Ukraine has set quotas for grain exports following a severe drought and heatwave this summer.
Deputy agriculture minister Serhiy Melnyk said the quota would remain in place until the end of the year.
The government will allow the export of two million tonnes of maize, 500,000 tonnes of wheat and 500,000 tonnes of barley.
Ukraine's quotas follow an export ban by Russia and could put further pressure on grain prices.
In August, the government reduced its forecast for the year's grain harvest to 39 million tonnes from 46 million in 2009.
Ukraine is the world's top producer of barley and sixth biggest of wheat.
News of the export quotas caused wholesale wheat prices to rise by 1% on Thursday.
Russia, the world's third-largest wheat exporter last year, already has a ban on exports until after next year's harvest.