Crews from Newquay, Perranporth, St Dennis, Wadebridge, Truro, Launceston, St Austell and Tolvaddon are on scene.
A spokesperson for Cornwall Fire and Rescue Service say they are using hosereel jets to put out the fire - but smoke is entering the A30 - close to the Carland Cross interchange.
Adding: "Please proceed with caution through this area. Devon and Cornwall Police and National Highways have been informed. A 50m cordon is in place and a road in the vicinity of the farm has been closed at this time."
Nearby residents are being asked to keep windows and doors closed.
RAF St Mawgan has reassured the public that increased activity levels at the base is part of a planned exercise
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