Did you tried feeding your brahmin? Perhaps he is startving and thus has not enough mattery to produce...
........
REALLY?!
Brahmin in bases don't shit. They just stand around.
If you need Brahmin shit, go to any village. There's tons of it laying around. You can even go to stables and get the original, just don't fuck with the herd, staff, or slaves (well, maybe the slaves).