A proper understanding of the ruminant digestive process and beef cattle basic nutrition is required for effective feed management. Energy is the first limiting nutrient, usually the largest portion of total mixed rations.
A cow’s energy requirements are met mostly by carbohydrates and, to a lesser extent, fats. A lack of energy will negatively affect performance more than a deficiency in any other nutrient.
Cattle require a variety of nutrients to thrive. In addition to the macrominerals, calcium, phosphorus, magnesium, potassium, and sodium, beef cattle have known dietary requirements for trace minerals such as copper, cobalt, iron, iodine, manganese, and selenium (see the fact sheet Ration Balancing).
Protein is an essential nutrient for all classes of beef cattle. It is required for the production of enzymes, milk proteins, and immunoglobulins, as well as muscle tissue and other organs and tissues in the body. Protein also provides a source of energy for the animal.
The metabolizable protein requirement of beef cattle depends on both the contribution of rumen undegradable protein and the digested protein from the feed. The metabolizable protein supply from the diet is also dependent on energy supplies, as ruminal bacteria need nitrogen and carbohydrates to complete their fermentation and liberate metabolizable protein.
A cow’s protein and energy demands increase significantly during pregnancy, calving, and lactation. The ration balancer should carefully manage the energy and protein content of the cow’s diet in order to meet this increased demand without reducing herd health or productivity.
Most natural protein supplements, grains, and byproduct feedstuffs contain sufficient concentrations of phosphorus to meet the needs of most classes of beef cattle. However, the phosphorus content of forages and other roughage materials declines rapidly after harvesting or dormancy. Therefore, if cattle are expected to consume dormant forages or high-concentrate feedstuffs, supplementation with a source of phosphorus may be necessary. Vitamin A is an essential nutrient for all classes of beef cattle, but the beta-carotene precursor of vitamin A in forages and other feedstuffs declines after dormancy or harvesting. Therefore, cattle consuming dormant forages or high-concentrate, high-grain, or corn milling byproducts may require supplementation with a source of vitamin A.
Water is essential for a variety of biological functions, including regulating body temperature, reproduction, and lactation. It also regulates metabolism and energy production, maintains the mineral balance of body fluids, buffers pH, and removes waste. In addition to its chemical role, water helps lubricate joints, cushion the nervous system, and transport sound while providing the medium to transport nutrients within the body.
Although cattle can get some of their water from high-moisture feed sources, such as silage and fresh pasture, or the oxidation of certain nutrients in the diet (such as calcium carbonate, which is used to make up tetany salt), it’s critical for the animal’s health that adequate amounts of clean drinking water be available at all times. A number of factors influence cattle’s water consumption and requirements, including ambient temperature, stage of production, and body weight.
Protein is another nutrient that’s important for beef cattle, but only when it’s in a form that can be absorbed and used by the animal. The metabolizable protein supply in a cow’s diet depends on both nitrogen from the rumen and carbohydrates for fermentation, so if there aren’t enough carbohydrate-supplying ingredients in the feed, it can impact the amount of metabolizable protein the animal receives.
Fats are another significant source of dietary energy for cattle. The dietary requirement for fats is less than that for carbohydrates, but many feed ingredients contain substantial levels of fat and thus can contribute significantly to the energy needs of an animal. Like water, fats can be a source of energy in the body, but it’s more expensive than carbohydrates to produce, so the body has to use other resources to meet its energy needs if the availability of fat is limited.
Cattle are classified as ruminants (with the exception of sheep) because they have a large fluid-filled digestive tract called a rumen which allows them to utilize roughage as a major source of nutrients. Ruminant animals eat grass, hay, and other forage, which are digested by microbes in the rumen to provide energy, protein, and fiber.
A major nutrient requirement for beef cattle is metabolizable protein, the source of amino acids used for growth and reproduction. Metabolizable protein is supplied by the microbial digestion of cellulose and hemicellulose in the rumen and starch in the true stomach and small intestine. This digestion produces volatile fatty acid byproducts that are absorbed across the gut wall to provide energy.
Vitamins are required in small amounts by beef cattle but play critical roles in a number of biological processes, including cell growth and repair, enzyme function, hormone production, skeletal development, and nervous impulse transmission. A deficiency in one of these vitamins may lead to disease or reduce animal performance.
Minerals are also required in relatively low concentrations by beef cattle. Calcium, for example, is a structural component of bones and teeth and is distributed throughout extracellular fluids and soft tissues. It functions in many important metabolic processes, including blood clotting, membrane permeability, muscle contraction, nerve impulse transmission, and cardiac regulation. Calcium requirements are met mainly by consuming roughages and feedstuffs with high concentrations of this mineral.
Trace minerals are essential in small amounts, including copper, iron, manganese, and zinc. They are required for a variety of biological functions, including iron transport and metabolism, enzyme activity, growth and reproduction, and resistance to stress. These minerals are commonly found in the form of inorganic salts or organic compounds such as cobalt, copper, iodine, phosphorus, potassium, sodium, sulfur, and zinc.
The immune system needs a good supply of protein to make antibodies that will fight bacteria and viruses. Calves receive their first antibodies through colostrum, which is produced in the mammary glands shortly after birth.
Forage provides the major source of dietary protein in beef cows. The quality of the forage will determine whether it can provide enough metabolizable protein to support the cow’s needs, and supplements may be needed for some classes of cattle. In addition to the rumen’s breakdown of dietary protein, microorganisms contribute a portion of the animal’s metabolizable proteins. The resulting metabolizable proteins contain the essential amino acids required by the animal. The nutritional protein requirement of cattle is usually expressed as a percentage of crude protein, calculated by multiplying the nitrogen concentration of the feed by 6.25.
Protein requirements decline during late gestation and through part of the lactation period. A supplemental level of protein may be necessary for cows in this phase, especially those with low body condition scores.
Cattle need a variety of minerals to maintain their health. Minerals are typically considered to be macrominerals when they are required in large amounts and expressed as a percentage of dietary dry matter, and microminerals when they are required in small amounts and expressed in parts per million or milligrams per kilogram of dietary dry matter. The seven macrominerals that are required for the health of beef cattle include calcium, magnesium (Mg), phosphorus, potassium, sodium (Na), chlorine (Cl), and sulfur.
Cattle also need water-soluble vitamins, particularly the B-complex vitamins. Rumen microorganisms degrade these unless they are fed in a form that is protected from rumen degradation. The lack of a source of water-soluble vitamins in the diet may result in nutritional deficiencies that include a rough coat, poor eyesight, and diarrhea. Vitamin A, which is stored in fat reserves and liver cells, is also important to the immune system.
Getting cattle to reach their production goals requires meeting energy and other nutrient requirements. Nutrient requirements vary for each stage of the cow’s production cycle, age, sex, breed, weight, and activity level. Energy and nutrient needs may be met from forage or by adding supplements.
Protein, fats, and minerals are important for optimum beef cattle health. Minerals are required in relatively small amounts, but a deficiency can significantly decrease growth and immune function. The concentration of minerals in forage varies based on soil and plant characteristics. Regular feed testing for minerals is good practice. Mineral interactions with water and other feedstuffs can also limit availability or absorption.
In addition to proteins, fats, and minerals, beef cattle require vitamins. Vitamins are required in very small quantities compared to other nutrients, but they have important roles in maintenance functions. B-complex vitamins are typically synthesized by microorganisms in the rumen at levels sufficient to meet the animal’s requirements. In addition, calf colostrum and milk usually contain adequate amounts of these vitamins. Newborn calves, however, may not have enough stores of B-complex vitamins to sustain them through a transition period when the dam’s dietary protein is low.
Energy is the nutrient that is required in the largest quantity. This is because it is used to perform work such as movement, growth, lactation, and reproduction. The nutrient sources that provide energy are cellulose and hemicellulose from roughages, starches from grains, and fats. During the cow’s production cycle’s late gestation and lactation periods, energy demands increase. As a result, the first limiting nutrient shifts from protein to energy. Managing the cow herd’s energy and other nutrient requirements is essential to achieving production and profitability goals.