A Six-Row Malt is derived from the kernels of six-row, instead of two-row, barley varieties. The term “six-row” refers to the morphology of the barley spike or head. Spikelets are arranged in an alternate pattern at each node along the rachis (stem) of the spike. In six-row varieties, the two lateral spikelets along with the central spikelet are fertile and produce a total of three kernels. The kernel count at each node on the rachis, therefore, is six. This arrangement of kernels gives the head a round appearance compared with the flat appearance of the two-row spike. It is only a single gene that distinguishes two-row from six-row barley varieties, but the two types each have their own separate breeding programs and thus constitute entirely different market classes of grain. In general, six-row barley is less plump and has a thicker husk and, after malting, will have lower extract yields, a higher protein content, and greater enzyme activity compared with two-row varieties. Because these characteristics are genetically determined, breeders can manipulate them. In theory, therefore, the differences between two-row and six-row barley traits can even be diminished. For example, breeders are now working on increasing the malt extract potential of six-row barley. The variety Morex is an example of this breeding direction. See morex (barley). The only characteristic that cannot be merged in any future barley variety is the kernel shape, because it is strongly influenced by the arrangement of kernels on the spike. In a six-row spike the lateral kernels will tend to be thinner, whereas in a two-row spike all the kernels will generally be more uniformly plump. Related to plumpness is husk content, which will generally be greater in thinner kernels than in plump ones. The result of this difference is a proportionally larger amount of flavor influencing husk-derived phenolic compounds in the finished beer. See phenolic.

Six-row malt is used primarily in North America breweries, whereas two-row malt is used most everywhere else in the world. The key regions for six-row barley cultivation are Mexico, the midwestern United States, and, to a lesser extent, the Prairie Provinces of Canada. In the United States, several large brewers use a blend of two- and six-row malts in their mashes. A number of factors may have contributed to the preference of six-row malt in North America. Historically, six-row barley varieties became more prominent because they are better adapted to growing conditions in the Midwest, where two-row varieties tend to be more susceptible to leaf diseases. Recently bred two-row varieties, however, have exhibited much greater resistance to leaf diseases as well. Many brewers in the United States, especially the large ones, also use enzyme-free adjuncts, such as rice or corn, in their mashes and, therefore, need the enzyme strength of six-row barley to obtain sufficient diastatic power for proper saccharification. See adjuncts and diastatic power. In craft breweries, where all-malt mashes are more frequently employed, two-row malts are often used as base malts, whereas caramel malts and roasted malts are frequently six-row varieties. The future of six-row barley supplies, however, is not certain because many farmers in the American Midwest are shifting toward other crops, often induced by the growing demand for biofuels.

See also two-row malt.