US farmers can grow more corn than ever before thanks to genetic modifications and improved planting techniques, but the crops are also increasingly vulnerable to drought, researchers said.

The study in the journal Science found that "densely planted corn appears to be unexpectedly more sensitive to water scarcity," raising concerns about future food supply as the planet warms.

The United States is the largest exporter of corn in the world, shipping about 40 percent of the world's corn.

In recent years, most commercially produced corn has been modified with new traits that make roots better able to access water and build in pest resistance.

That has allowed farmers to plant more corn and set the plants more closely together than they could in the past, resulting in higher yields.

But the plants have also become more susceptible to dry spells over the past two decades, which is a concern because most corn crops in the United States rely on natural rain, not irrigation.

If predictions for future climate in the midwestern US - known as the Corn Belt - are correct, then corn yields could fall 15 to 30 percent over the next half century, scientists said.

The study was led by David Lobell of Stanford University, and was funded by the National Science Foundation and the National Oceanic and Atmospheric Administration.

"Recent yield progress is overall a good news story. But because farm yields are improving fastest in favourable weather, the stakes for having such weather are rising," Lobell said.

"In other words, the negative impacts of hot and dry weather are rising at the same time that climate change is expected to bring more such weather."

Soybeans, which are often genetically modified in the United States but have not been more densely planted in recent years, have not become more susceptible to drought the way corn has, Lobell and colleagues said.

The study was based on an analysis of more than one million USDA crop insurance records between 1995 and 2012.

"The study raises a potentially grave concern beyond 2050, when yields are projected to decline at an accelerating rate," said an accompanying editorial by Donald Ort and Stephen Long of the University of Illinois.

Changing the way corn is planted can only go so far toward solving the problem, they wrote. The key lies in reducing harmful carbon emissions, caused mainly by burning fossil fuels, which boost global warming.

"The only real solution will be to avoid the CO2 emissions that will otherwise cause rises in temperature."

Source - http://www.nzherald.co.nz/