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Are you wasting soybean seed?

Farmers have historically overseeded soybean seed, primarily because it was cheap insurance. Twenty years ago, seeding costs with bin-run seed cost about $15 per acre, says Seth Naeve, a University of Minnesota Extension agronomist.

"With the cost of new technologies imbedded in the seed we plant now, seed can cost $60 per acre,” says Naeve. “It’s much more significant,”

How to maximize yields with the minimum amount of seed is one of the practices Naeve and other soybean researchers are examining in multi-state trials.

In research plots across the six participating states,

seeding rates varied from 25,000 to 275,000 plants per acre. "We know that in the South, 75,000 plants per acre will about maximize yield," says Naeve. "In northern states, we get that at about 125,000 plants. After 125,000, we get a slightly higher yield with each higher population. We refer to it as 'yield creep.'”

With each additional 50,000 plants per acre in seed, yield creep generates around one more bushel per acre.

"By the time we get to 275,000 plants, we have bumped yield by about an additional 4 bushels per acre,” he says. “But it takes two bushels of seed to get there. That doesn't pay,” he says. While Midwestern yields top out at 125,000 plants per acre, Naeve admits most farmers will -- and probably should -- plant up to 150,000 plants per acre. This will help ensure farmers end up with sufficient stands.  In studies in Iowa, Minnesota, and Michigan in 2009 and 2010, Naeve says 90% of maximum yields were achieved at 117,000 plants per acre. Obtaining the last 10% takes a 200,000 per acre seeding rate

Cumulative light interception Light interception is another factor researchers are studying.

"This is the energy from the sun that the plants harvest," says Naeve. "It's really our biggest limitation. While you might think it's only about row spacing, it's the integration of several factors including planting date, maturity factor, and density of the stand." The goal, Naeve says, is to find way ways to close rows--full canopy--as early as possible in the growing season. "It's the key to higher yields, to harvest more sunlight, earlier,” says Naeve.

Increased population impacts early canopying because higher populations can fill in a row gap earlier. While row shade aids weed control, a more significant benefit is an increased harvest of maximum sunlight. Over the growing season, a population increase of 25,000 to 275,000 plants per acre boosts total light interception from 23% to 84%.  "We need to try to get a high level of canopy at about the R1 (beginning flowering) stage of growth,” says Naeve. “For northern zones, that's about July 4. In our studies, with 25,000 plants per acre, we have about 25% canopy on July 4. At 175,000 plants, we can get 50% to 60% canopy on July 4."

One of the easiest ways to aid early canopy is to narrow row widths. Studies so far show a 2 bushel per acre yield advantage for narrowing rows from 30 to 20 inches. Narrowing rows from 20 to 10 inches boost yields another 2 bushels per acre.  One major contributor to soybean stand density is plant attrition. As soybean plants get closer together, some of them will not compete, and die while surrounding survivors fill in. While it's not fully understood, scientists know soybeans in 30-inch rows will crowd closely together and more will die off due to attrition.

"In narrow rows, as plants more spread out in the row, more of them live to harvest than in wider rows,” says Naeve. “There is less season-long stand attrition. That greater stand density harvests more sunlight." Soybean maturity levels can also impact cumulative sunlight collection. Longer-season varieties allow a longer yield accumulation period.

"Longer maturity means we get longer seed-filling windows around stress periods, and this may allow time for the plant to make up for excessive heat or dry weather,” he says. “But you have to balance that with the risk of frost, because seed filling happens later in the year."

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