manure has been used for centuries as a fertilizer for farming, as it is rich in nitrogen and other nutrients which facilitate the growth of plants. liquid manure from pig/hog operations is usually knifed (injected) directly into soil to reduce the unpleasant odors. manure from hogs and cattle is spread on fields using a manure spreader. due to the relatively lower level of proteins in grasses, which herbivores eat, cattle manure has a milder smell than the dung of carnivores–for example, elephant dung is practically odorless. however, due to the quantity of manure applied to fields, odor can be a problem in some agricultural regions. poultry droppings are harmful to plants when fresh but after a period of composting are valuable fertilizers.