Most beef cattle in the U.S. are finished on feedlots, bulked up with grains instead of the grass they evolved to eat. But more and more consumers are demanding grass-fed beef, saying it's healthier to eat and better for the environment. "Sales have soared from $17 million in 2012 to $272 million in 2016. And industry analysts say grass-fed beef could make up 30 percent of the market within 10 years," Alex Smith reports for NPR.

There's a problem with the growing trend of grass-fed beef though: the grass they eat is becoming less nutritious. Researchers at Texas A&M University and Boulder, Colo.-based ecology outfit Jonah Ventures studied cow manure collected from all over the country between 1994 and 2016 and found that crude protein in grass has dropped by almost 20 percent since the mid-90s. That's causing the cattle to gain less weight on grass than they would have in years past. The less-nutritious grass could hurt even conventional cattle, since many are grass-fed before finishing.

Jonah Ventures co-owner Joe Craine told Smith there are two theories on the cause for the drop. One is that rising carbon dioxide levels in the atmosphere could be the other reason, since increased CO2 levels are have been linked to fewer nutrients in other grass plants like rice and wheat. The other is cattle that are moved to feedlots don't defecate on the prairie anymore, delivering nutrients to the soil.

"Craine thinks this may be happening on a large scale in the prairie, and that it's just a matter of time before prairie grasses simply don't have enough protein to support grazing," Smith reports. The researchers studied cow manure collected from all over the country between 1994 and 2016. "Somewhere on the order of 50,000 cow pies got shipped to Texas for this study," Smith reports.