hi ej :)
cutting back fronds, from memory I read somewhere cut back in autumn/winter, when they have gone brown, helps to prevent asparagus beetle if you don't leave the fronds on. leave until they go brown to allow them to feed the crown underneath(same principle as daffs etc.?).
I planted my asparagus out this year (grown from seed), so have not had this experience yet!
don't know about feeding, will await other replies with interest :)