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Corn+Soybean Digest

Sizing up the situation, soybean seeds much smaller

Soybean producers need to recalibrate their drills and planters this year because soybean seed size is 10-20 % smaller than average, especially in northern Indiana.

Ellsworth Christmas, a Purdue University Cooperative Extension Service soybean specialist, says the reduced seed size is likely a result of late-season heat and moisture stresses in 2003.

"Soybean seed would normally run 3,200 to 3,400 seeds per pound," he says. "Many of those same varieties are running 3,800 to 4,000 seeds per pound this year."

Seed size will vary depending on location. Christmas says seed produced in southern Indiana may be near normal size, but seed from the northern part of the state will be small.

He said Group II beans, the group grown in the north, were in the middle of seed formation when moisture stress was most severe, causing the small seed size. Group III beans -- those grown in the southern part of the state -- were just beginning seed formation during the moisture stress, which resulted in seed abortion but less reduction in size.

The smaller size means farmers need to recalibrate their drills and planters. Failing to do so could lead to overseeding and plant stands that are too thick, Christmas says. Thick stands result in taller plants with small stems that are subject to lodging.

"If lodging occurs while the plants are green and growing, the leaf canopy is reduced and some plants will die, resulting in yield losses," Christmas says. "If lodging occurs after plants mature, losses will occur at harvest."

The smallness of the seed does not appear to have reduced germination and should have no negative effect on the seeds' vigor. Ultimately, the smaller seed may also be a way for farmers to save some money, Christmas says.

"The quality of soybean seed is very good this year," he says. "Producers need to get seed counts from their suppliers prior to finalizing the number of units needed. They may need to purchase 10-20% fewer units of seed in 2004 to plant the same acreage as last year."

Purdue experts recommend seeding rates of 200,000 seeds per acre for 7.5-inch rows, 165,000 for 15-inch rows and 130,000 for 30-inch rows. Christmas says this recommendation is for seed with at least a 90% germination rate.

"Always check the tag for germination rates and the number of seeds per pound before calibrating," Christmas says. "When you're calibrating drills, collect seed from all of the rows, since seeding rate varies widely from one row to another."

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