Japanese officials are considering a total ban on all shipments of beef cattle from Fukushima prefecture amid growing concerns that beef has already entered the food supply system from cattle that ate radioactive straw.

Agriculture Minister Michihiko Kano was quoted Tuesday saying authorities in all of Japan's 47 prefectures will be asked to check cattle feed for possible contamination.

Officials disclosed Monday that another 411 cattle from seven farms in Fukushima had been shipped to market after eating rice straw contaminated with radioactive cesium from the crippled nuclear plant in the prefecture. In some cases the contamination was more than 500 times the legal limit.

There is also evidence that cattle have been shipped after eating contaminated straw in two other prefectures. Altogether, more than 500 cattle are now believed to have been shipped after eating the contaminated feed.

The cesium is believed to have been emitted into the air in the early days after Japan's March 11 earthquake and tsunami, which knocked out cooling systems at the Fukushima Daichi plant, causing meltdowns in three reactors.

The government issued an order in March warning farmers not to use feed that had been stored outdoors in the prefecture, but some farmers are believed to have not seen the warning.

It remains unclear how much danger is posed by eating beef from cattle that consumed the straw.