DES MOINES, Iowa - This time, the presidential cattle call was held at the state fairgrounds - which seemed appropriate, given the topics.

Corn-based ethanol, immigration, genetically modified food, wind-energy tax credits, groundwater regulations, crop insurance, trade.

Over seven hours, nine potential Republican candidates addressed detailed questions about those and other issues important to the agricultural industry in Iowa, the state that produces about 10 percent of the nation's food, and, more important, holds the first 2016 nominating contest.

Some sharp policy differences emerged at the Iowa Ag Summit as the candidates tried to balance the needs of a powerful industry that depends on federal help with the passion for limited government that dominates the conservative GOP base.

Agribusiness mogul Bruce Rastetter posed the questions in solo interviews, including a pointed query on support for the federal renewable fuel standard, which requires blending corn-based ethanol and other biofuels into the nation's gasoline supply. The requirement forms a big part of Iowa's farm economy.

"The answer you'd like me to give is, 'I'm for the RFS.' Darn it, that would be the easy thing to do," said Texas Sen. Ted Cruz, who wants the requirement repealed. "But I tell you, people are pretty fed up, I think, with politicians that run around and tell one group one thing and tell another group another thing."

He, like many other conservatives in the party, believes that the fuel standard is a form of corporate welfare and that government "shouldn't pick winners and losers."

Former Florida Gov. Jeb Bush and former New York Gov. George Pataki sided with the free-marketeers, saying they wanted the fuel standard phased out, for ethanol to stand on its own.

"The markets are ultimately going to have to decide this," Bush said.

"I don't think the federal government has any business telling people what they ought to buy, whether it's health insurance or fuel," Pataki said.

New Jersey Gov. Christie said he supported the mandate for ethanol, and he blamed the Obama administration, which has delayed establishing the latest standard.

"It's the law," Christie said. "Certainly anybody who's a competent president would get that done in their administration."

Former Arkansas Gov. Mike Huckabee portrayed the renewable standard as an important national-security issue, a way of keeping the United States from depending on buying oil "from people who hate us."

Gov. Scott Walker of Wisconsin softened his past opposition to ethanol mandates, saying that without them, oil companies might block out the fuel. "I do believe it's an access issue, so it's something I am willing to go forward with," Walker said. "Right now, we don't have a free and open market."

And former Texas Gov. Rick Perry was also more favorable to the standard than he had been. In office, he sought waivers from the requirement in his petroleum-rich state, saying it was not economical and drove up food prices. On Saturday, he said it would be unfair to end the renewable fuel standard while oil companies continued to benefit from tax breaks.

If the RFS discussion seemed dry at times, it was a stark demonstration of the power of Iowa and other early-voting states to get would-be presidents to pay attention to their pet issues.

"It's a huge component of the Iowa economy - half the corn grown here goes to ethanol," said Bruce Phillips, 54, a real estate broker in Des Moines who also owns a farm near Pleasantville. "If the fuel standard went away, it would be very damaging. You'd be talking crisis in the heartland right there."

The candidates were unanimous in opposing the federal government's plan to regulate groundwater pollution as it does in waterways and agreed that genetically modified foods are safe.

They also supported leaving more agriculture decisions to the states. As Bush said, "We have to figure out how to rein in the top-down regulatory environment toward a more collaborative process. The first thing you do is change presidents," he said.

Differences among the candidates emerged on immigration. On his first trip of the year to Iowa, Bush defended his support for comprehensive immigration reform.

"If we want to be young and dynamic and growing again, where the debate isn't about who's taking from whom rather than having an expanding pie where opportunities exist for all of us, I think we need to fix this broken immigration system," Bush said. "Immigrants that are here need to have a path to legalized status."

He called for a guest-worker program to deal with labor shortages in agriculture and other sectors.

Others talked about the need to strengthen border protections and decried what they see as amnesty for illegal immigrants. "What do we do to stem the tide of people who are rushing over because they've heard that there's a bowl of food just across the border?" Huckabee asked.

At one point, a man stood up and began shouting as Christie was speaking. "I'm from New Jersey also!" Joe Mangino of Beach Haven, N.J., yelled as he stood, attacking Christie for the slow pace of recovery from Sandy. "I will not shut up."

Police escorted him and a companion out. Christie joked about importing protesters from back home.

"My people follow me everywhere; it's fabulous," he said. "I'm magnetic."

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@tomfitzgerald