Hundreds of thousands of Argentines rallied in the country's second-largest city on Sunday to protest grain export-tax hikes that farmers say are suffocating the sector. Gaucho hat-wearing farmers on tractors filled Rosario's main square. And they warned that if the government doesn't relent on the export restrictions during talks planned for Monday, they may return to the road blockages and protests that caused shortages of some food staples including beef and produce this spring. President Cristina Fernandez contends farmers are benefiting from a price boom and the profits should be redistributed to the poor. The farmbelt crisis has handed Fernandez the first political crisis of her five-month-old government, and led to the resignation of her first economy minister. Agricultural exports are the greatest source of revenue for Argentina, the world's third- largest soy producer. Soy is the crop most affected by the tax hikes, with duties rising as high as 45 percent. Angry farmers launched the protests in March, cutting off highways, disrupting delivery of farm goods and causing shortages in major cities. They later halted the blockades, but have maintained a ban on grain exports to pressure the government by choking off tax revenue. Negotiations have failed to defuse the crisis.