NEm and NEg can only be changed by adjusting diet TDN because the relationship between these energy values must be kept consistent. Diet TDN is used to predict microbial growth, which must be consistent with the energy value used to predict NEm available to meet maintenance, pregnancy, and lactation requirements and the energy value used to predict NEg allowable ADG. To get the diet NE value desired, the user adjusts TDN until the desired NE value is predicted. The subcommittee recognizes that the relationship between TDN, ME, NEm, and NEg may vary because of differences in amount of intake, rates of digestion and passage, and end products of digestion in the ME and their metabolizability. However, the relationships between them, as described in the preceding edition of this volume published in 1984, have also been used here for the reasons discussed in Chapters 1 and 10.
The concentration of nutrients needed for a given level of production depends on the actual DMI of the diet being fed to support the observed level of performance in a particular production setting. The DMI predictions are from equations developed from experimental feeding period averages as reported in published feeding trials involving wide variations in cattle type and stage of growth, as discussed in Chapter 7. Thus, predicted and observed values often differ in a specific production setting. Cattle fed feedlot finishing rations will typically consume 0 to 25 percent more early in the feeding period than predicted by these equations, which is compensated for by DMI of 0 to 25 percent less late in the feeding period. Further, as discussed in Chapter 7, concerning feed intake, most DMI prediction equations account for only 50 to 60 percent of the variation, leaving 40 to 50 percent to be accounted for by variations in local conditions such as feeding management, cattle type, and environment. The DMI adjusters allow the user to change the predicted DMI until it agrees with observed DMI; then the NE adjuster can be changed until predicted and observed performance agree.
Many factors can influence the NE derived from a diet for production, including variation in maintenance requirements, rates of digestion and passage, and metabolizability. If only DMI is adjusted, predicted and observed performance may not agree. For example, unrealistically high rates and efficiencies of gain may be predicted for calves consuming high-energy rations. Conversely, when these animals approach choice grade at the end of the finishing period, unrealistically low ADG may be predicted if only DMI is adjusted. Given these problems of prediction early and late in growth, limits were set on the weight ranges in the diet density tables at 55 percent of finished weight for the lightest weight and 80 percent of finished weight for the heaviest weight.
The primary use of these tables is intended to be for teaching the interactions of body size, stage of growth, diet energy density, and energy and protein requirements. The diet densities for CP and DIP may not be practical because the CP may have to be overfed to meet both DIP and UIP requirements. The user is encouraged to use the model with actual feed ingredients available for computing requirements for specific conditions. Despite their limitations as discussed in this section, simple guideline tables with diet nutrient concentration requirements for different classes of cattle are all that are needed in many situations and are provided at the end of the User’s Guide.
Tables 9–1 and 9–2 show daily requirements (Table 9–1) and diet evaluations (Table 9–2) for growing and finishing cattle. Inputs for Table 9–1 are for a 533-kg finished weight at 28 percent fat, a weight range of 200 to 450 kg, an ADG range of 0.50 to 2.50 kg, and breed code 1. Table 9–1 shows NEm, NEg, MP, Ca, and P required daily for maintenance and gain at six shrunk body weights, which represent six different stages of growth. All these requirements can be used directly to formulate dietary requirements for the specified level of performance, except the diet CP, DIP, and UIP required to meet the MP requirement. The CP intake needed can be estimated by dividing the total MP requirement in this table by 0.67, which is based on 80 percent of the MP from MCP and 20 percent from UIP. This approach was used in developing the guideline tables at the end of the User’s Guide. However, this assumes that the nitrogen difference between the diet CP and MP requirement will meet microbial requirements for DIP and tissue requirements for UIP. This approach, which was used in the preceding edition of this volume to compute CP requirements, has major limitations. For this edition, the dietary CP intake needed is computed in the model level 1 as a sum of the DIP needed for microbial growth plus the UIP needed above the MP required for maintenance plus gain not met by microbial protein. These variables are not directly accounted for when the CP required is determined as MP/0.67.
Table 9–2 shows the evaluation of five diets (rations A through E) with the diet evaluator for the same animal used in Table 9–1 between 55 and 80 percent of final weight. The diet concentration of eNDF, TDN, and CP and DIP as a percentage of CP were entered for each of the five diets, and all DMI and NE adjusters were set at 100 percent. The eNDF values are used to adjust microbial protein yield and are affected only when diet eNDF drops below 20 percent of diet DM. The feed eNDF values in Appendix Table 1 (the feed library) can be used to determine eNDF in the diet. The program first computed diet NEm and NEg values, DMI, energy allowable ADG, MP, Ca, and P required for that ADG, MCP from the TDN