Nutritional Requirements of Dairy Cattle

The cornerstone of dairy nutrition is managing feed intake relative to absolute nutrient requirements. Feed intake, typically defined as dry matter intake (DMI), and feed efficiency (milk production [absolute or component corrected] per unit of DMI) are key nutritional monitoring metrics. DMI 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, rumen degradable protein, and nutrient availability

• Cow physiological factors: age, body size, physiological state, body condition score, days in lactation, and production level

• Management factors: feed bunk management (feed delivery, availability, and consistency), grouping strategies, cow comfort, water availability and quality, 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 approximately 6–10 weeks into lactation, whereas DMI usually does not peak until 8–12 weeks into lactation. Severe postpartum negative energy balance will negatively impact body condition, resulting in greater risk for postpartum disease and reproductive inefficiency.

Intake regulation is a complex cerebral integration of signals from digestion products, gut distention, autonomic sensory inputs via the vagal nerve, and the milieu of metabolic regulatory hormones. Simplistically, intake is a function of the interaction between caloric intake and physical distention. Whether caloric status or physical distention has a greater impact on intake control depends upon the cow’s physiological state. 

Dietary nonstructural carbohydrate content, through its fermentation to propionate, influences intake capacity in late pregnancy and immediately after calving. As the cow approaches peak milk production, physical distention, a function of dietary NDF content (see the table Impact of Forage NDF on Forage Intake Capacity) is the primary factor at peak milk production. Intake capacity of NDF varies with physiological state, ranging from 0.6% to 1.3% of body weight. Lowest intake capacity occurs in late pregnant heifers (0.6%) followed by late pregnant cows (0.8%). Physical fill becomes less of an intake-controlling factor in later lactation. Intake following peak milk production should be monitored to prevent excess body condition accumulation (body condition score > 3.5 on a 5-point scale).

Dietary carbohydrates comprise a wide range of compounds from simple sugars to complex starch and nonstarch polysaccharides. Carbohydrates account for 60–80% of dietary dry matter for dairy cows. Carbohydrate fractions are segregated based on chemical measures and nutritional impacts (see image of plant carbohydrate fractions). Complex polysaccharides associated with the plant cell wall that are more slowly fermented are quantified by measures of neutral detergent fiber (NDF), and its subfraction, acid detergent fiber (ADF). These structural carbohydrates limit intake but stimulate chewing and rumination, which helps maintain rumen buffering and health and can increase milk butterfat composition.

Plant carbohydrate fractions

Image

Courtesy of Dr. Robert Van Saun.

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 carbohydrate (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.

Feed NFC content is calculated by subtracting the proportions (as dry matter) of NDF, crude protein, fat, and ash from 100%. NFC primarily consists of organic acids, sugars, starch, and neutral detergent soluble fiber (NDSF). In fermented feeds, fermentation acids (ie, lactate, acetate, propionate) 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.

The NSC fraction is enzymatically digested and determined by measuring released glucose units. 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 rumen fermentable and provide good sources of energy in the ruminant diet.

Balancing fiber and NFC/NSC 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 decreased 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 < 6.2, fiber digestion is decreased; at pH ≤ 5.5, fiber digestion is severely diminished, feed intake may be decreased, and rumen health is generally compromised. Dietary starch content has a measurable impact on NDF fermentation, and the NASEM report adjusts potential dietary NDF fermentability relative to dietary starch content.

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 fiber from nonforage sources. Thus, one variable in the assessment of dietary NDF adequacy is the proportion of NDF contributed 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 the 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, total NDF concentrations should be in the upper end of this range.

Unfortunately, all dietary NDF is not equivalent relative to these biological actions. Historically, NDF sources have been characterized relative to their ability to stimulate milk fat production (eg, effective NDF [eNDF]) or stimulate chewing (eg, physically effective NDF [peNDF]). A new descriptor of dietary NDF, physically adjusted NDF (paNDF), has been developed and incorporated into the NASEM 2021 report.

The desired outcome of dietary paNDF is to maintain rumen pH at 6.0 or 6.1 to facilitate fiber fermentation. Determination of paNDF dietary content is based on interaction of dietary starch content, forage NDF (fNDF), and fiber source fragility determined by the ratio of ADF:NDF. Essentially, this modeling system is predicting the amount of dry NDF material to be retained on the 8-mm sieve of the Penn State Particle Separator based on inputs of dry NDF material retained on the 19-mm sieve, dietary starch, forage NDF contribution, and fiber fragility. As dietary starch increases, it will adversely affect NDF digestibility due to pH effects. Fiber fragility is used to estimate NDF retention within the rumen, further stimulating rumination. Low-fragility fiber from grasses will be retained in the rumen longer than more fragile fiber from legumes. Contribution of NDF from forage will also promote more rumination and potential buffering.

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 and 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.

Dietary energy available for metabolic use is referred to as metabolizable energy (ME). In contrast to digestible energy (DE), the feed energy losses in urine and fermentation gases are subtracted. The efficiency of ME utilization varies based on the physiological 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 (NE_M), NE for gain (NE_G), and NE for lactation (NE_L). 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, NE_L, NE_M, and NE_G for some feedstuffs commonly fed to dairy cows.

Energy values for feeds are not directly measured by feed analysis laboratories but rather predicted using regression equations. Direct measurement of feed energy is only achieved using calorimetry methods; however, feeding a single feed is not practical. Feed energy values reported on laboratory feed analysis reports are estimates, usually based on formulas with ADF concentration and possibly crude protein as primary independent variables.

In the US, energy requirements of adult dairy cows are typically expressed in terms of NE_L. 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 kilogram 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.

Ruminant diets typically are low in total fat content (< 5% dry matter) due to the negative effects that polyunsaturated fatty acids (PUFAs) have on microbial fiber fermentation. Dietary fat is derived 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 decreased fiber digestion, indigestion with poor rumen health, suppression of milk fat concentration (ie, milk fat depression), or some combination.

Quantifying rumen unsaturated fatty acid load (RUFAL) is another method of evaluating dietary fat and potential impacts on rumen microbes or cow metabolism and fat production. Dietary PUFAs are naturally biohydrogenated by rumen microbes to generate saturated fatty acids, which accounts for ruminant body fat being predominately saturated. Dietary situations in which higher dietary PUFAs are being fed, coupled with lower rumen pH, result in an alternative biohydrogenation pathway in the rumen leading to bioactive conjugated linoleic acid (CLA) production. One of these CLA isomers (trans-10, cis-12 CLA) inhibits mammary de novo fat synthesis, resulting in milk fat depression (milk fat < 3%).

Supplemental dietary fat can provide additional concentrated energy to meet the lactating cow's energy needs. Adding fat within the first 3 weeks of lactation potentially can decrease feed intake, which is not desirable in early lactation. 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. Historically, the amount of total dietary fat consumed was suggested to be limited to the amount of fat produced (milk production × fat percentage); however, the contribution from dietary PUFA is more important and should be limited.

Lactating dairy cow protein requirements are based on amino acids required for maintenance and milk protein synthesis, with additional amino acids to support growth in the first two lactations. Ruminant animals derive from microbial protein most of the amino acids that support body metabolism. 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 supplemental dietary protein.

Dietary protein not degraded 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 (NPN) sources to support microbial protein synthesis in meeting their amino acid requirements. Ruminants consuming low-protein diets will efficiently recycle blood urea via saliva to the rumen to provide needed nitrogen sources to support microbial populations.

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 dynamic rumen model describing potential dietary nitrogen and carbohydrate pools available to rumen microflora. Based on dietary parameters and defined rates of passage and digestion interactions, microbial protein flow and dietary RUP fraction are predicted. Digestible amounts of microbial protein and RUP are predicted and collectively account for total metabolizable protein (MP) delivered to the small intestine and absorbed.

The crude protein system is relatively simple to use and historically has been the traditional means of formulating dairy cow rations. The table Estimated 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 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 (ie, 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 and must be estimated using dynamic rumen models.

See also the graphic depicting the relationship of dietary protein intake to metabolizable protein supply.

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. MCP, metabolizable crude protein; MP, metabolizable protein; N, nitrogen; RDP, rumen degraded protein; RUP, rumen undegraded protein.

Dietary ingredients vary in their proportion of RDP and RUP. In general, feeds with high moisture and high protein concentrations (eg, legume silages) have a high proportion of RDP. In contrast, feeds that have been processed, and especially those that have undergone drying or roasting, have relatively higher proportions of RUP. The percentage of RDP and RUP for a feed will always equal 100%; however, the proportion of each for any ingredient is not fixed and will be altered by changes in rates of passage through the rumen. Intake rate, body size, and dietary fiber amount and effectiveness will influence rate of passage.

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, with cows 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. High-forage diets in which the cow relies primarily on microbial protein may be deficient in histidine. Insufficient amino acids can be supplemented by feeding targeted RUP ingredients high in these amino acids or rumen-protected forms of these amino acids. The NASEM 2021 software estimates amino acid supply and efficiency of utilization for dairy cows on different diets. With typical feedstuffs, if the MP requirement is met and the dietary lysine:methionine ratio is approximately 3:1, then the amino acid requirements for milk production are likely being optimized.

The availability of high-quality water for ad libitum consumption is critical. Insufficient water intake leads immediately to decreased 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. For recommended formulas to estimate water consumption of lactating and dry dairy cows, see the table Free Water Intake Equations for Dairy Cows.

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 (2 inches) of length per cow at a height of 90 cm (3 feet) 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, anytime a cow drinks during a day. Many cows may drink simultaneously, especially right after milking, so trough volumes and drinking cup flow rates should be sufficient to ensure water availability 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 decreased 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, cleaning watering units more frequently is reasonable.

• 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 as mg/L or parts per million (ppm), which are numerically equivalent values (see the 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; however, calcium may add to the amount in the diet.

• Mineral content: Water can contain a range of mineral elements that are both essential nutrients and toxic elements. The table Safe Drinking Water Guidelines for Cattle lists potential elemental contaminants of drinking water with upper-limit guidelines.

• Inorganic contaminants: Beyond mineral elements, other inorganic contaminants of concern for ruminants include nitrates, nitrites, 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, to evaluate water for food-producing animals, identifying an appropriate laboratory that addresses this specifically is best. Contact the laboratory for directions on appropriate sampling technique and sample containers.

The table Safe Drinking Water Guidelines for Cattle lists potential elemental contaminants of drinking water with upper-limit guidelines.

The NASEM committee undertook many challenges in revising and improving upon feeding recommendations for dairy cattle. A new approach to defining a “requirement” was incorporated in the report, which better represents the data support for the requirement (see the table NASEM Requirement Definitions). Many of the trace mineral and vitamin requirements were defined as "adequate intake," indicating a lack of definitive information to adequately define a requirement. Energy requirement calculations were further refined with no significant change in values. However, the maintenance energy requirement was increased by 25%. This increase accounts for the majority of the increase in the dry cow energy requirement.

The use of metabolizable protein (MP) to define dairy cow amino acid requirements over crude protein is more strongly supported in this report. Metabolizable protein refers to all protein available for digestion in the small intestine, including microbial protein and protein not digested in the rumen. Rather than considering only the crude protein of a diet, MP is a more comprehensive description of the protein available to the cow. Improved characterization of feed protein fractions and amino acid composition is provided in compositional tables in the report. The modeling of protein supply has been modified from the NRC 2001 report, resulting in a complete modification of essential amino acid (EAA) recommendations and a focus on balancing diets for EAAs rather than MP. Although many improvements have been made to the MP prediction model, the report highlights much-needed research areas, especially relative to amino acid supply and utilization efficiency.

Similar to the modeling approach with MP focusing on EAAs, the feeding of fat focuses on compositional fatty acids. The new model of fat utilization allows for direct entry of fatty acid measurements rather than using ether extract. A new measure of dietary fiber has also been introduced—namely, physically adjusted neutral detergent fiber (paNDF). This parameter approach integrates the associative effects of dietary starch and forage type on the ability of NDF particle size to maintain a healthy rumen pH.

Refinements to macromineral requirements were made based on source bioavailability and significant mineral interactions. Among the trace minerals and fat-soluble vitamins, there were some notable changes to cobalt, copper, manganese, and zinc compared to the NRC 2001 report. Vitamin A and D requirements were increased for lactating cows, whereas vitamin E was increased in the close-up dry cow compared to NRC 2001.

Substantial changes were made in addressing nutrient requirements of the nonlactating pregnant cow.

Energy and protein requirements were based on a continuous growth model of the conceptus (uterus, placenta, and fetus; see graph of uterine growth in pregnancy). This approach more accurately describes the underlying gestational biology; however, it makes formulating a diet to match the changing requirements difficult. Unfortunately, insufficient data remain to include energy and protein required for mammary growth and colostrogenesis.

The model to predict DMI for the dry cow now accounts for dietary NDF content and its impact on decreasing intake capacity (see graph of predicted dry matter intake for dry cows). 

Uterine growth during pregnancy and associated energy and protein re...

Image

Courtesy of Dr. Robert Van Saun.

Predicted dry matter intake for dry cows based on dietary neutral de...

Image

Courtesy of Dr. Robert Van Saun.

Another improvement can be seen in revised models for the growing calf and heifer. Growth modeling was based on energy-allowable growth, which then directed protein requirements above maintenance. In contrast to the previous NRC 2001 report, prediction of retained energy is central to the new calf growth model in NASEM 2021. Studies documenting protein and fat composition of tissue deposited during calf growth provided additional data used to refine growth requirements. Calf mineral and vitamin requirements were provided on a factorial approach rather than a dietary composition. 

• National Academies of Science, Engineering, and Medicine (NASEM). Nutrient Requirements of Dairy Cattle. 8th rev ed. National Academies Press; 2021.

• Van Saun RJ, ed. In: Dairy nutrition. Vet Clin North Am Food Anim Pract. 2014;30(3):487-804.

• McNamara JP, Lucy MC, eds. In: Volume 100 special issue. J Dairy Sci. 2017;100(12):9892-10446.

• Swecker WS Jr, Van Saun RJ, eds. In: Ruminant trace mineral nutrition. Vet Clin North Am Food Anim Pract. 2023;39(3):385-558.

• Nolan J and Primary Industries Standing Committee. Nutrient Requirements of Domesticated Ruminants. CSIRO Publishing; 2007.

• Pond WG, Church DB, Pond KR, Schoknecht PA. Basic Animal Nutrition and Feeding. John Wiley & Sons; 2004.

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