Pictures taken on Friday show cows with "portholes" surgically inserted into their sides of bodies in a bid to allow access to their stomach contents in the research center owned by Sanders company, one of France's top providers of animal feed and a subsidiary of the French food research group Avril in northwestern France. /VCG Photo

French animal rights group L214 published footage on Thursday that was secretly shot during February and May, which captured workers putting their arms into cows' stomach through the porthole, and denounced the practice. /VCG Photo

By piercing a hole into cow's stomach, workers could gain regular access to the contents and deposit food samples or take them out, according to the video released by the animal rights group. /VCG Photo

The group said the practice, which had been used for decades by researchers, was aimed at perfecting the most effective form of feeding to increase cows' productivity, and criticized it as taking animals as "simple machines at our disposal." /VCG Photo

The group has launched a petition to end the "illegal experiments and the serious animal abuses" at the local farm. /VCG Photo