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.
Newquay man wins cheese race up Dorset's Gold Hill
Cornwall Air Ambulance has issued an urgent appeal as fuel costs rise
Full steam ahead for Lappa Valley’s biggest-ever gala
Motorcyclist seriously injured in A39 collision near Indian Queens
Newquay event to explore eclipses and space science
Early bird ticket release for charity beer festival
Newquay station car park to close temporarily for works
Police appeal after missing woman believed to have travelled to Newquay
Business pledges support for Paddle for Life event
Cornwall International Male Choral Festival returns with Newquay at its heart
Crantock Surf Life Saving Club plans backed by residents, the RNLI and national body
Para Surfing Roadshow set to return to Newquay
St Mawgan FC secure historic promotion as Newquay sides impress across sport
Reform UK retains Newquay council seat in by-election
New date set for English National Surfing Championships youth event after cancellation
Former Treviglas headteacher and choir figure Rob Spowart dies aged 78
New facilities officially opened at Treviglas Academy in Newquay
Seasonal parking charges to change at South Fistral car park in Newquay
Planning permission granted for Newquay community garden project
Rob Spowart - a tribute by Jim Christophers