Final answer:
The true biennial forage legume from the options provided is Sweetclover, completing its lifecycle in two seasons with a vegetative phase followed by a reproductive phase.
Step-by-step explanation:
The forage legume that is a true biennial is Sweetclover. Biennial plants, like Sweetclover, complete their lifecycle in two seasons, typically featuring a vegetative phase during the first season followed by a reproductive phase in the second season. This is unlike alfalfa and red clover, which are both perennials and complete their lifecycles in more than two years, and hairy vetch and crownvetch, which tend to be categorized as winter annuals or perennials depending on the climate and specific conditions. Sweetclover would focus on leafy growth in the first season and then produce flowers and seeds in the subsequent season if it were allowed to grow without harvesting.