No, they are not.
This, from the MD 2017 State & Local Tax Forms & Instructions, Subtractions:
ab. Income from U.S. Government obligations. Enter interest
on U.S. Savings Bonds and other U.S. obligations. Capital
gains from the sale or exchange of U.S. obligations should
be included on this line. Dividends from mutual funds that
invest in U.S. Government obligations also are exempt from
state taxation. However, only that portion of the dividends
attributable to interest or capital gain from U.S. Government
obligations can be subtracted.
