Variety selection is one of the best ways to manage iron-deficiency chlorosis (IDC) problems in soybeans, say North Dakota State University agronomists.

IDC symptoms include yellowing between the veins of the trifoliate leaves and can result in growing-point death. With precision planting and global positioning systems, it is may be possible in the future to plant two different varieties on the same field.

Growers can map their fields to show those areas that are susceptible to IDC, says Ted Helms, plant specialist. Producers need to map the field in a year when they grew soybeans on that field and a year when the IDC areas showed up as yellow soybeans in the early vegetative stages, he adds.

"Research shows that soil factors, such as pH, salt content and calcium carbonate levels, are associated with IDC symptoms, but these soil measurements cannot be used to map the IDC-prone areas of the field," Helms says. "There are genetic differences among varieties for IDC. Some soybean varieties are less susceptible to IDC because they are genetically bred for tolerance."

One way to evaluate the tolerance of a variety to IDC is to use visual ratings at the early vegetative and flowering stages, the agronomists say. Many different varieties are planted in replicated, side-by-side tests and the amount of yellowing is scored. Visual IDC symptoms are reported by North Dakota State University publication North Dakota Soybean Performance Testing 2010 (A-843) and also by seed companies. Growers use these ratings to identify the best variety for those fields that are known to have past IDC symptoms.

Research was conducted in 2006 and 2007 on North and South Dakota fields that had a past history of IDC. The same 18 varieties were planted on the portion of the field that had IDC and another area of that same field that did not.

The objective was to determine whether the varieties with the least amount of yellowing, due to IDC symptoms, also were the highest-yielding varieties on IDC areas. A second objective was to determine whether yield could be increased by planting two different varieties on the same field.

"Our results showed that when two varieties had the same visual IDC rating, one variety was higher yielding than the other variety on the IDC areas," Helms says. "For example, when we compare two varieties, each with a visual rating of 2.5, we found that one variety yielded 42 bushels per acre and the other variety yielded 32 bushels per acre on the IDC area of the same field."

Visual ratings for IDC are useful to identify a tolerant variety. However, visual ratings for IDC, combined with yield evaluation on IDC ground, was the best way to select a variety that had IDC tolerance as well as high yield on IDC areas. They also found that varieties that were not tolerant to IDC often were the highest-yielding variety to plant on the areas of a field that did not have IDC.

"This suggests that farmers could increase the yield of a field by planting an IDC-tolerant variety on the IDC areas of a field and plant a different, high-yielding variety on the areas of the field that do not typically have IDC," Helms says.

Yields on the whole field could be increased by 2.5 to 5 bushels per acre by planting two different varieties on the same field. This research suggests that planters that will use GPS coordinates to switch varieties as the planter travels across the field need to be developed and marketed.