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October 3, 2003

Agronomist: Early soybean harvest yields to weather's influence

WEST LAFAYETTE, Ind. – Average yields on early-maturing Indiana soybeans are as dreary as the weather that pounded the crops all season.

Farmers harvesting Group II soybeans report yields ranging from 11 bushels per acre to more than 50 bushels per acre, with most between 25 bushels and 35 bushels an acre, said Ellsworth Christmas, Purdue University Cooperative Extension Service agronomist.

Typical yield averages are around 45 bushels per acre.

Group II soybeans are grown in northern Indiana counties. Group III and IV soybeans, which mature later, are produced in central and southern Indiana. Because they develop on a different timetable than later-maturing soybeans, Group II varieties were hardest hit by ill-timed storms and late-season dry periods, Christmas said.

"There are two things that have contributed to Group II soybeans yielding significantly less than they've yielded in the past," he said. "The torrential rain on the Fourth of July weekend saturated soils over much of the state, resulting in root systems that deteriorated on those soybean plants. The nodules disintegrated or rotted, so it took another two to three weeks before the nodules on those plants were re-established to the extent that they could fix nitrogen for the plant."

Cooler air and soil temperatures also beset waterlogged soybean crops. Many plants failed to produce adequate carbohydrates to support root systems during this period, Christmas said.

Weather extremes even stymied soybean plants that did not shut down during the seed-producing phase, he said. Group II varieties are yielding grain that is smaller than normal.

"Another major cause of reduced yields is related to seed size," Christmas said. "I've had several questions about what impact this would have on yield.

"I'll give you one example: a soybean variety with a normal seed count of 3,000 seeds per pound and a yield of 45 bushels per acre. If you had the same number of seeds but a seed count of 4,000 seeds per pound, then the yield would be about 33.75 bushels per acre – or more than an 11 bushel per acre reduction in yield just because of reduction in seed size. A number of producers have told me that their seed counts were 4,000 per pound or more in early beans they've harvested."

Group III soybean yields might be a bit off, as well, Christmas said. The culprit is the same wet-then-dry growing season that hurt Group II varieties.

"The very dry conditions in late July and early August occurred at a time when the pods of Group III soybeans were fully developed," he said. "The plant, in an attempt to survive, aborted the seed. So we have some Group III fields with fully developed pods and no seeds within those pods."

Harvest of Group III and IV varieties will shift into high gear in the coming weeks. Christmas said those crops should post better yields than Group II varieties. Southern Indiana soybean fields might be in the best shape overall, he said.

"Harvest could get into full swing in central and southern Indiana this week if conditions are right, soils dry out and it warms up," Christmas said. "Right now we have soft fields, and they need to be firmer to support the weight of the combines."

It is too soon to say how much yields might suffer because of soilborne diseases and insect feeding, but the added stress on soybean plants won't help, Christmas said.

The two diseases most prevalent in northern Indiana this season were Sclerotinia or white mold, and charcoal rot.

White mold attacks the soybean plant's stem, covering it with a light-colored, fluffy growth. These lesions cause premature plant death. Charcoal rot leaves small black specks on roots and lower stems. The fungus can kill or substantially hinder a plant's development. Charcoal rot thrives in the kind of hot, dry weather that gripped Indiana in August.

Soybean cyst nematode feeding occurred in a few fields, but a relatively new pest, the soybean aphid, generated the most attention. The aphid, a leaf-attacking insect slightly larger than the period at the end of a sentence, swept across soybean fields in northern and central Indiana in July and August. Severe infestations of more than 500 aphids per plant were reported in some areas.

As of Sunday (9/28), 9 percent of the state's soybean crop was harvested, an identical percentage to the same period one year ago but 11 percent behind the five-year average, according to the Purdue-based Indiana Agricultural Statistics Service.

A September U.S. Department of Agriculture crop report estimated Indiana soybean growers would produce nearly 228 million bushels of soybeans this fall, down 3.4 percent from 2002. The state average soybean yield is projected at 43 bushels per acre – up 2 bushels an acre from last year.

Writer: Steve Leer, (765) 494-8415,

Source: Ellsworth Christmas, (765) 494-6373,

Ag Communications: (765) 494-2722; Beth Forbes,;

Related Web sites:
Purdue University Department of Agronomy
U.S. Department of Agriculture September Crop Production Report

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