The fertile, irrigated river bottoms of southeastern Nebraska have blessed Greg Peters and his son, Chris, with big, beautiful soybean plants.
A little too big.
"The beans have a tendency to get really tall, with a lot of space in their internodes," Greg Peters says of his fields, near DeWitt, Neb. "In a windstorm, they're so tall or heavy with pods, they start leaning and tend to lodge really badly."
Two years ago, one of his 15-inch row soybean fields produced a towering 6-foot-tall stand. "You step out into a field like that and just disappear," Greg recalls. "You had to get on the back of the pickup to look over the top of the field or get up on the pivot to see what was going on out there."
Most of those beans toppled over before harvest. Greg says that he lost between 15 and 20 bushels per acre to lodging in what should have been a bin-buster field. Chris estimates they lose close to 15% of their potential yields to lodged beans every year.
THE GENETIC FACTOR
Fortunately, scientists from Purdue University and the University of Nebraska have isolated and cloned a gene that could help farmers like the Peters.
The Dt2 gene produces soybean plants that are semi-determinate, and therefore, shorter. "With the shorter stature, there's less lodging potential," explains University of Nebraska agronomist Jim Specht. Semi-determinate soybean plants average about 17 nodes compared to the 22 or 24 nodes of current varieties.
Most soybeans in the United States are either determinate or indeterminate. Determinate beans are usually short and bushy, because they stop vegetative growth once flowering begins. Indeterminate soybeans continue growing after reproductive stages are under way and tend to be taller.
Southern growers typically plant determinate beans in varieties from maturity group V and up, Specht notes. The shorter growing season of northern states requires the extended growing habits of indeterminate soybeans. They are generally found in maturity group IV varieties and earlier.
For northern growers, efforts to increase fertility in their indeterminate soybean fields can sometimes result in overgrown beans instead of the intended higher yields.
Narrow row spacings can also contribute to the problem by forcing beans to grow up rather than out, Greg says. Yet, even widening rows to 30 inches hasn't tamed the Peters' leggy legumes.
"They get tangled up in the row beside where you are trying to cut, and the row dividers on the outside of your header can't separate them," Greg says. "Then they pile up on the outside row dividers, and they shatter out there when the reel goes around, so you have a yield loss."
A HAPPY MEDIUM
University of Nebraska soybean breeder George Graef developed the first high-yielding semi-determinate soybean variety using the Dt2 gene (named NE3001) for use in the northern U.S. several years ago. Since then, Specht says extensive university trials have shown that the Dt2 gene can produce a "Goldilocks" plant—not too big, not too short and with comparable yield.
"Even though it has fewer nodes, it has about the same number of pods per plant, which means it can yield as much as the indeterminate [varieties]," he says. "I don't think this is a bin-buster gene, but it offers an alternative architectural type for soybeans that might offer better usability in high, lodging-prone production environments."
Before the Dt2 gene discovery, which was financed by the United Soybean Board (USB), breeders could only get a semi-determinate hybrid by crossing indeterminate and determinate plants. The result was impractical because hybrids aren't used in soybeans, Specht says.
Now with the gene in hand, breeders can simply slip Dt2 into high-yielding indeterminate soybean varieties and start breeding with them.
Thanks to the cloning technique used by his Purdue colleague, molecular geneticist Jianxin Ma, the trait will be much easier for breeders to manipulate, Specht adds.
Cloning the gene produces a "perfect marker," a small gene sequence that breeders can look for when they chip the seeds of their test plants and run a genetic marker analysis. If they find the gene sequence in the plant, they'll know it will be semi-determinate and worth keeping for further breeding.
"Now that the DNA sequence for Dt2 has been published, commercial as well as public breeders can use it to manipulate their breeding populations to try to create semi-determinate derivatives of indeterminate high-yield cultivars, maybe for marketing in regions where lodging is a big problem or plant height needs to be controlled better," Specht says.
Specht says growers would only see minor differences in the field with a semi-determinate variety. A farmer might notice a darker green canopy in a semi-determinate field because the plants don't produce the small and lightly colored upper leaves of an indeterminate.
Semi-determinate plants won't look much shorter during the summer, Specht adds. Like a big-haired diva, they fluff their canopy tops up with broader, thicker leaves, which disguise their shorter stems. Only when they drop their leaves and go bald in the fall is the deception revealed.
Soybean lines derived from NE3001 are now undergoing more testing as part of a soybean gene mapping project called the Nested Association Mapping Project, also funded by the USB. Researchers have collected data on yield and other properties of the semi-determinates for three years. Specht says a summary of those results will be available to the public in the summer of 2015. It will be up to agricultural seed companies to decide if the Dt2 gene has a place in their product lineup.
Greg Peters says as long as any future semi-determinate soybean varieties are competitive on yield, oil and protein content, and disease resistance, he would be very interested. "There's a place for these beans because people are trying to get more fertility into their beans," he says. "Consequently, they get a lot of the plant growth and not necessarily increased yield."
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