Manure is basically a monoculture of whatever the cow ate - i.e. decomposing grass.

Worms and bugs need a balanced diet. So a farmer will, presumably, add crop and other agricultural waste to cultivate these helpful aerobics.

(I'm only a backyard horticulturalist. Gardening experts stress adding a balance of leaves, grass clippings and miscellaneous 'organic material' to one's vegie scraps for a healthy urban compost.)