Commonwealth Scientific and Industrial Research Organization, 1990; Agricultural and Food Research Council, 1993) were inability to use inputs available in a specific production setting in North America to mechanistically predict feed net energy values and supply of amino acids.

Level 1 should be used when limited information on feed composition is available and the user is not familiar with how to use, interpret and apply the inputs and results from level 2. Potential uses of level 2 are (Fox et al., 1995):

  • as a teaching tool to improve skills in evaluating the interactions of feed composition, feeding management and animal requirements in varying farm conditions;

  • to develop tables of feed net energy and metabolizable protein values and adjustment factors that can extend and refine the use of conventional diet formulation programs;

  • as a structure to estimate feed utilization for which no values have been determined and on which to design experiments to quantify those values;

  • to predict requirements and balances for nutrients for which more detailed systems of accounting are needed, such as peptides, total rumen nitrogen, and amino acid balances;

  • as a tool for extending research results to varying farm conditions; and

  • as a diagnostic tool to evaluate feeding programs and to account for more of the variation in performance in a specific production setting.

The equations for each level are presented in “pseudo code” form for convenience of programming them into any language. The data on which the equations are based are discussed in the appropriate section of the text.

In this revision, much more emphasis is placed on predicting the supply of nutrients, because animal requirements and diet are interactive, including calculating feed digestibility under specific conditions, heat increment to compute lower critical temperature, calculation of efficiency of ME use for maintenance, growth and lactation, and adjusting microbial protein production for diet effective NDF content. Therefore, accuracy of prediction of nutrient requirements and performance under specific conditions depends on accuracy of description of feedstuff composition and DMI.

In developing more mechanistic models for determining the nutrient requirements of beef cattle, the subcommittee considered recent models that describe some of all aspects of postabsorptive metabolism (Oltjen et al., 1986; France et al., 1987). The France model is mechanistic in its approach to metabolism but has received no, or limited, validation with field data. The Oltjen model was considered by the subcommittee and compared with predictions of the proposed models with respect to growth (see Chapter 3). For further presentation on alternative techniques to modeling responses to nutrients in farm animals, the reader is referred to the report of the Agricultural and Food Research Council (AFRC) Technical Subcommittee on Responses to Nutrients (Agricultural and Food Research Council, 1991).

REQUIREMENTS FOR BOTH LEVELS

The requirement section is subdivided into four main sections: maintenance, growth, lactation and pregnancy.

Maintenance

Maintenance requirements are computed by adjusting the base NEm requirement for breed, physiological state, activity and heat loss vs. heat production, which is computed as ME intake—retained energy. Heat loss is affected by animal insulation factors and environmental conditions.

ENERGY

Adjustment for previous temperature:

Adjustment for breed, lactation and previous plane of nutrition:

Adjustment for activity:

If on pasture:

otherwise

for growing cattle (used to compute heat increment):

for lactating cattle (used to compute heat increment):

adjustment for cold stress:



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