Independent Assessment of IDC Tolerance in Soybean Varieties
BY: Jennifer McCombe-Théroux, Production Specialist – East, MPSG and Laura Schmidt, Production Specialist – West, MPSG
Given the moisture situation this June across much of agro-Manitoba, you most likely saw some yellowing soybeans. After the first trifoliate (V1), soybeans need to acidify the area around their rooting zone to take up iron. Excess calcium carbonates, soluble salts and high nitrate levels can all buffer this and limit iron uptake into the plant. When excess moisture conditions occur like we had this year during early vegetative stages (V1–V4), the root zone becomes diluted and further aggravates any existing iron deficiency chlorosis (IDC)-contributing factors. This results in tell-tale IDC symptoms, where new leaves yellow and veins remain green.
Soybean varieties vary in their ability to acidify their rooting zone and as a result, variety selection remains the greatest tool when it comes to managing IDC. Choosing IDC-tolerant soybean varieties is critical on fields where risk of IDC development is moderate to high (Table 1).
Manitoba Pulse & Soybean Growers (MPSG) supports IDC evaluation through the regional variety trials in collaboration with Manitoba Agriculture. The IDC trials started in 2006 and have been on-going for 18 years. Dennis Lange, pulse and soybean specialist with Manitoba Agriculture, oversees and rates the IDC trial, emphasizing the importance of the information it provides.
“The IDC trial gives producers an unbiased representation of how a variety responds to IDC. It is done under the worst-case scenario with respect to carbonates and salts. The purpose of the trials is to find the varieties that are tolerant and the varieties that are susceptible. That will help growers make decisions on growing certain varieties based on field conditions such as their carbonate and salt levels going forward,” says Lange.
Each year, at an IDC-prone site near Winnipeg, Man. varieties are rated over a three-week period at V2–V3 growth stages. Three replicates of each variety are grown in single-row plots and rated weekly for a total of nine ratings of each line. Two check varieties are included for comparison, a tolerant line and a susceptible line. Visual IDC severity ratings are assigned from one to five in each plot – one equals healthy, green plants and five equals severe chlorosis and a stunted growing point.
It takes a trained eye to conduct these ratings, with consistency being a key benefit of these trials. Lange started rating these trials in 2011 and has been doing them for the past 13 years. The same person doing each rating, year after year, provides consistent, independent results for farmers and industry.
Ratings fluctuate with the weather and soil moisture conditions. To account for this, each variety is rated over three years and averaged in MPSG’s Soybean Variety Guide and Seed Manitoba.
When you flip open these publications, you’ll find IDC ratings and groupings. Varieties consistently rating 1.7 or lower are considered tolerant to IDC and varieties rating 2.3 or greater are susceptible. Anything falling between (1.8–2.2) is semi-tolerant. In 2023, for herbicide-tolerant soybeans, 41 varieties were semi-tolerant, 18 were tolerant and five were susceptible to IDC. For conventional soybeans, 17 were semi-tolerant, eight were tolerant and eight were susceptible to IDC.
Choosing a tolerant variety for IDC-prone fields can offer a large return on investment. Recently, these trials have been taken to yield by Kristen MacMillan, MPSG and University of Manitoba agronomist-in-residence, to explore the relationship between IDC score and soybean yield in Manitoba. Results have shown that when soybeans are grown in IDC-prone soils, each 0.1 unit increases IDC score results in an estimated yield loss of 1.5 to 2.8 bushels per acre on average. Within most fields IDC is quite variable. Often, less than a quarter of the field has the conditions for IDC to develop. This year, with excess soil moisture in June, IDC was much more widespread. Symptoms can show up as early as V1 but if plants recover by V5 or V6 stages, yield loss will be minimal. If symptoms persist beyond these stages, there will be more significant yield loss.
When looking to MPSG’s Soybean Variety Guide and Seed Manitoba to select a soybean variety, take note of the various agronomic characteristics such as herbicide tolerance traits, days to maturity, IDC ratings, and PRR and SCN resistance to choose the features needed for your farm.