Final answer:
The grass family (Poaceae) and the pea family (Fabaceae) contain most of the plant species used as forage, with alfalfa from the pea family being a particularly important perennial forage crop for livestock.
Step-by-step explanation:
The two plant families that contain nearly all of the plant species commonly used for forage are the grass family (Poaceae or Gramineae) and the pea family (Fabaceae or Leguminosae). The pea family is well known for its variety of edible crops which can be divided into pulse crops, like dry beans, and green vegetable crops, like green beans and soybeans. Similarly, the grass family is a major source of forage crops, such as wheat and other cereals.
Alfalfa (Medicago sativa), a member of the pea family, is a perennial herbaceous plant widely cultivated as a forage crop. Unlike the annual crops that are planted and harvested within a single growing season, alfalfa is a non-woody perennial that produces 'hay' for livestock feed. Other important perennial forage crops from the pea family include clover and soybeans.