Chapter 2 is a review of protein digestion and metabolism and presents the basis for considering metabolizable protein (MP), or amino acids absorbed from the gastrointestinal tract, and the utilization of MP in setting protein requirements for beef cattle. There is a discussion of factors affecting microbial protein synthesis, which includes consideration of needs for energy and degradable protein. A value for the maintenance requirement for protein, based on metabolic and endogenous losses of nitrogen, is proposed as well as an equation to estimate conversion of MP to net protein. Chapter 2 concludes with a section on validation of recommendations for protein requirements of both growing-finishing and breeding cattle.
A discussion of cattle size and body composition with reference to energy and protein begins Chapter 3. This discussion provides the basis for using mature size as a reference point to unify description of nutrient requirements across animals of different mature size and as affected by liveweight, age, and physiological state. To use a system of nutrient requirements based on a constant body fat composition, it is necessary to understand factors that affect rate of growth such as use of anabolic implants or ionophores, and a discussion of these is included. Chapter 3 also includes a discussion of compensatory growth and validation of the energy and protein requirement system. In addition, Chapter 3 considers predicting target weight gains for replacement heifers and discusses variables that affect nutrient requirements of breeding females. Chapter 3 concludes by describing a mechanism to predict energy reserves of beef cows through the use of body condition score, body weight, and body composition. It provides a relationship between condition score and percent fat in the body.
Unique considerations in setting nutrient requirements for breeding animals are considered in Chapter 4. It includes a discussion of factors affecting calf birth weight, energy, and protein requirements for gestation, and nutrient metabolism by the gravid uterus and placenta. In the discussion of lactation requirements, Chapter 4 reviews the literature on determining milk yield in beef cows and energy requirements for milk production. This chapter also describes factors affecting heifer development and breeding performance of mature cows and bulls.
Macromineral and micromineral requirements are presented in Chapter 5. Where possible, discussion of each mineral includes the role of the mineral in physiological processes of cattle, the bases for setting requirements of these nutrients, and relevant aspects about digestion, absorption, and metabolism. Signs of deficiency, factors affecting requirements, and toxicity and maximum tolerable concentrations in diets are discussed also. A table summarizing recommended dietary concentrations and maximal tolerable concentrations of some minerals is included in this chapter and differentiates recommendations according to physiological function. Sufficient information exists to specify higher levels of magnesium, potassium, sodium (salt), and manganese in diets for breeding cattle, particularly lactating animals, compared to growing and finishing cattle. Calcium and phosphorus requirements, as in the previous edition, are presented in equation format (in Chapter 7) to calculate recommended daily intakes for a comprehensive description of cattle types and management circumstances. Calcium requirements are similar to those established in the previous edition of this publication, but phosphorus requirements have been modified slightly from the previous edition and are discussed in the context of some recent studies on these minerals.
Maximum tolerable concentrations of other minerals have been listed in Chapter 5. In the case of chromium, molybdenum, and nickel, evidence that these minerals are essential to cattle has been presented, but there are insufficient data on which to base dietary requirements.
Requirements for vitamins and water have been considered in Chapter 6. Besides the fat-soluble vitamins, for which the evidence to support a required concentration in the diet is very strong, the literature on the water-soluble B vitamins is reviewed to document where supplementation of diets for beef cattle may be beneficial. A discussion of water requirements of beef cattle includes a table detailing these requirements as affected by ambient temperature and physiological function and liveweight.
Factors affecting feed intake of beef cattle are reviewed in Chapter 7. This chapter includes a review of how physiological factors affect feed intake. There is a section on prediction of feed intake by beef cattle and this includes validation of equations used for the models of requirements. There is a special section in this chapter to consider intake of all-forage diets.
Chapter 8 provides an overview of the effects of stress on nutrient requirements. Effects on energy, protein, mineral, and vitamin requirements are addressed.
Chapter 9 presents the application of new information to formulate equations and models for nutrient requirements. Tables of requirements generated by the model are provided for growing-finishing steers or heifers, for pregnant replacement heifers, for lactating cows, and for bulls. A step-by-step example of how to predict average daily gain and crude protein requirements is also presented.
Chapter 10 provides all of the equations used in the model plus a thorough description of the data contained in the feed library on the model disk.
Chapter 11 provides tables of nutrient composition of feedstuffs commonly used in beef cattle diets, including estimates of variation of nutrient content and discussion of processing effects.