Several factories have suspended production since Friday due to a lack of raw materials, Benoit Pietrement, head of the grain committee at farm office FranceAgriMer, told reporters.
The northwestern region of Brittany, a major area for livestock farming and feed suppliers, was particularly affected, he said.
A month-old public transport strike that has crippled rail services and rolling stoppages by dock workers have left companies in the European Union's biggest grain producer struggling to get grain to ports and to factories.
The cause of the production outages at some feed factories was the blocking of ports by protesters that have prevented lorries from transporting imported soybean and sunflower meal inland, Francois Cholat, president of SNIA, an association of French feed manufacturers.
"There are supply outages for soybean meal," Cholat said by telephone. "It's mainly due to lorries that are blocked from entering ports."
Unions have called a three-day stoppage by port workers this week from Tuesday, after a similar three-day walkout last week.
Soybean meal is widely used to provide protein in livestock feed rations, alongside cereals that offer energy.
SNIA had received reports of eight factories experiencing at least partial production stoppages, Cholat said, adding it was too early to estimate the impact on volumes and sales.
The outages risked disrupting deliveries to farmers, with the need to keep livestock fed raising the possibility of feed being brought in from neighbours such as Belgium, he added.
Rail strikes, meanwhile, were threatening to disrupt grain exports and dampen strong overseas demand for French wheat, FranceAgriMer said.
(Reporting by Gus Trompiz, writing by Sybille de La Hamaide, editing by Bate Felix and David Evans)
