Farmers drove hundreds of tractors and a lone cow to the heart of the European Union bureaucracy on Monday, pelting police with bottles and chickens and dumping milk and manure onto the streets of Brussels in a protest against collapsing milk prices. Over 2,500 farmers from across the EU burned tires and hay outside an emergency meeting of farm ministers. They sprayed milk from huge canisters, and the cow’s udder, on a square close to the meeting. The jittery cow was frightened by firecrackers, sprang loose and chased an office worker down the street before it was recaptured by the farmers.