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, according to the report.

The U.S. inventory of all hogs and pigs on Dec. 1, 2014, was 66.1 million head, the USDA said. This was up 2 percent from Dec. 1, 2013, and up 1 percent from Sept. 1, 2014. Iowa’s total herd inventory was up 700,000 head to 20.9 million head, the USDA said.

U.S. market hog inventory, at 60.1...