Big crops get bigger.

That old farming adage is holding true as the U.S. Department of Agriculture last week raised its forecast for record U.S. corn and soybean yields.

And there’s a good chance the final yield will be even larger, according to John Newton, American Farm Bureau Federation chief economist. Historically, U.S. corn and soybean final yield estimates tend to keep growing when the USDA raises its yield forecast from August to September, he said.

“Odds are favorable that this big crop will indeed get bigger once the combines start rolling,” he said.

The USDA has raised yield projections for...