* Shutdown of Bauna Field platform main reason for decline * Firm's output has been falling for past few years * Sub-salt region produced average 281,000 bpd in February SAO PAULO, Feb 28 (Reuters) - State-led oil company Petrobras said on Thursday that its oil production in Brazil fell to 1.965 million barrels a day equivalent in January, down 3.3 percent from December's average rate. The company said in a statement the shutdown of platform SS-11 for maintenance in the Bauna Field was the principal cause of the drop in production, but planned maintenance and operational problems with other platforms also factored into the monthly numbers. Petrobras has wrestled with falling output over the past years as the quality of its ageing fields declines over time. The company has begun to bring massive new subsalt fields onstream, however, which are expected to begin reversing the decline in output from 2014. The company also said on Thursday it produced an average 281,000 barrels per day (bpd) in the sub-salt region including the off-shore Santos and Campos basins in February. Petrobras highlighted a record output of 300,000 bpd for the region, achieved on Feb. 20.