An undercover video released by a French animal rights group showed live cows whose sides have been perforated with a "porthole" into their stomach to study their digestion, reports Euronews.
The images were filmed in February and May inside a private agricultural research facility in Saint-Symphorien, northwestern France, by campaigners at L214.
The "portholes" allow access to the rumen, one of the four stomachs of the animal, in a process invented in the 19th century and used in research centres across Europe to improve productivity.
L214 said the practice was "symptomatic of the way animals are considered as simple machines at our disposal" and has launched an online campaign to end it.
"As citizens, we call on the ministers for research and agriculture to immediately ban experiments aimed at increasing the productivity of animals," it said.
The facility belongs to a division of the French food research giant Avril which said the "six fistulated cows" there were monitored "extremely rigorously" by vets.