The practice has been in use for decades by researchers and the agricultural industry, though it is not widely known to the general public.

Known as cannulated or fistulated cows, the animals are fitted with a porthole-like device that can be opened, allowing direct access to the largest of their four stomachs in order to optimise and regulate nutrition. 

The L214 activist group published video footage it said was secretly shot between February and May at the Sourches Experimental Farm in northwestern France.

The site belongs to Sanders, one of France's top providers of animal feed and a subsidiary of the food research group Avril.

“They have pierced a hole into the cows' stomach so they can regularly access its content. Employees come regularly to open the porthole to deposit food samples or take them out,” says a video released by the group. 

“The aim is to perfect the most effective form of feeding so the cows produce as much milk as possible,” it says, describing the animals as little 
more than “milk producing machines” that put out some 27 litres (seven gallons) per day.

L214 said it had filed a complaint with the regional prosecutor over the “illegal experiments and the serious animal abuses” at the farm. 

“For Sanders and those involved in intensive livestock production, which is the norm in France, these animals are nothing more than production machines, a basic raw material at our disposal,” the video said.

Fabrice Belargent, prosecutor in the Mans region, confirmed to AFP he had received the L214 complaint by email.

Footage of the animals was widely shared on social media, prompting a sharp rebuke from Avril, which said it “deplores the manipulation of images filmed at night for the purposes of sensationalism”. 

It said the procedure had been “used for many years in research on animals” and was currently being used “on six cows (at the farm) in the context of a research study designed to develop alternative practices”. 

The aim is to “improve the digestive health of millions of animals, reduce the use of antibiotics, and lower the nitrate and methane emissions linked to livestock farming,” it said.

As Europe's second-largest milk producer after Germany, France has some 3.6 million dairy cows housed at more than 61,700 dairy farms, with the industry accounting for nearly 300,000 jobs across France, official figures show.

In 2018, the industry produced 23.9 billion litres of milk. 

A typical farm has 52 cows and produces 330,000 litres of milk every year.

L214 takes its name from an article in the French rural code which in 1976 recognised for the first time that animals are sentient beings.