"The prices are going up, but not everywhere. So we now need a joint push to get other food stores on board," said the president of the DBV union, Gerd Sonnleitner.
He warned that unless all stores put up prices the nation's 100,000 dairy farmers would step up a protest that has seen them pour milk into ditches instead of delivering it to dairy plants.
"We reserve the right to take further protest action," Sonnleitner told ZDF television.
In a first victory for the dairy farmers, Lidl on Wednesday announced that it would increase the price of a litre of milk by €0.10 ($0.15) to €0.71 from Monday. The price of 250 grammes (one cup) of butter will go up by €0.20 on its shelves, the discounter said.
For just over a week, tens of thousands of dairy farmers across Germany have refused to deliver to plants that pasteurise milk and make cheese and other products that they sell to retailers. In some cases, they have blocked access to plants with tractors, prompting police to intervene.
Europe's biggest dairy, the Sachsenmilch Molkerei in eastern Leppensdorf, has been blocked since Sunday, as have other major milk hubs. On Thursday afternoon, protesting farmers were expected to drive their tractors to Berlin's historic Brandenburg Gate to highlight their battle for higher prices.
The BDM dairy farmers' federation, which called the strike, wants farmers to be paid €0.43 per litre, compared to €0.28-0.34 at present, to offset rising energy costs which they say are swallowing their profits.
The protests have angered the Organisation for Economic Cooperation and Development, which warned that images of milk being thrown away would offend developing nations battling growing food shortages.