PALMERSTON NORTH, New Zealand -- On a typical day, researchers in this college town coax hungry sheep into metal carts. They wheel the fluffy beasts into sealed chambers and feed them grass, then wait for them to burp.

The exercise is part of a global effort to keep sheep, deer, cows and other livestock from belching methane when they eat and regurgitate grass. Methane is among the most potent greenhouse gases, and researchers now believe livestock industries are a major contributor to climate change, responsible for more...