Animal feed can be divided into four groups - forages (e.g. grasses); cereals and other home-grown crops; compound feedingstuffs; and products and by-products of the human food and brewing industries.
Forage crops generally have a high fibre content, and are usually used on the farms where they are grown. The exceptions are cereal straw and grass dried after cutting, which are traded and may be transported many miles. Cut grass can also be turned into silage by storing it in plastic sheeting, which excludes air and promotes fermentation of the sugars under anaerobic conditions. Silage can also be made from cereal crops such as maize and wheat, which are harvested whole and then chopped into fragments. Approximately 15 million tonnes of silage dry matter were produced in Great Britain in 2004.
Cereals and other home-grown crops
These are feeds with a high energy and/or protein content. They may be fed to livestock on the farm where they are grown or bought in from outside, and may be fed either singly or as compound feeds (manufactured, often pelleted, mixtures of single feed materials, minerals and vitamins).
For ruminants, feeds of this type are necessary to supplement fresh or conserved feeds that do not provide sufficient nutrients for the animals. Non-ruminants (such as pigs and poultry) are unable to digest forages, and so their diets consist almost entirely of these feeds, together with the products and by-products described below. Around 20 million tonnes of feed materials and feedingstuffs are used annually in Great Britain, approximately two-thirds in the form of compound feeds.
Products and by-products of the food and brewing industries
By-products of the food and brewing industries (sometimes described as 'co-products') which remain after the production of food for human consumption are widely used as feeds for livestock. These include the residues of vegetable processing, such as rape and soya meal, maize gluten feed (the residue of starch extraction from maize or corn), spent grains from brewing and malting, and by-products of the baking, bread-making, and confectionery industries.
Food products which were intended for human consumption but which are rejected at the factory as sub-standard, or are broken or misshaped, or have passed their 'best before' dates, or are surplus to market requirements, can also be fed to animals.
The quality of these products is high because their ingredients were selected for use in the human food chain, but their use on farms will be determined by their costs relative to those of other feeds. However, if these products were not recycled into feed - which is not in itself a new practice - they would have to be disposed of in some other way, such as landfill or composting.