Soybean seeding rates should be adjusted for cool, wet soils

As farmers take advantage of the extra time from a long spell of rain to tune up planting equipment, one Purdue Extension soybean specialist says growers need to pay attention to seeding rates – especially with the cold, wet weather in the Midwest.

“April 2011 has been much cooler and wetter than this time last year, so as farmers take advantage for final equipment tune-ups, I want to remind them that planting should be based on soil and environmental conditions,” Shaun Casteel said. “As farmers tune up their planters, drills and air-seeders, they need to consider seeding rates.”

Casteel said that many of the soybean lots planted in 2010 were large seeds. That isn’t the case this year.

“Soybean seeding rates need to be adjusted by seed size rather than weight,” he said. “Planter settings used last year will probably drop more seeds per acre with this year’s seed lots and germination scores fluctuate, as well,” he said.

Total seeding rate is calculated by dividing the variable seeding rate by the germination percent. For example, seeding at 140,000 viable seeds for a variety with 90% germination would require 155,000 total seeds per acre. This same variety at 3,100 seeds per pound would require 50 pounds of seeds per acre, whereas 2,500 seeds per pound would require 62 pounds per acre.

“More difficult conditions for planting and seedling establishment will push the seeding rate higher,” Casteel said. “A general rule of thumb is to estimate that 85% to 95% of the viable seeds planted will emerge and establish a stand, depending on accuracy of seed placement, residue issues and other field conditions.”

Casteel and his colleagues have found that in their small-plot research, harvest stands near 100,000-120,000 plants per acre optimize yield. Soybean yields do not effectively increase with plant stands above 120,000 plants at harvest.

“This is lower than traditional thoughts of soybean plant stands and certainly lower than the target of many farmers,” he said.

Farmers also need to keep in mind that planting into cold and wet soil can cause problems such as sidewall compaction in the seed furrow, emergence issues involving cold temperatures and seedling blights, and favorable conditions for sudden death syndrome infection during early growth.

“Planting dates are a good reference, but they should not override field conditions,” Casteel said.

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