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.
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.
"This process is used worldwide for research purposes only," Avril Group said in a response to the L214 investigation. "This is currently the only solution to study the digestion of plant proteins."
It added: "This analysis is essential for many advances in breeding and, in particular, to improve the digestive health of millions of animals, reduce the use of antibiotics and reduce emissions of nitrates and methane related to livestock."
It also criticised L214 for unnecessary "illegal intrusion" at the facility, which it said "regularly opens its doors to the public."