The finding addresses a question that has surfaced in U.S. research and among Prairie growers managing high-pH soils: does choosing a low IDC score come at the cost of yield where chlorosis is not present?

“We didn’t know the answer to this question until just now,” U of M researcher Kristen MacMillan said during a presentation at Ag Days in Brandon.

WHY IT MATTERS: Understanding how variety selection affects yield helps farmers make more confident decisions in challenging soil conditions.

“It’s a highly visual condition,” said MacMillan, who is also Manitoba Pulse and Soybean Growers’ agronomist in residence.

“We’re choosing varieties based on their visual response, but what is the actual yield correlation to that?”

IDC is a common issue in calcareous, high-pH soils, where iron becomes chemically unavailable to the plant.

The condition causes yellowing between the veins of young soybean leaves, typically appearing in patches and lasting from mid-June into July. While symptoms may only persist for a few weeks, early-season stress can reduce yield potential.

How much yield does IDC cost?

To understand how IDC severity translates into yield loss, MacMillan collaborated with Manitoba Agriculture to analyze six years of data from single-row plots rated annually for IDC response near Winnipeg. Those plots were taken through to harvest to compare visual scores with final yield.

The analysis confirmed that yield declines as IDC scores increase in affected areas.

MacMillan reported a yield loss of roughly 1.5 to 2.8 bushels per acre for every one-point increase in IDC score. In practical terms, a two-point difference between varieties could mean a three- to six-bushel swing in IDC-prone zones.

Is there a yield trade-off?

The more pressing question, however, was whether selecting for low IDC scores sacrifices yield elsewhere in the field.

Many growers report IDC often affects only portions of a field, yet variety decisions are made for the entire field. Research in North Dakota and South Dakota has identified yield trade-offs in some soybean populations, where selecting for IDC tolerance reduced yield under non-IDC conditions.

To test whether that trade-off exists under Manitoba conditions, MacMillan established paired trials in IDC and non-IDC areas of the same field using identical varieties. She also identified a common set of varieties grown over three years to ensure consistency in comparisons.

“What we’re finding is that yield is also negatively correlated with IDC score or not related at all. So this is good news,” she said.

In other words, varieties with strong IDC tolerance performed well in affected areas without yielding less in unaffected parts of the field.

MacMillan also examined whether precision-planting different cultivars in IDC and non-IDC zones would provide an advantage.

Because no yield penalty was detected, splitting varieties within a field offered little benefit in most scenarios. Only in fields almost entirely affected by IDC did a particularly strong cultivar show a measurable advantage.

“If IDC is a constraint in your fields, continue choosing varieties with low IDC score and high yield,” she said.

For growers managing high-carbonate Prairie soils, the data reinforce current practice: selecting IDC-tolerant varieties remains the most reliable defence without sacrificing yield potential where chlorosis never appears.

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Plant soybeans too early and you can run into frosts, adverse soil conditions and other problems that jeopardize plant establishment and early development. It could even mean a washed-out field requiring replanting.

Plant soybeans too late, though, and you could be sacrificing yield, since there are fewer days before the crop’s critical seed pod filling period in summer.

Seth Naeve, a soybean agronomist and an agronomy and plant genetics professor with the University of Minnesota, has studied the benefits and risks associated with early soybean planting in northern climes such as Minnesota. He shared some of his research findings with farmers at the CropConnect 2025 conference held in Winnipeg in February.

A good decision

In an interview with Grainews, Naeve explained the main yield constraint for northern-grown soybeans is our shorter growing season. He says the argument for earlier planting dates is in how they can help soybean crops get a jump on germination and plant establishment when fields warm up enough for that to happen.

Naeve’s planting date research includes soybean field trials conducted at four Minnesota locations in 2023 and 2024. The findings showed, on average, planting as early as possible was “a good decision,” according to Naeve.

“We have this challenge in the north in that we have good long days in the spring, but there is just not enough heat there at the same time,” he says.

READ MORE: Check out our Soybean Guide.

“Soybean is actually quite tolerant of planting early and sitting in the soil for a long period of time,” Naeve adds. “Soybeans tend to not come out of the ground until the ground warms up significantly enough … to support germination and emergence. By that time, we tend to be outside of the risk of imminent frost late in the spring.”

Naeve noted a range of management tactics can be useful for lowering risks associated with early soybean planting.

“We can increase populations. We can use a fungicide seed treatment. We can plant shallow or deep, depending on where the moisture is and what the weather forecast looks like. So, there are a lot of things we can do to help reduce the risk that we’re imposing on these soybeans by doing this real early planting,” he says.

Naeve points out the value of early soybean planting may be limited at the earliest end of the seeding window — but it is an incremental value that accumulates over time.

Say, if a farmer decides to hold off planting for a day but is then unable to get back into the field for two weeks — that 14 days can make a significant difference in yield potential, Naeve says.

According to Naeve, the relatively low cost of seed is another risk management consideration for early-planted soybeans.

“Soybeans are relatively inexpensive to replant. If a farmer has a limited number of days to plant, it may be the best risk situation for them is just to plant soybeans early and hold off on some of their other crops like corn (so) they can be then planted under a little bit better conditions,” he explains.

Naeve had one proviso with respect to early-planted soybeans. Findings from the Minnesota study suggest yield responses to planting date are closely tied to how productive the soil is.

In high-productivity environments with good fertility and adequate moisture, early planting with full-maturity cultivars maximized yield. In low-productivity environments, early planting offered no yield advantage.

“If it gets dry or there is low fertility, this trumps any kind of advantage we get from early planting. It basically caps the yield,” Naeve says, noting stressors such as pest infestations and hail damage are also yield-limiting factors.

“If there are other problems that cap our yields, there’s just nothing that we can do. We can’t add more fertilizer. We can’t put on a fungicide. We can’t plant earlier. We can’t do any of those things if the yields are going to be capped by something else.”

Critical period

Naeve discussed some other aspects of his research, which include examining when northern-grown soybeans are maximizing their yield potential.

Naeve says field experiments in three Minnesota locations in 2022 and 2023 indicated the critical yield period was the R4 stage (when rapid pod growth and seed development starts) up to just before the R7 stage (when pods and seeds begin to turn their mature colour).

The testing also identified the R5.5 stage, which typically occurs in late August in Minnesota, as the most important time for soybean yield establishment.

“That corresponds to this idea that farmers have had forever that August makes beans, or late August makes beans, or rains in August make beans. All those things are very true, and we have some data now that really shows why that’s true,” Naeve says.

“If you can support that soybean crop during that most critical time, then it can really make a lot of yield for you,” he adds.

“The way I talk about getting a good crop is, we basically build the machine early in the year, and then at the end of the year, we’re just making seed. If we can get a really nice, large machine built and if we do have good conditions late in the year, then we can really take advantage of it.”

Iron deficiency chlorosis

Naeve also offers some recommendations for managing iron deficiency chlorosis (IDC), a nutrient deficiency that causes yellowing of soybeans leaves and stunted plant growth. In extreme cases it can result in serious yield losses in soybean crops.

He referred to a study by a master’s student of his, Maykon da Silva, called “Strategies for IDC from a Systems Approach: Variety Selection, Iron Chelates and Seeding Rates.”

The research, which involved experiments at 10 Minnesota field sites in 2021 and 2022, examined trade-offs and interactive effects between soybean varieties, populations and iron chelate rates across a range of IDC levels.

Iron chelate products can help mitigate IDC in soybean by maintaining iron in a soluble form that can be available to the plant early in the season.

Naeve notes FeEDDHA, or ortho-ortho iron chelate, is commonly used for this purpose, but he cautions not all of these products are the same. He recommends farmers look for products containing higher levels of iron in the ortho-ortho form, because they are generally more effective against IDC.

The IDC testing results suggest soybean producers would be wise to consider applying iron chelates to any fields with a history of IDC, he says.

The study, he adds, showed better crop results with iron chelates used as in-furrow treatment rather than a foliar application.

“There are a number of factors that all go together to make (foliar applications) not very good practice,” Naeve says. “It’s better just to put (iron chelate) in the soil where the plant can access it.”

Soybean foliage doesn’t absorb minerals such as iron chelate nearly as well as the plant’s root system, he says — and soybeans seriously afflicted with IDC don’t have much leaf material left in any case.

Naeve says findings from the Minnesota study also suggest choosing a variety with the highest rating for IDC control offers the best defence for soybean crops.

“It also comes with relatively little cost. Those (IDC-tolerant) varieties don’t necessarily have a yield penalty, so just picking the right variety is of utmost importance” — and this can be particularly true in fields with severe IDC conditions, he says.

Naeve stresses it’s also important to select “proven” soybean varieties that have been adequately tested.

“I think practically speaking, the biggest risk here is that farmers grow new varieties that are relatively untested for IDC. That’s one of the bigger issues, and that’s a problem we see with industry rolling out these varieties quite quickly.”

Naeve says he’s found that to be particularly true with new herbicide traits in soybean. “Those first varieties that come out tend to really lag for IDC tolerance and/or resistance.

“Certainly, in the United States, we have so many new varieties come out and sometimes they get pushed to the market without good testing,” he says. “I know Canadians have a much better program for releasing and testing soybean varieties than we do in the U.S. It is much more conservative, so hopefully those get weeded out better than what we have here.”

Another finding from the Minnesota study was that bumping up seeding rates to increase soybean populations can be a beneficial practice for managing IDC. The research shows a higher seeding rate had a small but consistently positive effect on yield.

“I think if they’re concerned about IDC, farmers can increase their seeding rates a little bit,” Naeve says. “It’s a small benefit, but it is large enough to pay for the additional seed cost.”

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Since the 2017 peak, soybean area has dropped, with western Canadian area at 1.49 million acres this year. That drop since 2017 has been mostly due to challenging growing conditions and strong prices for other crops. Although genetic improvement in soybeans has continued over the past seven years, economic conditions have reduced soybean area.

Most of the increase in soybean area has been driven by the development of suitable varieties for the growing conditions in the northern U.S. and southern Prairies. The improvement in genetics has resulted in the expansion of cropped area. Climate change has also played a role in the soybean acreage expansion.

There have been many studies that have examined the changes to the climate in Western Canada over the past couple of decades. A review of the research on climate change in the agricultural growing regions in Canada was conducted by Mapfumo et al. in the Canadian Journal of Plant Science in 2023. The authors examined the research to date of the parameters that are important to producing crops.

Of the factors examined the in the review, the three most critical parameters for soybean production are growing season precipitation, growing season length and air temperatures during the growing season. The research indicates that changes in all three parameters have occurred over the past 100 years.

Some rain must fall

Every farmer knows precipitation is the critical element for producing crops on the Prairies. The weather during the past two growing seasons certainly proves the point that “rain makes grain.” Research indicates that growing season precipitation has increased since 1900. In Alberta’s agricultural region it has increased by 18 mm over the period beginning in 1900. In another study, Prairie rainfall has increased by 39.2 mm over the growing season in the period from 1956 to 1995.

The increase in precipitation is welcome news for soybean production in the coming years. Soybean yields are generally directly related to the amount of moisture received during the growing season. Rains are very critical during August for soybeans, when the crop is going through its reproductive growth stage. The general increase in growing season precipitation should translate to increased precipitation during August.

Change of season

Growing season length is the second most critical factor in producing crops in Western Canada. This is particularly true for soybeans, which are vulnerable to an early-season frost. Western Canada has seen its growing season increase by between three and 12 days, using data from 1920 to 2020.

Frost dates have also changed in Western Canada with the median spring frost earlier by 11.1 days and the fall frost dates were later by 9.4 days. A longer growing season allows soybeans to expand their footprint in the Prairies.

Growing by degrees

The heat experienced over the past two growing seasons has certainly brought temperatures to the forefront in terms of risk to crops. The maximum air temperatures have been increasing by 2.4 to 3.6 C during the period from 1950 to 2010.

The increase in maximum temperatures is probably the largest concern for soybean production in the future. Although temperatures during the growing season are expected to be higher than current levels, soybean yields are not likely to be significantly impacted by the higher temperatures.

The warmer temperatures are also expected to increase the growing degree days (GDD). GDDs have increased by 95 days in the Prairies over the period between 1948 to 2016. Another study in Alberta indicated GDDs increased by 77.5 (6.2 per cent) during the period from 1901 to 2002. The increase in growing degree days is essential in expanding the soybean area in Western Canada.

In conclusion, climate change is expected to expand the soybean growing area in Western Canada. With the increase in area, soybean production in Western Canada during the coming years. Economic factors will play a large role in determining the extent of the acreage expansion. The drop in soybean area over the past five years has been largely due to deteriorating prices for soybeans relative to competing crops.

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The unusual heat and dryness has been linked to the El Nino weather pattern, which is also causing a severe drought that is draining Amazonian rivers and disrupting grains shipments in the North of Brazil.

In the south of the country, El Nino has slowed soy planting as excess rains make farmers in third biggest producer Rio Grande do Sul afraid to start sowing, crop agency Emater-RS said.

Aprosoja-MT said extreme weather conditions could increase costs for farmers who have to replant their soy and could cause sowing of Brazil’s second corn and cotton to miss the ideal climate window in the Center-West.

So far, however, the government and analysts still believe Brazil will reap a record soy crop between 162 and 164 million tons in 2023-24.

According to Aproclima, Aprosoja-MT’s climate monitoring project, some regions in the south of Mato Grosso have recorded temperatures above 44 C. Accumulated precipitation over the last seven days was below average in Mato Grosso, adding up to less than 20 millimetres of rain over areas where it rained the most, LSEG data shows.

Mato Grosso accounts for nearly 30 per cent of soybean production in Brazil, the largest global producer and exporter of this commodity.

In recent days, dry weather has pushed back soy planting there. Further delays are likely as the prospect of rain for next week are not ideal.

“We have many places in need of replanting, but the assessment will be done after it rains and when it rains,” Fernando Cadore, president of Aprosoja-MT, said in a statement sent to Reuters. “With high temperatures… the situation is very worrying, extremely adverse,” he added.

— Roberto Samora is a commodities editor for Reuters in Sao Paulo; writing by Ana Mano.

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But some early winter storms caused him to abandon the late autumn fieldwork necessary to seed the yellow grain in the spring. That will force him and other farmers to devote more of his acreage to soybeans than expected next year.

As soybeans are far more affected by the U.S. trade war with China than corn, the trend adds to economic risk across a U.S. farm belt the Trump administration recently said would get up to US$12 billion in aid to make up for trade related losses.

China resumed buying U.S. soybeans in December as part of a trade war truce following a meeting between Chinese President Xi Jinping and U.S. President Donald Trump, but the amount of purchases remains well behind the pace of previous years. It is unclear how much U.S. soy China will actually buy with a record Brazilian harvest just weeks away.

Even with China back in the market for the first time in six months, the record soybean supplies littering the countryside have weighed on prices — soy futures were down 8.2 per cent so far this year — and made corn a more attractive option to growers.

Corn futures have risen 7.9 per cent during 2018 and were on track for their biggest yearly gain since 2012. China historically has imported very little corn.

“With some of these corn prices and the corn yields we’ve had, I wanted to plant some continuous corn (but) the window (to prepare the ground) closed on us too tightly,” Johnson said.

The severe weather across the U.S. Midwest in October and early November meant farmers left nearly 11 million acres of corn and soybeans from Kentucky to North Dakota standing in their fields as of the end of November — the fourth-highest amount ever.

Crops left in the ground during winter can still be harvested once the snow melts but face severe yield loss as soybean pods split and corn stalks are often knocked down, and mould is another risk. That will further squeeze farm profits after a bleak year of low prices and few buyers.

Analysts are now casting doubt on the U.S. Department of Agriculture’s early November forecast that 2019 corn plantings will rise about three million acres, to 92 million. At that time, soybean acres were seen falling 7.5 per cent, to 82.5 million.

“Because of what happened during fall tillage… maybe you do not plant as much corn,” said Don Roose, president of U.S. Commodities in West Des Moines, Iowa.

The weather problems also could cut into the bottom lines of seed and fertilizer companies. Corn requires more tilling and fertilizer applications that cannot be completed with unharvested crops or frozen ground.

Seed companies like DowDuPont that earn more money from corn seeds could see their businesses affected by the changes. Two-thirds of DowDuPont’s net seed sales are corn and only 20 per cent are soybeans. The company said in October it would take a US$4.6 billion charge to write down the value of its agriculture business, which has been hurt as Brazilian farmers increased soy planting to meet China’s demand for South American soybeans and reduced corn output.

Bayer, the German drugmaker that bought U.S. seed company Monsanto, has a much bigger market share in corn seeds and related crop protection products than in the soy market, and would also would benefit from rising corn acreage.

DowDuPont and Bayer did not respond to requests for comment.

Back on his 4,000-acre farm in central Illinois, Johnson had planned to boost corn seeding by 400 acres in 2019 before adverse weather sidelined him in November. After some heavy rain, early snow and hail, he parked his tractor and other equipment in the shed for the winter as there was little benefit to plowing through a sloppy field.

“We’ve learned that turning over mud does not do much good,” Johnson said.

— Mark Weinraub is a Reuters commodities correspondent based in Chicago.

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Corn futures advanced on spillover support from wheat, while soybeans advanced on follow-through buying from Monday’s strong gains.

Wheat recovered all of its prior-session losses that stemmed from rain in the U.S. Plains, where a large share of the milling wheat crop has suffered under a severe drought this year.

The U.S. Department of Agriculture said just 36 per cent of the U.S. winter wheat crop was in good-to-excellent shape as of Sunday, compared with 52 per cent at the same point last year.

“We got some rain, but the market is worried that maybe it’s a little bit too late to have an impact at this point,” said Ted Seifried, analyst with Zaner Ag Hedge.

Dry weather in parts of Canada, eastern Australia and southern Russia, all major wheat exporters, also gave prices a lift.

Chicago Board of Trade July soft red winter wheat was up 14-1/4 cents, or 2.8 per cent, at $5.21-1/2 a bushel, while K.C. July hard red winter wheat was up 14 cents, or 2.7 per cent, at $5.40-1/2 a bushel (all figures US$).

Both ended well above all key moving averages but closed below session highs.

Better-than-expected U.S. soybean planting progress capped the market after the USDA said 56 per cent of the crop had been seeded as of Sunday, well ahead of the five-year average of 44 per cent.

Corn planting was 81 per cent completed, in line with the average pace, USDA said.

CBOT July soybeans were up 5-1/4 cents at $10.20-1/2 a bushel, settling near its 100-day moving average. The contract failed to breach chart resistance at its 50-day moving average.

CBOT July corn added two cents to $4.04-3/4 a bushel.

— Karl Plume reports on agriculture and agribusiness for Reuters from Chicago; additional reporting by Michael Hogan in Hamburg and Naveen Thukral in Singapore.

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Conditions and progress in the southern portion of the province vary — some considerably — compared to those in eastern Ontario, particularly where it comes to planting corn and soybeans. If there’s one common theme for both areas, it’s that most growers would like to see some rain in the coming week.

Across much of southern Ontario, the winter wheat crop is the outstanding story at this point of the season, with lush growth and a healthy overall grading from advisers, extension personnel and retailers. Based on figures from Agricorp, it’s estimated that growers managed to plant about one million acres of winter wheat in 2015 (800,000 acres insured, with an assumed 200,000 uninsured).

Of course, appearances can be deceiving with winter wheat; the “windshield survey” often hides problems deeper in the canopy, from phosphorus or sulphur deficiencies to drainage issues.

“But we’re really nitpicking and trying to find faults because the wheat crop does look terrific,” said Alan McCallum, an independent certified crop adviser from Iona Station, southwest of London. Most of the wheat he’s seen hasn’t grown too tall, he said, likely due to the cooler spring temperatures in the south thus far.

“In a lot of cases, it’s reaching the flag-leaf stage, and it’s a pretty good-looking crop — the disease pressure is pretty low.”

As for corn and soybean planting, the cool start to the spring has delayed that task so far this season.

“In my immediate area, there’s still a decent percentage of corn yet to be planted,” McCallum said. “The clay soils were still pretty tacky down at two and three inches, and there are pockets around the region that still have a ways to go on corn planting.”

A few soybean fields planted earlier in May were germinating following sufficient rains late during the week of May 9 and early the following week.

Eastern Ontario

In the east, the conditions are almost reversed. By the end of last week, corn planting was expected to be all but complete, with soybeans at roughly 70 to 75 per cent finished before the holiday weekend. It’s been a fast start, but some concerns that come with those ideal conditions, said Paul Hermans.

“The two-week forecast is dry, with hardly anything here,” said Hermans, DuPont Pioneer’s agronomist for eastern Ontario and the Maritimes. “My only concern right now is that conditions are so dry, and I hope the growers that did this last bit of planting got their seed into moisture.”

He’s also interested in watching earlier-planted fields for any signs of delayed emergence or damage caused by last week’s cold snaps across much of the province.

— Ralph Pearce is a field editor for Country Guide at St. Marys, Ont. Follow him at @arpee_AG on Twitter.

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Agricorp, the province’s crop insurance and farm funding agency, said Monday it would give Essex County farmers until July 7, 2015 to plant their soybeans for this crop year, and until July 10 to report final planted acres.

For growers elsewhere in the province, the reporting deadline for acres is Tuesday (June 30).

Affected farmers who are “struggling to get their soybeans in the field” will, however, need to keep their production insurance coverage active by calling Agricorp ahead of the June 30 deadline.

Other planted acres and unseeded acres still must be reported by Tuesday, and eligible Essex County growers must also “verify their intentions” to plant soybeans before the July 7 deadline.

Agricorp said it made its decision to extend the deadline after “assessing the risks and consulting with industry.” Planting dates are extended “very rarely, on a crop or location basis, and under specific circumstances where it makes sense.” — AGCanada.com Network

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