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A percentage of agricultural gross production is used for animal nutrition purposes. These products can be used directly, i.e. grasses, silages and grains, other feedstuffs of animal or plant origin do not reach feedstuff status until several steps of processing (to provide human food) have been carried out. These products are generally called by-products, i.e. soy bean meal, rice bran, fish meal and dried distiller’s grain. Finally manufactured feedstuffs exist; these feedstuffs mainly involve feed additives (i.e. amino acids, enzymes, minerals, vitamins) that help improve the quality of diet compositions based on primary products (direct use) and by-products.
A detailed characterization and evaluation of the different feeds is achieved by initially describing the main ingredients. These main ingredients comprise energy (fats, starch and sugar), protein and fiber. The additives are synthesized for special nutritional purposes and have unique characteristics.
The main objective when formulating a diet is to combine different feedstuffs in order to meet the energy and protein requirements of the animal. Primary feed products used in pig and poultry are cereal grains. The list below shows that cereal grains are good for supplying energy with 10.0–13.4 MJ-ME/kg for poultry and 11.4–14.5 MJ-DE/kg for pigs.
But in comparison to protein rich feeds like leguminous seeds (see table) the crude protein content of grains is rather low (ca. 11 percent). It is also important to keep the fiber content of feedstuffs in mind when feeding pigs and poultry. While limited amounts of fiber can have a positive dietary effect on digestion high contents have adverse effects, because fiber is indigestible for these animals.
By-products originate from the industrial processing of primary products. The by-product feedstuffs listed below are associated to the milling industry (wheat bran, corn gluten meal), oil industry (soybean meal, rapeseed meal), meat industry (feather meal) and fish industry (fishmeal). Other industrial sectors are the brewing and distilling industry as well as sugar, milk and baking industry.
Comparing the primary product with the originated by-product shows that processing influences the ingredient composition. Wheat bran and corn gluten meal have a higher CP and CF and lower energy content because the main objective of the milling industry is to remove the starchy endosperm for flour and the germ for oil. In the same way removing the high energetic oil from soybeans and rapeseeds leaves a by-product with a higher CP and CF and lower energy content. Fish and feather meal are leftovers of the meat and fish industry. These protein sources of animal descent are valuable protein-rich by-products not acceptable for human nutrition.
The class of feed additives comprises various ingredients. Basically the additives can be divided into a group with a nutritive value and a group without a nutritive value. Amino acids, minerals (macro- and microminerals) as well as vitamins belong to the group of additives with a nutritive value because they themselves are essential ingredients in complete diets. Without them an imbalance would be on hand, which in turn could result in reduced animal performance or even in severe symptoms of deficiency. The second group influences the nutritive value of a diet indirectly. Adding organic acids, different enzymes (phytase, non starch polysaccharide (NSP)-enzymes) or probiotics improves the palatability of the diet, availability of ingredients, feed conversion and a healthy balance of the digestive tract’s microflora etc. Thereby these additives themselves are not necessary to supply the nutritive requirements of an animal.