By Dave Franzen
Iron deficiency chlorosis (IDC) is a widespread and destructive disorder of soybeans. Farmers often ask, "How much does IDC reduce yield?" Or, "if a crop is yellow, and greens back up, how much was my yield hurt?" My research, going back almost 20 years, shows that there is a very strong relationship between the severity of chlorosis at the 5-6 trifoliolate stage and final yield.
In our studies, we use a 1-5 chlorosis scale. A rating of 1, means that the plants are normal, and dark green. A rating of 2, means that the upper leaves have yellowed a little, but the veins and tissues are the same color. The chlorosis does not have an "interveinal" nature. A rating of 3, means that the upper leaves have interveinal chlorosis. That means, the veins are green, and the tissues between the veins are yellow. A rating of 4, means that the upper leaves have interveinal chlorosis, and necrosis (dead spots) is setting in, but the growing point does not appear to be damaged. A rating of 5, means that there is interveinal chlorosis on the upper leaves, there is necrosis setting in, and the growing point is visibly damaged.
According to research we conducted in 1998, 1999, and 2000, it was estimated that somewhere between 9 and 19 bushels per acre were lost per unit of chlorosis at the 5-6 leaf stage (Figure 1), depending on the year. Additional work we did in 2009, using three varieties and two rates of FeEDDHA (0 and 3 lbs/A), gave an estimate of about 13 bushels per acre lost per unit of chlorosis (Figure 2).
So, if there is a small degree of chlorosis early, and the crop fully recovers before the 5-6 trifoliolate stage, there probably isn't much yield loss. However, chlorosis that persists to the 5-6 trifoliolate stage definitely reduces yield, even if there is recovery later.
The most effective chlorosis control measure is a resistant variety.
If variety selection is not enough to eliminate chlorosis, then a resistant variety should be planted with 2-3 lbs per acre of a high-quality FeEDDHA product applied in-furrow at planting. The quality of FeEDDHA fertilizer varies from manufacturer to manufacturer, however. The best FeEDDHA products in the marketplace have about 80% of the iron in the most effective form, the ortho-ortho isomer. Fertilizing a chlorosis-susceptible variety with FeEDDHA may improve the yield of the susceptible variety, but the yield will still be less than if a resistant variety had been planted (Figure 2).