Some 150 dairy cows arrived overnight from Friday to Saturday for the first milking at the farm named the "Mille Vaches".
The unprecedented operation has a capacity of more than 1,000 cows, even though it is only currently authorised to milk 500.
Critics say the farm is illegal and threatens the quality of agriculture with industrial-scale factory farming.
About 40 farmers picketed the site on Saturday.
One of them, Pierre-Alain Prévost of the Confédération Paysanne union told RFI the farm raises a number of social and environmental concerns.
Campaigners say its smell and the manure it produces will cause nuisance to the surrounding area and that the cows will be living in exceptionally crowded conditions.
They also claim the opening has been rushed to dodge a legal case filed on 5 September at a court in nearby Amiens.