After all my bow-making adventures, I still haven't done a sinew backing. I know deer, elk, and moose is best but is regular cattle sinew all that bad because I have some?
Cow sinew will work well also but it has a lot more grease in it that has to be removed and I think it is harder to shread into smaller threads.
I've been using cow sinew, lately. Its been working very well for me. My most favorite for silkiness is Buffalo but don't have a source for that anymore. Whitetail is my next favorite behind buffalo.