Feed requirements are based on the need for specific amounts of various classes of nutrients. Each nutrient fulfills specific roles in growth, production or metabolism. Nutrient classes are defined by their chemical structure or by their function in metabolism.
Energy provides the body with the ability to do work. In beef cattle rations energy is usually expressed as % Total Digestible Nutrients (TDN). Work includes growth, lactation, reproduction, movement and feed digestion. Energy is the nutrient required by cattle in the greatest amount. It usually accounts for the largest proportion of feed costs. The primary sources of energy for cattle are cellulose and hemicellulose from roughages and starches from grains. Fats and oils have a high energy content but usually make up only a small part of the diet.
Protein is one of the main building blocks of the body. It is usually measured as %4 Crude Protein (CP). It is a major component of muscles, the nervous system and connective tissue. Protein is composed of chains of amino acids. Adequate dietary protein is essential for maintenance, growth, lactation and reproduction. Protein is composed of several fractions which vary in their solubility in the rumen. Rumen soluble protein is digested by microbes in the rumen. Rumen insoluble protein passes intact through the rumen to the lower digestive tract. A portion of this bypass (or escape) protein is digested in the small intestine.
Various minerals are required for growth, bone formation, reproduction and many other body functions. Those that are required in fairly large amounts are called macrominerals. They include sodium (salt), calcium, phosphorous, magnesium and potassium. Those that are required in very small amounts (micro or trace minerals) include iodine, copper, zinc, sulphur and selenium. Mineral content is affected by the type and quality of the feedstuff. Adding supplementary minerals to the ration is usually required to ensure that the proper amounts of these elements are available to the animal. The type of supplementary mineral mix required is determined by the feeds in the ration and the animal’s requirements. Problems caused by deficiencies of some minerals are shown in Table 1.
Vitamins are biological compounds which are active in extremely small amounts. Vitamins of concern in beef cattle nutrition include Vitamin A, Vitamin D and Vitamin E. They are usually reported in International Units (IU’s). Fresh forage is a good source of Vitamins A, D and E. Vitamin content of well preserved hay is initially high, but declines over time. Silages usually contain low amounts since the fermentation process destroys most of the vitamins. Grains usually contain relatively low amounts of these vitamins.
Vitamin A is essential for normal growth, reproduction and maintenance. Insufficient Vitamin A is associated with lowered fertility in both bulls and cows. Vitamin D is required for proper development of bone. Vitamin D deficiency in calves results in bowing of the leg bones (rickets). In older animals bones become weak and easily fractured. Vitamin E, along with selenium, is required for proper development of muscle tissue. Lack of Vitamin E and/or selenium causes nutritional muscular dystrophy, commonly called white muscle disease. It is most common in young calves. Prevention of white muscle disease may be achieved by injecting calves with Vitamin E/selenium at birth, injecting pregnant cows with Vitamin E/selenium, or feeding cows supplementary Vitamin E and selenium.
The level of B vitamins in beef cattle diets is not usually of concern, although some special situations exist. The rumen microbes manufacture large amounts of these vitamins, which are then available for absorption by the animal. The B vitamins are of importance in the young calf which has not yet developed a functional rumen. Cattle which have been severely stressed have a depleted rumen microbe population and may benefit from supplemental B vitamins.
Beef cattle can utilize a wide variety of feedstuffs. Feeds are classified into groups based on their nutrient content and physical form. Most common feeds can be placed in one of the following groups:
• high in fibre (cellulose and hemicellulose) and usually low to intermediate in energy
• protein content varies widely, depending on the plant species and stage of maturity
• examples are hay, grass, grain hulls, oilseed hulls
• high in energy and relatively low in fibre
• most have a moderate protein content
• examples are corn, barley, oats
• high in protein, usually high in
• energy variable fibre content
• examples are soybeans, canola meal
4. By products
• variable nutrient content
• may contain a high level of moisture
• examples are distillers grains, sweet corn cannery waste, bakery waste, grain screenings, apple pomace
A list of the energy and protein content of some common feeds is contained in Table 2.