Autumn fruiting Raspberry - which mulch to use?

I have a row of Autumn Bliss raspberry they have been in about 3 years but last year the crop was poor (small berries) I think I need to top dress with something?  What should I use?  Well rotted horse manure or compost or perhaps BFB.  Advise please.

I make a slurry with neat horse dung ( no straw etc.) and mulch them with that.

Thanks  stompy and Hobnails will carry out your recommendations this w/end.

am planning on using grass cuttings, didn't know not to let it touch the stems so thanks for the tip Yorkie.