Soybeans — it might be OK to go easy on the inoculant

MPGA tests also show little or no benefit in using fungicides

Root cyst nematode infection on soybeans

Plant your soybeans on wheat or corn stubble and aim for 140,000 to 150,000 plants per acre.

Those were two of the recommendations from research results delivered by Manitoba Pulse Growers production specialist Kristen Podolsky to a meeting of the Brokenhead River Agricultural Conference here last week.

Podolsky also said you might not need to use in-furrow granular inoculants, and those yellow canola volunteers might not be as bad as they look.

The recommendations for plant establishment are based on work by Ramona Mohr at AAFC in Brandon. While optimum yield was achieved at an actual plant stand of 160,000 plants per acre, because of the extra seed cost the total economic return was higher at 140,000 plants per acre when calculated both at $9 and $10 per bushel.

Podolsky said 30 on-farm tests showed that average expected seed survival is 74 per cent with an air drill and 82 per cent with a planter, so an actual plant stand of 140,000 to 150,000 can be achieved with a seeding rate of 190,000 to 210,000 per acre with an air drill and 170,000 to 190,000 with a planter.

An app for calculating plant stands is available at

Soy on soy is risky

Podolsky said that based on rotation trials by Yvonne Lawley of the University of Manitoba at three sites in 2013 and 2014, planting soy on soy was either the big winner or the big loser.

At Carman in 2013, it yielded 65 bushels per acre compared to 53 on corn and 52 on wheat. But at Kelburn in 2014, soy on soy yielded only 36 bushels versus 45 on corn and 47 on wheat.

While trials of soy on canola produced similar yields to corn or wheat in most cases, Podolsky warned of the risks in the long term. They include susceptibility to the same root rot and sclerotinia diseases, herbicide residue concerns and suppression of beneficial mycorrhizae on soybean roots.

Another risk is herbicide-resistant canola volunteers, and the increasingly common sight of a yellow canopy above Manitoba soybean fields. “I’ve been in fields where you have to guess what the crop is this year,” Podolsky said.

On the other hand, the problem is usually esthetic, she said. Deciding on control requires determining the density. Research by Rob Gulden of the University of Manitoba from 2012-13 showed that yield loss per canola volunteer per square metre ranges from 0.5 to as high as 4.9 per cent.

Podolsky said five per cent is an “action threshold” for yield loss, and that control becomes economic at 2.4 volunteers per square metre in narrow rows and 4.2 plants in wide rows, though she was cautious on the latter figure.

“This narrow-row threshold is pretty solid, on the wide row we want to do more work.”

To try and reduce the volunteer seed bank, Podolsky suggested patch spraying, and harrowing or light tillage early after soybean harvest to encourage the seeds to germinate and be killed by frost.

2008-20012 soybean data
2008-20012 soybean data

Fungicides and inoculants

MPGA on-farm tests in 2014 have yielded some good news for soy growers looking to shave input costs.

At nine of 10 locations in 2014, there was no significant statistical difference in soybeans grown with or without fungicides. Podolsky noted that most disease pressure on soy is on the roots rather than above ground, but that tests will be repeated in 2015 and 2016 to see if the recommendation to skip fungicide holds.

There is also good news if you’ve grown soybeans for more than two years — you can probably skip that second inoculant now that Manitoba soils are becoming colonized with the soy rhizobia. MPGA tests at seven locations in 2014 showed no significant difference between beans grown with liquid inoculant only and with a second application of in-furrow granular inoculant.

“Overall we’re finding that the longer you grow soybeans, the more likely you can back off,” Podolsky said. “Maybe in a few years we’re at the point where we can’t use any inoculant but we’re not there yet.”

Nematodes at the door

While there’s good news for now on soybean cyst nematode, the No. 1 yield robber in the U.S., it may not last long. Tests across Manitoba last season found no evidence of the pest, but it’s been moving north and is already being found at low levels in North Dakota near the border.

“It could very well be in Manitoba — we just haven’t scouted the right field yet,” Podolsky said.

One problem in identifying the nematode’s presence is that yields can be reduced 10 to 15 per cent before any symptoms are visible.

Podolsky encouraged producers to choose varieties rated “R” for the nematode, and to scout their own fields for the pest. Mario Tenuta at the University of Manitoba has been running a scouting program, and encourages producers who’ve received a positive result from a private lab to contact him for further testing.

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