The animals associated with man cover the spectrum from herbivores, the plant eaters (ruminants, horses and small animals such as rabbits and guinea pigs); omnivores, which eat all types of foods (pigs and poultry); to carnivores, which eat chiefly meat (dogs and cats). Under the control of man these major classes of animal still pertain, but the range of foods that animals are now offered is far greater than they might normally consume in the wild (for example, ruminants are given plant by-products of various human food industries and some dog foods contain appreciable amounts of cereals). Nevertheless, plant and plant products form the major source of nutrients in animal nutrition.(agrilearner)
The diet of farm animals in particular consists of plants and plant products, although some foods of animal origin such as fishmeal and milk are used in limited amounts. Animals depend upon plants for their existence and consequently a study of animal nutrition must necessarily begin with the plant itself.
The major components of feedstuffs are moisture, lipids, protein, fibre, carbohydrate, minerals and vitamins.
Moisture (water) is an important diluent of the nutrients in feedstuffs. It is necessary to know the moisture content of raw materials and compound feeds as a check on their feeding requirements, for use in calculating analytical data on a dry matter basis and also because moisture has an important function in determining the form of the diet. It also has an effect on its stability and its shelf life.
Lipids and Fatty Acids
In feed stuff chemistry the words fat, lipid and oil are sometimes used synonymously. Tables of feed composition often refer to the crude fat level, by which is meant the material which can be removed from the feed by ether extraction. The term ‘oil content’ is also often used in this context. The term crude lipid content can also be used. The word lipid is a general term which covers sterols, waxes, fats, phospholipids and sphingomyelins. Many of the vitamins are fat soluble and will be extracted by ether – thus the term crude lipid content. The words oil, fat, and wax, reflect the increasing melting points of these lipid components.
Fats are the fatty acid esters of glycerol and are the primary means by which animals store energy. Fish are able to metabolize lipids readily particularly when deprived of food, as during the migration of salmon, for example. Phospholipids are components of cellular membranes. Sphingomyelins are found in brain and nerve tissue compounds. Sterols are important components of, or precursors of, sex and other hormones in fish and shrimp. Waxes form important energy storage compounds in plants and in some animal components.
Proteins and Amino Acids
Proteins are large complex organic compounds which perform an essential role in the structure and functioning of plants and animals. Animals cannot synthesise them from simple inorganic materials, unlike plants, and have to rely on ingesting them through their diet (either from plants or from other animals which already contain them) or on their synthesis by gut bacteria. Dietary protein is therefore essential for all animals. The ‘optimum’ dietary level of protein is that which produces maximum growth. However protein acts as an energy source as well as a tissue builder and excessive levels of dietary protein may form an expensive way to supply energy . The optimum dietary protein level may not be the most economic to use.
Proteins are composed mostly of amino acids linked with peptide bonds and cross linked between chains with sulphydral and hydrogen bonds. There are twenty major amino acids. The amino acid composition of proteins from different sources varies widely. Some proteins have none of certain amino acids. Some amino acids can be synthesised by animals; those that cannot be synthesised are called essential (essential in the diet) amino acids or EAA’s. For fish and crustaceans the EAA’s are arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine.
The carbohydrates, which include starches, sugars, cellulose and gums containing only the elements carbon, hydrogen and oxygen, are usually the cheapest source of energy in foods and feeds. Fish and shrimp, however, vary in their ability to digest carbohydrate effectively. Many fish appear to be able to utilize simple carbohydrates, such as sugars, more effectively than complex starches; the reverse appears to be true for shrimp and prawns but this observation may be confused by the beneficial effect that carbohydrates tend to have on the structural integrity of the feed, caused by the binding quality of starches. Carnivorous fish such as salmon and trout and, particularly, marine fish are not efficient converters of carbohydrate. Channel catfish, like shrimp, appear to be able to utilize complex carbohydrates more readily than simple sugars. Channel catfish and carp can utilize quite high levels of dietary carbohydrate; the natural diet of grass carp is very high in this component.
Fish use much less energy for protein synthesis than do warm-blooded farm animals because they do not need to maintain a constant body temperature, need less energy to maintain position and move, and because the excretion of ammonia uses less energy in protein breakdown and excretion.
However, excess or insufficient dietary energy levels result in reduced growth rates. Energy needs for maintainance and movement will be fulfilled before energy is used for growth. Thus if the energy/protein ratio is too low, protein will be used to satisfy energy requirements first; what is left will be available for growth. Fish and shrimp eat primarily to satisfy energy requirements, so a diet with excess energy content will inhibit food intake and also reduce the protein available for growth. Excess dietary fat also leads to high body fat in cultured fish, low dress-out yield and poor shelf life in market size animals.
Mineral elements are important in many aspects of fish and shrimp metabolism. They provide strength and rigidity to bones in fish and the exoskeleton of crustacea. In body fluids they are involved mainly with the maintenance of osmotic equilibrium with the aquatic environment and in the nervous and endocrine systems. They are components of enzymes, blood pigments and other organic compounds. They are essentially involved in the metabolic processes concerned with energy transport.
Most of the seven major ‘minerals’ – calcium (Ca), phosphorus (P), potassium (K), sodium (Na), chlorine (Cl), magnesium (Mg) and sulphur (S) -and fifteen trace elements – iron (Fe), zinc (Zn), copper (Cu), manganese (Mn), nickel (Ni), cobalt (Co), molybdenum (Mo), selenium (Se). chromium (Cr), iodine (I), fluorine (F), tin (Sn), silicon (Si), vanadium (Va), and arsenic (As) reported essential for terrestrial animal life are also believed to be required by fish. However, only seven, (Ca, P, Mg, Fe, Zn, I, and Se) have been shown to be required or utilized by salmonids. It can be assumed that the following elements at least are also essential for body functions: Na, Mo, Cl, Mn, Co, and probably Cr and F.
Fish and crustacea can absorb minerals by other routes than from the digestion of food – through the ingestion of seawater and through exchange from their aquatic environment across body tissues such as skin and the gill membranes. Minerals are therefore probably not so important a component of the diet of fish and shrimp as they are in that of other animals.
Calcium is absorbed by fish from seawater but freshwater is low in calcium. However, since most feedingstuffs, particularly animal proteins, have high levels of calcium, calcium deficiency in fish through dietary insufficiency is most unlikely. On the other hand both seawater and freshwater contain very little phosphorus so this element is important from a dietary point of view.