In recent minecraft versions bonemeal can be used in dispensers on plants, grass blocks, etc. Chicken manure doesn't work that way, dispensers just drop it. I think this is a bug or a missing feature that should be in the mod.

Its not a bug, I would have to actually add that feature into the dispenser to handle manure the same way. I will have to look at it. I haven't messed with dispenser handler in a while