Look to more than calendar for best time to plant soybeans

Soybeans don’t like cold and they are very susceptible to spring frost

Now is the time to plant soybeans in Manitoba according to the calendar, but date is just one of four factors to consider, says Terry Buss, a farm production advisor with Manitoba Agriculture, Food and Rural Development (MAFRD) based in Beausejour.

The others are soil temperature, the weather forecast for 24 hours before and after seeding and the farmer’s risk tolerance, Buss said May 7 during MAFRD’s Eastman webinar.

Some farmers started seeding soybeans at the end of April. That’s too early, according to Buss because the risk of poor germination due to cold soil temperatures or frost after emergence are too high. Even seeding last week was risky, he said.

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When Buss emailed farmers about the webinar on soybean planting one replied: “I had to laugh as I read the title of the next webinar while rolling my last 70 acres of beans. lol.”

The seeding date for soybeans in Manitoba is a compromise. Seed early and the crop is at risk due to cold soil or frost; seed late and yield potential is reduced and fall frost is possible before the crop fully matures.

Crop insurance records show soybeans seeded mid-May still have the potential to reach maximum yields. By then soil temperatures are usually close to the recommended temperature of 10 C. And by the time the crop emerges the risk of frost has declined, but not disappeared.

In much of eastern Manitoba there’s still a 50 per cent chance of temperatures hitting 0 C or lower by May 25, Buss said. (There are exceptions such as Altona where May 14 to 19 there’s a 50 per cent chance of frost.)

Data show newly emerged soybeans can only tolerate temperatures of 1.1 to -1.7 C at the cotyledon to unifoliate leaf stage, Buss said. If hardened by cool weather they can take -2.2.

After being frozen “patience, patience, patience,” is key, Buss said.

“There is no way anyone can go out the day after a frost and really be sure the crop is dead,” he said.

“Frankly a lot of the frosted soybeans I’ve seen in my area (in the past) haven’t needed to be replaced. They recovered from the damage.

“Give it three to five days before you look for signs of recovery.”

If it stays cool and cloudy for a few days it could take a week.

While the growing point on a soybean is above ground and can be badly damaged or killed by frost, growth can sometimes resume at the base of the cotyledons.

The ideal soybean stand should have at least 150,000 plants per acre and some farmers aim for 170,000 or even more. Seventy-five thousand plants per acre is considered the minimum population after a frost, Buss said. Yield potential will be cut by 10 to 20 per cent, but is still higher than the potential from a re-seeded soybean field.

Planting soybeans into cold soil is also asking for trouble, Buss said. The Manitoba Pulse & Soybean Producers recommends a soil temperature of at least 10 C taken at 10 a.m. two days in a row. And then only plant if the soil temperature is expected to not drop lower than 8 C for two days after planting.

Soil temperature is critical during the first 24 hours after seeding, Buss said. During that time if the seed absorbs cold water (the temperature will be the same as the soil’s) the cotyledons and embryo can be damaged resulting in poor seedling vigour.

“Really cold water — 4.5 C — cause outright germination failure and seedling death,” Buss said.

And the damage can sneak up on you. The soil could be warm at seeding time but drop off with cold weather or a cold rain.

With one in three acres seeded to soybeans in the Beausejour area farmers might not be able to seed all their soybeans at the optimum time, Buss said. Each farmer has to assess the risk.

For those pushing the planting window Buss has the following advice:

  • Finish seeding other crops first, most of which will tolerate cold soils and frost better than soybeans.
  • Select the driest, blackest fields for early soybean planting.
  • Don’t cut the seeding rate. Some might even boost it but high seed costs discourage it.
  • Take soil temperatures and pay attention to the weather forecast for before and after seeding.
  • Spread the risk. Don’t plant all your soybeans early.
  • Seed shallow because soil temperatures decline the deeper you seed.

“Know when to park your planter,” Buss said. “There are times when you’ve got to stop and you’ve got to stop before it’s already cold.”

Farmers who seed soybeans last week need to monitor those fields closely. And pay even more attention to the ones seeded two weeks ago, Buss added.

About the author

Reporter

Allan Dawson

Allan Dawson is a reporter with the Manitoba Co-operator based near Miami, Man. Covering agriculture since 1980, Dawson has spent most of his career with the Co-operator except for several years with Farmers’ Independent Weekly and before that a Morden-Winkler area radio station.

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