Sheep droppings which fall on grass as they graze are quickly broken down by rain, insects, birds (including domestic poultry) then they are taken down into the soil by earthworms. At that point their nutrients are released to the soil, to help the grass grow better next year.
If the manure is building up in certain places, such as around their hay feeder, or where they sleep, then periodically clear it away, stack it for a year, and use it on the veg garden. If there's not much, then add it to the compost heap.