Foliar rescue treatments don’t correct iron deficiency chlorosis in soybeans, reports Jay Goos, North Dakota State University soil science professor.

NDSU researchers have tested many sources of iron — both ferrous and ferric forms, different complexes and chelates, and different adjuvants — and haven’t found any that increased yield.

"The main reason for the lack of yield response to a foliar spray of iron is the lack of mobility of iron in the soybean plant," he says.

An NDSU demonstration illustrates the lack of movement: Researchers sprayed iron fertilizers on chlorotic soybeans at the first trifoliate stage, but covered the tip of the center leaf with a Post-It note during the spraying process to prevent the spray droplets from touching the leaf. A week later, the soybean leaves only greened up where a droplet actually landed on the leaf. None of the iron moved to the tip of the center leaflet that was covered up during spraying, and none of the iron moved to the next set of leaves.

"You can green up the leaves of a young soybean plant with an iron spray, but the effect will very short-lived, as the iron does not help leaves that are forming," Goos says.

The only successful IDC rescue treatment was demonstrated by Gyles Randall, University of Minnesota Extension soil specialist, in the 1970s. In the 1970s, most soybeans were grown in 30-inch rows, and were cultivated.

Randall demonstrated that 2-3 pounds per acre of a high-quality FeEDDHA product, dissolved in water and spray-banded right on top of the rows (not sprayed over the total soil surface), could help rescue the crop. Randall's treatment would only work if applied very early, no later than the second trifoliolate stage. An application could be combined with cultivation, with the operator switching on and off the pump while cultivating the soybeans depending on whether there were symptoms.

DOESN’T SPREAD: This chlorotic soybean plant was sprayed at the first trifoliate stage with an iron fertilizer and an adjuvant in an attempt to correct iron deficiency chlorosis. After a week, the iron hadn’t moved to the unsprayed portion of the first trifoliate leaf or to the developing second trifoliate leaf.

"The concept worked, because FeEDDHA has both leaf and soil activity for reducing IDC. Production systems are different today, with few farmers growing soybeans in 30-inch rows, and even fewer doing mechanical cultivation. However, the principle is still valid, and could be adapted to today's production systems, especially for soybeans grown in 22-30 inch rows," Goos says

Best option today
Today, an aggressive effort to manage IDC includes selecting the most IDC-resistant varieties, and placing an effective iron fertilizer with the seed at planting in the "hottest" parts of the field, Goos says.

"To date, we have only identified three soil-applied iron fertilizers capable of reducing IDC in soybeans. These materials are FeEDDHA, FeEDDHSA, and FeHBED. However, for all three of these products, always be careful to choose the material with the highest quality, as some commercial products contain non-effective isomers and condensates."

Selection of a resistant variety is still the foundation of fighting IDC.

NDSU evaluates dozens of soybean varieties every year for resistance to IDC. Iron fertilizers can reduce chlorosis, but can't turn a weak variety into a strong variety, Goos says.

Can seed company IDC rating be believed?
Selecting varieties that are resistant to iron deficiency chlorosis is the best way to reduce the impact of IDC, Goos says.

But beware of how seed companies rate their varieties

Goos screens dozens of varieties ever year for IDC resistance, and he recently the compared ratings he gave to the varieties with the ratings the companies gave their varieties. In some cases, there was little correlation.

Of the four he compared, "One company, company A, published IDC scores that correlated well with NDSU's measurements. Two companies, B and C, rated all of their varieties at the top of the scale, which might cause a farmer to plant a susceptible variety on IDC-prone soil. There was no correlation at all between NDSU's IDC score and the IDC scores for the varieties of Company D," he says. 

Goos recommends using more than one company's IDC score when selecting a variety for IDC-prone land.  "Using NDSU's ratings is a good place to start," he says.

You can find the ratings at bit.ly/2017IDCTrial.