 100 cc soil at planting. Many would believe that a population like this is nothing we should really worry about. That was true 10 years ago when there was a yield drag between SCN resistant and SCN susceptible varieties. However, that is not the case anymore. In fact, it is anything but. The problem at this location is that some varieties are building the SCN numbers from 151 to more than 10,000 eggs per 100 cc within five months. Yes, the long-term consequences may be much larger than you really think. So, please sample your fields this fall if they are going to be planted to soybean next spring and start managing SCN when the numbers are low.

Many are still very focused on seed price, which should not be a criterion at all when selecting a variety. Spending time on variety selection is very rewarding. The best way to select SCN-resistant varieties today is by 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.

Figure 1. Effect of variety selection of soybean yield when taking SCN resistant and SCN susceptible variety into consideration at a high yielding location. Whiting, 2005.

Figure 2. Correlation of soybean yield and SCN number at harvest. Whiting, 2005.