The world has a lot of cows—more than 1.5 billion. And every day, US slaughterhouses turn nearly 20,000 of them into steaks, hamburgers, and roast beef. But before they become patty-worthy, each cow spends a lifetime chewing grub and burping greenhouse gases. Cows love grass. So scientists at Aarhus University in Denmark are engineering “super grass” they hope can cut methane without starving the cows of other vital nutrients. Through genetic selection, they’re looking for DNA in fodder that could be engineered to yield less gas build up in the rumen. Or, if not grass, how about meddling directly with the gut itself? Last year, a crew of Penn State researchers began adding 3-nitrooxypropanol, or 3NOP, into cow feed. Cows still get to burp, but the 3NOP binds enzymes to the bovine’s gut and prevents the gut microbes from producing methane. In experiments, cows with altered guts emitted 30 percent less gas than usual.