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The biggest difference between group IV soybean products and groups V—VIII products more typically grown across the Southeast is their indeterminate growth habit. Indeterminate products start flowering earlier and plant growth continues as plants continue to flower. Determinate soybean products produce most of their plant height before they start flowering. Planting indeterminate soybeans early (mid-April to early May)allows for earlier vegetative growth, reproductive growth, and earlier seed maturity.
"One advantage of planting indeterminate soybeans is they produce flowers for one to three weeks longer than determinate products," says University of Florida agronomist David Wright. “If they go through drought or heat stress, they have a chance to come back and still have the potential for higher yields as they continue to put on more flowers and pods. Determinate products do not have that potential to compensate if stress causes pod loss," he says. (D. Wright, University of Florida, 03/27/17)
Because indeterminate soybeans typically have a more upright (fewer branches) growth habit, Wright recommends planting in narrow rows to encourage plants to cover row middles as quickly as possible. “Many growers in the south plant indeterminate soybeans in twin 30– or 36-inch rows."
A key to maximizing soybean yield potential, and thus profits, is producing plants capable of capturing as much sunlight as possible from every acre of land. That does not mean producing the largest plants possible.
"Our goal is to produce plants that are touching in the middles and about three feet tall," says North Carolina State University Extension soybean specialist Jim Dunphy. “This will give us a leaf area index (LAI) of four, with four layers of leaves per plant and about four acres of leaves per acre of ground. We can get enough sunlight to all four leaf layers to be productive. There is no need for taller plants. It costs more in photosynthate to keep a fifth layer of leaves growing than the fifth layer will generate. If we can get a LAI of four with a variety that starts reproductive growth earlier in the season, then the plants can capture a few more hours of sunlight and maybe produce higher yields," Dunphy says. (J. Dunphy, North Carolina State University, 03/24/2017)
Both Wright and Dunphy caution that any stress, particularly drought stress, early in the growing season can slow plant growth and make a LAI of 4 more difficult to achieve. “The biggest thing that can slow or stop development of a LAI of four is lack of water,“ Dunphy says. “Planting on less productive or sandy, drought-prone soils makes it harder to get a LAI of 4 without irrigation. Most of the very high yields we see from early-planted group IV soybeans are produced on highly productive soils with irrigation," Dunphy says.
"We see very high yields in our state yield contests where the earlier maturing soybeans were planted on highly productive soils or had access to irrigation and the soybeans achieved a LAI of four," Dunphy says. “If another field did not get that LAI of 4, we don’t see that entry in the yield contest. I think farmers are realizing that the soybean product they select for the yield contest may not be suitable for most of their farm," he says.
Soybean product selection can also be a key to the success of an early soybean production system. “The indeterminate soybeans bred in the South are typically taller than the products we brought down from the Midwest," says Dunphy. He encourages growers considering early maturing group IV soybeans to look at tables showing how they perform in various yield environments. Growers should also consider shattering and lodging ratings, disease and insect resistance, and herbicide tolerance when selecting soybean products.
Wright emphasizes that shattering tolerance may be particularly important in an early soybean production system. “If we plant in mid-April, these soybeans will mature in mid to late August," Wright says. “Once these soybeans dry down, growers need to be prepared to get them out of the field before late August and September rainfall pushes the moisture content back up. Several rainfall events after the soybeans have matured can significantly reduce the quality and yield potential," he says.
If indeterminate soybean plants are still green after seeds have matured, growers should consider using a desiccant to dry the plants before harvesting.
“It may be more economical to harvest when the moisture content is a little high and then dry down the soybeans, rather than waiting for them to dry down naturally," Wright says. “Leaving them in the field too long after they mature leaves them susceptible to shattering or damage from excess moisture. Growers should consider their harvesting capacity and the ability to get early maturing soybeans out of the field in a timely manner as they decide how many acres they want to plant," he says.