Barley can be divided by the number of kernel rows in the head. Three forms have been cultivated; two-row barley (traditionally known as Hordeum distichum), four-row (Hordeum tetrastichum) and six-row barley (Hordeum vulgare). In two-row barley only one spikelet is fertile; in the four-row and six-row forms, all three are fertile.
Two-row barley is the oldest form, wild barley having two rows as well. Two-row barley has a lower protein content than six-row barley and thus a lower enzyme content. High protein barley is best suited for animal feed or malt that will be used to make beers with a large adjunct content. Two-row barley is traditionally used in English ale style beers. Six-row barley is common in some American lager style beers, whereas two-row malted summer barley is preferred for traditional German beers. Four-row is unsuitable for brewing.
Barley is widely adaptable and is currently a major crop of the temperate and tropical areas.
Of interest perhaps: Am helping to set up a new site, dedicated to the growing of ancient cereal varieties and bread making: www.oxfordlocalbread.org