January 5, 2006 | By: Laura Skillman

As soybean seed costs increase, farmers may be considering lowering the number of seeds planted per acre but are concerned about losing yield.

However, ongoing research at the University of Kentucky College of Agriculture shows such concern might be unfounded. Seeding rates can be reduced from historical levels without reducing yield potential, said Jim Herbek, UK agronomist.

Soybeans are known to have a tremendous ability to compensate not only with branching but also in pod numbers, seed numbers and seed size. They sense the need to branch when they are not in a crowded area, while in more crowded plantings, they won’t branch nearly as much. This branching effect helps them maximize yields at varying plant populations, he said. 

Studies across the United States have shown that plant populations can go below 100,000 plants per acre and attain maximum yields, Herbek said. A common goal on many farms is 150,000 plants per acre using seeding rates of 175,000 seeds per acre. As a result, most people are over seeding.

A seeding rate study in Lexington was conducted during 2003 and 2004 using 15-inch rows. Seeding rates varying from 17,000 to 190,000 seeds per acre showed that maximum yields could be attained with planting rates below 100,000 seeds per acre if final stands between 40,000 and 90,000 plants per acre were achieved, he said.

This study also seemed to indicate that early maturity varieties and double-cropped plantings might need slightly higher seeding rates than later maturity varieties and full-season plantings.

“We also started this study in western Kentucky in 2005 using 15-inch rows and seeding rates ranging from 50,000 seeds to 225,000 seeds per acre,” he said. “On a late Group III variety planted in May, we ended up with a final stand of about 80 to 85 percent at all seeding rates. What we found out was for this variety there was no particular yield advantage between final plant stands of 44,000 plants per acre and 180,000 plants per acre which really surprised me.”

Another somewhat later maturing Group IV variety was also studied at the same seeding rates. This variety had a better emergence rate of about 85 to 90 percent with final plant populations ranging from the mid-40,000s to about 200,000 plants per acre. Again, there was no statistical difference in yield, Herbek said.

Using lower seeding rates shows little impact on yields while lowering seed costs, and it generally results in less plant lodging, he said. Possible negatives include a more difficult harvest because the plants are bushier and closer to the ground, requiring combines to be operated at slower speeds and lower to the ground. Herbek said he does not think weed problems will be an issue in fields with lower plant populations because of the weed control technology available today.

Dropping seeding rates from 175,000 to 150,000 seeds per acre using 50-pound bags averaging 2,800 seeds per pound can result in a savings of $5 per acre.

“Cutting it to 125,000, which I think is realistic, can save $10 an acre in seed costs, and 100,000 seeds per acre would result in $16 per acre in savings,” he said.

Based on UK research to date, Herbek said populations for full-season soybeans can be lowered from currently used rates and still attain maximum yields. More research needs to be done to determine what rates producers will be comfortable with year in and year out. More work also needs to be done on seeding rates for double cropping and for early maturing beans to determine if lower seeding rates can be justified in these two situations.

“We really need to look at this more over different environments and stress and no stress situations,” he said. “We hope to be able to continue this work over the next several years.”

The soybean population research is supported by the Kentucky Soybean Promotion Board.


Jim Herbek, (270) 365-7541 ext. 205