When the Fed buys securities on the open market, it causes the price of those securities to rise. Bond prices and interest rates are inversely related. The Federal Discount Rate is an interest rate, so lowering it is essentially lowering interest rates. If the Fed instead decides to lower reserve requirements, this will cause banks to have an increase in the amount of money they can invest. This causes the price of investments such as bonds to rise, so interest rates must fall. No matter what tool the Fed uses to expand the money supply interest rates will decline and bond prices will rise.