Lorry drivers are blocking roads in a number of Brazilian states in protest against the high cost of fuel and tolls.
Deliveries of soya and corn to one of Brazil's main ports, Paranagua, have slowed, the authorities said.
The protest has affected production in many rural areas and has led to localised shortages of fuel.
Farmers in the main meat, soya bean and milk producing regions of the country say their silos are full.
The road leading to Brazil's busiest port, Santos, is one of those blocked.
Police clashed with drivers, fired tear gas and arrested seven people, but failed to clear access to the port.
The government has invited the protest leaders for a meeting in Brasilia to try to reach an agreement.
The protest, which began last week, has now spread to many central and southern states: Rio Grande do Sul, Parana, Santa Catarina, Mato Grosso, Mato Grosso do Sul, Goias, Minas Gerais and Sao Paulo.
The situation is particularly worrying in western Mato Grosso state, which accounts for most of the Brazilian production of soya beans.
Some 50% of the soya produced in the state has already been harvested, according to Folha de Sao Paulo newspaper.
But much of the output is waiting in silos or lorries to be transported to other parts of the country or exported.