Craig Myhre, a farmer in western Wisconsin, is trying to make a living off 600 acres of crops and a small herd of beef cattle. He also hires himself out to harvest other farmers’ fields, earning money to make payments on his combine.

It’s still tough to make ends meet, despite putting in 12- to 16-hour days. In 2015, he added yet another job, as a mail carrier.

“We’re constantly doing something around here to keep things moving,” said Mr. Myhre, who is 50 years old. His wife is a physician’s assistant, and sometimes climbs into the seat of a combine herself. Together, they are raising five children and trying to maintain a farm in Wisconsin’s rolling hills that has been in Mr. Myhre’s family since 1952. Sometimes, that means missing his sons’ high-school football games, and staying home while friends in town take vacations. “I struggle to pay myself sometimes,” he said.

Most U.S. farm households can’t solely rely on farm income, turning what was once a way of life into a part-time job. On average, 82% of U.S. farm household income is expected to come from off-farm work this year, up from 53% in 1960, according to the U.S. Department of Agriculture.

Off-farm work has become more important since a slump in prices for corn, wheat and other farm commodities over the past five years has cut total U.S. farm income in half. Last week, the USDA said income from farming is expected to fall further over the next decade. Now, picking up work in construction or truck driving is required for many farmers to fund seed and fertilizer purchases, and keep current on loan payments for tractors and land.