The U.S. pig herd is expanding, and producers in Iowa are leading the way.

The U.S. breeding herd on Dec. 1, 2014, at 5.97 million head, was up 4 percent versus a year ago, according to the U.S. Department of Agriculture’s (USDA) hogs and pigs report. The breeding herd in­­ventory in Iowa was up 40,000 head to 1.01 million head,...