During lactation, dairy cows have very high nutritional requirements relative to most other species ( see Table: Feeding Guidelines for Large-Breed Dairy Cattle). Meeting these requirements, especially for energy and protein, relative to intake capacity 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.
Feed Intake
The cornerstone of dairy nutrition is managing feed intake relative to absolute nutrient requirements. Feed intake (dry-matter intake) and feed efficiency (milk production [absolute or component corrected] per unit of dry-matter intake) are key nutritional monitoring metrics. Dry-matter intake is influenced by the following factors:
Feed compositional factors: neutral detergent fiber (NDF) content, quality of ensiled feeds (excessive moisture and fermentation products), maturity (lignification) of forage, palatability attributes, and nutrient availability
Cow physiologic factors: age, body size, physiologic state, body condition score, days in lactation, and production level
Management factors: feed bunk management (feed delivery, availability, and consistency), grouping strategies, cow comfort, and heat abatement strategies
Lactating cows should be managed to maximize intake rapidly after calving to minimize the severity and duration of negative energy balance experienced. Milk production and associated energy requirements generally peak at ~6–10 weeks into lactation, whereas DMI usually does not peak until 8–12 weeks into lactation. Postpartum negative energy balance severity will negatively impact body condition, resulting in greater risk for postpartum disease and reproductive inefficiency. Dietary starch content influences intake capacity immediately after calving, while dietary NDF content ( See table: Impact of Forage NDF on Forage Intake Capacity) is the primary factor at peak milk production. Dietary NDF provides a physical fill factor influencing intake, which depends on rate of passage and NDF digestibility. Intake capacity of NDF varies with physiologic state, ranging from 0.6% to 1.2% of body weight. Lowest intake capacity occurs in late pregnant heifers (0.6%) followed by late pregnant cows (0.8%). Intake following peak milk production should be monitored to prevent excess body condition accumulation (body condition score >3.5 on a 5-point scale).
Carbohydrates
Dietary carbohydrates comprise a wide range of compounds from simple sugars to complex polysaccharides. They account for 60%–80% of dietary dry matter for dairy cows. Carbohydrate fractions are segregated based on chemical measures and nutritional impacts (see Plant carbohydrate fractions). Complex polysaccharides associated with the plant cell wall and resistant to microbial fermentation are quantified by measures of neutral detergent fiber (NDF) and its subset of acid detergent fiber (ADF). These structural carbohydrates will limit intake but stimulate chewing and rumination, which helps maintain rumen buffering and health and can increase milk butterfat composition.
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 nonfiber carbohydrates (NFCs). 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 NFCs.
Courtesy of Dr. Robert Van Saun.
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 organic acids, sugars and starch, and neutral detergent soluble fiber (NDSF). In fermented feeds, fermentation acids also contribute to the NFC fraction. The sum of sugars and starch is referred to as nonstructural carbohydrates (NSCs), which should not be confused with NFCs. Fiber compounds associated with the secondary plant cell wall that are not digestible by mammalian enzymes but that are solubilized by neutral detergent are defined as NDSF. Although pectins, beta-glucans, and galactans are associated with the secondary plant cell wall, they are highly fermentable and provide good sources of energy in the ruminant diet.
Balancing fiber and NFC fractions to optimize energy intake and rumen health is a challenging aspect of dairy nutrition. To increase the energy supply, dietary NDF concentrations are usually reduced by adding starch and other sources of NFCs. 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 <6.2, fiber digestion is reduced; at pH ≤5.5, fiber digestion is severely diminished, feed intake may be reduced, and rumen health is generally compromised.
Recommended minimum NDF concentrations depend on the source and physical effectiveness of the NDF and the dietary concentration of NFCs. 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). 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; however, they provide more energy per mass unit of NDF than diets with less fermentable NDF. Feeding management schemes such as total mixed rations result in lower minimum NDF concentrations than feeding dietary components individually.
Energy
Dietary energy available for metabolic use is referred to as metabolizable energy (ME). 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 US, 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. The table Dry Matter, Energy, Crude Protein, Fiber, and Nonfiber Carbohydrate Concentrations of Some Feedstuffs Commonly Fed to Dairy Cattle has typical values for ME, NEL, NEM, and NEG, for some feedstuffs commonly fed to dairy cows.
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 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.
In the US, 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 the table Maintenance Energy Requirements for Cows of Various Body Weights. Energy requirements per kg of milk produced at various milk fat concentrations are given in the table Dietary Net Energy Requirement for Milk Production.
The required dietary energy concentration is a function of the energy requirement and 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. 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.
Fats
Ruminant diets typically are low in total fat content due to the negative effects that fatty acids, especially polyunsaturated fatty acids, have on microbial fiber fermentation. Dietary fat can come from three sources:
Endogenous fats: forage lipids that include glycolipids, pigments, cutins, and waxes
Vegetable fats: polyunsaturated fats from oilseeds such as soybean, corn, canola, sunflower, and flaxseed
Rumen inert fats: saturated animal fats, calcium soaps, and prilled fats
Typically, each type of fat source can be supplied in the diet at 2%–3% of dry matter up to a total of 8%–9% total fat. 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.
Another method of evaluating dietary fat and potential impacts on rumen microbes or cow is quantifying rumen unsaturated fatty acid load (RUFAL). Unsaturated fatty acids are biohydrogenated by rumen microbes to generate saturated fatty acids. An intermediate in this biohydrogenation process, especially under lower rumen pH conditions, are alternative conjugated linoleic acids (CLA) that can inhibit mammary de novo fat synthesis, resulting in milk fat depression. The trans-10 CLA compound has been primarily associated with milk fat depression.
Supplemental fat can provide additional concentrated energy to meet lactating cow energy needs. Adding fat within the first 3 weeks of lactation has shown a negative effect on intake; thus, it is not recommended. The addition of fat after this period may improve milk fat content, milk production, or reproductive efficiency; however, response to dietary fat supplementation is not consistently predictable. The amount of total fat consumed from all three sources should be limited to the amount of fat produced (milk production × fat percentage).
Protein
Protein requirements of lactating dairy cows are based on amino acids required for maintenance and milk protein synthesis. Ruminant animals derive most of their amino acids to support body metabolism from microbial protein. Microbial protein is of high biological value and highly digestible. Mixed microbes contain between 45% and 60% crude protein. Therefore, dietary formulations are directed to ensure optimum microbial growth to minimize the need for expensive dietary protein supplements.
Dietary protein not used by rumen microbes can be potentially digested in the abomasum and resultant amino acids absorbed in the small intestine. This fraction of dietary protein is termed rumen undegradable protein (RUP), in contrast to the dietary protein fraction degraded in the rumen (rumen degradable protein [RDP]) and used by the microbes. To their advantage, ruminants are able to use low-quality protein or nonprotein nitrogen sources to generate microbial protein to meet their amino acid requirements.
Two systems of describing the dietary protein supply and requirements for dairy cows are in general use:
Crude protein system: based on dietary nitrogen converted to protein equivalent using 6.25 multiplier factor (assumes protein is 16% nitrogen); does not account for differences in availability to rumen or cow
Metabolizable protein system: based on a model predicting dietary nitrogen and carbohydrates availability to rumen microflora and predicted microbial protein flow and dietary rumen undegraded protein fraction and its digestibility to account for total protein available and absorbed
The crude protein system is relatively simple to use and has provided a traditional means of formulating dairy cow rations. The table Recommended Minimum Dietary Protein Concentrations for Dairy Cows at Various Levels of Production 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. It was developed in recognition of the need to provide dietary nitrogen to support microbial growth (RDP) in addition to dietary RUP to collectively meet cow amino acid needs. Unlike crude protein, MP cannot be directly measured in a feed ingredient via laboratory analysis.
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, RDP, and RUP. 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.
Relationship of dietary protein intake to metabolizable protein supply
Diagram showing the relationship of dietary protein intake to metabolizable protein supply. The two branch points (indicated by 1 and 2) constitute the major variables relating the dietary crude protein supply to the metabolizable protein supply. The first branch point represents the proportion of protein that is degraded in the rumen. This branch point is influenced by inherent properties of the protein and the rate of ingesta passage through the rumen. The second branch point represents the proportion of nitrogen from degraded protein that is recaptured as microbial protein. This is influenced by the microbial growth rate, which depends on the supply of rumen available energy. Nitrogen that is not recaptured as microbial protein is absorbed from the rumen as ammonia and converted to urea by the liver. Some urea is recycled back to the rumen; however, a large portion is excreted in urine. RUP, rumen undegraded protein; RDP, rumen degraded protein; N, nitrogen; MCP, metabolizable crude protein; MP, metabolizable protein. |
Dietary ingredients vary in their proportion of RDP and RUP. 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.
The specific amino acid requirements of dairy cows are not as well understood as those of swine or poultry. Most research has focused on methionine and lysine as first limiting amino acids in typical dairy cattle diets, especially during early lactation. These amino acids can be supplemented by feeding targeted RUP ingredients high in these amino acids or rumen-protected forms of these amino acids. Software is available that estimates the amino acid supply for dairy cows on different diets. With typical feedstuffs, if the MP requirement is met and the dietary lysine-to-methionine ratio is ~3:1, then the amino acid requirements for milk production are likely being optimized.
Water
The availability of high-quality water for ad libitum 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:
where 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 sodium intake 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. Every cow group should have a minimum of two watering stations to prevent a high social order cow from blocking a single water source.
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. Water can be evaluated by its organoleptic properties (color, taste, and smell) or quantification of dissolved or suspended contents. Factors affecting water quality include the following:
pH: A wide pH range from 5 to 9 seems acceptable to cattle; extremes of the pH range may be of concern for palatability.
Microbiological contamination: Bacterial counts that may cause digestive issues in ruminants have not been well documented. No correlation has been found between bacterial contamination level and cow performance; however, if bacteria are present, it is reasonable to clean watering units more frequently.
Total dissolved solids (TDS): Also referred to as total soluble salts, TDS 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).
Hardness: A measure of calcium and magnesium content in water; not equivalent to TDS; generally not shown to affect cow performance, although calcium may add to the amount in the diet.
Mineral content: Water can contain a range of mineral elements that are both essential nutrients as well as toxic elements; the table 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.
Inorganic contaminants: Beyond mineral elements, other inorganic contaminates of concern for ruminants include nitrates, nitrite, and sulfates. 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.
Organic contaminants: A wide range of organic compounds such as herbicides, insecticides, and pharmacological agents may contaminate water sources. Industrial pollution from mining and gas drilling may increase solvents, fuels, methane, and other surfactants or chemicals.
Water testing is readily available; however, for evaluating water for food-producing animals it is best to identify an appropriate laboratory that addresses this specifically. Contact the laboratory for directions on appropriate sampling technique and sample containers.
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.
Minerals
Calcium and Phosphorus Requirements of Dairy Cattle
Of the macromineral requirements of dairy cattle, calcium and phosphorus are most often considered due to their roles in skeletal structure, metabolism, and milk. Calcium requirements of lactating dairy cows are high relative to other species or to nonlactating cows because of the high calcium concentration in milk. Phosphorus requirements are approximately half of calcium requirements; phosphorus is lost to milk as well as recycled via saliva to the rumen to support microbial growth needs. Phosphorus is considered the first limiting mineral on a forage-based feeding program due to the low phosphorus content of forages (<0.21% dry matter), especially without fertilization. Phosphorus has been associated with infertility in cattle; however, this only occurs in extremely low forage phosphorus (<0.15% dry matter) conditions. Of greater concern is oversupplementation of phosphorus and its environmental impact.
Due to its tight homeostatic control, blood calcium concentration is very consistent within cows of specific age groups. Blood phosphorus concentration in preweaned calves is higher than in adult cows, with a decrease in concentration when phosphorus starts to be recycled through the saliva to the rumen. Consequently, blood calcium-to-phosphorus ratio is approximately 1.2–1.5 in younger animals, compared to 1.5–2.0 in older animals. Dietary calcium and phosphorus supplementation amounts and ratio are important in young growing animals in support of bone development and in lactating cows for milk production. Early lactation often results in a period of negative calcium and phosphorus balance resulting in bone mobilization.
Legume and grass forages have similar phosphorus content but divergent calcium content, with legumes having much higher calcium levels. Depending on the amount of legume forage in the diet, calcium may need to be supplemented to support lactation. More recent requirement models account for differences in availability of a mineral element depending on ingredient source. Inorganic mineral sources have the highest availability (75%–90%), though there is some variability among different mineral sources. Forages have the lower availability for calcium (30%) and phosphorus (64%) resultant of chelating compounds such as phytic acid and oxalic acid found in plant tissues. Mineral requirements are presented as total and available with the difference being the mineral availability from various dietary ingredients being considered.
Beyond their support for growth and lactation, nutrition of calcium and phosphorus is of great interest relative to parturient hypocalcemia (ie, milk fever). The role of each mineral has been researched extensively in determining methods of preventing parturient paresis. Initially, dietary calcium-to-phosphorus ratio was considered, though this usually resulted in supplementing more dietary phosphorus to balance against the high endogenous dietary calcium. Research indicates that restricting dietary calcium at or below the available requirement 2–3 weeks prior to calving induces the homeostatic system to up-regulate calcium influx to counter subsequent colostrum and milk calcium losses. 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). Dietary calcium content with the DCAD approach to milk fever prevention has been debated and not well defined. Some research has suggested a higher dietary calcium is necessary to compensate for increased urinary loss. Others suggest between 0.9% and 1.2% (dry-matter basis) calcium is sufficient. Lower calcium diets have been fed successfully with DCAD diets. A potentially greater issue to address is dietary phosphorus content. Recommended diet phosphorus concentration in dry cow diets ranges from 0.25%–0.35% (dry-matter basis). High dietary phosphorus (>0.5%) may promote hypocalcemia whereas some new research suggests low diet phosphorus (0.2% dry matter) may be protective.
Serum concentrations of calcium and inorganic phosphorus are of value in assessing the short-term homeostasis of these minerals; however, they are of little value in assessing long-term nutritional status. Bone ash concentrations are the best way to assess longterm calcium and phosphorus nutritional status.
Other Macromineral Requirements of Dairy Cattle
Other macrominerals of consequence to dairy cows include sodium, chloride, potassium, magnesium, and sulfur. Sulfur is required primarily to provide substrate to rumen microbes in generating sulfur-containing amino acids as part of microbial protein generation. The ratio of nitrogen to sulfur in microbial protein is 14.5:1. A typical recommendation for ruminant diets is to maintain nitrogen-to-sulfur ratio between 10:1 and 12:1. A more biological approach is to estimate sulfur needed for microbial protein synthesis based on dietary metabolizable energy and protein availability. Sulfur generally is not deficient in the diet unless forages are grown on sulfur deficient soils. Recommended dietary concentration is 0.21%–0.25% dry matter.
The other macrominerals serve important biological roles as electrolytes located either extracellularly (sodium and chloride) or intracellularly (potassium and magnesium). These electrolytes have roles in acid-base balance, cell membrane electrical potentials, nerve conduction, and active transport.
Sodium needs to be supplemented in the diet as sodium chloride or common salt or provided as free-choice salt. Forage contains very little sodium (<0.05% dry matter) unless contaminated with salty water. Cows have an appetite to consume salt and will seek it out; thus, salt is used as a carrier for trace mineral supplements. Signs of severe salt deficiency include licking and chewing on fences and other environmental objects, urine drinking, and general ill thrift.
Insufficient dietary sodium results in reduced feed intake with subsequent reductions in animal performance. Milk production is reduced within 1–2 weeks of removing supplemental salt from the diets of lactating cows. Completely withholding salt from dry cow diets to prevent udder edema at calving is not a good practice. Additional salt is necessary during heat stress. Sodium requirement ranges from 0.15% (dry cow) to 0.23% (lactating cow) dry matter.
Chloride typically follows sodium in biological systems, commonly added to diets as salt (sodium chloride); additional chloride without sodium is used in diets to supplement more anions to reduce DCAD for dry cow diets. Chloride requirement ranges from 0.25%–0.29% dry matter.
Potassium plays an important role as the primary intracellular cation. Supplementing potassium in the diet is typically not necessary given the high potassium content of forages (grasses and legumes), especially with excess potassium soil fertilization. Excess dietary potassium plays an important role in acid-base status that interferes with calcium homeostasis. Additional dietary potassium may be appropriate in early lactation and during heat stress to counter metabolic acid load. Potassium requirement ranges from 0.6%–1.2% dry matter.
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. Magnesium requirement is between 0.21% and 0.35% dry matter, depending on dietary potassium.
Trace Mineral Requirements of Dairy Cattle
The trace minerals typically supplemented or measured in dairy cow diets include cobalt, copper, iron, manganese, selenium, iodine, and zinc. Trace minerals are required in minute amounts on the order of micrograms or nanograms per day. These minerals play important biological roles as components of metalloenzymes influencing metabolic reactions, immune function, and antioxidant status (see table).
Ruminants are challenged in regard to trace mineral nutrition. Although there is great geographic variation in forage trace mineral status, most are marginal to low, generally resulting in a deficient diet without supplementation. Additionally, rumen microbial environment can alter these metal cations to alter valence and thus absorption efficiency at the small intestine or by generating chelating ligands to prevent absorption.
Another concern relative to trace minerals is a documented interrelationship between the minerals that can influence dietary availability. Excess zinc intake will reduce copper availability. High dietary sulfur can inhibit copper and selenium availability. High dietary molybdenum will reduce copper availability, especially when combined with higher dietary sulfur. Like the macrominerals, inorganic mineral sources are more available; however, they are also more prone to rumen interference.
Inorganic selenium (sodium selenite or sodium selenate) sources are adversely affected in the rumen where the selenium ion is reduced from a +4 or +6 state to elemental selenium or selenide. Only the selenite and selenate selenium forms are absorbed at the small intestine. Use of selenomethionine (organic selenium) is not susceptible to this reduction and thus is more bioavailable.
Dietary copper requirement depends greatly on the concentration of interfering substances. These include primarily sulfur and molybdenum; however, iron, zinc, and calcium may also interfere with copper availability. The absorption efficiency for dietary copper in ruminants is normally quite low, 4%–6%. However, with increasing concentrations of dietary sulfur, molybdenum or a combination, absorption efficiency may be reduced to ≤1%.
Iodine is readily absorbed from the diet and across skin and concentrated in the thyroid gland. Various dietary compounds or toxic plants contain substances that interfere with iodine uptake or utilization of iodine at the level of the thyroid gland. These compounds are termed goitrogens and their presence in the diet requires additional iodine to be supplemented.
Trace mineral nutritional status is best assessed by determining hepatic mineral concentration, whereas the liver is the primary storage site for all trace minerals except iodine. Blood analysis can be used with understanding of confounding issues limiting interpretation.
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.
Copper status of cattle can be assessed best from liver and only from serum copper concentrations when multiple samples are collected. Liver concentrations <50 mg/kg dry tissue or serum concentrations <0.3 mcg/mL indicate inadequate copper status.
Normal serum zinc concentrations are 0.7–1.3 mcg/mL. Concentrations <0.4 mcg/mL are considered deficient.
Adequate serum iron concentrations are 110–150 mcg/dL. However, these concentrations decrease quickly in inflammatory disease, and such changes in serum iron concentrations should not be interpreted as being due to a dietary deficiency.
Vitamins
Vitamin A Requirements of Dairy Cattle
The biological form of vitamin A, 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 beta-carotene, which is found in plants and is particularly abundant in fresh forages. Beta-carotene is labile; its concentrations in forages are not constant but diminish with time in storage. Therefore, measurement of beta-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:
Adult cows (lactating and dry)—110 IU/kg body wt, which is ~4,400 IU/kg dry diet
Growing heifers—80 IU/kg body wt, which is ~2,500 IU/kg dry diet
Vitamin A requirements are increased with low forage diets, high corn silage diets, poor-quality forages, and infection.
Vitamin A deficiency is associated initially with night blindness followed by poor growth, poor coats, and suppressed immunity. In adult cattle, vitamin A deficiency is associated with retained placentas and impaired fertility.
Calves are born with low body stores of vitamin A and depend on colostrum consumption to supply hepatic vitamin A. Minimal fetal vitamin A status may cause stillbirth and weak calves. The National Research Council 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.
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, 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, usually decreasing to ~150 ng/mL within 1 week after calving.
Vitamin D Requirements of Dairy Cattle
Vitamin D is necessary for the absorption and metabolism of calcium and phosphorus. 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 supplementing 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 serum concentrations of 25-hydroxycholecalciferol. Adequate concentrations are 20–50 ng/mL, with concentrations <5 ng/mL indicating deficiency.
Vitamin E Requirements of Dairy Cattle
Natural sources of vitamin E are derived from plant oils and are designated RRR-alpha-tocopherol or d-alpha-tocopherol, based on stereoisomer characteristics of their chemical structure. 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) 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 physiologic stage:
Close-up dry period (last 3 weeks)—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 (4,000 IU/d close-up dry period; 2,000 IU/d early lactation) 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. Synthetic supplements are designated all-rac-alpha-tocopherol, or dl-alpha-tocopherol. The natural source supplements seem to have much greater biological activity.
Serum concentrations may be used to assess vitamin E status in dairy cattle. Serum concentrations of 2–4 mcg/mL are generally adequate. Dry cow serum concentrations <3 mcg/mL have been associated with higher susceptibility to mastitis. 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.
Other Vitamin Requirements of Dairy Cattle
Supplementation of other compounds such as vitamin K, B-complex vitamins, and vitamin C is not typical in most ruminant diets. Cows, unlike humans and primates, synthesize vitamin C adequately. Vitamin K and the B-complex vitamins are believed to be sufficiently synthesized by rumen and intestinal microbes to meet the cow’s needs.
Some vitamins may be provided via natural feed ingredients. There may be cases in which B-vitamins might be supplemented, primarily during disease conditions and especially in conditions of altered rumen fermentation. Supplementation of rumen-protected B-vitamins (specifically, biotin, niacin, and choline) may benefit health and performance of high-producing dairy cattle See also Feed Additives in Dairy Cattle).
