Watch the Alt Bass Country online guitar lesson by Vicki Genfan from 30 Strumming Patterns You MUST Know
This is a must know pattern for country music. The bass line alternates between the root and the 5th. If you're playing a chord with the root on the 5th string, you'll alternate between the 5th and 4th strings. If you're playing a chord with the root on the 4th string (like the D chord and its variations) you can use your ear to decide - alternate between the 4th and 5th string or just stay on the 4th string.
For some real fun, once you've mastered this and can hit your bass notes pretty accurately try speeding this up and you'll find more of a bluegrass feel.