Iron Deficiency Chlorosis (IDC) has been a soybean production problem for many years. George Rehm reported in the January 4, 2007 Crop Enews, results from research investigating the effect of cover crop during the early part of the growing season to increase iron uptake in 2006. The theory is that cover crop is reducing the amount of soil nitrate. Excessive soil nitrate concentrations inhibit the conversion of ferric iron (Fe+++) to ferrous iron (Fe++). In 2007, two sites were established to reproduce the same results as in 2006. One site was in Kandiyohi County while the second location was in Yellow Medicine County. The treatments in 2007 were nitrogen levels of 0, 100, and 200 pounds per acre and with or without an oat cover crop. The cover crop was planted at a rate of 1 bushel per acre the day before the soybean was seeded. A glyphosate tolerant variety soybean was planted in 30 inch wide rows. When the oats were 8 to 10 inches in height, they were killed with an application of glyphosate.

The pictures below were taken July 5, 2007. At that time, the application of fertilizer nitrogen to increase the soil nitrate-N levels had reduced the soybean growth and increased the incidence of iron deficiency chlorosis symptoms. This can be seen by comparing the left hand picture in Figures 1, 2, and 3 to each other. The use of the oat cover crop planted on the day before soybean was planted increased the soybean growth and reduced the iron deficiency chlorosis symptoms. These effects are similar to what occurred in 2006. In 2006, the cover crop reduced the soil moisture at one site to the point there were no grain yield differences. It will be interesting if these differences shown in plant growth will be measured in grain yield in fall 2007.

Because of the soil moisture reduction in 2006 at the Chippewa County site, a second experiment was conducted in 2007 to determine when the best time to kill the companion crop during the growing season. The treatments included no cover crop and kill the cover crop, oats, when it was 6 inches tall, 12 inches tall, when headed. The soybean growth was the least when no cover crop was planted. The growth increased and the color of the soybean became greener as the length of time of kill increased. The pictures in Figure 4 were taken July 5, 2007. At this time, waiting until the oats were headed had no affect the soybean growth. As the growing season extends into August, the soil moisture use by the cover crop may be a factor in the grain yield potential of the soybean.

This is a mid-season report on the IDC work. The practice of using a cover crop looks promising. Stay tuned for the harvest results.

Figure 1. Soybean with 0 pounds N per acre application, without and with a cover crop.

Figure 2. Soybean with 100 pounds N per acre application, without and with a cover crop.

Figure 3. Soybean with 200 pounds N per acre application, without and with a cover crop.

Figure 4. Soybean with (A) no cover crop, (B) cover crop killed at 6 inch height, (C) cover crop killed at 12 inch height, and (D) cover crop killed at heading.

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