Slowing grass growth at dairy pastures in New Zealand, crucial feeding grounds for cows, is raising the prospect of tighter-than-expected milk supply and may offer a window for U.S. exports.

“We are not sure what it will do to milk production, but it’s definitely something to keep an eye on,” Kyle Schrad, vice president of global dairy and food operations for INTL FCStone in Chicago, said in a telephone interview on Monday.

The Pasture Growth Index, which measures grass conditions and growth potential, is near the lowest in more than 21 months amid dry weather in New Zealand, the world’s largest dairy exporter. The measure runs from zero to one, and while it typically declines this time of year for seasonal reasons, over the past few weeks it’s “fallen off far more significantly” than normal, Schrad said. Any drop in New Zealand’s milk production “should help bolster demand for U.S. exports,” he said.