Appearing in the top U.S. corn-producing state, Trump took several shots at Senator Ted Cruz of Texas, who is giving Trump a strong challenge, especially in the campaign for the Feb. 1 Iowa caucuses, the nation's first nominating contest for 2016.

"My primary opponent is totally opposed to ethanol and the ethanol industry because he's with the oil industry," Trump said.

Joining with the ethanol lobby, Trump has criticized Cruz for opposing the federal Renewable Fuel Standard, which sets the minimum amount of corn-based ethanol and other biofuels that must be included in the nation's gasoline supply.

The junior senator from Texas has said that he supports ethanol, but doesn't want to see the federal government pick "winners and losers" in the energy sector.

Trump also continued a line of attack against Cruz from earlier in the week, suggesting that nominating him might be a dangerous thing for Republicans because Cruz was born in Canada. "He's got to straighten out his problem," Trump said. "You can't have that problem and go and be a nominee."

The reality television star refrained from repeating his comments about former President Bill Clinton's sexual transgressions, although a state senator who introduced him at Saturday's event couldn't resist. Brad Zaun, who is backing Trump, referred to the former president, and husband of Democratic front-runner Hillary Clinton, as an "accused rapist."

Trump noted that the first-place finisher in recent Iowa caucuses had not ultimately won the party's nomination, something he said he wants to change. In 2012 Iowa narrowly picked former Senator Rick Santorum of Pennsylvania, and in 2008, former Arkansas Governor Mike Huckabee won by a wide margin.

"You haven't been good at picking the winners, folks,"Trump said. "If you pick me, you're going to pick a winner."

A Real Clear Politics average of recent state polls shows Trump running second in Iowa behind Cruz, but the New Yorker did nothing to lower expectations for his performance in the state, and said he would be spending "a lot" of time there before Feb. 1.

"We've got to win Iowa," he said. "Otherwise, we're wasting our time, folks."