The Right Time for Soybeans

Reprinted from Country Guide – March 12, 2015 by Anne Cote

If there’s one thing soybeans won’t tolerate, it’s late planting

The optimum seeding time for soybeans throughout most of Manitoba is the end of May. It isn’t a date you want to miss, but you’ll also want to get the right seed. Whether you’ve grown soybeans before or are planting them for the first time, you’ll have the most success if you choose carefully among the different varieties that are available from your seed dealer, says Dennis Lange, farm production adviser at Manitoba Agriculture, Food and Rural Development (MAFRD).

“First and foremost is variety selection,” Lange says. “If you plant a short-growing-season variety in a short-season zone, you can maximize the yield potential of that variety,” he explains. “If you plant a long-season variety in a short-season zone, then that variety doesn’t have the opportunity to reach its potential yield and full maturity… Know your zone.”

Kristen Podolsky, production specialist at Manitoba Pulse Growers, looks at days-to-maturity as the single most important factor when choosing a seed variety. The short-season zone in Manitoba is in the southwest and northwest, both areas where acreage for soybeans is expanding. The long-season zone is generally in southeast Manitoba, south of Highway 23. In between those zones is classified as mid-season.

With soybeans, these zones are rated on heat days rather than sunlight hours, rainfall or soil type. It means that if you choose a variety simply for its yield potential without paying attention to the number of days to maturity, you’ll put your crop at risk of fall frost damage.

But if you’re running out of time to get those acres in, it is possible to plant the long-season varieties on the first seeded acres and a mid- or short-season variety on the last acres, says Lange. Optimum planting times in Manitoba are between May 10 and May 30 with the soil at 10 C. At this temperature the seeds will take two or three weeks to germinate. If the temperature rises to 13 C or 15 C, seeds will germinate in as few as seven days. But if the temperature is below 10 C, the seed will sit in the ground and when it finally emerges, the plants won’t be as vigorous as those planted at the warmer temperature.

The length of time between planting and maturity (i.e. when 95 per cent of the pods are brown), varies between 113 days in short-season varieties up to 121 days in mid- to long-season lines. Lange says once the plants have reached maturity, frost isn’t a major issue but a heavy frost before then can damage the yield.

Lange says that’s why it’s important for growers to know their region, their capabilities and the weather outlook. Flexibility is key. When the weather and calendar are working against you, he suggests planting the long-season varieties on the first fields and then using the mid- and short-season varieties on subsequent fields.

Flexibility extends to equipment as well. Lange says it’s a matter of choice but a first-time soybean grower can use the same planting equipment they have for cereal crops, whether it’s a planter or air seeder. He says there’s no reason to go out and buy additional equipment when you already have reliable planting equipment on the farm. Lange notes growers all have their own preferences on what’s important on their field and most are great at modifying the equipment they have to meet their needs. “Once you figure out what place soybeans have in your crop rotation, it always comes down to the nut behind the wheel. By that I mean the person driving the equipment,” Lange says.

Lange says row spacing is also a matter of individual preference. Wider rows require less seed than narrow rows. In a 15-inch or eight-inch row, the target is to get 200,000 to 210,000 seeds in the ground to ensure 170,000 plants develop. With 22- to 30-inch rows, it’s possible to seed 180,000 plants and still have as many as 160,000 reach maturity because the germination rate is usually higher in these rows. There’s really not a significant yield difference between row choices but weed control is more difficult and takes more attention in the wider rows while quicker row closure in the narrow rows can reduce weed control work, he says.

“Ideally, if you have between 140,000 to 165,000 plants established… you’ve got the ultimate range to maximize your yield,” Lange says. Even if early damage on seeds reduces the final plant count to 80,000 plants, it’s not a crop failure, just more weed control work. The important thing at this stage is to have a good hard look at what caused the low germination rate and learn from it.

Podolsky says seeding rates for soybeans should be calculated by determining the risks for failure to emerge including germination rates, presence of soil pathogens, possible insect damage, and damage to seed that may have occurred due to handling and seeding equipment. She says to achieve a target plant stand of 150,000 plants per acre, for example, the seeding rate should reflect the expected survival rate of the seed once all the risk factors have been calculated.

Manitoba Pulse is conducting on-farm studies to create a data base that will provide growers with more precise information on these risk factors, Podolsky adds. “In the past three years, over 60 on-farm research trials have been conducted and some of these trials have looked at seed survival rates in air seeders and planters.”

Plus, adds Podolsky, “There’s a 65 to 75 per cent seed survival rate with an air seeder… not as high as we’d like to see. That’s just what the reality is.”

Lange notes the reduced germination rates when seeds are planted with an air seeder is likely due to damage on seeds that are already dry.

However, Podolsky says, there’s a higher survival rate with planters — 72 to 82 per cent. Another benefit to using a planter is improved seed placement and depth control. The target plant stand of 140,000 to 160,000 or more plants is the same across row spacings.

But soybeans are adaptable so using an air seeder and adjusting the seeding rate from that of other crops is feasible. “Many studies show the yields on narrow rows planted with a drill or seeder are the same or higher as those done with a planter,” Podolsky says.

Proper inoculation to induce nitrogen development in the soil is another factor vital to the health of the soybean crop, Lange adds. Over time he’s noticed some growers lay a granular inoculant down through a separate tube, placing it a bit over from the seed to provide a little extra yield insurance. The granular inoculant becomes available later in the growing season when the pod is filling and can make up for deficiencies earlier on.

Both Lange and Podolsky note the number of acres given to soybeans in Manitoba has been expanding in recent years. Podolsky says the Red River Valley has been growing soybeans for a little more than a decade, but the acreage has doubled in the past four years with most of the expansion coming from the mid- and short-season zones.

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