No. 2 Article 7/April 9, 2010

Soybean Planting, Variety Selection, White Mold, and Seed Treatments

Selecting appropriate genetics is always the first and best way to increase your yield and profits. I suspect many of you decided during the winter months what soybean varieties to plant for all or most of your acres. However, you might need to purchase a final few bags, or you may develop additional needs as the spring progresses. If so, don't rush your final selection. Continue to take the necessary time to select varieties with the appropriate maturity, soybean cyst nematode resistance, disease resistance, and overall agronomic characteristics to match the needs of your fields and farming operation. Use the yield and lodging information generated by the University of Illinois Variety Testing program. If you don't find the varieties you use on your farm, nominate a variety to be tested in the future, sponsored by the Illinois Soybean Association. Testing results from 2009 are summarized in Figure 8. Average yields ranged from 48.6 bu/acre for Region 4, Maturity Group 3 to 67.2 bu/acre for Region 5, Maturity Group 4. Within each region-by-maturity group category, yields ranged 12% to 27%.


Figure 8. 2009 soybean yield data from the University of Illinois Variety Testing Program. Bars represent yield for 90% of the varieties tested in each testing region by maturity group category. Region 1 is in northern Illinois; Region 5 is in southern Illinois.

Additional variety information, including SCN resistance data and disease ratings, are provided by the Varietal Information Program, funded by the Illinois Soybean Association; they can be found at www.vipsoybeans.org. I want to point out that there were a lot of problems with white mold (Sclerotinia stem rot) in the northern regions of Illinois last year. There are greenhouse white mold tolerance ratings available on VIPS for varieties that were grown in Region 1 testing locations last year. The Region 1 yields are also available in the same table. The tolerance ratings were generated by a greenhouse screening technique, so the yield column does not reflect the yield in an environment with that disease rating. Furthermore, the greenhouse technique may not perfectly reflect a variety's white mold tolerance under field conditions, but it likely provides some indication. These ratings might help guide final planting decisions or provide or confirm suspicions of causes of poor performance from particular varieties you planted last year if white mold was an issue in your fields.

Once you have decided on your seed, you may be left with another decision--should you treat it? More specifically, should it be treated with a fungicide seed treatment, insecticide seed treatment, inoculants, or other potential micronutrients or products? These are not easy issues, and Carl Bradley will be providing results from his 2009 soybean seed treatment studies. The overlying agronomic principle to keep in mind is that you want to establish at least 100,000 healthy and uniformly spaced soybean plants to maximize yield.

The necessity of seed treatments to accomplish that establishment goal depends on a couple of things. First is the likelihood of needing to ward off seedling disease and early-season insect feeding where a field has a history of these problems or conditions that favor them. Second, how many seeds you plant and into what field conditions are important factors. The fewer seeds you plant, the more important it becomes to have every one become a productive plant. In this regard, seed treatment is a form of insurance, and that is certainly how it is marketed. Historically, it was much easier to plant many more seeds (up to twice as many) than the number of plants needed. However, seed prices have escalated to a point that brings that strategy into question, so it is hard to develop a hard-and-fast rule. A grower who still finds comfort in planting at high seeding rates (say 170,000 seeds/acre or greater) is probably not likely to find much value in soybean seed treatments. However, for a grower who is reducing seeding rates to the newer economic optimum ranges (say 130,000 to 150,000 seeds/acre), seed treatment has greater potential to be a good investment.

The use of soybean inoculant, Bradyrhizobium japonicum, is yet another issue. On one hand, inoculants are fairly inexpensive; on the other hand, they rarely provide large yield increases in most Illinois soils. I think it's fair to say that in general you should only expect at most an extra couple of bushels, and more likely an extra bushel or less. In 2009 I tested several inoculants at three locations in Illinois and found no statistically significant differences. In a few cases the inoculants might have paid for themselves by increasing yield by a partial bushel of extra yield, but that could not be separated from random chance alone. Since I only have one year of results, I will not present more detail here, but I will be testing more inoculants, in more locations, this year.

The rule of thumb for inoculating soybean seed is to do so if a field has not had soybean grown in the last five years. The questions continually arise for our growers who are planting more corn in their rotations and may have many fields that are on rotations of 3 or 4 years of corn between soybean. We do not have good data to evaluate the benefit of inoculants versus number of previous years in corn as a curve. All I can say is that is makes sense that the more years the field has previously grown corn, the greater the likelihood that inoculants will pay for themselves and may boost your fields' average yields. So inoculation is a cheap investment and provides some level of insurance for proper nodulation, but don't expect too much on most Illinois soils.--Vince M. Davis

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