Alfalfa, or lucerne, lined the roadsides in many places during my recent trip across Canada’s Prairies. In a few places, it was also cultivated as a crop for hay.
Medicago sativa is suspected to be native to south-central Asia. With a long history of cultivation spanning millennia, its precise origin is uncertain. The primary purpose for cultivation, both now and when it was first domesticated, is for livestock. It has many advantages, including: highest yields per hectare, high nutritional quality, drought-resistance, nitrogen fixation, a long life-time (usually between 4 and 20 years), and a tolerance to being cut back multiple times in a year. Partly because of these properties, it was the most cultivated forage legume in the world (at least in the early 2000s).
More on production and use of alfalfa, via Wikipedia:
Alfalfa is widely grown throughout the world as forage for cattle, and is most often harvested as hay, but can also be made into silage, grazed, or fed as greenchop. Alfalfa usually has the highest feeding value of all common hay crops. It is used less frequently as pasture…Its primary use is as feed for high-producing dairy cows, because of its high protein content and highly digestible fiber, and secondarily for beef cattle, horses, sheep, and goats. Alfalfa hay is a widely used protein and fiber source for meat rabbits. In poultry diets, dehydrated alfalfa and alfalfa leaf concentrates are used for pigmenting eggs and meat, because of their high content in carotenoids, which are efficient for colouring egg yolk and body lipids. Humans also eat alfalfa sprouts in salads and sandwiches…Fresh alfalfa can cause bloating in livestock, so care must be taken with livestock grazing on alfalfa because of this hazard.
Pollinator.ca has a excellent write-up on the pollinators of Medicago sativa; in short, alfalfa leafcutter bees and wild bees are excellent, but honeybees are only adequate (apparently they do not like getting hit in the head by the floral parts when tripping the pollination mechanism).