For the Guida dei Vini d'Italia over the years, we've always had discussions about context when translating this term, and we've agreed that it has to be translated in various ways according to English usage rather than choosing a single term.
Here I think I would go for vine biomass (or perhaps even vine-pruning biomass) though it would be more helpful if the client could explain just what sort of biomass it is, as I'm not 100% sure. Can you really make textiles out of grape must??

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Note added at 50 mins (2018-05-13 12:07:01 GMT)
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apparently yes, https://onlinelibrary.wiley.com/doi/full/10.1002/jsfa.3013 you can at least use the pomace for creating dyes