Soybean School: Preventing those yellow patches next year

Most of the yellow patches in soybean fields in Western Canada and the northern U.S. have disappeared as the plants have recovered, or turned necrotic and died, but one of the big questions heading into harvest is: what toll did widespread issues with iron deficiency chlorosis (IDC) earlier in the season take on yields?

The yellow patches associated with IDC were likely more prevalent than in recent years due to a drier growing season following wet conditions last year.

“With wet soil and a high water table come soluble salts, and when the soil dries up, those salts are left on the surface. That’s a contributing factor,” explains Kristen MacMillan, research agronomist with the University of Manitoba, in this Soybean School episode.

Compounding the issue were cool temperatures slowing growth when the plants were stressed by not only IDC, but herbicide application, as many plants were yellow at weed control timing.

Research in the U.S. suggests yield loss from IDC is minimal unless the plants are affected until the fifth or sixth trifoliate stage.

“I would ask the question whether that holds true for Manitoba where we have very calcareous soils and we see iron chlorosis maybe earlier and for a longer duration of time,” says MacMillan.

As soybeans take a more prominent place in prairie crop rotations, some of the lingering yellow patches may also be due to root rot — specifically, phytophthora, she notes.

Although root rots haven’t been as prevalent as in wetter years, a low level of infection in spring can show up in August, especially when plants are under drought stress, as is the case in some areas.

Looking ahead to next year, variety selection remains the primary tool for control, and MacMillan says both IDC and phytophthora susceptibility are critical criteria to consider.

“What we need to really focus on are those two agronomic factors: iron chlorosis rating — the lower the rating, the better —  and secondly, phytophthora root rot resistance. About half the commercial varieties we have available have a good IDC rating and have some genetic resistance, but half of the soybean varieties we grow don’t have either one or both of those packages.”

Keep in mind, she says, you can grow the highest-yielding variety in the seed guide, “but if you put it in a field that’s prone to IDC, it’s the IDC rating that matters, not the high yield necessarily.”

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