Antibiotics have been used in livestock diets since the early 1950s. They and other growth stimulants are non-nutritive substances added to animal feeds to treat diseases, to improve the efficiency of feed utilization and feed acceptance, or to improve the health or metabolism of the animal in some way. The use of antibiotics can be broadly divided into two categories, therapeutic and subtherapeutic, in which the distinction purely depends on the amount added to the feed. In therapeutic use, enough antibiotics are used to control bacterial infections within an individual or animal population; in subtherapeutic use, antibiotics are given in relatively low doses to enhance the performance (typically growth and feed efficiency) of the animals. The addition of subtherapeutic antibiotics to the diets of young pigs improves growth performance by 10 to 15 percent or more. Because the subtherapeutic use of antibiotics does not completely eradicate bacteria populations, over time the practice leads to antibiotic-resistant bacteria, which forces the search for new antibiotics for both livestock and humans. Some countries in the European Union have banned the subtherapeutic use of antibiotics, and there is growing pressure in the United States to ban the subtherapeutic use of penicillin and tetracyclines, the most important antibiotics for human use. The most commonly used antibiotics in feed are chlortetracycline, oxytetracycline, bacitracin, penicillin, and tylosin.

Other growth enhancers added to the feed do not have a disease-reducing affect but rather change the animal’s metabolism. Most are related to hormones produced by the animals. These include lasalocid for cattle and sheep, melengestrol acetate and monensin for cattle, and ractopamine for swine. Several ear implants are approved for delivering hormones or other drugs to feedlot cattle in the United States. For example, these products typically increase daily gain by 10 to 15 percent and feed efficiency by 5 to 10 percent. Other countries, particularly those in the European Union, severely restrict or prohibit the use of implants in meat-producing animals because of opposition from some consumer groups.

Determination

Digestion and balance experiments measure the degree to which the various components of a feed are absorbed and retained by the animal body. Proteins are composed of varying quantities of amino acids, which are ultimately used by the animal to develop muscle and other body tissues. Microbes in the rumens of cattle and sheep can synthesize amino acids from the various nitrogen sources in their feed. In contrast, monogastrics such as pigs and poultry must have the proper amounts of the essential amino acids provided in their diet. Thus, ruminants typically have simple protein requirements, while monogastrics have amino acid requirements.

Protein and amino acid requirements are expressed as the amounts of digestible protein or amino acids needed for growth or other body functions, either as a percentage of the diet or as the total grams or units required per day. The amounts of energy needed are measured as digestible energy (DE), metabolizable energy (ME), net energy (NE), or total digestible nutrients (TDN). These values differ with species. The gross energy (GE) value of a feed is the amount of heat liberated when it is burned in a bomb calorimeter. The drawback of using this value is that a substance such as wood and corn may have a similar GE but vastly different nutritional values because some or all of it passes through the body without being digested. Furthermore, some of what is digested gets excreted in the urine as urea. Still more energy is lost from the heat of digestion and as gas produced by bacteria in the digestive tract. This loss is appreciably greater in ruminants than in pigs, chickens, and other monogastric animals. The work of eating, digesting, and metabolizing food may also be subtracted from the food energy, resulting in the NE value, or useful energy value of a feed that can be used for production (growth, reproduction, or milk) or for maintenance (basal metabolism, activity, keeping warm). The TDN value of a feed represents the sum of the digestible protein, digestible ether extract (fat) times 2.25, digestible nitrogen-free extract (carbohydrate), and digestible crude fibre. Its use as a measurement factor has declined in recent years because it considers neither the fermentation and heat losses during digestion and metabolism nor the fact that energy is utilized more efficiently for maintenance or milk production than for growth and fattening.

Optimization of nutrient-cost ratio

Feed costs vary widely from season to season; it is often possible for producers to realize substantial savings through wise selection of the feed ingredients used to formulate complete diets. It is much easier for large commercial feed companies with widespread operations to take advantage of regional variations in feed prices than it is for individual relatively small-scale livestock producers who must rely on locally produced feeds.

Least-cost formulation of feed mixtures makes it possible to use computers to select the correct amounts of competitively priced feed ingredients that will combine to fully satisfy the nutrient requirements of a specific type of animal at a particular stage of development. When used by a qualified nutritionist, computer programs can successfully formulate diets that yield maximum production at the lowest possible cost.