Its negative intercept is not biologically logical. The main fallacy is that it assumes a constant efficiency at all TDN concentrations. This is misleading because it suggests that both intake of TDN and concentration of TDN yield change in a similar direction. The equation underpredicts BCP production with low-TDN intakes commonly fed to beef cows and stocker calves. TDN intakes can be low either because body weight is low (young cattle) or because TDN concentration in the diet is low. Low-TDN diets might reduce passage rate and microbial efficiency; conversely, a lower intake of a higher TDN diet might give maximum microbial efficiency. The average BCP value for the data set from which Eq. 2–2 (>40 percent forage) was developed (National Research Council, 1985) is BCP=12.8 of TDN intake. This should not be interpreted, however, as a constant.
The value 13 g BCP/100 g TDN for BCP synthesis is a good generalization but it does not fit all situations. At both high- and low-ration digestibilities, efficiency may be lower but for different reasons. Logically, the higher digestibility diets are based primarily on grain. High-grain finishing diets have lower rumen pH values and slower microbial turnover, which leads to lower efficiency for converting fermented protein and energy to BCP.
Eq. 2–1 (<40 percent forage; National Research Council, 1985) predicts about 8 percent BCP as a percentage of TDN on a 10 percent roughage diet. Spicer et al. (1986) found a somewhat higher value (10.8 percent of digestible organic matter). These researchers used the lysine to leucine ratio as the bacterial marker; purines were used as the marker by the Subcommittee on Nitrogen Usage in Ruminants (National Research Council, 1985). Russell et al. (1992) proposed that microbial yield is reduced 2.2 percent for every 1 percent decrease in forage effective neutral detergent fiber (eNDF) below 20 percent NDF. This gives values similar to those proposed in Ruminant Nitrogen Usage (National Research Council, 1985).
The synthesis of BCP is also likely to be lower on low-quality forage diets. With slow rates of passage, more digested energy is used for microbial maintenance—including cell lysis (Russell and Wallace, 1988; Russell et al., 1992). Therefore, the efficiency of synthesis of BCP from digestible energy is reduced. To summarize previous reports (Stokes et al., 1988; Krysl et al., 1989; Hannah et al., 1991; Lintzenick et al., 1993; Villalobos, 1993), BCP averaged 7.82 percent of total tract digestible organic matter; the range was 5 to 11.4 percent. The range of total tract organic matter digestibilities was 49.8 to 64.7 percent, and BCP synthesis efficiency was not related to digestibility differences. Intake levels may have been sufficiently low to influence rate of passage and microbial efficiency. The difficulty in obtaining absolute results (Agricultural and Food and Research Council, 1992) makes it difficult to estimate BCP synthesis efficiency in low-quality diets. Most of the beef cows in the world are fed such diets during mid-gestation, so it is important to have more accurate estimates. Russell et al. (1992) predicted an efficiency of 11 percent of TDN for diets containing 50 percent TDN.
A review of the international literature (Agricultural and Food and Research Council, 1992) reveals that BCP synthesis was 12.6 to 17 g/100 g TDN. Some of the differences are compensated for by predicted differences in bacterial true protein (BTP) content and in intestinal digestibility of BTP. Because developers of many of the systems have based their systems on the summarized literature, many of the systems have a similar data base; consequently, values do not vary much from Burroughs et al. (1974) value of 13.05 percent of TDN. Therefore to simplify the NRC (1985) system, 13 percent of TDN was used here for diets containing more than 40 percent forage. For diets containing less than 40 percent forage, the equation of Russell et al. (1992) is used—2.2 percent reduction in BCP synthesis for every 1 percent decrease in forage eNDF less than 20 percent NDF. This provides consistency between model levels 1 and 2.
Currently there are no generalized empirical equations to predict BCP synthesis efficiency at low passage rates. Level 1 of the model with this publication assumes 0.13 efficiency on all forage diets; however, the user is able to reduce that efficiency value in the model. The data reviewed suggests that this value is as low as 0.08 with intakes of low TDN (50 to 60 percent) diets at 1.9 to 2.1 percent of BW. Low values may also be expected with low (limited) intakes of higher energy diets. Level 2 of the model estimates lower synthesis of BCP because of the low predicted rates of passage.
The consequence of using 0.13 BCP synthesis efficiency in level 1 and in the tables is that the BCP supply may be overestimated. Subsequently, DIP requirement would be overestimated and the UIP requirement would be underestimated. This would have little impact on the CP requirement.
Many factors affect efficiency of BCP synthesis (National Research Council, 1985; Russell et al., 1992). Compared to ammonia, ruminal amino acids and peptides may increase the rate and amount of BCP synthesized. In most cases, natural diets contain sufficient DIP to meet microbial needs for amino acids, peptides, or branched-chain amino acids. Deficiencies have not been reported in practical feeding situations. Type of carbohydrate (structural vs nonstructural) may also affect microbial maintenance requirements because of differences in rates of fermentation (microbial growth rate) and rates of passage and because of effects on rumen pH. Level of intake as it changes rate of passage and pH is important. Lipids provide little if