Soybean growers tempted to chase moisture by planting deeper than five centimetres risk trading one emergence problem for another, according to newly published Manitoba research that puts local numbers behind long-standing seeding-depth advice. University of Manitoba researcher Kristen MacMillan tested depths from 0.6 to 5.7 cm (about a quarter-inch to 2.25 inches) over six site-years at Carman, on sandy loam to loam, and Arborg, on clay, from 2017 to 2019, with narrow rows and a double-disc plot seeder at eight target depths.

The range is fairly forgiving. Yield was expected to peak at 3.2 cm, and depths from 1.9 to 5.1 cm returned harvests within 5% of the predicted maximum. Outside it the bin was lighter: seeding at 0.6 cm cut yield by an average 14%, and at 5.7 cm by 9%. The losses had different causes. At 0.6 cm stands averaged only 82,000 plants an acre — 41% establishment — as seeds began germinating and then dried out when surface moisture vanished; at 5.7 cm stands reached about 146,000 plants an acre (73%), but emergence was slow and vigour reduced, with yellowing, lost cotyledons and distorted seedlings.

The study grew out of what MacMillan saw in the 2010s as Manitoba soybean acres expanded and farmers planted very shallow in dry springs and very deep in wet ones without knowing the cost. Confirming current practice is a valid result, she said — "farmers and agronomists observe and make decisions even if the science is lacking or not readily available" — and much of the work has already reached farmers through extension and Manitoba Pulse and Soybean Growers (MPSG) recommendations; publishing it puts the evidence in the scientific record.

MPSG production agronomist Terry Buss says the findings match the field. "If we're shallower than three-quarters of an inch, seed can easily get stranded in drying soil. If we're deeper than two, we start having high mortality because the soybeans are having a very difficult time coming out of the ground," he said; he recommends a slightly narrower 1.9 to 4.4 cm, because soybeans' large cotyledons make them less forgiving of deep seeding than field peas. Dry seedbeds this spring made depth a frequent topic: some farmers planted unusually deep and got thin stands, and drills behaved differently as soil texture changed field to field, while rolling can push soil into deep seed trenches. "What we learned this year was that we really need to be checking depth field to field," Buss said — stop and dig.

MacMillan's next questions are whether precision seeding equipment that adjusts down-pressure row by row, or sensors that read soil moisture and temperature, actually improve stands, yield or profit over standard gear, along with coarse soils, wet conditions, residue and seeding date. She also points to a major shortfall in investment in applied agronomy research in western Canada — one reason, she said, she decided to publish.

Source: Manitoba Co-operator