Breeding corn to boost yields has made it more vulnerable to heat

A century of breeding corn to boost yields in the US Midwest may have also made the crop more vulnerable to the hotter temperatures expected with climate change.

The amount of corn grown in the US more than quintupled during the 20th century due to a combination of breeding, agricultural intensification and favorable temperatures. But hotter and drier weather projected to arrive due to climate change threatens to slow or even reverse those gains.

“It’s fairly severe,” says Patrick Schnable at Iowa State University. “If you look at middle-of-the-road projections, corn yield goes down.” The worst scenarios project as much as a 50 per cent decrease in yield by 2100.

To investigate whether corn breeders can develop more hardy variants, Schnable and his colleagues looked at data from corn-growing trials in four Midwestern states conducted between 1934 and 2014, along with temperature data from the same years. The trials involved nearly 5000 different varieties, enabling the researchers to track the influence of both climate and breeding on yield.

They found that after decades of breeding, corn varieties became more tolerant of moderately hot temperatures between 32˚C and 34˚C (89.6˚F and 93.2˚F). However, many varieties became less tolerant of severe heat above 38˚C (100.4˚F), suggesting a genetic trade-off between breeding for a 20th-century climate and a 21st-century one.

“The trade-off in there is bad news if you’re in a high heat area,” says team member Aaron Kusmec at Iowa State University, though exactly why it occurs is unclear, he says.

Such severe heat is rare in the Corn Belt, but could become more frequent with climate change, says Ethan Butler at the University of Minnesota. The fact that corn adapts differently to moderate and severe heat shows that “the exact magnitude of warming is going to make a really big difference”, he says.

While the trade-off suggests breeding varieties that can tolerate both moderate and severe heat will be more challenging, the amount of genetic variation in response to temperature means careful breeding or genetic engineering could address this vulnerability. “Maize is so adaptable,” says Schnable. “It’s pretty extraordinary.”