Soybean Plant Mortality May Be Greater This Year

Take weather into account when you are determining seeding rate for soybeans.

Published on: May 8, 2007

This spring's weather should be accounted for when determining soybean planting rates, in terms of seeds per acre, points out Palle Pedersen, Iowa State University Extension soybean agronomist. His research has shown that planting as early as April 25 in ideal soil conditions can result in maximized yields.

This year has not lent to ideal soil conditions and his recommendation for a final soybean plant population of 100,000 plants per acre will require slightly more seeds to be planted. He suggests a seeding rate on the upper end of his 125,000 to 140,000 plants per acre recommendation. This is due to expectation that disease pressure will be greater this year.

Delayed planting affects bean yield

"Looking back at the last two years, we've been spoiled with nearly perfect spring planting conditions for most of Iowa," says Pedersen. "There were areas in 2005, especially north central Iowa, that were very wet in the spring. But in general, we have been very fortunate with our planting conditions recently. Corn and soybean planting have been completed in record time. That will not be the case in 2007."

For southern and central Iowa this spring, the optimum time to plant soybeans to get maximum yield, has already passed, he says. In many areas, soil moisture has been excessive with standing water in some fields. Many farmers still haven't finished corn and want to start planting beans as soon as possible, maybe with fields a little on the "wet side" to reduce yield loss from delayed planting.

"That is not recommended," says Pedersen. "Even though some farmers are getting desperate to get back into the field, it is highly recommended to wait until soil is dry before taking heavy machinery into the field. Soil compaction contributes to reduced root growth and can affect the crop through the year. Yield can be reduced significantly, and long-term consequences can be considerable."

How to determine bean seeding rate

The wet soil conditions many farmers are dealing with this year will influence the soybean stand. Over the last four years, ISU has studied seeding rate recommendations in Iowa. "Our data are very consistent and show that a final stand of 100,000 plants per acre is enough to maximize your yield and return. The challenge is determining what seeding rate will give a final stand of 100,000 plants per acre because that varies so much with equipment, operator, soil type and seedbed conditions.

"Based on our research from previous years with saturated soils and wet seedbed conditions like those of this spring, we can, on average, expect a plant mortality of 30% to 40% compared to 20% to 25% under more perfect seedbed conditions," says Pedersen. Plant mortality is the difference between seeding rate and final stand at harvest, in percent.

Good year to use fungicide seed treatment

"My recommendations for growers using a planter in Iowa, regardless of row spacing, is to use a seeding rate of 125,000 to 140,000 seeds per acre to get a final stand of 100,000 plants per acre," he says.

The lower end of the scale (125,000 seeds per acre) is what you should aim for if you have perfect seedbed conditions. The high end of the scale (140,000 seeds per acre) is for the areas in central Iowa where field conditions this year are far from optimum, adds Pedersen.

A fungicide seed treatment also would be a wise suggestion this year, especially one that will protect against Pythium and Phytophthora root rot, he says. "The wet conditions that we currently are facing will favor seeding diseases that will reduce our stand. Phytophthora root rot could be a common problem this year with the very wet and potentially warm seedbed conditions." For more information, go to www.soybeanmanagement.info.