"Pig in the Pen" is a breakdown tune, usually played at MM=108-135. Here is the skeletal 8-measure melody in the key of G.
Solos usually run 16 measures, so the the chord progression is repeated in the second eight measures.
This example is single note oriented, accenting the blues aspect of bluegrass in measures 9-16. It is meant to demonstrate the placement of typical riffs, and so is busier than I usually play. The first four measures contain a bluegrass approach to the melody, while measures 5-6 are made up of a typical G-scale run.
A couple of subtleties may be worth examining. In measure 2, the choice of an F# instead of the more expected G note creates a descending D6 arpeggio over a G chord. Regardless of the theory, it just sounds good to me. I originally ended measure 5 with one E quarter note, but I found that I kept starting measure 6 with an up bow, which was uncomfortable in context. So I chopped the quarter into two eighths and got the bow directions to my satisfaction.
There is nothing intrinsically wrong with beginning a measure with an up bow- it happens often - but some phrases seem to be easier when begun with a down bow. When I am improvising I can often sense when my bowing is getting out of whack, so I add or drop a slur or note to get back on track. You do not have time to think about this as you are playing. Bowing becomes more of a feel or attitude with experience, and you will eventually sense when your slur patterns are not appropriate. You must do a lot of careful listening to other players and to yourself to develop this sense.
I like to play the first two G notes of measure 9 in #191 a bit on the staccato side. Draw out the slide that follows. Notice the mix of F# and F natural notes in measures 11 and 15. Measures 13-14 are related to a classic lick I first heard played by Chubby Wise on a mid-1940's Bill Monroe recording, Can't You Hear Me Calling. It is one of those bluesy, repeating phrases that works over I and IV chords. Stress the slides. An extended fill-lick closes out this mini-production. It is not unusual for a couple of stray measures to be added to a solo while waiting for the vocalist to begin.