Sorghum is a staple food crop of 500 million people in the semi-arid tropics and among the cereals ranks 5th in production and 4th as a staple after rice, wheat and maize (FAO, 2011). It is crucially important to food security in Africa as it is grown in the drier and resource poor areas, where its capacity to better tolerate drought, high temperature, and low fertility make it a preferred crop to maize. In Australia sorghum is an important crop in New South Wales and Queensland; producing 1.5 to 3 million tonnes of grain per year, which is an important stockfeed for the poultry, pig and intensive beef industries.
Cereals provide over 50% of human calorific intake, and form the staple for almost two-thirds of the world’s population (FAO, 1995). In addition cereals provide the major feed grains for intensive animal production and biofuels. Hence cereals are the most important plant family for the world economy, food security and bio-renewables. With the rapidly growing world population, and the uncertainty of climate change, the necessity to increase cereal productivity without drawing upon more of the world’s resources (space, fertility, chemical and water) is paramount. Sorghum is capable of producing the some of the highest biomass per unit area of the cereals, rivalling the productivity of sugarcane, with a lower water and input requirement. It is significantly more drought and heat tolerant than the other cereals.
In traditional agriculture, tall varieties of sorghum are grown for multiple purposes including various uses of grain, fodder, extract of sweet syrup (by crushing stems in the same manner as with sugar cane) and use of the panicles for broom heads. In developed agriculture, particularly in the Americas and Australia, sorghum has been selected and bred for specific end uses – grain sorghums as short plants suited to mechanised agriculture and taller types for forage, fodder and silage production.
Throughout Africa, sorghum is consumed in a range of foods. It is manually milled. The milled grain can then be used to make porridges, gruels, various flatbreads, cooked whole grains or beverages.
Sorghum grain produced in the developed world is used primarily as animal feed; most commonly fed to poultry, pigs and feedlot cattle. It can be fed as broken grain or a meal produced with a hammer-mill or similar device, but it is more often used in complete-ration pellets (which sometimes include enzymes to aid in digestion of the sorghum).
All grain sorghum is the product of cultivated types of Sorghum bicolor (L.) Moench., a species from Africa which has spread throughout the world. There have been at least two, possibly more, domestications of sorghum. There is little doubt that the original domestication occurred in Ethiopia or Sudan. Evidence from DNA sequence analysis suggests that there has been at least one other more recent domestication in West Africa. The cultivated types have been further classified, mostly on the basis of the shape of heads and grain, into five races: bicolor, kafir, caudatum, durra and guinea. From southern Africa through to East Asia, traditional farmers have grown and selected sorghum suited to their production environments and their individual and-uses.
The genus Sorghum has at least 20 species extending from Africa through Asia and including 12 species endemic to Oceania, but only Sorghum bicolor has been domesticated as a grain crop. .Botanically sorghum belongs to the grass family and the tribe Andropogoneae. This is a tropical and subtropical group of grasses which also includes sugarcane (Saccharum spp.) and maize (Zea mays). Like all the grasses in this tribe, sorghum has a C4 photosynthetic pathway, which makes sorghum inherently more water-use-efficient than temperate cereals.
Sorghum has a small genome and was the second major crop to have a full genetic sequence published. This has opened many opportunities for further research into basic biology which in turn enables greater progress in applied agricultural research particularly plant improvement.
Although essentially a tropical and sub-tropical species, cultivated sorghum is also grown to 40o latitude as a warm season crop. It needs soil temperatures over 15oC for crop establishment. Crop growth can occur from these temperatures to just over 40oC. Flowering and seed-set require temperatures between 20oC and 39oC, the most sensitive process being pollen production with higher and lower temperatures leading to sterility.
Sorghum is adapted to a wide range of soil conditions. As with other cereal grains, in most circumstances nitrogen will be the limiting nutrient in grain production. Sorghum will form mycorrhizal associations with fungi that improve the nutrient efficiencies of elements that are present in the soil but fertilization with phosphorus, potassium and other minor nutrients will often still be necessary to remove limitations to yield.
Many traditional landrace varieties are sensitive to photoperiod, requiring days shorter than a certain maximum before they will flower and produce grain. Such varieties may fail to flower and produce grain when grown at different latitudes or even similar latitudes with different temperature regimes.
The agronomy of sorghum production in mechanized agriculture is quite different to that in traditional production systems. In developed agriculture sorghum is grown as a row crop at plant densities suited to the available moisture. Nitrogen fertilizer may be applied entirely pre-planting or split between a “starter” application and a later targeted “top-up” when the likely yield is more predictable. Herbicide application may be used to desiccate the sorghum plant and grain in preparation for harvest.
Generally maize is produced in higher rainfall areas and sorghum is cultivated in more marginal environments. From 2004-2013 Africa produced 40% of the world’s sorghum, the Americas 37%, and 20% was produced in Asia and Oceania. Grain sorghum is grown in over 100 countries but of these, only 14 produce about a million tonnes or more annually. In recent decades the Unites States has produced most, other major producers are Nigeria, India and Mexico.