Alfalfa has been known since ancient times. It's rich in protein, and an excellent fodder for cattle. Furthermore, cultivating it fertilizes the ground thanks to the small tubers in its roots, which give a home to bacteria that fix nitrogen from the ground and transform it into nutrients.
The alfalfa flower offers a lot of nectar, but it does so clumsily: when the bee reaches the flower a mechanism goes off that hits the bee and traps it in a cloud of pollen. The bee abandons alfalfa as soon as 'kinder' flowers appear.