Forage and Grain Crops
Agronomy 105's Weed ID
Alfalfa
Alfalfa is a perennial legume that has been called the "queen of the forages."  It is common to find roots of alfalfa extending 15 feet or more into the soil.  Alfalfa is a very heavy water user and thus is seldom grown in drier areas of the United States without irrigation due to its habit of depleting water from the soil profile.
The flowers of alfalfa are usually a light purple or violet color, however there are also white, yellow and blue flowered varieties.  Alfalfa should be cut for hay when 10% of the plants are in bloom.  In the seedling stage alfalfa looks very much like sweet clover and the leaf margin serrations are the best way to tell the two species apart. 
Sweet clover leaves have serrations around the entire leaf whereas alfalfa leaves are only serrated near the leaf tips. 
The kidney shaped seeds of alfalfa are yellowish-brown to greenish-brown in color and often have one blunt or rounded end.