Betting on Shorter Beans

Gene Discovery Could Shrink, Stabilize Northern Soybean Varieties

Emily Unglesbee
By  Emily Unglesbee , DTN Staff Reporter
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The semi-determinate soybean varieties on the right are noticeably shorter than the indeterminate variety on the left. Scientists hope the shorter stature will help prevent lodging for soybean growers in high-yielding environments. (Photo courtesy of Jim Specht, University of Nebraska)

ST. LOUIS (DTN) -- Year after year, the fertile, irrigated river bottoms of southeastern Nebraska have blessed Greg Peters with big, beautiful soybean plants.

A little too big, in fact.

"The beans have a tendency to get really tall, with a lot of space in their internodes," the DeWitt, Neb., farmer and chairman of the Nebraska Soybean Board of Directors told DTN. "In a windstorm, they're so tall or heavy with pods, they start leaning and tend to lodge really badly."

Just a couple years ago, one of Peters' fields spawned towering, 6-foot-tall beans in 15-inch rows. "You step out into a field like that and just disappear," Peters recalled. "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. In what should have been a bin-buster field, Peters estimated that he lost 15 to 20 bushels per acre to lodging.

Fortunately, scientists from Purdue University and the University of Nebraska have isolated and cloned a gene that could help farmers like Peters in the future.

The gene, called Dt2, produces soybean plants that are semi-determinate, and therefore shorter. "With the shorter stature, there's less lodging potential," University of Nebraska agronomist Jim Specht explained to DTN. On average, Specht said the plants sport about 17 nodes, compared to the 22 or 24 nodes that current varieties can reach.

THE DETERMINATE FACTOR

Most beans 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 throughout the season, even after reproductive stages are underway, so they tend to be taller.

Determinate beans are typically found in southern regions and in varieties from maturity group V and up, Specht noted.

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The shorter growing season of more northern states requires the extended growing habits of indeterminate soybeans, and they are generally found in varieties from maturity group IV and earlier, Specht said.

"When we try to grow determinate varieties in the North, we only get about 12 nodes," he explained. "In the North, we have to combine our vegetative period with our reproductive period -- they have to overlap. In the South, with the longer season, they don't."

Ironically, the conditions that produce some of the best soybean yields -- irrigated fields, fertile river bottom soils, or a wet growing season -- can also produce overgrown indeterminate soybeans that are prone to collapse in windstorms or topple over from the weight of their own pods.

Narrow row spaces can contribute to the problem by forcing beans to grow up rather than out, Peters said. Widening his rows to 30 inches didn't tame the leggy legumes either. Come harvest time, the sprawling plants still cause a problem.

"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," he said. "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

Using the Dt2 gene, UNL soybean breeder George Graef developed the first high-yielding semi-determinate soybean variety (named NE3001) for use in the northern U.S. several years ago. Since then, Specht said 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," he said. "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 funded by the United Soybean Board (USB), growers could only get a semi-determinate hybrid by crossing indeterminate and determinate plants. The result was ultimately impractical, because hybrids aren't used in soybeans, Specht said.

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 added.

Cloning the gene produces a "perfect marker," a small gene sequence that breeders can look for when they chip the seeds of their tests 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 concluded.

IN THE FIELD

Specht said growers would only see minor differences in the field with a semi-determinate variety. During the growing season, 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 added. 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 another USB-funded project known as the Nested Association Mapping Project. For three years, researchers have collected data on yield and other properties of the semi-determinates, and a summary of those results will be available to the public in the spring of 2015, Specht said. Then it will be up to agricultural seed companies to decide whether or not the Dt2 gene has a place in their line-up of soybean varieties.

Peters said as long as any future semi-determinate soybean varieties were 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 and consequently they get a lot of the plant growth and not necessarily increased yield," he said.

Emily Unglesbee can be reached at emily.unglesbee@dtn.com

Follow Emily Unglesbee on Twitter @Emily_Unglesbee

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Emily Unglesbee