With the fifth round of talks to renegotiate the North American Free Trade Agreement underway, Florida fruit and vegetable growers are pressing for changes.

They complain that subsidies in Mexico have flooded the US market with cheap competition under the landmark 1994 trade pact and want it amended -- unlike some corn growers and ranchers, who have benefited from the deal and prefer it to remain as it is.

All sides are trying to make their voices heard as negotiators from the United States, Canada and Mexico gather in Mexico City to continue the US-initiated effort to revamp the deal.

Under NAFTA, exports to Canada and Mexico of food produced in the United States have jumped to $39 billion this year from $8.7 billion in 1992. Imports of Mexican and Canadian agricultural products also have surged to $44.5 billion last year from $6.5 billion in 1992.

The lowering of tariff barriers made it easier for US corn producers to sell their crop to new markets.  

"Our exports of corn have grown tremendously. We worked hard to build domestic and export markets," said Mark Recker, president of the Iowa Corn Growers Association, who favors remaining in the trade agreement. 

"If we pull out of NAFTA, tariffs suddenly go up, and then we can't compete," he warns. "Mexico is already looking to South America or other sources for their corn."

- Blueberries, watermelons, eggplant - 

But blueberries and broccoli in Georgia, watermelons in Texas, grapes and asparagus in California, and strawberries, peppers, tomatoes, squash and eggplants in Florida have all seen production fall.

"Since NAFTA's passage in the 1990s, there has been a steady decline in both the acreage and production value in regions around the United States," said Lisa Lochridge of the Florida Fruit and Vegetable Association. 

And "The pace of that decline has increased significantly over the past five to 10 years as the Mexican government has provided subsidies for its farmers to grow produce for the US marketplace," she said.

Fruit and vegetable growers are pushing for a clause in the trade deal that makes it easier to launch a lawsuit against subsidies or dumping in another country. 

But Paul Mastronardi, president of produce company Sunset Grown, warns that those concerns do not represent the majority of farmers and the strategy is problematic.

"If we start to choose small subsections of each industry for different rules and standards, then we are exposed to retaliation from the NAFTA partners."

- Disagreements among ranchers -

US cattle ranchers disagree on whether NAFTA has helped or hurt their sector 

Hope Pjesky, who raises cattle in Oklahoma, said at a time when agricultural commodity prices are particularly low, US farmers cannot afford to lose customers.

For example, while beef tongue does not appeal to many American cooks, it is widely used in Mexico, and that can add "between $250 and $300 in value to each calf at the end of his life," she said.

But the Ranchers-Cattlemen Action Legal Fund, United Stockgrowers of America (R-CALF USA), the largest federation of cattle producers in the country, says NAFTA is mostly detrimental to American ranchers. 

"Under NAFTA, US ranchers are forced to absorb Canada and Mexico's overproduction" and that is "driving down domestic cattle prices even when global US exports are growing," the group said.

- Remove barriers -

Like the ranchers, the farming organizations are not on the same page. 

The American Farm Bureau Federation is clear: "Any renegotiation must protect the gains achieved in agricultural trade and work to remove remaining barriers to trade with Canada and Mexico."

But the National Farmers Union, which represents 200,000 farmers, is in favor of the renegotiation "to restore the United States' sovereignty over farm and food policy."

Juliette Majot, director of the Institute for Agriculture and Trade Policy (IATP), these positions reflect different visions of agriculture. 

Defenders of the NAFTA promote a system where "the only way to keep your head above water is to get big, to consolidate your land and to do more intense farming," she said.

According to the institute, from 1992 to 2012, the United States lost 22 percent of its small farmers -- those with less than $350,000 in annual gross farm income -- while the number of those earning more than $1 million a year doubled