Barley (Hordeum vulgare L.) is one of the main cereal crops. With 150 million tons produced in 2009, barley grain production ranked fourth behind maize, rice and wheat (FAO, 2011). Developing countries account for about 25% of the total barley harvested area (Akar et al., 2003).
Barley is an annual, erect and tufted grass, up to 50 to 120 cm high. It has a strong fibrous root system, with seminal roots that grow as deep as 1.8-2.1 m and anchor the plant, and adventitious roots that explore the upper soil for moisture and nutrients (Ecocrop, 2011; UC SAREP, 2006). The stems are made of 5 to 7 hollow, cylindrical internodes. The leaves are linear and lanceolate, up to 25 cm long, placed opposite their neighbours along the stem (Ecocrop, 2011; Duke, 1983). In barley, the sheaths, ligule and auricles of the leaves surround the stem (Ecocrop, 2011). Spikes are variable in size (short or long) and shape (lax or compact). They may also be awned or awnletted or awnless; they are borne at the end of the culms. Depending on barley type, 1 or 3 spikelets are alternately borne at each node along the rachis of the spike. In domesticated barley, all spikelets are fertile: depending on the number of spikelets, there are barleys with 2 or 6 rows of grains (Cecarelli et al., 2006). Barley grain is an ellipsoid, grooved, 0.7-0.9 cm long caryopse that may be white, blue or black in colour and hairy at the end (Ecocrop, 2011; Duke, 1983).
There are thousands of cultivated barley landraces and hundreds of cultivars. Cultivars can be classified according to several factors: the number of rows of grains (2-row and 6-row), compactness of spikes, hull adherence (hulled or naked barley), presence or size of awns (awned, awnletted or awnless varieties), growth habit (winter or spring barley) and colour (white, blue or black kernels) (Cecarelli et al., 2006; CFIA, 2005; OECD, 2004). End-use may also be a way to classify barley (OECD, 2004). The average yield for barley grain is 2.7 t/ha but there are large differences between countries, from yields as high as 8.39 t/ha in Belgium to yields as low as 0.6 t/ha in Morocco and 0.2 t/ha in Lesotho (FAO, 2011).
Barley grain has three major uses: livestock feed, raw material for alcohol and starch production, and food (OECD, 2004).
Barley is of utmost importance for livestock feeding, which accounts for about 85% of barley production. Six-row barleys, which have higher protein content, are a valuable feed ingredient (OECD, 2004). Two-row barleys contain more starch and less protein and are thus preferred for brewing (barley with more than 11.5% protein causes beer cloudiness) (Monfort et al., 2005).
Barley grain is used for the production of alcohol (beer, whisky and ethanol), non-alcoholic beverages (barley tea, breakfast beverages) (OECD, 2004).
Food products include starch flour, flakes and pearled barley and it is a staple food in several countries including Morocco, India, China and Ethiopia (OECD, 2004).
The by-products of barley grain processing are used as feed: brewer’s grains, brewer’s yeast, malt culms (barley sprouts and rootlets), barley distillers and solubles, hulls, bran and barley feed (the by-product of pearl barley production) (OECD, 2004).
Barley forage can be fed to livestock as pasture, hay or silage.
Barley straw is also used as fodder for ruminants and as bedding material (OECD, 2004, Akar et al., 2003).
Barley is grown in more than 100 countries: the 10 main barley producers (Russian federation, France, Germany, Ukraine, Canada, Australia, Spain, Turkey, UK and USA) account for 75% of the total world production. Barley importers include countries that use it primarily for feed, as Saudi Arabia (29% of exported barley), Iran and Syria, and beer producers like Belgium and the Netherlands (FAO, 2011; OECD, 2004).
Though its origin is still debated, it is believed that wild barley is an indigenous plant in the Middle East and adjacent regions of North Africa. It was domesticated more than 7000 years ago (Cecarelli et al., 2006; OECD, 2004). Its cultivation spread from the Middle East to northern Africa, and moved South along the Nile, into Ethiopia, as early as 5000 BCE (Cecarelli et al., 2006). It spread to northern and western Europe and later to North America, Australia, and southern Africa (Ellis, 2002).
Modern barley is the most widely distributed cereal crop (Ecocrop, 2011; Duke, 1983). Barley is cultivated from 70°N (in Norway) to 17°N (around tropical Timbuktu, Mali). It can be found down to 53°S in southern Chile (Ecocrop, 2011). Barley is commonly found at higher latitudes, higher elevations and in drier places than cereals such as wheat and oats. Thanks to its short growing season (some cultivars mature in 60-70 days), it can be found at higher latitudes than wheat (Duke, 1983) and it does well in areas that are too hot or dry for wheat because it ripens before harsh conditions occur. In the tropics, barley is found at higher elevations than other cereals (above 1800 m and up to 4500 m in the Himalayas) (Ecoport, 2011).
Optimal growth conditions are an annual rainfall ranging from 190 mm to 1760 mm, average day temperatures of about 20°C and soil pH between 4.5 and 8.3 (Duke, 1983). Barley thrives on well-drained, fertile loams or light clay soils and does better on alkaline than on acid soils. Low pH may induce aluminium toxicity (Duke, 1983). Barley does not withstand waterlogging but has good salt tolerance (up to 1% salt in the soil)(Duke, 1983).
Barley is grown as a winter crop in areas with comparative mild winters, as in Mediterranean basin and India (Duke, 1983). In other regions, such as the highlands of Ethiopia, barley can be cropped twice a year. Barley can be sown with wheat in mixed cropping systems, examples being in Eritrea and northern Ethiopia (Cecarelli et al., 2006).