Summary1
Since the publication of the first booklet on animal nutrient requirements in 1945 (the dairy section consisted of 21 pages), the National Research Council’s series on the Nutrient Requirements of Dairy Cattle has become an essential tool for students, teachers, researchers, and the dairy industry. The seventh revised edition was more than 380 pages. The information, equations, and, more recently, the software included in these publications have helped the dairy industry improve the efficiency of nutrient utilization, reduce the environmental impact of milk production, and improve the health of dairy cows. The updates to this publication generally reflect incremental improvements that match the pace at which new data are generated. Occasionally, some larger-scale changes are made when versions are updated, but the data needed to make these larger changes may have required numerous years to accumulate.
The overarching task for the committee (see Appendix A for the complete Statement of Task) was to develop “a comprehensive analysis of recent research on the feeding and nutrition of dairy cattle including . . . amounts of amino acids, lipids, fiber, minerals, vitamin and water needed by . . . dairy cattle.” The committee’s goal was to develop a report that would improve the accuracy of predicting animal performance from nutrient inputs and to consider variables that affect requirements. Other specific charges to the committee included developing accurate feed composition tables, reviewing the literature on the value of coproducts of biofuel production for dairy cattle, improving methods to estimate energy values of feeds, and reviewing and proposing strategies to minimize nutrient losses in manure and reduce greenhouse gas production. Last, the committee was charged with updating and expanding the previous computer model.
The committee responsible for producing the eighth revised edition of the Nutrient Requirements of Dairy Cattle committed substantial time and effort to reviewing scientific literature; collating, screening, and analyzing data; and finally synthesizing this report and accompanying software. The seventh revised edition served as the starting point, but the committee revised all chapters and added some new sections. Per its charge, the committee evaluated all of the equations and updated or completely replaced them when appropriate. This summary is only an outline of the major changes and updates. To appreciate the full scope of the changes, see the specific chapters.
In Chapter 1, for the first time in the Dairy Requirement series, the meaning of requirement as used in this publication is defined. The term Adequate Intake (AI) is introduced, which is used when adequate data were not available to define an actual requirement. The importance of variation in feed and diet nutrient composition and in animal responses is discussed.
Chapter 2 discusses feed intake regulation in dairy cattle at the physiological and biochemical level. Dietary and management factors that influence intake are also discussed. This chapter contains revised equations to estimate intake of growing heifers and lactating cows. A change from the previous version is that some equations contain not only animal factors, such as body weight and milk production, but also dietary factors. The software provides intake estimates using animal only or animal plus dietary factors.
Energy supplies and requirements are discussed in Chapter 3. The net energy system was retained, but the method to estimate dietary concentrations of net energy was modified, with the greatest change from the previous edition being the discount factor related to level of feed intake. Digestible energy is calculated on a feed basis, but metabolizable and net energy are only calculated for the complete diet. Energy requirements for maintenance were increased by about 20 percent, and smaller refinements were made to energy requirements for pregnancy, growth, and lactation.
Chapter 4 discusses fat metabolism with an emphasis on quantifying factors that affect digestibility of fatty acids. Digestibility coefficients for feeds and common fat supplements were rederived based on newer data. The chapter also includes
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1 Minor factual corrections were made subsequent to the prepublication version of this report on pp. 41, 64, 74, 87, 257, and 434. The changes were made to provide additional clarity on equations and assumptions relative to the model used in this report.
a review of recent literature on production and reproductive responses to fat supplementation. Last, dietary factors affecting milk fat concentration and milk fatty acid profile are discussed along with potential human health responses.
Major dietary carbohydrates (predominantly starch, simple sugars, and neutral detergent fiber) are discussed in Chapter 5. Because carbohydrates are the predominant energy source for dairy cattle, factors affecting both ruminal and total tract digestibility are discussed in detail. Recommendations for total dietary neutral detergent fiber (NDF) and forage NDF concentrations have been updated and are based on dietary starch concentration. A new physically adjusted NDF (paNDF) system is introduced to incorporate diet particle size into fiber recommendations.
Chapter 6 discusses protein and amino acid metabolism, responses, and requirements. This chapter arguably represents the greatest changes from the previous version. The equations to generate microbial protein were revised extensively, and essentially all estimates of rumen-degradable and digestible rumen undegradable protein were updated. Similar to the previous edition, protein requirements were factorialized, but in this revision, requirements for essential amino acids are included. Requirements are presented for net essential amino acids and net protein, which can be converted to metabolizable amino acids and protein using variable efficiencies. Rather than a requirement for milk protein synthesis, response functions were developed that estimate protein yields based on intakes of specific amino acids and energy.
Mineral requirements are in Chapter 7. For most minerals, requirements are expressed on an absorbed mineral basis, and both absorption coefficients and requirements were reevaluated for all minerals. For the first time, absorption coefficients for some minerals (magnesium, phosphorus, and copper) are adjusted based on user inputs rather than using ingredient-specific constants. Generally, most mineral requirements were refined rather than undergoing substantial changes. For some trace minerals, AI rather than requirements is used.
Vitamins are discussed in Chapter 8. AIs are presented only for vitamins A, D, and E, but other vitamins are discussed in detail. The AI for vitamin D is greater than the requirement presented in the previous version, and the AI for vitamin A includes an adjustment for milk yield.
Chapter 9 consists of a discussion of factors affecting water intake, quality guidelines for water, and effects of water composition (e.g., salinity, concentrations of other minerals, and nitrate) on water intake and cow production and health. The chapter includes an extensive review of available equations that can be used to predict water intake, and the software now includes estimated water intake as an output.
Calf nutrition and management are discussed in Chapter 10. Calves are generally defined as animals less than 18 percent of mature body weight or about 125 kg for a Holstein calf. Animals larger than that are considered heifers and are discussed in Chapter 11. The nutrient requirement system for calves was completely overhauled. Equations for requirements for metabolizable protein, metabolizable energy, and minerals were derived from newer data and more accurately predict animal response than those in the previous version. An equation to estimate starter intake was also derived. Maintenance requirements are now adjusted for environmental temperatures outside the thermoneutral zone of the calf. Equations to estimate the metabolizable energy value of feeds when fed to young calves are included.
Chapter 11 is dedicated to a discussion of growing replacement heifers, which are defined as animals with a body weight >18 percent of mature weight (approximately 125 kg) until either 60 or 21 days prepartum depending on user inputs. Structural errors in equations to predict body composition were eliminated, and the overall equations were simplified. A much larger database was available to develop body composition equations than was available to the previous committee. Hence, estimates of changes in body composition (fat and protein) will be more accurate, resulting in more accurate estimates of energy and protein requirements for growth.
Chapter 12 discusses physiological and metabolic changes that occur as cows or heifers transition from the gestating state to the lactating state. Nutrient requirements unique to this class of animals are in this chapter along with equations to estimate intake during the dry and late-gestation period. The equations to estimate intake have been improved compared to the last version and incorporate some dietary factors in the prediction. The chapter includes an up-to-date review of health disorders that occur in the peripartum period and nutritional and management practices that can be used to reduce the prevalence of those disorders. The impact of colostrum synthesis in late gestation on nutrient needs is discussed but not incorporated into the model.
Various dairy production systems are discussed in Chapter 13, including organic milk production, grazing systems, and robotic (or automatic) milking systems. The emphasis in this chapter is on how the various systems may influence nutrient requirement or nutrient supply differently from what is discussed in previous chapters. For example, organically managed dairy cows do not have different requirements than other cows, but because of economics and available ingredients, supply of nutrients may differ. Energy requirements are greater for grazing cows than cows housed in confinement (assuming equal body weight and milk production) because of the energy expended walking and gathering food (pasture). These equations were changed from the previous version because of a larger database, and the equations should be more accurate. Best nutritional management practices for automatic milking systems are discussed, as is the effect of genetically modified feeds on milk production and composition.
The direct environmental impacts of milk production and mitigation strategies are discussed in Chapter 14. Equa-
tions to estimate methane production (also used to estimate metabolizable energy) for both nonlactating and lactating animals are included in the text and the computer model. Equations to estimate nitrogen and phosphorus excretion in manure are included (excretion of many other minerals is estimated based on estimated mass balance). Equations do not include functions related to mitigation methods, but the chapter contains information on current best practices to reduce the environmental impact of dairy farming.
Material in Chapters 15, 16, and 17 is not directly used in the model; however, these chapters are up-to-date reviews on important topics relevant to the nutrition and management of dairy cattle. Production methods, nutrient composition, and animal responses to important by-product feed ingredients are in Chapter 15. Numerous by-products are discussed, but feed ingredients derived from biofuel production are emphasized, including various types of distillers grains. Chapter 16 discusses commonly used feed additives that are allowed to be fed to dairy cattle in the United States (as of 2020). Some additives such as monensin, essential oils, and direct fed microbial can affect digestibility and metabolic efficiency in some situations, but these effects were not included in the model. Users can use the information in that chapter to manually adjust output if desired. Chapter 17 discusses naturally occurring compounds found in some feedstuffs that can have adverse effects on dairy cattle. Examples include alkaloids (e.g., fescue toxicity), mycotoxins, and gossypol. Toxicities derived from consumption of feeds contaminated with certain bacteria such as Clostridium botulinum and Escherichia coli O157:H7 are also discussed. Effects of anthropogenic agents and excess mineral intake are not included.
Chapter 18 is a new addition. It includes information on assays needed for inputs in the model. The chapter includes appropriate citations with detailed descriptions of recommended assays along with potential sources of error. It also includes chemically based assays (e.g., crude protein), mechanical-based assays (e.g., particle size to calculate physically adjusted neutral detergent fiber), and biologically based assays (e.g., in situ protein digestibility kinetics).
The feed composition data in Chapter 19 is completely new and was generated specifically for this publication. Data were provided by four major commercial laboratories (see Acknowledgments) and then underwent a rigorous screening method to identify mislabeled feeds. Many of the mean nutrient concentrations for individual feed ingredients in Chapter 19 will be similar to other feed composition tables reported in previous revisions. However, the standard deviations may differ greatly from other tables. Because of the rigorous screening that was applied to these data, the standard deviations found in this chapter are likely more accurate representations of the true variation in nutrient composition.
Chapter 20 lists all of the equations in the model used to calculate nutrient supplies, requirements, and responses. For some equations, additional text is included describing how the equations were derived. However, for most equations, that information can be found in the specific nutrient chapters. Model evaluation is also discussed in this chapter.
Last, Chapter 21 includes tables of recommended nutrient supplies and dietary concentrations for different classes of dairy cattle. Because numerous factors affect requirements and nutrient supply, the data in those tables represent very specific situations. Nutrient supplies are calculated assuming “typical” diets with model-estimated dry matter intake. Requirements are for very specific animal conditions (e.g., milk yields, body weight, growth rates). The software, with user-specific dietary and animal inputs, will yield more accurate assessments of nutrient supply and requirements than the tables.
