The following plants are forage grasses except

The plant that is not a forage grass among the given options is Calopogonium mucunoides. Forage grasses are plants that are primarily grown to be used as food for grazing animals, such as cows, sheep, and horses. Pennisetum purpureum, Panicum maximum, and Axonopus compressus are all examples of forage grasses that are commonly used for this purpose. On the other hand, Calopogonium mucunoides is a legume that is used as a cover crop or for soil improvement, but not typically used as a forage grass for grazing animals. Therefore, Calopogonium mucunoides is the plant that is not a forage grass among the given options.