"多年生牧草" can be translated into English as "perennial forage grass" or "perennial forage herbage".
"Perennial" refers to plants that live for more than two years, which aligns with the meaning of "多年生" (multi-year growing).
"Forage grass" or "forage herbage" specifically denotes grass or herbage used for grazing or feeding livestock, corresponding to "牧草" (forage grass).
Both expressions are commonly used in agricultural and livestock-related contexts.