Amber waves of grain are an iconic image of American agricultural bounty, but the genetic improvements that have increased wheat yields year after year may have hit a wall, according to new research. That could present new challenges to farmers and policymakers trying to find ways to feed a growing world population that shows no signs of plateauing.
The U.S. harvests around 68 million metric tons of wheat each year. There are two ways to increase this amount: One is to change farming practices by, say, increasing the amount of cultivated or irrigated acreage. The other is to breed the crop to be more prolific by introducing attributes that make it mature at ideal times, resist fungal infections, and divert more energy into making grain. Starting in 1959, wheat yield increased by about 1.1% per year, thanks to these breeding efforts—a boost known as “genetic gain.” But in 1984, a few scientists noticed that in the 10 previous years, the average yield improvement had slowed, indicating that genetic gain was potentially leveling off. Ever since, genetic gain has continued to steadily drop.
To better quantify what was happening, Robert Graybosch, a geneticist at the University of Nebraska, Lincoln, and C. James Patterson, a geneticist at Oregon State University, Corvallis, analyzed data collected by the U.S. Department of Agriculture over the last 50 years. They found that genetic gain began slowing down in the late '80s, as scientists had suspected, and now appears to have come to a halt. With current breeding techniques, wheat may finally have reached the upper limit of its potential yield, Graybosch says. "Now it's sort of reshuffling cards from the same deck."
Why has genetic gain come to a standstill? One of the primary reasons, says Graybosch, is that pathogens are likely evolving more quickly than breeders can keep up with. Another possibility, says Kulvinder Gill, a wheat geneticist at Washington State University, Pullman, who was not involved with the study, is genetic bottlenecks. For example, "dwarfing" genes that allow the plant to divert more energy to producing grain are widely used to increase yield—but this also means that breeders have been ignoring nondwarf varieties, thereby restricting the gene pool. Selecting for varieties that are resistant to particular pathogens has created a similar bottleneck, says Allan Fritz, a wheat geneticist at KansasStateUniversity in Manhattan who was also not involved with the work.
Directly altering the DNA of wheat—so-called genetic modification—could again spur genetic gain in crops, but Graybosch doesn't expect that anytime soon. The wheat's genome is very complex, he notes, and the public tends to frown on genetically modified foods.
Still, wheat production isn't going to slow down or drop off, says Graybosch, whose team reports its findings in an upcoming issue of Crop Science. The study only shows that the rate of improvement is going down, not yield itself. But as the fourth largest producer of wheat (behind the European Union, China, and India), American farmers will likely have to turn to nongenetic means of raising their yields, such as increasing irrigation and planting more land in order to keep up with a growing world population. "The only way to make more wheat as a nation is to have better production practices," Graybosch says.