As soybean growers plan how many seeds to drop in the ground this year, University of Illinois Extension soybean specialist Vince Davis says the current economic environment will play a much bigger factor in this decision than ever before.

“Soybean seeding rate is a decision that has become more economically critical as soybean seed prices have increased with improved varieties, the addition of biotechnology traits, and the increasing popularity of soybean seed treatments,” Davis says.

Soybean seeding rates in Illinois have typically ranged from 150% to 200% of the number of plants needed at harvest (75,000-100,000 plants/acre) to maximize yield. Some believe high seeding rates provide insurance against conditions that reduce soybean emergence.

“Historically, the cost of seed was a relatively minor expense,” Davis says. “Today, higher seed costs have increased interest in reducing seeding rates in order to maximize economic return.”

So, what’s the magic number this year?

Davis suggests the best way to determine a desired seeding rate is to have an understanding of your operation including your seedbed preparation, the accuracy of your planting equipment and the quality of your seed.

“The best rule of thumb is that a minimum of 100,000 uniformly spaced plants/acre can maximize yield for a very wide range of conditions,” Davis says. “Determine how many seeds you should start with by dividing your desired plant stand establishment by the percentage of viable seeds planted, and then divide again by the percentage of your expected emergence rate.”

Growers who have a history of scouting their plant stand establishments following planting should be able to estimate the expected percent stand establishment to use specific to their equipment, seedbed preparations and soil/planting conditions.

Data were recently compiled from three different experiments representing 50 site years of data in a variety of locations throughout Illinois, several different growing seasons ranging from 1998 to 2009, multiple row spacings (7.5, 15 and 30 in.) and varying fungicide seed treatment applications.

“Results showed there is just as much yield variation at the high seeding rates and the low seeding rates, suggesting there is not that much greater insurance for starting with higher seeding rates,” Davis says.

These data were also modeled to a quadratic equation to find the seeding rate that maximized yield. The magic number was approximately 200,000 seeds/acre to maximize yield. Davis said this explains why growers have historically planted such high seeding rates.

“However, research shows that seed costs have increased from less than 15¢/1,000 seeds 15 years ago to nearly 45¢/1,000 seeds today,” Davis says. “Where a recommendation should remain under today’s economic environment can be pretty wide given a number of conditions. Many growers can reduce their seeding rates some, but they should remain in a zone they are comfortable with. If seed germination and planting conditions are good, seeding rates above 140,000 are probably good in most environments.”

For more information about seeding rates, read the April 29th edition of The Bulletin online.