The Food and Agriculture Organisation's (FAO) price index, which measures monthly price changes for a basket of cereals, oilseeds, dairy, meat and sugar, averaged 196.6 points in August, down 7.3 points or 3.6 percent from July.
The figure was 7.9 points or 3.9 percent below August 2013.
FAO raised its forecast for global cereal production for 2014 to 2.512 billion tonnes, 14 million tonnes higher than its previous forecast.
The agency also raised its world wheat output forecast to 716.5 million tonnes, versus 707.2 million tonnes previously foreseen, and predicted world cereals stocks at the end of the 2015 season would be 616 million tonnes, 12 million tonnes above its previous forecast.