Strategies to enhance the performance of pigs and poultry on sorghum-based diets

Grain sorghum is grown for consumption by both human and animals; sorghum-based diets are offered to ruminants, pigs and poultry. Sorghum is included in animal diets primarily as an energy source, being largely derived from starch. However, the efficiency of utilisation of energy from sorghum is variable and this may be problematic for animal production. Starch granules are surrounded by kafirin protein bodies and both are embedded in the glutelin protein matrix in the sorghum endosperm. Protein–starch interactions in the sorghum endosperm may limit starch hydrolysis and its availability. The digestibility of protein/amino acids in sorghum is usually inferior to the other cereal grains. Kafirin, which is the dominant protein fraction in sorghum, is poorly digested and deficient in basic amino acids, especially lysine. Sorghum contains more phenolic compounds and phytate than the other cereal grains and both phenolics and phytate may impede digestion by directly or indirectly binding with protein and starch. As considered in this review, various feed processing technologies have been evaluated to improve sorghum utilisation in pigs and poultry. Sorghum varieties with a hard endosperm tend to be more popular in breeding programmes due to their insect resistance and high yield. The texture of sorghum grains varies with the proportions of corneous and floury endosperm. The extent of particle size reduction and its uniformity following grinding is critical to growth performance in pigs and poultry. Sorghum is especially vulnerable to hydrothermal processes which markedly reduce the in vitro pepsin digestibility of sorghum proteins. Thus steam-pelleting, steam-flaking and wet-extrusion, which involve heat and moisture, may lead to undesirable physico-chemical changes in sorghum including disulphide linkage formation in kafirin protein bodies. Dry-extrusion where heat is generated by friction may enhance starch digestibility by gelatinising starch and disrupting sorghum structures without the addition of moisture. Combining reducing agents with hydrothermal processes may enhance the solubility and digestibility of sorghum protein by either cleaving disulphide linkages or preventing their formation. The inclusion of exogenous enzymes in pig and poultry diets is an established practice to improve performance of monogastric species and phytate-degrading enzymes are of particular relevance due to the relatively high phytate contents in sorghum. Additional strategies including irradiation may also have potential to enhance nutrient utilisation in sorghum. Pigs and poultry may respond differently to any strategy due to fundamental differences in gastrointestinal structure and physiology, which is particularly true of grain particle size.