Study Finds Corn Increasingly Sensitive To Drought

Corn is the number one crop grown by American farmers, accounting for more than 95% of total production and use. The crop, which was a human invention and cannot be found in the wild, is also one of the most dependent on ideal weather conditions to grow. In a world with an ever-warming climate, these two factors do not play well with one another.

Now scientists from Stanford University have found another “oh, goody” moment to insert into the conversation. A new study, published in Nature Food, has found that the staple crop has become significantly more sensitive to drought conditions.

New technologies are able to help raise yields in a variety of weather conditions. That’s the good news.

“The bad news is that these technologies, which include some specifically designed to withstand drought, are so helpful in good conditions that the cost of bad conditions are rising,” said study lead author David Lobell, the Gloria and Richard Kushel Director of the Center on Food Security and the Environment at Stanford. “So there’s no sign yet that they will help reduce the cost of climate change.”

Corn production in the U.S. is a seemingly unstoppable juggernaut. Despite concerns about resistant weeds, a changing climate and many other factors, the industry has set record yields in five of the last seven years. Likely drivers of these bumper crops include changes in planting and harvesting practices, such as adoption of drought-tolerant varieties, and changes in environmental conditions, such as reduced ozone levels and increased atmospheric carbon dioxide concentrations that generally improve the water-use efficiency of crops.

As climate change intensifies, however, the cost to maintain crop yields will likely increase. A variety of factors enter into the equation, such as increased crop water needs due to increased plant sowing density. What is clear is that despite robust corn yields, the cost of drought and global demand for corn are rising simultaneously.

The Stanford scientists akin it to “a baseball slugger whose home run totals rise despite missing more curveballs each season.”

To accomplish the study, scientists used county soil maps and satellite-based yield estimates, among other data, to examine fields in the Corn Belt, the nine-state region of the Midwest which account for roughly two-thirds of the nation’s corn production. By comparing fields along gradients of drought stress each year, researchers could identify how sensitivity to drought is changing over time.

Even within a single county, they found a wide range of soil moisture retention, with some soils able to hold twice as much water as others. As might be expected, there were generally higher yields for soils that held more water. They found yield sensitivity to soil water storage in the region increased by 55 percent on average between 1999 and 2018, with larger increases in drier states.

The results made it clear that soil’s ability to hold water was the primary reason for yield loss. In some cases, soil’s ability to hold an increased amount of moisture was three times more effective at increasing yields than an equivalent increase in precipitation.

The Stanford report comes less than a month after a pair of studies out of the University of Illinois found that as the climate trends warmer and drier, global food security increasingly hinges on crops’ ability to withstand drought, and scientists and producers aren’t really focusing on the right metrics when measuring crop-relevant drought. Researchers said that atmospheric dryness (called “demand” in the studies) was not taken into account nearly enough.

“Plants have to balance water supply and demand. Both are extremely critical, but people overlook the demand of the equation, especially in the U.S. Corn Belt,” said Kaiyu Guan, principal investigator on the reports. “If you only consider rainfall and soil moisture, which is how most people think about drought, that’s mostly describing the supply side. Of course if you have low soil moisture, plants will be stressed by how much water they get. But the supply is often pretty sufficient, especially here in the U.S. Corn Belt…however, the demand side from the atmosphere can also severely stress plants. We need to pay more attention to that drought signal.”

The first study, “Redefining droughts for the U.S. Corn Belt: The dominant role of atmospheric vapor pressure deficit over soil moisture in regulating stomatal behavior of maize and soybean,” published in Agricultural and Forest Meteorology, used data from seven sites across the Corn Belt to conclude Vapor Pressure Deficit (VPD) accounts for nearly 90% of the changes in crop stomatal conductance, a proxy for drought stress, and approximately 85% of changes in gross primary productivity, a measure of productivity.

In the other study, “Connections between hydrological cycle and crop yield in the rainfed U.S. Corn Belt,” published in the Journal of Hydrology, Guan’s team focused on grain yield. Yield depends on many factors related to water cycles, but the researchers found that VPD explains the biggest proportion of variability in crop yield and also provides the earliest warning for yield loss when comparing with other water cycle metrics and traditional drought indices.

Stanford researchers believe that to better understand how climate impacts to corn are evolving over time, there should be increased access availability to field-level yield data that are measured independently of weather data, such as government insurance data that were previously available to the public but no longer are.

“This study shows the power of satellite data, and if needed we can try to track things from space alone. That’s exciting,” said Lobell. “But knowing if farmers are adapting well to climate stress, and which practices are most helpful, are key questions for our nation. In today’s world there’s really no good reason that researchers shouldn’t have access to all the best available data to answer these questions.”