Corn yields in the U.S. have become more sensitive to drought, not less, over the past 18 years, according to a study published today in Science. The report warns that if the trend persists, yields could fall substantially by midcentury as higher temperatures subject corn plants to more water stress.
Corn yields aren't falling yet; on the contrary, they've been steadily rising year after year, even as the average yields of other major grains like wheat and rice have been almost stagnant since 2000. Companies like Monsanto, Syngenta, and DuPont have invested enormous sums in developing varieties that purport to be drought tolerant—and overall yields have indeed improved even during drought conditions, although not in the disastrous year of 2012.
David Lobell of Stanford University in California and colleagues found, however, that most of that yield gain has come not through drought tolerance but through genetic improvements that allow higher planting densities. Farmers in the Corn Belt now typically grow 30,000 corn plants per acre, compared to 24,000 plants per acre during the 1990s.
"There's a lot of marketing out there telling people that we can adapt to drought," says Lobell. "But the data shows that we're actually getting more sensitive. Everything driving yield gains is also driving more plant stress. It's a little foolish to think we can magically adapt away from these constraints."
A recent report by the Intergovernmental Panel on Climate Change concluded that climate change is already having more negative than positive effects on crop yields globally, including on wheat and corn. By weight, corn is the largest grain crop in the world. More than a third of it, and half of all exports, is produced in the American Midwest.
Lobell and his team analyzed field-level yield and weather data in Corn Belt states from 1995 to 2012. They calculated the environmental stress that each field was under in each year. The biggest predictor of stress, they found, is the "vapor pressure deficit," or VPD, which is the difference in humidity between the inside of the plant and the outside air. It determines how fast the plant loses water to the air as it takes in carbon dioxide to fuel its photosynthesis.
Although overall yields have increased since 1995, the researchers found that yield growth has been far lower in fields where VPD was high—an indication that corn yields had become more drought sensitive over time, rather than more drought tolerant. The disparity was greatest in the drier Corn Belt states of Nebraska, South Dakota, and Kansas, the target market for drought-tolerant corn varieties.
The most likely cause of the increased sensitivity, Lobell says, is the very increase in planting densities that has allowed yields to increase. More plants typically translate into higher yields, but also less soil, less sunlight, and less water per plant.
In any case, the increased sensitivity to VPD is not a good sign for the future. VPD goes up with temperature, and climate models project that VPD in the Corn Belt in July, when corn plants are most sensitive to it, could to rise by 20 percent over the next 50 years. Lobell and his colleagues calculate that yields could fall 15 to 30 percent—unless new corn varieties or agronomic techniques can manage to offset the trend.
More Bushels Needed
The current average yield in the Corn Belt is around 170 bushels per acre. By some estimates, says Donald Ort of the U.S. Department of Agriculture's Agricultural Research Service and the University of Illinois, Urbana, yields will have to rise well above 250 bushels per acre by 2050 to meet projected demand for feed grain and biofuel. With both VPD and corn's sensitivity to it rising, Ort worries there won't be enough water.
"We're not even talking about how to get 250 bushels," Ort says, "but how we're going to maintain the yields we have. This thing you hear from Monsanto that they'll deliver 300-bushel corn in the Midwest—well, the data show no, you won't, unless you have irrigation." Currently, only about 10 percent of the U.S. corn crop is irrigated.
Monsanto's newest drought-tolerant corn varieties, which were just released to farmers last year, were not taken into account in Lobell's study. But Lobell stresses that such new technology faces an uphill battle.
"This paper is a reality check on the biophysical limits on what we can produce, particularly when conditions are not good," he says. "We can't underestimate the challenges. It is very difficult to make these systems more productive."
This story is part of National Geographic's special eight-month Future of Food series.