3. NUTRIENT SOURCES - COMPOSITION OF FEEDSTUFFS AND FERTILIZERS

Although the nutrient content of individual feed ingredients varies widely from country to country or region to region (depending on local climate, alimentary regime, stage of maturity on harvesting, harvesting/post-harvesting processing method, and storage) some generalizations can be made regarding the chemical composition of individual feedstuffs. For specific national feed composition tables readers should refer to the United States - Canadian Tables of Feed Composition (NRC, 1982), the Latin America Tables of Feed Composition (McDowell et al., 1974), the Arab and Middle-East Tables of Feed Composition (Kearl et al., 1979), and the Food Composition Tables for Use in Africa (USDHEW/ FAO, 1968), East Asia (USDHEW/FAO, 1972), the Near East (FAO/USDA, 1982), Great Britain (ARC, 1976), the Philippines (Castillo and Gerpacio, 1968), Brazil (ANFAR, 1985), and the Tropics (Platt, 1962; Springhall, 1969; Gohl, 1981).

3.1 Cereal grains and by-products

The name ‘cereal’ is given to those members of the grass family (Gramineae) which are cultivated for their seed (ie. grain), and include barley, maize (indian corn), millet, oats, rice, rye, sorghum, and wheat. Cereal grains and their by-products are usually used as dietary energy sources for monogastric farm animals, including fish and shrimp.

The mature cereal grain consists of three main parts: 1) the germ or embryo; 2) the starchy endosperm (or food reserve tissue surrounding the embryo); and 3) the outer skin or pericarp (also known as the bran; Figure 3). In some cereal grains (oats, millet, barley and rice) the pericarp is surrounded by a close fitting fibrous husk or hull (such as the flowering glumes of oats and barley) which is not normally removed from the grain on threshing, and as such these cereals are known as the ‘coarse’ cereal grains.

Figure 3. The cereal grain

The protein level in cereal grains is low, ranging from 8 to 12% of the dry matter for most grains, with lysine and threonine generally being the first and second limiting essential amino acids, respectively. Cereal grains are rich sources of carbohydrate (60–80% by weight of the seed); the major part of the carbohydrate present being in the form of starch granules within the endosperm. Cereal starches usually contain about 25% amylose and 75% amylopectin. The lipid or oil content of cereal grains varies with species and maturity (1–8% by weight of the seed); the highest oil levels being found in the germ or embryo. Cereal oils are normally unsaturated, with linoleic and oleic acid being the predominant fatty acids present, and consequently are prone to rapid oxidation and rancidity on storage. The crude fibre content of cereal grains is highest in those species which contain a fibrous husk or hull (ie. ‘coarse’ cereal grains - oats, barley and rice). Although cereal grains contain little calcium, they are good sources of phosphorus; the major part of the phosphorus being present in the form of phytates. Cereal grains are deficient in vitamin D, C and provitamin A (ie. carotenes; with the exception of yellow maize) but are good sources of vitamin E and most B vitamins (Tacon, 1987). Cereal vitamins are usually concentrated in the aleurone layer or outer rim of the endosperm and within the germ of the grain. As with most plant feedstuffs, cereal grains may also contain a variety of endogenous anti-nutritional factors (see Table 2).

The average proximate and essential amino acid composition of cereal grains and their by-products is shown in Table 5 and 6 respectively.

¹ The data presented represents the mean values from various sources, including: Allen (1984); Bath et al., (1984); Bolton and Blair (1977); Branckaert, Tessema and Temple (1976); Cooley (1976); Devendra (1979); Gohl (1981); Hastings (1973); Hickling (1971); Janssen (1985); Ling (1967); McDonald, Edwards and Greenhalgh (1977); Miller (1976); NRC (1982, 1983, 1983a).

¹ The data presented represents the mean values from various sources, including: Allen (1984); Bolton and Blair (1977); FAO (1970); Gohl (1981); McDonald, Edwards and Greenhalgh (1977); and NRC (1982, 1983).

3.2 Oilseeds and by-products

Oilseeds differ from cereals in that lipid replaces carbohydrate as the major food reserve within the plant seed. Important oilseeds include soya, cotton, groundnut (peanut), sunflower, rape, linseed (flax), coconut, sesame, castor, palm kernel, safflower, mustard, niger and crambe. Although some oilseeds can be used in their whole or ‘full-fat’ form within animal feeds, the majority are used in the form of defatted oilseed cakes and meals; the extracted oil being used for human consumption, animal feeding or within industrial/pharmaceutical preparations. Oilseeds can be defatted by using either mechanical pressure to force out the oil (hydraulic or expeller process) or through dissolution by solvent extraction with hexane, alcohol or trichloroethylene. Some seeds such as groundnut, cotton and sunflower are enclosed in a close fitting fibrous hull which usually requires removal by cracking and riddling (a process also known as decortication) prior to oil extraction. Oilseeds which are defatted by mechanical pressing methods are called press cakes, and on grinding are usually termed oil-meals.

Compared with the cereal grains, the oilseeds and their oil extraction products are rich sources of protein (20 to 50% by weight) and relatively poor sources of carbohydrate. Although the biological value of oilseed proteins is generally higher than that of cereal protein, the essential amino acid pattern of oilseed proteins is usually imbalanced; lysine, methionine and threonine usually being limiting and tryptophan and arginine being in excess of dietary requirements (Tacon, 1987). The oil content of oilseed cakes and meals varies according to the oil extraction method employed, ranging from below 1% within solvent extracted oilseeds to 8% within hydraulically pressed oilseed cakes. Oilseeds fats or oils may inturn be classified according to their fatty acid composition as drying, semi-drying, or non-drying oils and fats that are normally solid at room temperature (Table 7). Oilseeds are generally poor sources of calcium, vitamin E and provitamin A (ie. carotenes), but are good sources of phosphorus (mainly in the form of phytates) and B vitamins (Tacon, 1987). As with the cereals most oilseeds also contain a variety of endogenous antinutritional factors which, unless destroyed or deactivated, can seriously reduce their feed value to fish or shrimp (Table 2; Liener, 1975; Tacon and Jackson, 1985). The average proximate, essential amino acid, and fatty acid composition of the major oilseeds and their by-products is shown in Table 8, 9 and 10 respectively.

¹ From Hilditch and Williams (1964)

¹ The data presented represents the mean values from various sources, including: Allen (1984); Bolton and Blair (1977); Branckaert, Tessema and Temple (1976); Capper, Wood and Jackson (1982); Cooley (1976); Devendra (1979); Fetuga, Babatunde and Oyenuga (1975); Gohl (1981); Gogus (1975); Godin and Spensley (1971); Hastings (1973); Hickling (1971); Janseen (1985); Ling (1967); Miller (1976); NRC (1982, 1983); Oyenuga (1975); Platt (1962); Springhall (1969); and Stosic and Kaykay (1981).

¹ The data presented represents the mean values from various sources, including: Allen (1984); Bolton and Blair (1977); Capper, Wood and Jackson (1982); Fetuga, Babatunde and Oyenuga (1975); FAO (1970); NRC (1982, 1983); Oyenuga (1975); and Stosic and Kaykay (1981).

¹ Based on the data of Godin and Spensley (1971) & Oyenuga (1975)