During lactation, dairy cows have very high nutritional requirements relative to most other species (see Table: Feeding Guidelines for Large-Breed Dairy Cattle a). Meeting these requirements, especially for energy and protein, is challenging. Diets must have sufficient nutrient concentrations to support production and metabolic health, while also supporting rumen health and the efficiency of fermentative digestion.
Feeding Guidelines for Large-Breed Dairy Cattle a
Under nearly all practical management conditions, dairy cows and growing dairy heifers are fed ad lib. Thus, voluntary feed intake is the major limitation to nutrient supply in dairy cattle. Feed intake is usually characterized as dry matter intake (DMI) to compare diets of variable moisture concentrations. DMI is affected by both animal and feed factors. Body size, milk production, and stage of lactation or gestation are the major animal factors. At peak DMI, daily DMI of high-producing cows may be 5% of body wt, and even higher in extremely high-producing cows. More typical peak DMI values are in the range of 3.5%–4% of body wt. In mature cows, DMI as a percentage of body weight is lowest during the nonlactating, or dry, period. In most cows, DMI declines to its lowest rate in the last 2–3 wk of gestation. Typical DMI during this period is <2% of body wt/day, with intake rates depressed more in fat cows than in thin ones. Feed intake during this period has an important relationship to postpartum health, with low DMI and associated prepartum negative energy balance increasing the risk of postpartum disease. After calving, DMI increases as milk production increases; however, the rate of increase in feed consumption is such that energy intake lags behind energy requirements for the first several weeks of lactation. Milk production and associated energy requirements generally peak around 6–10 wk into lactation, whereas DMI usually does not peak until 12–14 wk into lactation. This lag in DMI relative to energy requirements creates a period of negative energy balance in early lactation. Cows are at greater risk of metabolic disease during this period than at other times during their lactation cycle. Management and nutritional strategies should be designed to maximize DMI through the period of late gestation and early lactation.
Feed factors also affect DMI. Total ration moisture concentrations >50% generally decrease DMI, although this may be related more to fermentation characteristics than to moisture per se, because high-moisture feeds for dairy cattle are typically from fermented (ensiled) sources. Rations high (>30%) in neutral detergent fiber (NDF) may also limit feed intake, although the degree to which this occurs is related to the source of NDF. Environment also affects feed intake with temperatures above the thermal neutral zone (>20°C [68°F]), resulting in reduced DMI. Monitoring DMI, when possible, is a useful tool in diagnosing nutritional problems in diets of dairy cows.
Energy requirements for lactating dairy cows are met primarily by carbohydrate fractions of the diet. These consist of fibrous and nonfibrous carbohydrates. Fibrous carbohydrate proportions are generally measured as NDF and expressed as a percentage of dry matter. Nonfiber carbohydrate (NFC) proportions are calculated by subtracting the proportions (as dry matter) of NDF, crude protein, fat, and ash from 100%. Nonfiber carbohydrates primarily consist of sugars and fructans, starch, organic acids, and pectin. In fermented feeds, fermentation acids also contribute to the NFC fraction. The sum of sugars and starch is referred to as nonstructural carbohydrate (NSC), which should not be confused with NFC. Balancing fiber and NFC fractions to optimize energy intake and rumen health is a challenging aspect of dairy nutrition.
In general, fiber in the diet supports rumen health. Fiber in the rumen, especially fiber from forage sources that have not been finely chopped or ground, maintains rumen distention, which stimulates motility, cud chewing, and salivary flow. These actions affect the rumen environment favorably by stimulating the endogenous production of salivary buffers and a high rate of fluid movement through the rumen. Salivary buffers maintain rumen pH in a desirable range, while high fluid flow rates increase the efficiency of microbial energy and protein yield. Fiber, however, delivers less dietary energy than NFC. Fiber is generally less fermentable in the rumen than NFC, and rumen fermentation is the major mechanism by which energy is provided, both for the animal and the rumen microbes. Therefore, diets with high NDF concentrations promote rumen health but provide relatively less energy than diets high in NFC.
To increase the energy supply, dietary NDF concentrations are usually reduced by adding starch and other sources of NFC. This increases the rate and extent of rumen fermentation, which leads to greater energy availability. Increased ruminal fermentation also leads to the increased production of volatile fatty acids, which tends to lower rumen pH. At rumen pH values <6.2, fiber digestion is reduced; at values ≤5.5, fiber digestion is severely diminished, feed intake may be reduced, and rumen health is generally compromised. There is a reciprocal relationship between NFC and NDF proportions, so the adverse effects of high dietary NFC may be especially evident as cud chewing and salivary flow may be simultaneously diminished because of reductions in dietary NDF.
Recommended minimum NDF concentrations depend on the source and physical effectiveness of the NDF and the dietary concentration of NFC. Fiber from forage sources is, in general, more effective at stimulating salivation and cud chewing than is fiber from nonforage sources. Thus, one variable in the assessment of dietary NDF adequacy is the proportion of NDF coming from forages. Minimum NDF concentrations in the diets for high-producing cows are 25%–30%. When fiber sources from forage make up ≥75% of the NDF, then total NDF concentrations in the lower end of this range may be acceptable (see Table: Recommended Minimum NDF Concentrations Based on Proportion of NDF Coming from Forage Sources a). When a smaller portion of total NDF is derived from forage sources, then total NDF concentrations should be in the upper end of this range. Maximum recommended NFC concentrations are 38%–44%. Diets with higher NFC concentrations will benefit from higher proportions of NDF coming from forage sources. These recommendations must be viewed as broad guidelines rather than strict rules. Factors including the total fermentability of the diet as well as the fermentability of the NDF influence the NDF requirement. Diets with highly fermentable NDF sources require higher total concentrations of NDF but provide more energy per mass unit of NDF than diets with less fermentable NDF. Feeding management schemes such as totally mixed rations result in lower minimum NDF concentrations than feeding dietary components individually (see Nutritional Requirements of Dairy Cattle).
Recommended Minimum NDF Concentrations Based on Proportion of NDF Coming from Forage Sources a
Dietary energy is usually measured in megacalories (Mcal) or megajoules (MJ). When the energy in a given feedstuff is expressed in terms of the Mcal or MJ actually available for metabolism, heat production, or storage in the animal, the term metabolizable energy (ME) is used. The efficiency of utilization of ME varies based on the physiologic functions supported, which include body maintenance, growth, and lactation. The net energy (NE) system takes into account the differences in efficiency of ME utilization for each of these processes and assigns a separate NE value to individual feedstuffs based on each of these energy-requiring processes, ie, body maintenance, growth, and lactation. Thus, in the USA, in which the NE system is typically used, energy values of feedstuffs for ruminants are expressed as NE for maintenance (NEM), NE for gain (NEG), and NE for lactation (NEL). This system is cumbersome and nonintuitive and has many computational disadvantages compared with alternative systems based directly on ME. However, the NE system has the major advantage of more equitably comparing the energy values of forages to concentrates when used in ruminant diets. Dry Matter, Energy, Crude Protein, Fiber, and Non-Fiber Carbohydrate Concentrations of Some Feedstuffs Commonly Fed to Dairy Cattle a has typical values for ME, NEL, NEM, and NEG, for some feedstuffs commonly fed to dairy cows. The values in these and other published tables are estimates of the energy delivered to lactating cows consuming feed at three times the maintenance consumption rate, ie, three times more feed than they would consume were they not in production. The listed values are typical averages for the feeds; the actual values for individual feeds may vary considerably, especially for forages. Laboratory analyses of feeds and forages are always advisable for both comparative evaluation and ration balancing. Values for ME and NE cannot be measured directly by typical laboratory analyses. These and any other energy values on a laboratory report are estimates, usually based on formulas with acid detergent fiber concentration as the primary independent variable. Many contemporary computer programs for ration evaluation or balancing in dairy cows do not rely on laboratory estimates of feed energy concentrations. Rather, they estimate the contributions of individual feeds to the energy supply based on feed characteristics, intake rates, and estimated rates of passage through the rumen. Such programs are frequently referred to as "models." When using programs of this type, the estimated energy values of individual feeds will diminish with increasing rates of feed intake.
Dry Matter, Energy, Crude Protein, Fiber, and Non-Fiber Carbohydrate Concentrations of Some Feedstuffs Commonly Fed to Dairy Cattle a
In the USA, energy requirements of adult dairy cows are typically expressed in terms of NEl. This applies to pregnant dry cows as well as lactating animals. Maintenance requirements for mature cows of various mature body weights are given in Maintenance Energy Requirements for Cows of Various Body Weights. Energy requirements per kg of milk produced at various milk fat concentrations are given in Dietary Net Energy Requirement for Milk Production.
The required dietary energy concentration is a function of the energy requirement and the feed intake rate. Calculated requirements for dietary energy concentration typically are very high in early lactation because the rate of milk production is high relative to the feed intake rate. However, the ration energy density concentrations required to meet the energy requirement of cows in very early lactation may be too high to be compatible with adequate dietary fiber concentrations (see Carbohydrates). In general, diets with energy concentrations >1.71–1.76 Mcal/kg do not contain adequate fiber to support good rumen health and function. Thus, dairy cows in early lactation typically cannot meet their energy requirements and are expected to lose weight. During the first 3 wk of lactation, dairy cows commonly have rates of negative energy balance in the range of −5 to −10 Mcal/day. The risk of metabolic disease increases with the degree of negative energy balance, although there is great variability among individual cows in the capacity to adapt to negative energy balance without incurring metabolic disease. Feed intake, rather than milk production, is generally the most limiting factor influencing energy balance in early lactation dairy cows. Thus, nutritional management strategies that result in rapid increases in feed intake rates after calving are the most beneficial in terms of both cow health and productivity.
Supplemental fats can be added to increase energy concentration. Fat concentrations in typical dairy diets without supplemental fat are usually low, ~2.5% of dry matter. Supplemental fats may be added to attain a total ration fat concentration of ~6% of dry matter. Fats in ruminant diets can induce undesirable metabolic effects, both within the rumen microbial population and within the animal. Ramifications of these effects include reduced fiber digestion, indigestion and poor rumen health, and suppression of milk fat concentration. The major benefit of supplemental fat in ruminant diets is that dietary energy concentration can be increased without increasing the NFC concentration.
Fats may be supplemented from vegetable sources such as oil seeds, animal sources such as tallow, and specialty fat sources that are manufactured to be rumen inert, ie, not interact with the metabolism of rumen microbes. Supplemental fats from vegetable sources generally have a relatively high proportion of unsaturated fatty acids. Unsaturated fats adversely affect rumen microbial activity. In addition, these fatty acids are extensively converted to saturated fatty acids in the rumen. When fed in excessive dietary concentration, intermediate products from the saturation process may escape the rumen and be absorbed by intestinal digestion. Some of these products are trans-fatty acids, some of which directly suppress mammary butterfat synthesis. Supplemental fats from animal sources are more saturated and thus less detrimental to microbial activity and less apt to result in suppression of butterfat synthesis. Rumen-inert fats are designed to have little or no effect on rumen microbial activity and mammary butterfat synthesis. In general, when supplementing fats to dairy diets, up to 400 g (~2% of diet dry matter) may be added as vegetable fats, particularly if the fats are added as oil seeds, which tend to be less detrimental than free oils. An additional 200–400 g may be added from highly saturated or preferably rumen-inert sources, generally not to exceed a total of 6.5% fat in the total dietary dry matter.
The protein requirements of lactating dairy cows are high because of the demand for amino acids for milk protein synthesis. Two systems of describing the dietary protein supply and requirements for dairy cows are in general use: the crude protein system and the metabolizable protein system. The crude protein system considers only the total amount of dietary protein, or protein equivalent from nonprotein nitrogen sources. Crude protein values are based on the measurement of total dietary nitrogen and the assumption that protein is 16% nitrogen. The crude protein system is relatively simple to use and has provided a traditional means of formulating dairy cow rations. Recommended Minimum Dietary Protein Concentrations for Dairy Cows at Various Levels of Production a provides general guidelines for the required crude protein concentration of diets for large- and small-breed dairy cattle at various levels of production. It can be used for general evaluations of the protein adequacy of dairy diets. The metabolizable protein (MP) system is more complex than the crude protein system, and it was developed in recognition of the fact that not all crude protein provided to cows may be available for absorption as amino acids.
Recommended Minimum Dietary Protein Concentrations for Dairy Cows at Various Levels of Production a
MP refers to amino acids absorbed from the small intestine and available for metabolism. MP in ruminants is derived from two sources: microbial protein synthesized in the rumen and dietary proteins that escape rumen degradation. Protein escaping rumen degradation is referred to as rumen undegraded protein (RUP), while protein that is broken down in the rumen is referred to as rumen degraded protein (RDP). Both sources are important and must be considered in diet evaluation and formulation.
RUP passes unaltered through the rumen and forms a direct source of protein for intestinal digestion and amino acid absorption. Nitrogen from RDP, in contrast, must be incorporated into newly synthesized microbial protein before it will provide amino acids available for intestinal absorption. The efficiency with which RDP is recovered as microbial protein depends on the growth rate of the rumen microbes, which in turn depends on the supply of fermentable energy sources in the rumen. Thus, diets with sufficient RDP and relatively high energy concentrations will result in high yields of microbial protein, which will become available for intestinal digestion and absorption as MP. Calculations that balance dairy diets for MP must consider the complex interrelations among fermentable energy sources, RUP, and RDP. In general, specialized software, commercially available, is necessary to formulate dairy diets using the MP system. Even with such software, many variables must be estimated with uncertainty. Therefore, calculations of MP supply must be recognized to be approximations.
The relationship of dietary protein intake to metabolizable protein supply
Dietary ingredients vary in their proportion of RUP and RDP. In general, feeds with high moisture and high protein concentrations, eg, legume silages, will have a high proportion of RDP. In contrast, feeds that have been processed and especially those that have undergone drying will have relatively high proportions of RUP. The proportions of RUP and RDP in diets and individual ingredients are not fixed but can vary somewhat depending on intake rate. At high rates of feed intake, the rate of feed passage through the rumen is high; thus, there is less opportunity for rumen protein degradation than with the same feeds at lower intake rates. Therefore, on the same diet, RUP proportions are higher in animals with high rates of feed intake than in those with low rates of feed intake. Animals most likely to benefit from supplements selected for high RUP proportions are those with relatively high protein requirements and relatively low rates of feed intake. Cows in very early lactation and young, rapidly growing heifers are the primary examples. Supplements formulated for high RUP proportions are commonly known as rumen bypass protein supplements; however, even with these types of supplements, some portion of the protein is degraded in the rumen.
Along with overall protein requirements, dairy cows, as all other animals, have specific amino acid requirements. However, evaluating dairy cow diets relative to amino acid requirements is more difficult than making similar evaluations of diets for monogastric animals. This is because the amino acid supply for dairy cows and other ruminants is a combination of the amino acids provided by the microbial protein and the RUP. Microbial protein has an excellent amino acid profile, and diets with a large supply of microbial protein typically meet amino acid requirements if MP requirements are met. In some cases, however, high-producing dairy cows may benefit from the selection of RUP sources with specific amino acid profiles, or from adding rumen-protected forms of specific amino acids. Software is available that estimates the amino acid supply for dairy cows on different diets. The first limiting amino acids in typical dairy cow diets are lysine and methionine. With typical feedstuffs, if the MP requirement is met and the dietary lysine:methionine ratio is ~3:1, then the amino acid requirements for milk production are probably being optimized.
The availability of high-quality water for ad lib consumption is critical. Insufficient water intake leads immediately to reduced feed intake and milk production. Water requirements of dairy cows are related to milk production, DMI, ration dry matter concentration, salt or sodium intake, and ambient temperature. Various formulas have been devised to predict water requirements. Two formulas to estimate water consumption of lactating dairy cows are as follows:
Note: FWI is free water intake (water consumed by drinking rather than in feed), DMI is in kg/day, milk is in kg/day, Na is in g/day, and temperature is in °C. Water consumed as part of the diet contributes to the total water requirements; thus, diets with higher moisture concentrations result in lower FWI.
Providing adequate access to water is critical to encourage maximal water intake. Water should be placed near feed sources and in milking parlor return alleys, because most water is consumed in association with feeding or after milking. For water troughs, a minimum of 5 cm of length per cow at a height of 90 cm is recommended. One water cup per 10 cows is recommended when cows are housed in groups and given water via drinking cups or fountains. Individual cow water intake rates are 4–15 L/min. Many cows may drink simultaneously, especially right after milking, so trough volumes and drinking cup flow rates should be great enough that water availability is not limited during times of peak demand. Water troughs and drinking cups should be cleaned frequently and positioned to avoid fecal contamination.
Poor water quality may result in reduced water consumption, with resultant decreases in feed consumption and milk production. Several factors determine water quality. Total dissolved solids (TDSs), also referred to as total soluble salts, is a major factor that refers to the total amount of inorganic solute in the water. TDS is generally expressed in units of mg/L or parts per million (ppm) which are numerically equivalent values (see Table: Guidelines for Total Soluble Salts (Total Dissolved Solids) in Drinking Water for Cattle). TDS is not equivalent to water hardness, which is a measure of the amount of calcium and magnesium in water. Water hardness has not been shown to affect dairy cow performance.
Guidelines for Total Soluble Salts (Total Dissolved Solids) in Drinking Water for Cattle
Other inorganic contaminants that affect water quality include nitrates, sulfates, and trace minerals. Concentrations of nitrate (expressed as nitrate nitrogen) <10 mg/L are safe for ruminants. At concentrations >20 mg/L, cattle may be at risk, especially if nitrate concentrations in the feed are high. Water with nitrate concentrations >40 mg/L should be avoided. General recommendations for sulfate concentrations in drinking water are <500 mg/L for calves and <1,000 mg/L for adult cattle. The specific sulfate salts present in water may affect the response of cattle; iron sulfate is the most potent depressor of water intake. Concentrations of Potentially Toxic Nutrients and Contaminants in Drinking Water Generally Considered Safe for Cattle lists potential elemental contaminants of drinking water with upper-limit guidelines.
Concentrations of Potentially Toxic Nutrients and Contaminants in Drinking Water Generally Considered Safe for Cattle
Calcium requirements of lactating dairy cows are high relative to other species or to nonlactating cows because of the high calcium concentration in milk. Thus, inorganic sources of calcium, such as calcium carbonate or dicalcium phosphate, must be added to the rations of lactating dairy cows. For the first 6–8 wk of lactation, most dairy cows are in negative calcium balance, ie, calcium is mobilized from bone to meet the demand for milk production. This period of negative calcium balance does not appear to be detrimental so long as there is sufficient dietary calcium such that bone reserves can be replenished in later lactation. The availability of dietary calcium for absorption varies with dietary source. Dietary calcium from inorganic sources is generally absorbed with greater efficiency than that from organic sources. Furthermore, cows in negative calcium balance absorb calcium more efficiently than cows in positive calcium balance.
When calculating calcium requirements, newer nutritional models take into account the variability in calcium availability from different sources. This availability generally ranges from 75%–85% for inorganic calcium supplements to a low of 30% for forage sources of calcium. This approach makes it difficult to generate general recommendations for total dietary calcium concentrations across various diets. Generally, diets with large portions of forage from legume sources will have minimum calcium concentration requirements in the range of 0.71%–0.75%, while diets with forages from primarily grass (including corn silage) sources will have minimum calcium concentration requirements in the range of 0.42%–0.47%.
Two approaches are taken with respect to the calcium supply for dry cows, each with the objective of preventing milk fever, or parturient paresis (see Parturient Paresis in Cows). One approach is to place cows in a calcium-deficient state during the last 2–3 wk of gestation; the rationale is to stimulate parathyroid hormone secretion and skeletal calcium mobilization before calving. This makes calcium homeostatic mechanisms more responsive at the time of parturition, allowing cows to maintain serum calcium concentrations during lactation. This approach requires diets with calcium concentrations near 0.3% of dry matter. Such diets are difficult to formulate with available feedstuffs while still meeting other nutritional requirements. Another approach is to feed an acidifying diet, usually referred to as a diet with a low or negative dietary cation-anion difference (DCAD). The low-calcium diet approach is not additive with the DCAD approach to milk fever prevention. When low-DCAD diets are fed, total dietary calcium concentrations should be near 0.9%, which is substantially greater than the requirement for a dry cow on a conventional diet.
Phosphorus nutrition for lactating dairy cows has dynamics similar to those of calcium. The efficiency of phosphorus absorption is affected by physiologic state and dietary source. As is the case with calcium, most dairy cows in early lactation are in negative phosphorus balance. Phosphorus mobilized from bone early in lactation is replaced during later lactation when feed intakes are higher. Young animals and animals in negative phosphorus balance absorb phosphorus more efficiently than do older animals or animals in positive phosphorus balance. Phosphorus from inorganic sources is more available than that from organic feed sources.
Judiciously balancing diets to meet, but not exceed, phosphorus requirements is important for dairy cow performance and environmental stewardship. Excess phosphorus excreted in feces is one of the major pollutant risks associated with livestock production. Newer nutritional models account for variation in phosphorus availability from different sources, but there is less variation in availability among phosphorus sources than among calcium sources. In general, concentrates when fed to ruminants have a phosphorus availability of 70%, and forages close to 64%. Inorganic mineral supplements are usually rated at 75%–80% availability, but rock phosphate is very low, ~30%. Total dietary phosphorus concentration requirements for most dairy diets will be in the range of 0.35%–0.4%, and for dry cows, 0.3%–0.35%. Phosphorus supplementation for dry cows is seldom necessary.
The dietary calcium:phosphorus ratio is not of particular importance in ruminants. Ratios from 7:1 to 1:1 are acceptable, so long as the total amount of each element meets the dietary requirements.
Serum concentrations of calcium and inorganic phosphorus are of value in assessing the short-term homeostasis of these minerals but of little value in assessing longterm nutritional status. Bone ash concentrations are the best way to assess longterm calcium and phosphorus nutritional status.
Other macrominerals required in dairy cow diets include sodium, potassium, chloride, magnesium, and sulfur. Of these, sodium generally needs to be supplemented, typically as sodium chloride or common salt. Insufficient dietary sodium results in reduced feed intake with subsequent reductions in animal performance. Signs of severe salt deficiency include licking and chewing on fences and other environmental objects, urine drinking, and general ill thrift. Milk production is reduced within 1–2 wk of removing supplemental salt from the diets of lactating cows. Completely withholding salt from dry cow diets in an effort to prevent udder edema at calving is not a good practice. Maintenance requirements for sodium in nonlactating cows are estimated at 1.5 g/100 kg body wt/day, with gestation requirements estimated at an additional 1.4 g/day after 190 days of gestation. For large-breed dairy cows, this results in a sodium requirement of ~9–10 g/day. Unsupplemented dry cow diets seldom provide sodium at >3 g/day. Therefore, daily supplementation of dry cow diets with a minimum of 6–7 g of sodium per day (~15–16 g of salt) is important. Additional salt is necessary during heat stress. Although salt should be supplemented to dry cows in required amounts, excessive salt supplementation is unnecessary and may contribute to udder edema at calving.
Supplemental magnesium may need to be fed with diets containing high proportions of grass forages, especially those consisting of rapidly growing pasture grasses. Such forages typically have low magnesium concentrations as well as high concentrations of potassium and organic acids, which interfere with the availability of dietary magnesium. Magnesium oxide is the typical magnesium supplement in ruminant diets.
Dairy cattle, like other animals, have no dietary requirement for inorganic sulfur. The dietary requirement for sulfur reflects only the dietary requirement for sulfur-containing amino acids. In ruminants, rumen microbes can synthesize sulfur-containing amino acids from nonprotein sources of nitrogen and sulfur. Dairy cow diets most likely to require supplemental sulfur are those with low protein concentrations and those with supplemental nonprotein nitrogen. In general, a nitrogen:sulfur ratio of 15:1 is recommended in ruminant diets.
Recommended dietary concentrations for typical dairy cow diets are: sodium (0.23%), chloride (0.29%), potassium (1.1%), magnesium (0.21%), and sulfur (0.21%).
The trace minerals typically supplemented or measured in dairy cow diets include cobalt, copper, iron, manganese, selenium, iodine, and zinc. Of these, selenium and copper are the trace minerals most likely to be deficient. Several areas of North America, Europe, and other continents are characterized by growing conditions that result in feeds with low selenium concentrations. In these areas, livestock feeds need to be supplemented with selenium. Sources of supplemental selenium include sodium selenite, sodium selenate, and selenomethionine. The latter source is typically referred to as organic selenium.
Selenium deficiency is known to cause myopathies in calves, which may affect cardiac or skeletal muscle (ie, white muscle disease, see Nutritional Myodegeneration). In adult cattle, selenium deficiency appears to suppress immune function and especially neutrophil function. It also increases the risk of retained placenta, although feeding selenium in excess of requirements does not prevent this condition. Dietary selenium requirements in dairy cattle are estimated at 0.1–0.3 mg/kg diet dry matter. In the USA, 0.3 mg/kg dry matter is the maximum legal concentration of supplemental selenium in dairy cattle diets.
The selenium status of cattle can be accurately assessed from blood or serum concentrations. Whole blood concentrations of 120–250 ng/mL or serum concentrations of 70–100 ng/mL in adult cattle indicate adequate selenium status.
Recommended dietary copper concentrations in cattle diets are 10–15 mg/kg diet dry matter; however, the dietary copper requirement depends greatly on the concentration of interfering substances. These include primarily sulfur and molybdenum, but iron, zinc, and calcium may also interfere with copper availability. The absorption efficiency for dietary copper in ruminants is normally quite low, 4%–5%. However, with increasing concentrations of dietary sulfur and/or molybdenum, absorption efficiency may be reduced to ≤1%.
Copper deficiency is characterized by loss of hair pigmentation, loss of hair around the eyes, anemia, and general ill thrift and suppressed immunity. In severe cases, persistent diarrhea may also occur.
The copper status of cattle can be assessed from liver or serum copper concentrations. Liver concentrations <20 mg/kg dry tissue or serum concentrations <0.5 mcg/mL indicate copper deficiency. Because the liver is a physiologic storage site for copper, copper concentrations in the liver will be reduced before serum concentrations.
Dietary manganese deficiencies in dairy cattle are less common than deficiencies of copper or selenium. Signs include poor growth and skeletal deformities in newborn calves and reproductive abnormalities, including anestrus, in adult cows. Recently recommended dietary manganese concentrations for cattle are 15–25 mg/kg; previous recommendations have been as high as 40 mg/kg dry matter.
Recommended zinc concentrations in the diets of dairy cattle and calves are 23–63 mg/kg dry matter. Signs of zinc deficiency include reduced feed intake and general ill thrift. Parakeratosis, particularly around the nostrils and lower legs, and weakening of the hoof horn are signs of prolonged zinc deficiency. Normal concentrations of serum zinc are 0.7–1.3 mcg/mL. Concentrations <0.4 mcg/mL are considered deficient.
Iron deficiency is extremely rare in adult cattle, because iron is ubiquitous in the environment and the endogenous concentrations of iron in most feedstuffs will more than meet requirements. Signs of iron deficiency are primarily anemia and low serum iron concentrations. Adequate serum iron concentrations are 110–150 mcg/dL. However, these concentrations drop quickly in the presence of inflammatory disease, and such changes in serum iron concentrations should not be interpreted as being due to a dietary deficiency. Suckling calves are the only group of cattle generally at risk of iron deficiency and to which supplemental iron need be provided.
Iodine deficiency occurs with some frequency in cattle and is primarily manifest by goiters in newborn calves. The required dietary iodine concentration is generally ~0.2 mg/kg of dietary dry matter. However, dietary iodine concentrations of 0.6 mg/kg are recommended as a safety factor because of the potential presence of goitrogenic substances in common protein supplements.
Preformed vitamin A, or retinol, does not exist in any plant material, so there is no vitamin A in natural diets for dairy cattle. Vitamin A activity from natural sources comes primarily from β-carotene, which is found in plants and is particularly abundant in fresh forages. β-carotene is labile; its concentrations in forages are not constant but diminish with time in storage. Therefore, measurement of β-carotene concentrations in feeds is not practical and seldom done. Recommended vitamin A consumption rates for various classes of cattle are based on providing supplemental vitamin A, which is derived from commercial sources: for adult cows (lactating and dry)—110 IU/kg body wt, which is ~4,400 IU/kg dry diet; for growing heifers—80 IU/kg body wt, which is ~2,500 IU/kg dry diet. Conditions that can increase dietary vitamin A requirements in adult cows include low forage diets, high corn silage diets, poor quality forages, and infection.
The vitamin A status of cattle may be assessed via serum or hepatic vitamin A concentrations. The liver stores vitamin A for release during periods of insufficient dietary intake, thus making liver the ideal tissue for nutritional assessment. For adult cattle receiving diets with recommended supplemental vitamin A concentrations, hepatic vitamin A concentrations are 300–1,100 mg/kg dry tissue (expressed as retinol). Clinical signs of vitamin A deficiency do not occur until these reserves have been substantially depleted. Adequate serum vitamin A concentrations in adult cattle are 225–500 ng/mL, with values usually dropping to ~150 ng/mL within 1 wk of calving.
Calves are born with low body stores of vitamin A and depend on colostrum consumption to supply hepatic vitamin A stores. The NRC recommends dietary vitamin A concentrations for young calves at ~9,000 IU/kg diet dry matter. Most milk replacer diets have substantially higher concentrations of vitamin A, possibly because vitamin A requirements may be increased by infectious diseases, especially those affecting the respiratory or enteric epithelium.
Vitamin A deficiency is associated initially with night blindness followed by poor growth, poor hair coats, and suppressed immunity. In adult cattle, vitamin A deficiency is associated with retained placentas and impaired fertility.
Vitamin D is necessary for the absorption and metabolism of calcium and phosphorus. Recent research suggests that vitamin D may also be necessary for immune cell function. Vitamin D3 (cholecalciferol) can be formed by the solar irradiation of skin or vitamin D2 by the solar irradiation of forages. However, reliance on natural vitamin D formation is considered unreliable, and vitamin D requirements are based on recommendations for supplement addition to diets. The recommended rate of vitamin D supplementation for adult dairy cows is 30 IU/kg body wt, which would be supplied by diets with ~1,000 IU/kg dry matter.
Vitamin D status can be assessed via blood serum concentrations of 25-hydroxycholecalciferol. Adequate values are 20–50 ng/mL, with concentrations <5 ng/mL indicating deficiency.
Vitamin E is present in relatively high concentrations in fresh forages. Thus, cattle receiving pasture or fresh-cut forages may require little vitamin E supplement. In contrast, vitamin E degrades in stored forages, so dairy cattle on typical confinement-reared diets require supplemental vitamin E.
Vitamin E functions to protect cellular membranes from oxidative damage. Clinical manifestations of deficiency include nutritional myopathy (white muscle disease, see Nutritional Myodegeneration) in young calves and diseases in older cattle including retained placenta and increased susceptibility to environmental mastitis.
Recommended rates of vitamin E intake vary based on gestation stage: terminal dry period—1.8 IU/kg body wt, which is ~90 IU/kg dry matter; lactation—0.8 IU/kg body wt, which is ~30 IU/kg dry matter. Much higher concentrations are occasionally supplemented when environmental mastitis is a particular problem. Vitamin E is essentially nontoxic, and there is little risk of oversupplementation.
Vitamin E supplements may be natural or synthetic. Natural sources of vitamin E are derived from plant oils and are designated RRR-α-tocopherol or d-α-tocopherol, based on stereoisomer characteristics of their chemical structure. Synthetic supplements are designated all rac-α-tocopherol, or dl-α-tocopherol. The natural-source supplements appear to have much greater biologic activity.
Blood serum vitamin E concentrations may be used to assess vitamin E status in dairy cattle. Serum concentrations of 2–4 mcg/mL are generally adequate. However, in addition to vitamin E nutritional status, these concentrations are influenced by the total concentration of serum lipid, with higher serum lipid concentrations resulting in higher vitamin E concentrations. Serum lipids are generally low in late gestation and high in the period of peak feed intake. To compensate for this fluctuation, serum vitamin E concentrations are sometimes expressed as a ratio, with some serum lipid component, such as cholesterol or triglyceride, used as the denominator.