The downgrade follows a similar move by Standard & Poor's last week, which lowered Fonterra's credit rating one notch to A-, citing much the same reasons.
Fonterra chief financial officer Lukas Paravicini said the revised ratings won't impact on the cooperative's approach to farmer payout.
Last week, the company reset the annual interest paid on its $35.1 million of perpetual shares listed on the NZX's debt market to 4.94 per cent from January 10. The notes currently pay annual interest of 4.69 per cent.
Fonterra's net finance costs jumped to $518 million in the year ended July 31 from $366 million a year earlier.
The rating agency Fitch said it anticipates famers' level of debt to the value of milk solids production will remain above its historic average, but that suppliers should be able to service their finance costs aided by industry bodies.BusinessDesk