After the 2003 growing season, when corn yields boomed and soybean yields bombed, there were some dire comments on soybean yield progress. For example, a Purdue University report says, “Corn has taken giant leaps forward in average bushels per acre, while soybean yields have advanced in baby steps.”

Between 1930 and 2003, the Purdue report says average corn yields jumped nearly sevenfold, from 20.5 bu./acre to 142.2 bu./acre. In that same 73-year period, average soybean yields nearly tripled, from 13 bu./acre to 33.4 bu./acre. Over the past 10 years, national soybean yields have hovered around 40 bu./acre.

“We're looking at about a 0.4 bu./acre per year average increase for soybeans, vs. 1.5 bu./acre/year for corn,” says Jeff Volenec, Purdue agronomist.

But Jim Specht, a University of Nebraska research agronomist and soybean breeder, points out that the yield trend lines for corn and soybeans have been nearly parallel since 1972. Although corn has had a 3:1 yield advantage, that ratio has not changed much over the past 32 years.

“Thus, the yield potential of new soybean varieties is, in relative terms, keeping pace with the rise in yield potential of the new corn hybrids,” says Specht.

A study by University of Illinois ag economist Gary Schnitkey shows a similar conclusion. Over most of Illinois, the corn-soybean yield ratio from 1972 through 2003 averaged 3.24:1 and exhibited no trend up or down. This suggests, says Schnitkey, that corn yields relative to soybean yields have been stable over that period.

University of Illinois data also show, as might be expected, that soybean prices, nationally, in years of down yields rose well above the previous year, thus cushioning the lower yields.

Mike Snyder, an independent crop consultant at Ashland, OH, points out that although top-end soybean yields haven't increased greatly in recent years, there have been great strides in tolerance to weather extremes.

“When we are under extreme conditions, such as too wet or too dry, yields have been much better than they used to be,” he says. “That became obvious when some growers started planting older varieties for use in tofu (Beeson 80, Amcor, etc.). I saw very clearly how much more tolerant the newer varieties were to weather extremes and associated disease complexes.”

Soybean yields, of course, are a result of genetics and environment. People who most influence genetics are public and private soybean breeders. The Corn And Soybean Digest asked the soybean research directors of four major seed companies for their take on what has happened and what will happen in soybeans genetics.

Although soybean yields haven't increased as much or as fast as breeders and producers would like, this isn't because of a slowdown in genetic improvement, says Alan Walker, soybean commercial breeding director for Monsanto. Rather, it's due to the combination of more diseases, nematodes, insects and lack of water.

Walker notes that U.S. soybean yields rose 1.4%/year from 1975 through 2002 due to both genetic gain and improved cultural practices. Soybean breeders and plant physiologists attribute 70% of the yield improvement to genetic gain and the remainder to cultural practices.

“Genetic gain is measured by comparing new varieties to other varieties for yield in the same tests,” Walker explains. “This encompasses all the environmental factors (drought, iron deficiency chlorosis, etc.) and pest factors (diseases, soybean cyst nematode [SCN], root-knot nematode, aphids, etc.) that occur in those fields. Genetic gain for yield includes genes for yield, per se, plus defensive genes that protect varieties.”

Walker says the average genetic gain for newly released Monsanto varieties has been 1.34% annually over the past five years. This translates into a fraction over 0.5 bu./acre annual genetic improvement.

John Soper, Pioneer's director of soybean research, also notes that soybean yields have risen, on average, by nearly 1.5 bu./acre/year since the early 1970s, although there have been multi-year plateaus of flat yields within that period. He says those flat periods have been caused by weather and diseases.

“In our internal research, comparing new varieties with those that are two to three years old, we're seeing an average yield gain of 0.5 bu./acre/year,” he reports.

Soper says Pioneer uses both traditional breeding practices and molecular marker technology in soybean breeding (as do several of the major seed companies).

“Use of molecular markers is especially valuable in identifying defensive traits such as resistance to soybean cyst nematode,” he notes. “Ten years ago, using traditional breeding only, we would cross a resistant line with a normal line and then take the highest yielders and test them for resistance. Maybe one in 10 would qualify.

“Today, using molecular markers, we can test thousands of lines, already known to have cyst resistance, for yield,” Soper says. “With this approach, the rate of yield increase is double the historic rate.”

What's more, current SCN-resistant varieties don't take a yield hit on non-SCN soils as was the case in the past.

“We're looking now to extend the use of molecular markers to identify genes that specifically increase yield,” Soper reports.

But genetically improving yield is more complex than improving some of the other soybean traits, explains Hunt Wiley, soybean research director for Dairyland Seed Company.

“Many genes are involved in yield, whereas single-gene traits are often associated with pest problems,” Wiley says. “One single-gene example is weeds controlled by the Roundup Ready trait. In contrast, there are complex pressures on yield, such as drought, flooding, excess heat/cold, standability, diseases, etc., that require many genes to maximize performance.”

Nevertheless, says Wiley, a company such as his finds higher-yielding varieties by generating thousands of new genetic crosses each year and hundreds of thousands of their progeny. Promising lines are put through severe testing conditions for diseases and SCN. Those lines that exhibit acceptable defensive traits are tested at multiple locations for yield stability.

John Thorne, soybean development director for Syngenta Seeds, recalls that the average U.S. soybean yield was about 27 bu./acre when he began as a breeder in 1970. It advanced to 34 bu./acre in the mid-'80s and pushed to 38 bu./acre in 2000 and surrounding years. He sees the average pushing to 40 bu./acre for 2004. “The increases may not have been dramatic, but they have been consistent,” he says.

As an example of yield progress, Thorne notes that a 1976 NK release, S1492, a premier performer for its day, yielded 82% of Syngenta's current varieties in 2003 comparisons.

Thorne also makes this observation: “Soybean acreage has increased significantly in recent years. That means acres with less than optimum soybean yield potential (such as on the fringes of the Corn Belt) are now being planted with soybeans.”

Many of those areas grow early maturing bean varieties and that also tends to reduce national yield averages.

“By using advanced technology, we're able to test many more lines than in the past, test them in more environments, and collect and analyze the data quickly and efficiently,” Thorne points out. “These factors will have a long-term positive impact on yields.”

Exciting Exotics

There's a concern among some scientists that the gene pool for new soybean varieties is too limited. But help is coming.

The National Soybean Research Laboratory, located at the University of Illinois, is developing exotic material for use in domestic soybean breeding.

“We are using dozens of exotic lines in our breeding programs and continue to bring in more exotic material,” says Randall Nelson, USDA-ARS geneticist. “In the past two years we've released two experimental lines, each derived from crossing two Chinese varieties.”

Those lines have performed well in regional testing. One finished first out of 34 entries at 11 locations, while in another test the other came in third among 24 entries at 15 sites.

“These are the best results we've had so far,” says Nelson. “We now have lines derived from exotic germplasm that yield as well as the best public varieties of the same maturity.”

Good News On Pest Resistance

A University of Missouri soybean breeder, in cooperation with the USDA Agricultural Research Service, has developed a soybean line that offers resistance to both soybean cyst nematode (SCN) and southern root-knot nematode. At the same time, a team of University of Illinois researchers has identified a single-gene source of resistance to the soybean aphid.

Grover Shannon with the University of Missouri at Portageville, MO, is the breeder responsible for the nematode-resistant line, designated S299-3181. It is an edible soybean but could be used for commercial varieties. It has broad resistance to SCN (many races), yields well and doesn't shatter. Commercial breeders, using molecular marker technology, could accelerate its incorporation into commercial soybean varieties.

SCN is the most destructive soybean pest in the U.S. Root-knot nematode is the second most destructive pest in the southern U.S.

The resistance to soybean aphids should be easily crossed into commercial varieties by 2008, according to University of Illinois soybean breeder Brian Diers. Dairyland Seed Company expects advance lines ready by 2007 if there are no yield constraints. Monsanto is looking at 2009 or 2010. Syngenta and Pioneer are not projecting a date at this time.