The H5N8 strain of aviation influenza has been rampaging through the region since November, hitting 95 farms with a mortality rate of 30 percent.
It is "much more deadly" than the H5N1 virus that has spread around the world from the Far East and hit France with dire consequences in 2006 and 2015, according to Jean-Luc Guérin of France's National Veterinary School in Toulouse.
The current outbreak, spread by wild birds, has affected all of Europe but is particularly serious for south-west France because the region is the centre of foie gras production and on a migration route to Africa.
France is the world's biggest foie gras producer and exporter.
Preventive culls to be organised
An official decree published Thursday authorised preventive culls of free-range ducks and geese, who are at risk through contact with wild birds, in 150 local authority areas, mostly in the Gers department where more than half of affected farms are situated.
More than 300,000 ducks have already been slaughtered but more than a million birds are under threat in the area covered by the decree.
A small group of farmers and hunters demonstrated outside regional administrative buildings in Auch, in the Gers, on Thursday morning, complaining that duck and goose farming could be wiped out and accusing the authorities of responding too late to the latest outbreak.
Exports to Asia stopped
The cost of the cull is to be paid by the government but farmers claim they only received 70 percent of compensation for their losses after cleansing operations that followed last year's outbreak.
The foie gras producers' association predicts losses of 75-80 million euros from the slaughter and cleansing of premises.
France had hoped to be granted flu-free status, which is needed to export outside Europe.
The H5N8 outbreak means that it cannot do so, blocking access to important markets such as Saudi Arabia, China and Japan.