Fodder crops may be classified as either temporary or permanent crops. The former are cultivated and harvested like any other crop. All three types are fed to animals, either as green feed, as hay, i.e. crops harvested dry or dried after harvesting, or as silage products. Silage, or ensilage, refers to green fodder preserved without drying by fermentation that retards spoiling. Some fodder crops are components of compound feeds.
Grasses contain crude fibres, crude protein and some minerals.
Legumes are particularly rich in proteins and minerals. Root crops are high in starch and sugar and low in fibre, making them easy to digest. The fibre content of most fodder crops consists of cellulose, a complex carbohydrate polysaccharide that is indigestible for humans, but which is a good source of energy for animals, and particularly ruminants. For example, aggregations are reported in terms of metabolizable energy, digestible nutrients, starch equivalent, protein equivalent, or grain equivalent. The FAO list includes 17 primary crops.