Argentina's farmers are facing a sharp increase in transportation costs after fuel prices were hiked in recent months, dismaying a sector that has been one of center-right President Mauricio Macri's biggest allies.

Shortly after taking the reins of Latin America's third-largest economy in December, free-market proponent Macri dropped the trade and currency controls applied by his leftist predecessor, Cristina Fernandez.

He eliminated corn export duties and reduced soy export taxes, winning the favor of the farming sector.

But his government, blaming exchange-rate corrections, has also overseen a jump in fuel prices, up 31 percent so far in 2016.

That has boosted the cost of ground transportation for Argentina's grains producers, most of whom rely on trucks to transport their harvest to the country's ports.

"As of today, fuel prices have already neutralized the gains of the tax reduction," Eliseo Rovetto of the Argentine agrarian federation told Reuters.

Although Macri has won plaudits on Wall Street for his economic policies, the rapid adjustments are causing pain for many Argentines and sending his approval ratings down.

The fuel cost situation is especially troublesome for farmers in Argentina's northern provinces, many of whom have to cover as much as 1,200 km (750 miles) to reach the country's ports. Nearly 80 percent of agricultural products are shipped from the central port of Rosario.

Argentina's farm belt is concentrated in the country's central provinces, but about 11 percent of the 20 million hectares (49 million acres) of soy plantations for the 2015/16 season are in the north, as well as 19 percent of corn plantations, according to the Buenos Aires grains exchange.

For farmers in the north, the cost of shipping to Rosario can represent over half of what the shipment is worth, said Ernesto Ambrosetti, an economist at Argentina's Rural Society, which represents large farm owners.

Argentina is the world's top exporter of soymeal livestock feed and the third-biggest exporter of raw soybeans.