RIO DE JANEIRO  Farmers in Argentina began cutting off grain exports on Thursday after the breakdown of talks to resolve a conflict over government agricultural policies.

Farmers north of Buenos Aires in Gualeguaychú, a hot spot during crippling strikes in March, once again blocked major roads used by trucks to take corn, wheat and soybeans to other South American countries.

In Córdoba, Entre Ríos and Santa Fe Provinces, farmers also gathered on roads to prevent trucks from passing. Alfredo de Ángeli, a leader in the Argentine Agrarian Federation, said the protests would continue until next Thursday. But unlike during the 21-day strike in March, which caused widespread food shortages and crippled exports, farm leaders have vowed that this time they will not block food deliveries to Argentine citizens.

“We will only stop grains and international exports,” Mr. de Ángeli said in a television interview.

The protests threatened to exacerbate global food shortages and to damage Argentina’s economy, which relies on agriculture for more than half its export earnings. Argentina is the world’s second largest corn exporter and third largest soybean supplier.