Nutrient Requirements of Beef Cattle

Without question, water is the most important nutrient to cattle. A water deficiency will result in death far more quickly than a deficiency of any other nutrient. Restricting water intake can decrease the animal's resiliency to environmental stressors, as well as its immune function, feed intake, growth, milk production, and fertility. Water plays important roles in the transport of essentially all substances throughout the body, the digestion and absorption of other nutrients, reproduction, structural integrity, and nearly all metabolic processes in the body.

The water requirements of beef cattle are met by three major sources: drinking water, water consumed through feedstuffs, and metabolic water, which is produced by chemical reactions in the body during the digestion and metabolism of nutrients.

Beef cattle and other ruminant production animals consume proportionally more water per unit of their body weight than do nonruminants such as pigs and chickens. This high consumption of water is necessary because proper function of the rumen depends on a considerable amount of water. Nonetheless, the water requirements of beef cattle have not been well defined.

The terms "water requirements" and "voluntary water consumption" are often considered equivalent and therefore are often used interchangeably. The reason, in part, is that water consumption is influenced by a number of different factors. Water consumption can be affected by the animal's size, stage of production, genetics, amount of activity, daily feed intake, and feed composition.

Environmental conditions such as temperature, humidity, precipitation, wind, and solar radiation also affect the amount of water required by cattle. In addition, several factors related to water quality influence the animal's willingness to voluntarily consume water; examples include salinity, concentrations of nitrates and other potentially toxic elements, and cleanliness.

As a general rule of thumb, beef cattle consume 0.01–0.04 gallons of water per pound of body weight (0.06–0.20 L/kg)—ie, ~4–80 gallons (15–300 L) per animal per day.

Cattle should be provided with continuous, free-choice access to clean water. Limiting the water intake of beef cattle has no production benefit and only negative repercussions. Restricting access to water or providing access to only low-quality water that the animal refuses can negatively impact the animal's productivity, health, and well-being.

Before being used for cattle, water sources should be routinely screened for salinity (often measured as total soluble salts), nitrates, and other potentially toxic mineral elements. Potentially toxic concentrations of mineral elements in water have been established for aluminum, arsenic, boron, cadmium, chromium, cobalt, copper, fluorine, lead, manganese, mercury, nickel, selenium, vanadium, and zinc. Note that some of these, including cobalt, copper, manganese, selenium, and zinc, are mineral elements required in the diet of cattle and play roles in important metabolic functions. Nonetheless, they can become toxic at certain concentrations, and their concentrations, along with those of other minerals, should be considered in the development of mineral supplementation programs.

Consumption of excess amounts of water can also be problematic for beef cattle. In most situations, cattle that have been provided with continuous, free-choice access to clean water will not overdrink. However, periods of water restriction followed by unlimited access can result in overconsumption, which often leads to digestive upset and diarrhea, or scours. If water has been restricted, it should be reintroduced gradually to avoid this problem.

In addition, overconsumption of water through the intake of high-moisture feedstuffs such as lush green pasture forages, high-moisture by-product feeds, or silages can result in a similar condition. In this situation, supplementing cattle with a low-moisture source of dry matter (eg, dry hay) helps to dilute the moisture content of the animal's diet, slows the passage rate, and often is sufficient to ameliorate the condition.

Dietary energy requirements, mature beef cow

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Courtesy of Dr. Jason Smith.

As its name indicates, energy is the source of fuel to the body. It is necessary to drive metabolic processes that support or maintain the animal, enabling it to grow, reproduce, and lactate. Cattle growth and productivity are energy-dependent processes. Therefore, an energy deficit affects productivity more quickly than do deficits of other nonwater nutrients.

Common signs of energy deficiency include loss of weight or body condition, decreased growth performance, reproductive failure, and impairment of the immune system. Carbohydrates and, to a lesser extent, fats provide most of the dietary energy to cattle. Protein also contributes to meeting the animal's energy demands, although to a much lesser extent.

During fermentation, bacteria in the rumen produce enzymes that break down carbohydrates into their simple sugar building blocks. The bacteria then engulf the sugar molecules and partially metabolize them. The by-products of this process are excreted by the bacteria into the ruminal environment. One of these by-products is a class of compounds known as volatile, or short-chain, fatty acids, which are the major source of energy for cattle. As a result, any process or factor that influences ruminal fermentation can affect the amount of energy liberated from the diet and therefore made available to the animal.

Fibrous carbohydrates, the major carbohydrates found in forages and many by-product feedstuffs, range from being poorly digestible to highly digestible. In contrast, grains, which typically contain a relatively high amount of starch, tend to be easily digested, and therefore usually provide more energy than do most low-starch feedstuffs. However, because starch is fermented much more rapidly and extensively than are most fibrous carbohydrates, the risk of digestive upset is much higher for grain-based rations than for roughage-based rations that contain more fibrous carbohydrates.

Although far more energy dense than carbohydrates, dietary fats typically make up a much smaller portion of the diet of beef cattle. Therefore, they contribute less to the animal's overall energy consumption than carbohydrates do. Beef cattle have relatively low dietary requirements for essential fatty acids, and typically these fatty acid needs are met by the inherent fat content of common feedstuffs (~1%–3.5% of dietary dry matter).

Nonetheless, because fat is a substantial source of energy, ingredients that are high in fat or that provide more fat than most feedstuffs provide are often supplemented or included in the diet of beef cattle. In these situations, care should be taken to avoid overfeeding fat.

For cattle fed roughage-based rations, the total dietary fat content should not exceed 5% of dry matter, because fiber digestion may begin to be compromised at greater inclusion amounts. In addition, the fat content of grain-based diets should not exceed ~6.5% of dietary dry matter, particularly if the source of fat contains a high proportion of unsaturated fatty acids, such as plant-based oils.

Different feedstuffs can differ substantially in the amount of energy they provide. The energy content of feedstuffs for beef cattle is quantified as total digestible nutrients (TDN) or as net energy. Usually expressed as a percentage of dietary dry matter, TDN represents the amount of digestible energy that the feedstuff provides to the animal, which is the amount of energy that was consumed but not lost through the animal's feces. One pound of TDN represents ~2 megacalories (Mcal) of digestible energy (1 kg of TDN = 4.4 Mcal of digestible energy).

Net energy is expressed in megacalories per unit of dry weight (Mcal/cwt, Mcal/kg, or Mcal/lb) and represents the amount of energy available to be used by the animal for maintenance (net energy for maintenance; NE_m) and gain (net energy for gain; NE_g).

The energy requirements of various production classes and sizes of beef cattle are outlined in the tables Energy Requirements of Growing and Finishing Beef Cattle and Energy Requirements for Maintenance of Mature Beef Cows.

In general, fresh, actively growing forages typically are capable of meeting and often exceeding the maintenance energy requirements of beef cattle, assuming access is not limited and forages are abundant. In this situation, excess energy is stored in the form of as glycogen in muscle and as lipids in fat, and is reflected in the animal's overall body condition.

Mature or dormant forages, crop residues, and other relatively low quality forages may not meet the maintenance energy requirements for all classes of cattle. Therefore, they are best reserved for cattle with relatively low requirements, such as middle-gestation dry cows. To meet the higher energy requirements of other classes of cattle, cattle consuming these low-quality forages often require supplementation.

For growing cattle, the rate of growth (often referred to as average daily gain) is proportional to the amount of energy consumed in excess of maintenance requirements.

Sometimes the animal's environment can substantially affect its energy requirements. For example, the energy requirements of cattle increase by 1%–2% for each effective degree below the animal's lower critical temperature after the effective temperature drops below the animal's thermoneutral zone. Having a wet hair coat may also increase nutrient requirements, by ~5%–6% under similar effective temperature conditions. In addition, depending on its depth and extent in the animal's environment, mud may increase the total energy requirements of cattle by upwards of 10%–15%.

Dietary crude protein requirements, mature beef cow

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Courtesy of Dr. Jason Smith.

Protein plays important roles in all aspects of beef cattle nutrition. Therefore, a protein deficiency may present as any impairment to normal maintenance, growth, reproductive, or immune processes. Protein is an important component of the diet because 1) it supplies ruminal bacteria with a source of nitrogen, which they use to make their own protein, and 2) it supplies cattle with a source of amino acids, which they use as the building blocks to assimilate or repair protein in the body.

The protein requirements of beef cattle are expressed in units of metabolizable protein, the amount of protein that is absorbed and therefore available to be metabolized by the animal. Contributors to the metabolizable protein supply of cattle include microbial protein and "rumen undegradable protein" (RUP), which is digested and absorbed in the small intestine. Collectively, these two protein components supply all of the essential amino acids, ie, the nutrients actually required by the animal.

Microbial protein is synthesized by ruminal microorganisms as a normal part of fermentation. Bacteria degrade the fraction of dietary protein that is subject to degradation in the rumen, commonly referred to as "rumen degradable protein" (RDP), through a series of steps. Once degraded, the bacteria use the remaining ammonia to assemble their own amino acids and proteins. Eventually, these bacteria wash out of the rumen and are digested as a source of protein (and therefore amino acids) in the intestine.

Because fermentation depends on an energy substrate, the amount of carbohydrates supplied by the diet can limit microbial protein synthesis. Similarly, because ruminal bacteria use RDP as a source of nitrogen (through ammonia) to synthesize their own protein, the amount of dietary nitrogen available in the rumen can also limit microbial protein synthesis.

The remaining protein fraction that was not degraded in the rumen, ie, the RUP is subject to digestion and absorption in the small intestine. Feedstuffs differ in the intestinal digestibility and therefore availability of the RUP fraction.

Collectively, RDP and RUP account for the animal's supply of amino acids. Dietary protein requirements, expressed in amount of crude protein (which is different from how protein concentrations are most commonly expressed in nutrient analyses and on feed tags or product labels, , are outlined for various classes and sizes of beef cattle in the following tables:

• Metabolizable Protein Requirements of Growing and Finishing Beef Cattle

• Crude Protein Requirements of Growing and Finishing Beef Cattle

• Metabolizable Protein Requirements of Mature Beef Cattle

• Crude Protein Requirements of Mature Beef Cattle

Because microbial protein in the rumen makes up a large portion of the metabolizable protein supply and depends on both nitrogen and carbohydrates as substrates, energy and protein nutrition are interdependent. A deficiency in RDP (or, more correctly, rumen available nitrogen) has the ability to influence the supply of energy to cattle because ruminal bacteria require nitrogen to complete fermentation and liberate energy from the diet. Likewise, ruminal bacteria cannot synthesize protein without a carbon backbone that is made available by carbohydrates during fermentation.

Therefore, supplementing cattle with amounts of protein that exceed the requirements of the ruminal microorganisms and the animal will not meaningfully change productivity or performance. Dietary protein concentrations should be sufficient to meet the needs of the animal but also match the amount of energy provided by the ration, because cattle growth and productivity are energy-dependent processes.

Another unique aspect of the ruminant digestive system is the ability of ruminal bacteria to use nonprotein nitrogen (NPN) such as urea as a source of nitrogen to assemble microbial protein and supply cattle with amino acids. This process, however, requires a source of readily fermentable carbohydrates. Cereal grains such as corn, wheat, and barley are ideal sources of carbohydrates to enable the use of NPN. Low-quality roughages do not provide sufficient fermentable carbohydrates to enable the conversion of NPN to microbial protein. Therefore, cattle that are fed primarily low-quality roughages are not expected to benefit from NPN supplementation without supplemental energy.

Urea and other sources of NPN can become toxic because of excess amounts of ammonia if ruminal conditions do not allow its conversion to microbial protein. Urea toxicosis is most common in cattle that consume low-quality roughages, in hungry cattle in poor condition that are offered urea-rich feed, and in young calves that lack a fully functional rumen and therefore cannot detoxify the ammonia.

In addition, cattle that consume urea or other supplemental sources of NPN should not be fed raw soybeans, because soybeans contain urease, an enzyme that converts urea to ammonia. The combination of raw soybeans and urea or another source of NPN may result in a rate of conversion from urea to ammonia that could lead to ammonia toxicosis.

Calves and other cattle that are consuming raw soybeans should never be fed supplemental sources of urea, which can be present in some protein blocks, tubs, liquid feeds, range cubes, and commercially blended feeds. The feed tag, a nutritionist, or a product representative should be consulted to determine whether a supplemental feed contains urea. For more information on NPN poisoning and urea toxicosis, see Nonprotein Nitrogen Poisoning.

Beef cattle have known dietary requirements for various minerals, including calcium, phosphorus, potassium, magnesium, sodium, sulfur, cobalt, copper, iodine, iron, manganese, selenium, and zinc (see the table Dietary Mineral Requirements and Maximum Tolerable Amounts for Most Classes of Beef Cattle). Qualitatively, beef cattle require the same mineral elements that dairy cattle require. However, the amounts required by beef and dairy cattle differ.

Minerals play crucial roles in almost all metabolic processes that affect beef cattle productivity. Failing to meet the mineral requirements of cattle has performance and economic consequences such as impaired growth and development, low reproductive performance, impaired immune function, problems with hoof health and structural conformation, and increased sensitivity to antinutritive factors and some toxins such as the ergot alkaloids implicated in tall fescue toxicosis. Deficiencies of many minerals can result in disease.

Nearly all feedstuffs are deficient in one or more of the minerals required by beef cattle. Forages alone almost never meet the complete mineral requirements of cattle and therefore almost always require supplementation. Similarly, total mixed rations should be formulated to meet the mineral requirements of cattle after considering the content and relative availability of minerals in the feedstuffs that make up the ration, which rarely meet all mineral requirements without the addition of specific mineral ingredients.

Mineral toxicoses can also be problematic, and some feedstuffs may provide cattle with excess minerals, particularly when they are supplemented in relatively high amounts or otherwise make up a major portion of the diet. Certain minerals antagonize (limit) the absorption of other minerals. Often these conditions are the result of failing to meet the requirements of one or more minerals while also exceeding the maximum tolerable amounts of others.

Some minerals (eg, selenium) may accumulate in certain plants or feedstuffs under specific environmental conditions and result in toxicoses when overconsumed by cattle. These mineral imbalances are often described as being isolated to certain regions. However, the variation in the mineral content of forages or other feedstuffs within a region is often as great as the variation across regions.

The mineral content of forages is influenced by soil mineral concentrations and environmental conditions, as well as by forage type and stage of plant maturity. Attempts to correct forage mineral deficiencies or toxicoses through soil amendments have been widely unsuccessful and would be economically infeasible in most situations. Rather, mineral supplementation remains the most economical means of addressing forage mineral deficiencies and meeting the mineral requirements of cattle grazing pasture, on range, or consuming primarily harvested roughages.

Mineral supplements should be selected primarily on the basis of their ability to complement the mineral content of the forage base. Other important factors to consider include availability, physical form, supplementation method, palatability, anticipated rate of consumption, and ability to serve as a vehicle of delivery for other valuable technologies, such as ionophores or feed-through fly control.

The consumption of mineral supplements should be monitored, and the feeding area should be moved as necessary to achieve the targeted amount of consumption. If there is any uncertainty, the feed tag, product label, or a nutritionist should be consulted. Care should also be taken to avoid mineral toxicoses when feeding more than one supplement or using an injectable source of minerals. The mineral contribution of all sources should be considered.

White salt and trace-mineralized salt are often confused as being adequate mineral supplements for grazing cattle; however, they are not adequate substitutes for a complete mineral supplement. Most trace-mineralized salt products consist of more than 90%–95% salt, with only trace (very low) amounts of very few other minerals. Trace-mineralized salt is also rarely fortified with those minerals and vitamins that are likely to be deficient in forages.

The sodium concentration of trace-mineralized salt limits consumption to the extent that cattle meet their requirement only for sodium, but not for other minerals. Cattle that are provided both trace-mineralized salt and a complete free-choice mineral supplement often underconsume the complete supplement. As a result, mineral imbalances and deficiencies can develop because the requirements for other minerals are not being met.

The reason cattle often are deficient in certain minerals is that they actively seek out salt and consume it if it's available. Salt is also an effective intake limiter because cattle typically consume only a certain amount of salt over a given period of time. Most complete free-choice mineral supplements contain a sufficient amount of salt to satisfy the animal's desire. For this reason, a complete free-choice mineral supplement is always recommended for grazing or forage-fed beef cattle, and a supplemental source of salt should be provided only in addition to a complete mineral supplement when recommended by the supplement’s feed tag or a nutritionist.

Buffet-style mineral supplementation programs, which allow cattle to voluntarily consume various mineral ingredients separately from one another, should also be avoided. This type of supplementation program increases the risk of deficiencies in some and toxicoses of other various minerals, potentially at the same time. Cattle voluntarily consume an adequate amount of only a limited few mineral ingredients because the others have characteristics that make them unpalatable. 

Macrominerals are those required by cattle in relatively large amounts, which are typically expressed as a percentage of dietary dry matter. The macrominerals required by beef cattle are calcium, phosphorus, magnesium, potassium, sodium, and sulfur.Microminerals, which are also commonly referred to as "trace minerals," include those required by cattle in relatively small amounts, which are typically expressed in parts per million (ppm) or milligrams per kilogram (mg/kg) of dietary dry matter. The microminerals for beef cattle are cobalt, copper, iodine, iron, manganese, selenium, and zinc.

Vitamins are organic compounds that differ from the other nutrients in terms of chemical composition and function in the body. Most vitamins act as coenzymes or cofactors that catalyze or are otherwise involved in metabolic reactions within the body.

Vitamins are found in very low quantities in feedstuffs in either their active (water-soluble) or inactive precursor (fat-soluble) forms, but beef cattle require them in only relatively low amounts, compared to other nutrients. Nonetheless, they play critically important roles in the maintenance of the animal, contributing to normal biological processes required for survival and conservation of mass, as well as for growth and development, health, and resilience to stress.

As with other nutrients, vitamin deficiencies can result in a specific disease or condition.

Vitamins are typically classified by their solubility in either water or fats.