The world's most pampered cows are finding that a key supplement in their carefully designed diet is getting harder to come by thanks to thousands of pigs falling sick in China.

Japan's wagyu, known for its soft, fatty and marbled meat, is dependent on China for supplies of rice straw, an important part of the gourmet cattle's high-calorie, high-protein diet.

Since the outbreak of African swine fever in China, a large portion of the vital feed has been banned from entering Japan, and the Agriculture, Forestry and Fisheries Ministry fears that eventually no supplies will be available as the deadly disease spreads.