Data set 3 included ADG predicted, using model level 2, from independent trials with 96 different diets fed to a total of 943 Bos indicus (Nellore breed) steers and bulls in which ME intake and body composition were determined (Lanna et al., 1996). FSBW was determined from final EBW fat content. For the NRC 1984 and the present systems, the r2 was 0.58 and 0.72, and the bias was -20 percent and -2 percent respectively.
These validations indicate that given the accuracies obtained, problems with predicting net energy and protein requirements and SWG are likely to include one of the following:
choosing the wrong FSBW
short-term, transitory effects of previous nutrition, gut fill, or anabolic implants,
variation in NEm requirement,
variation in ME value assigned to the feed because of variations in feed composition and extent of ruminal or intestinal digestion,
variation in NE and NEg derived from the ME because of variation in end products of digestion and their metabolizability, and
variations in gut fill.
In recent studies, abomasal infusion of high-quality sources of amino acids significantly increased nitrogen balance in steers, despite the fact that they were fed diets balanced to optimize ruminal fermentation and to provide protein in excess of NRC requirements (Houseknecht et al., 1992; Robinson et al., 1994). These studies indicate that protein accretion was constrained by quantity and/or proportionality of amino acids absorbed.
Amino acid requirements for tissue growth are a function of the percentage of each amino acid in the net protein accretion and thus depend on the accuracy of prediction of protein retained. Ainslie et al. (1993) summarized various studies that have determined essential amino acid content of tissue protein in selected muscles (Hogan, 1974; Evans and Patterson, 1985), in daily accretion (Early et al., 1990), or in the whole empty body (Williams, 1978; Rohr and Lebzien, 1991; Ainslie et al., 1993). In a sensitivity analysis with model predicted vs first-limiting amino acid allowable gain, the average of the three whole empty body studies gave the least bias (Fox et al., 1995). These average values (average of Williams, 1978; Rohr and Lebzien, 1991; Ainslie et al., 1993) are as follows (g/100 g empty body protein); arginine, 3.3; histidine, 2.5; isoleucine, 2.8; leucine, 6.7; lysine, 6.4; methionine, 2.0; phenylalanine, 3.5; threonine, 3.9; and valine, 4.0. Tryptophan values were not given because of limitations in assay procedures.
A number of recent studies have evaluated tissue amino acid requirements by measuring net flux of essential amino acids across the hind limb of growing steers (Merchen and Titgemeyer, 1992; Byrem et al., 1993; Boisclair et al., 1994; Robinson et al., 1995). The proportionality of individual amino acid uptake did not markedly change when protein accretion was increased by infusing various compounds (bovine somatotropin, cimaterol, or casein). The proportions of the essential amino acids in the net flux in these studies followed the same trends as suggested by the tissue composition values listed above.
The above studies and the data previously cited in this section suggest that both quantity and proportionality of amino acid availability are important to achieve maximum energy allowable ADG. In a first NRC attempt to accomplish this for cattle, the model level 2, as described in Chapter 10, has been provided to allow the user to estimate both quantity and proportion of essential amino acids required by the animal and supplied by the diet. The critical steps involved are the prediction of microbial growth and composition; amount and composition of diet protein escaping ruminal degradation; intestinal digestion and absorption; and net flux of absorbed amino acids into tissue. Because of limitations in the ability to predict each of these components, the estimates of amino acid balances provided should be used only as a guide. The subcommittee has taken this step to provide a structure that is intended to stimulate research that will improve the ability to predict amino acid balances, which should lead to increased efficiency of energy and protein utilization in cattle.
Net daily tissue synthesis of protein represents a balance between synthesis and degradation (Oltjen et al., 1986; Early et al., 1990; Lobley, 1992). Lobley (1992) indicated that a 500-kg steer with a net daily protein accretion of 150 g actually degrades and resynthesizes at least another 2,550 g. Thus, balancing for daily net accretion accounts for only about 5.5 percent of the total daily protein synthesis. Protein metabolism is very dynamic, and a kinetic approach is needed to accurately predict amino acid requirements. Small changes in either the rate of synthesis or degradation can cause great alterations in the rate of gain, and the relative maintenance requirement changes with level of production. Lobley (1992), however, concluded that the precision of kinetic methods is critical; a 2 percent change in synthesis rate would alter net protein accretion 20 to 40 percent, and many of the procedures are not accurate within 4 to 5 percent. When combined with a system that has limitations in predicting absorbed amino acids from microbial and feed sources, errors could be greatly magnified with an inadequate mechanistic metabolism model. Given present knowledge, the subcommittee decided that protein and amino acids required for growth should be based on net daily accretion values that have been actually measured. Maintenance requirements for protein have been measured with metabolism trials (Institut National