Nutrient Requirements of Beef Cattle

ByJason Smith, PhD, Texas A&M AgriLife Extension, Department of Animal Science
Reviewed/Revised Aug 2023

Beef cattle productivity is highly dependent on nutrition and the ability of the animal's diet to meet nutrient requirements. The animal's phenotype (what can be measured or observed) is the result of an interaction between its genetics and the environment. Nutrition often makes up a large portion of the environmental component and therefore has a substantial ability to affect the animal's phenotype.

To survive and be productive, beef cattle require water, energy, protein, minerals, and vitamins. Beef cattle have known (quantified) dietary requirements for many nutrients. A deficiency in any of these can affect the growth, development, reproduction, and health of beef cattle. Although the effect may be greater for some nutrients than for others, the response is often proportional to the magnitude of the deficiency. It is important to recognize that cattle require certain absolute amounts (not percentages or concentrations) of nutrients, so even if a feed or supplement contains a relatively high amount of a nutrient, its ability to meet requirements depends on the amount consumed.

The sections that follow provide an overview of the nutrient requirements of beef cattle, as well as some considerations for meeting those requirements. The amounts of nutrients that cattle require—including water, energy, protein, and some macrominerals—are influenced by various animal-related factors and environmental conditions. The animal-related factors include weight, age, stage and extent of production, body composition, genetics, and amount of physical activity. The environmental conditions include temperature, humidity, windchill, ultraviolet radiation, precipitation, and mud.

Water 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.

Energy Requirements of Beef Cattle

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).

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Table

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; NEm) and gain (net energy for gain; NEg).

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%.

Protein Requirements of Beef Cattle

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.

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Table
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Table

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:

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.

Mineral Requirements of Beef Cattle

Table

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.

Calcium

Calcium is the most abundant mineral element in the body of any mammal. It functions as a structural component of bones and teeth, is distributed throughout extracellular fluids and soft tissues, and is involved in vital functions such as blood clotting, membrane permeability, muscle contraction, transmission of nerve impulses, cardiac regulation, secretion of certain hormones, and activation and stabilization of certain enzymes. Most roughages contain relatively high concentrations of calcium.

In addition to meeting calcium requirements, the diet of beef cattle should contain a calcium:phosphorus ratio of between 1:1 and 2:1, preferably between 1.5:1 and 2:1 to minimize the risk of urinary calculi.

Phosphorus

Phosphorus is the second most abundant mineral element in the body of beef cattle. The vast majority of it is found in the bones and teeth; the remainder is distributed across the soft tissues. Phosphorus plays critical roles in metabolic processes that affect structural development, growth, reproduction, and lactation.

Most natural protein supplements, grains, and by-product feedstuffs contain relatively high concentrations of phosphorus. Forages often contain sufficient concentrations to meet the phosphorus requirements of most classes of cattle; however, mature and dormant forages often require supplementation. Rations that contain high amounts of grain or corn milling by-products generally do not need to be supplemented with additional sources of phosphorus.

Magnesium

Magnesium is a component of numerous enzymes in beef cattle, plays roles in various critically important biochemical processes in the body, and is responsible for nerve impulse transmission. A lack of muscle control is obvious in cases of magnesium deficiency. Other clinical signs of magnesium deficiency include excessive nervousness, decreased feed intake, seizures, abnormal frothing at the mouth, and muscle twitching. Severe cases of magnesium deficiency result in a condition known as hypomagnesemia, more commonly referred to as "grass tetany," described later in this chapter.

Feedstuffs vary in their magnesium content and therefore often require magnesium supplementation.

Potassium

Potassium is the major cation in intracellular fluid in beef cattle and therefore plays an important role as an electrolyte to maintain acid-base balance and regulate osmotic gradients. It also plays important roles in the nervous and muscular systems.

Potassium deficiencies are rare in grazing cattle; when they do occur, however, they usually occur in cattle grazing dormant forages. Instead of deficiencies, high concentrations of forage potassium are often implicated in mineral imbalances such as that observed with hypomagnesemia. Potassium deficiencies may occur in cattle fed high-grain diets if not supplemented with an exogenous source of potassium, and they are expected to result in small decreases in feed intake and average daily gain.

Sodium

Sodium is the major cation in extracellular fluid in beef cattle. Therefore, like potassium, it plays an important role as an electrolyte to maintain acid-base balance and regulate osmotic gradients. It also plays important roles in the nervous and muscular systems. Clinical sodium deficiencies are rare, and requirements are easily met by giving cattle a complete free-choice mineral supplement that contains salt, or by including salt at ~0.2%–0.5% of the dietary dry matter in total mixed rations.

Sulfur

Sulfur is a major component of several amino acids and B-complex vitamins in beef cattle, and it is required by ruminal microorganisms. Signs of a sulfur deficiency include decreased feed intake and feed efficiency, loss of body weight or condition, and a general unthrifty appearance. However, sulfur deficiency is rare, and cattle rarely require sulfur supplementation. Rather, sulfur toxicosis is more common than deficiency, and high concentrations of dietary sulfur are often implicated in trace mineral deficiencies through antagonism. For more information on sulfur toxicosis, see Polioencephalomalacia in Ruminants.

Cobalt

Cobalt plays a major role in the synthesis of vitamin B12 by ruminal microorganisms in beef cattle, and thus it plays important roles in fermentation, digestion, and the availability of dietary energy. Therefore, clinical signs of cobalt deficiency include decreased feed intake and feed efficiency, weight loss, poor immune function, and general unthriftiness. Cobalt toxicosis is rare and generally unexpected in beef cattle.

Copper

Copper functions as an essential component of many enzyme systems in beef cattle. Therefore, it is involved in various processes that affect cattle productivity, such as those of the immune and reproductive systems. In addition to various signs that copper deficiency shares with other micromineral deficiencies, clinical signs of a copper deficiency include decreased growth rates, impaired immune function, a rough or dull and discolored hair coat, lack of hair shedding, and reproductive failures.

Feedstuffs commonly fed to beef cattle are often deficient in copper and therefore require supplementation. Copper is readily antagonized by molybdenum and sulfur, so copper supplementation requirements are elevated when diets contain relatively high concentrations of either molybdenum or sulfur.

Although copper is a critically important trace mineral required by beef cattle, it has a relatively small margin of safety, so overfeeding copper should be avoided. Certain breeds of cattle, such as those influenced by dairy breeds or Bos indicus, appear to be more sensitive to high dietary copper concentrations than are Bos taurus beef breeds.

Note that the amount of copper required by cattle is often lethal to sheep. Therefore, care should be taken to ensure that sheep do not consume mineral supplements or other feeds formulated to meet the copper requirements of cattle.

Iodine

Iodine plays an integral role in thyroid hormones in beef cattle; therefore, it is critically important to energy metabolism. Signs of iodine deficiency in beef cattle include lack of hair at birth, decreased fertility, and retained placentas. The iodine content of feedstuffs commonly fed to beef cattle has not been well characterized and therefore is largely unknown.

Goitrogens (antinutritive factors or compounds that bind to iodine and render it unavailable to the animal) inherently increase iodine requirements. Known goitrogens include a number of brassicas and clovers, as well as unroasted soybeans, cottonseed, and many of their by-products.

Iron

Iron plays a critically important role in blood gas transport in beef cattle. Signs of iron deficiency include anemia and pale-colored mucous membranes.

Most feedstuffs contain sufficient amounts of iron to meet the dietary requirements of beef cattle without supplementation. Therefore, iron deficiency is rare. Most cases of iron deficiency are secondary to a heavy parasite load or some other cause of excessive blood loss.

High dietary amounts of iron antagonize the absorption of some trace minerals, such as copper and manganese. Therefore, high iron intake may increase copper and manganese requirements.

Manganese

Manganese is a component or activator of various enzymes in beef cattle and therefore plays roles in some key metabolic processes. Signs of manganese deficiency include abortions, stillbirths, skeletal abnormalities, impaired growth, and infertility.

Feedstuffs commonly fed to beef cattle are often deficient in manganese and therefore typically require manganese supplementation. High iron intake can antagonize manganese absorption and therefore increase manganese requirements.

Selenium

Selenium is a component of the enzyme glutathione peroxidase in beef cattle and therefore acts as an antioxidant. Common signs of selenium deficiency include unthriftiness, weight loss, decreased immune response, and decreased reproductive performance.

White muscle disease in calves (see Nutritional Myodegeneration), which is characterized by degeneration and necrosis of the skeletal and heart muscles, results from a deficiency in selenium and/or vitamin E.

Although selenium is a critically important trace mineral required by beef cattle, high selenium intake can result in toxicosis, even under practical feeding conditions. In the US, the selenium content of commercial feeds is highly regulated. As with iodine, the selenium content of feedstuffs commonly fed to beef cattle has not been well characterized. However, selenium toxicoses and deficiencies appear more regionally specific than are toxicoses and deficiencies of other trace minerals.

Zinc

Zinc is a component of various enzymes in beef cattle that play key roles in nutrient metabolism, immune function, growth, structural development, and reproduction. Signs of zinc deficiency include decreases in feed intake, feed efficiency, and growth; impaired hoof and skin health; decreased fertility; and impaired immune function.

Feedstuffs commonly fed to beef cattle often do not contain sufficient amounts of zinc to meet the animal's dietary requirement for zinc, so it must often be supplemented. Feeding elevated amounts of zinc (more than the defined requirements of beef cattle) may aid in hoof health in certain situations. In addition, feeding elevated amounts of zinc to finishing cattle managed to achieve a high level of growth and often improves growth performance and carcass transfer.

Vitamin Requirements of Beef Cattle

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.

Water-Soluble Vitamins

Water-soluble vitamins are soluble in, and thus absorbed with, water. They include the B-complex vitamins and vitamin C (ascorbic acid). The B-complex vitamins include vitamins B1 (thiamine), B2 (riboflavin), B3 (niacin or nicotinic acid), B4 (choline), B5 (pantothenic acid), B6 (pyridoxine), B7 (biotin or vitamin H), B9 (folic acid), and B12 (cobalamin).

With the exception of vitamin B12, water-soluble vitamins are not stored in the body for extended periods of time. Under normal conditions, rumen microorganisms can synthesize B-complex vitamins as by-products of ruminal fermentation and at concentrations sufficient to meet the requirements of beef cattle. Therefore, they do not need to be supplied in the diet. Newborn and suckling calves are exceptions because they do not have a fully functional rumen; however, the dam's milk typically contains enough B-complex vitamins to meet their requirements.

Cattle also possess the ability to synthesize vitamin C in their tissues, so they have no known dietary requirement for vitamin C.

Even if provided in the diet, water-soluble vitamins would be expected to be degraded by rumen microorganisms unless fed in a form protected from rumen degradation. Therefore, water-soluble vitamins are rarely supplemented to beef cattle, and when they are supplemented under normal conditions, they are not expected to result in noticeable changes in the animal.

However, major digestive upsets, starvation, and nutrient deficiencies can affect rumen fermentation to the extent that a B-complex vitamin deficiency may develop. Similarly, treatment with antimicrobials may bring about enough of a change in the rumen microbial populations to affect ruminal fermentation and therefore inhibit microbial B-vitamin synthesis.

Fat-Soluble Vitamins

Table

Fat-soluble vitamins are soluble in, and thus absorbed with, fats. They include vitamins A, D, E, and K. In contrast to most water-soluble vitamins, some fat-soluble vitamins can be stored in the body for extended periods of time. However, only vitamin A is thought to be stored in the substantial quantities required to meet the needs of the animal for an extended period of time.

Fat-soluble vitamins exist in feedstuffs in their inactive precursor forms, which must be converted to their active forms before they can perform specific functions in the body. In addition, vitamins A and E are not synthesized by rumen microorganisms or the animal, so they must be provided to cattle in the diet.

The (current) vitamin requirements of beef cattle are outlined in the table Dietary Fat-Soluble Vitamin Requirements of Beef Cattle.

Vitamin A

The major inactive precursor to vitamin A (retinol) in feedstuffs is beta-carotene. Fresh, green forages and roughages typically contain sufficient amounts of beta-carotene to meet the vitamin A requirements of beef cattle.

The vitamin A precursor content of forages and other feedstuffs begins to decline rapidly after dormancy or harvest. Therefore, cattle that consume dormant forages or a substantial amount of harvested roughages or other previously harvested feedstuffs should be supplemented with a source of vitamin A to avoid a deficiency.

Because excess vitamin A is stored in the liver, cattle that consume a diet deficient in vitamin A may not begin to show signs of deficiency for several weeks. Newborn calves, which have small stores of vitamin A, depend on colostrum and milk to meet their dietary needs. If the dam is fed a ration low in carotene or vitamin A during gestation, such as throughout the winter, signs of deficiency may become apparent in the newborn calf within the first few weeks after birth, even if the dam appears healthy.

Vitamin A deficiency is most common in beef cattle if they are fed primarily harvested feeds, if they graze dormant forages, or during drought conditions if they are not supplemented with vitamin A to meet their requirements. The most common signs of a vitamin A deficiency include night blindness; decreased growth performance and fertility; abnormal bone development; aborted, stillborn, or blind calves; and retained placentas. Although injectable vitamin A may be a short-term solution to a clinical deficiency, the longterm correction of a vitamin A deficiency should be achieved through supplementation and nutritional management.

Vitamin D

Vitamin D is normally produced in sufficient quantities to meet requirements by the animal in response to sunlight. Ultraviolet-B radiation from sunlight converts provitamin D found in the skin of animals (7-dehydrocholesterol) or in harvested plants (ergosterol) to active vitamin D. As a result, vitamin D deficiency is rare in beef cattle. However, deficiency may develop when cattle are raised without exposure to a substantial amount of sunlight, such as in northern latitudes during the winter or completely or partially indoors (raised in barns or turned out only at night).

The most common clinical sign of a vitamin D deficiency, "rickets," is characterized by abnormally developed, weak, brittle bones. Other signs of vitamin D deficiency include swollen, immobile joints; decreased feed intake and growth performance; and stillborn, lethargic calves. Allowing cattle direct exposure to sunlight, feeding them sun-cured forages such as hay, and supplementing them with vitamin D are all methods to prevent vitamin D deficiency.

Vitamin E

The major inactive precursor to vitamin E that naturally exists in feedstuffs is alpha-tocopherol. Vitamin E serves a primary role as an antioxidant; however, dietary requirements are difficult to quantify because the functions of vitamin E are related to those of selenium. Dietary requirements for vitamin E can be influenced by the animal's selenium status and dietary selenium concentrations.

The major clinical sign of a vitamin E deficiency is white muscle disease in calves, which can be easily prevented through adequate vitamin E and selenium nutrition of the dam. Other negative impacts of vitamin E deficiency have not been well documented. Because of its role as an antioxidant, however, vitamin E may be capable of improving calf health in certain situations, particularly when supplemented at relatively high concentrations during or soon after stressful events such as weaning and transportation.

For more information on the interrelationships of vitamin E and selenium in reproduction and in the etiology of various myopathies and the predisposition of a relative thiamine deficiency, see Nutritional Myopathies in Ruminants and Pigs. (Also see Polioencephalomalacia.)

Vitamin K

As with the B-complex vitamins, synthesis by bacteria in the rumen and hindgut during fermentation provides quantities of vitamin K sufficient to meet the requirements of beef cattle. Common feedstuffs also contain relatively high quantities of vitamin K. Therefore, vitamin K typically does not need to be supplemented to beef cattle under normal conditions.

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