2
Protein

The previous edition of Nutrient Requirements of Beef Cattle (National Research Council, 1984) expressed protein requirements in terms of crude protein (CP). In 1985, the Subcommittee on Nitrogen Usage in Ruminants (National Research Council, 1985) presented an excellent rationale for expressing protein requirements in terms of absorbed protein, a rationale adopted in 1989 by the Subcommittee on Dairy Cattle Nutrition (National Research Council, 1989). Since then absorbed protein (AP) has become synonymous with metabolizable protein (MP), a system that accounts for rumen degradation of protein and separates requirements into the needs of microorganisms and the needs of the animal. MP is defined as the true protein absorbed by the intestine, supplied by microbial protein and undegraded intake protein (UIP).

There are basically two reasons for using the MP system rather than the CP system. The first is that there is more useable information about the two components of the MP system—bacterial (microbial) crude protein (BCP) synthesis and UIP, which allows more accurate prediction of BCP and UIP than was possible in 1984. The second reason is that the CP system is based on an invalid assumption—that all feedstuffs have an equal extent of protein degradation in the rumen, with CP being converted to MP with equal efficiency in all diets. The change from the CP system to the MP system was adopted in the Nutrient Requirements of Dairy Cattle (National Research Council, 1989) and by the Agricultural and Food Research Council (1992). Crude protein can be calculated from the sum of UIP and degraded intake protein (DIP), both of which are determined in both levels of the model. The table generator presents MP requirements in amounts required per day and checks diet adequacy when crude and degradable protein levels are entered. In addition to this, estimates of daily crude protein requirements can be obtained by dividing MP amounts by a value between 0.64 and 0.80, depending on degradability of protein in the feed. The coefficients of 0.64 and 0.80 apply when all of the protein is degradable and undegradable, respectively.

Protein requirements are best determined using model levels 1 or 2. Model level 1 uses UIP and DIP values of feeds from the feed library. Level 2 is mechanistic and uses rates of protein degradation of various protein fractions to estimate DIP and UIP. BCP synthesis is estimated from rates of digestion of various carbohydrate fractions. In both cases, rates of passage are also used. Level 2 also includes supply and requirements for amino acids.

MICROBIAL PROTEIN SYNTHESIS

Bacterial crude protein (BCP) can supply from 50 percent (National Research Council, 1985; Spicer et al., 1986) to essentially all the MP required by beef cattle, depending on the UIP content of the diet. Clearly, efficiency of synthesis of BCP is critical to meeting the protein requirements of beef cattle economically; therefore, prediction of BCP synthesis is an important component of the MP system. Burroughs et al. (1974) proposed that BCP synthesis averaged 13.05 percent of total digestible nutrients (TDN). In Ruminant Nitrogen Usage (National Research Council, 1985), two equations were developed to predict BCP synthesis—one for diets containing more than 40 percent forage and one for diets containing less than 40 percent forage. Both equations are more complex than that of Burroughs et al. (1974). Both forage and concentrate intakes (percent of body weight) are needed to calculate the less than 40 percent forage equation

Eq. 2–1

The more than 40 percent forage equation was developed primarily for dairy cattle:



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Nutrient Requirements of Beef Cattle: Seventh Revised Edition, 1996 2 Protein The previous edition of Nutrient Requirements of Beef Cattle (National Research Council, 1984) expressed protein requirements in terms of crude protein (CP). In 1985, the Subcommittee on Nitrogen Usage in Ruminants (National Research Council, 1985) presented an excellent rationale for expressing protein requirements in terms of absorbed protein, a rationale adopted in 1989 by the Subcommittee on Dairy Cattle Nutrition (National Research Council, 1989). Since then absorbed protein (AP) has become synonymous with metabolizable protein (MP), a system that accounts for rumen degradation of protein and separates requirements into the needs of microorganisms and the needs of the animal. MP is defined as the true protein absorbed by the intestine, supplied by microbial protein and undegraded intake protein (UIP). There are basically two reasons for using the MP system rather than the CP system. The first is that there is more useable information about the two components of the MP system—bacterial (microbial) crude protein (BCP) synthesis and UIP, which allows more accurate prediction of BCP and UIP than was possible in 1984. The second reason is that the CP system is based on an invalid assumption—that all feedstuffs have an equal extent of protein degradation in the rumen, with CP being converted to MP with equal efficiency in all diets. The change from the CP system to the MP system was adopted in the Nutrient Requirements of Dairy Cattle (National Research Council, 1989) and by the Agricultural and Food Research Council (1992). Crude protein can be calculated from the sum of UIP and degraded intake protein (DIP), both of which are determined in both levels of the model. The table generator presents MP requirements in amounts required per day and checks diet adequacy when crude and degradable protein levels are entered. In addition to this, estimates of daily crude protein requirements can be obtained by dividing MP amounts by a value between 0.64 and 0.80, depending on degradability of protein in the feed. The coefficients of 0.64 and 0.80 apply when all of the protein is degradable and undegradable, respectively. Protein requirements are best determined using model levels 1 or 2. Model level 1 uses UIP and DIP values of feeds from the feed library. Level 2 is mechanistic and uses rates of protein degradation of various protein fractions to estimate DIP and UIP. BCP synthesis is estimated from rates of digestion of various carbohydrate fractions. In both cases, rates of passage are also used. Level 2 also includes supply and requirements for amino acids. MICROBIAL PROTEIN SYNTHESIS Bacterial crude protein (BCP) can supply from 50 percent (National Research Council, 1985; Spicer et al., 1986) to essentially all the MP required by beef cattle, depending on the UIP content of the diet. Clearly, efficiency of synthesis of BCP is critical to meeting the protein requirements of beef cattle economically; therefore, prediction of BCP synthesis is an important component of the MP system. Burroughs et al. (1974) proposed that BCP synthesis averaged 13.05 percent of total digestible nutrients (TDN). In Ruminant Nitrogen Usage (National Research Council, 1985), two equations were developed to predict BCP synthesis—one for diets containing more than 40 percent forage and one for diets containing less than 40 percent forage. Both equations are more complex than that of Burroughs et al. (1974). Both forage and concentrate intakes (percent of body weight) are needed to calculate the less than 40 percent forage equation Eq. 2–1 The more than 40 percent forage equation was developed primarily for dairy cattle:

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Nutrient Requirements of Beef Cattle: Seventh Revised Edition, 1996 Eq. 2–2 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

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Nutrient Requirements of Beef Cattle: Seventh Revised Edition, 1996 any energy for ruminal microorganisms, and the energy obtained from protein fermentation is minimal (Nocek and Russell, 1988). Further, ensiled (fermented) forages may provide less energy for microorganisms than comparable fresh or dry feeds (Agricultural and Food Research Council, 1992), but this has not been documented for silages and high-moisture grains in the United States. Carbohydrate digestion in the rumen is likely the most accurate predictor of BCP synthesis, and this mechanism is used in model level 2. However, for feedstuffs used for beef cattle few good data are available for rates of digestion and of passage of the different carbohydrates potentially digested in the rumen. More accurate values are available for TDN, and laboratory predictors of TDN can be used to estimate BCP synthesis. Therefore TDN is used as the indicator of energy availability in the rumen for level 1. The Agricultural and Food Research Council (1992) found that total tract digestible organic matter intake was the most precise indicator of BCP synthesis when nitrogen intake was adequate. Digestible organic matter and TDN are roughly equivalent in feedstuffs and diets. The requirement for rumen degradable protein (including nonprotein nitrogen [NPN]) is considered equal to BCP synthesis. This assumes that the loss of ammonia from the rumen as a result of flushing to the duodenum and absorption through the rumen wall is equal to the amount of recycled nitrogen. A number of factors affect each of these fluxes of nitrogen (National Research Council, 1985) but rather complex modeling is needed (Russell et al., 1992) to account for them. Simply put, a deficiency of ruminal ammonia encourages recycling and an excess encourages absorption from the rumen. Therefore, a balance (rumen degradable protein in diet equal to BCP synthesis) minimizes both recycling and absorption. Few studies have attempted to titrate the need for rumen degradable protein. Karges (1990) found 10.9 percent of TDN as rumen degradable protein was needed to maximize gain in beef cows, presumably to maximize BCP synthesis; Hollingsworth-Jenkins (1994) found only 7.1 percent DIP was needed to maximize gain. These values are smaller than the value of 13 percent used in this publication to calculate BCP synthesis. Optimum use of rumen degraded protein (including nonprotein nitrogen) would logically occur if protein and carbohydrate degradation in the rumen were occurring simultaneously. This is not the case in many diets. Protein degradation of many of the forages, for example, is rapid and degradation of energy-yielding components of NDF is much slower. With grains (for example, corn and sorghum) the opposite is true—slow protein degradation and rapid starch degradation. This results in low ruminal ammonia levels from high-grain diets postfeeding and high levels from forage diets, which is influenced by CP levels. The ruminant compensates by recycling nitrogen. An excellent example of this is how cow performance is similar with protein supplementation either three times per week or once per day (Beaty et al., 1994). More basic studies with animals (Henning et al., 1993; Rihani et al., 1993) suggest little or no advantage to synchrony of energy availability and protein breakdown. Cattle also compensate by eating numerous meals per day such as in the feedlot. Use of NPN is appropriate in high-grain diets (National Research Council, 1984, 1985; Sindt et al., 1993) because of the rapid rumen degradation of starch. The value of NPN in low-protein, high-forage diets is less clear (Rush and Totusek, 1975; Clanton, 1979). Reduced gains when using urea as opposed to a “natural” protein may be the result of insufficient UIP rather than the faster rate of ammonia release in the rumen. Until more information is available, it is advisable to use caution when using urea in low-protein, high-forage diets. Russell et al. (1992) have demonstrated the need for amino acids and peptides for optimum BCP synthesis, and this concept is used in model level 2. A lack of amino acids or peptides is unlikely to be a problem in typical diets for beef cattle. Adequate MP in finishing diets can be accomplished by adding urea (Sindt et al., 1993). Fiber-digesting bacteria use primarily ammonia for BCP synthesis (Russell et al., 1992), so amino acids/peptides should not be limiting in the rumen. However, these fiber-digesting bacteria may require branched-chain volatile fatty acids (National Research Council, 1985), which would be supplied by amino acid degradation. A need for rumen degradable protein (other than NPN) might occur in diets containing mixtures of forage and grain such as “step-up” rations for finishing cattle (Sindt et al., 1993). Digestibility of protein is important—for both BCP and UIP. In this publication, the value of 80 percent digestibility of BTP (National Research Council, 1985) is used. UIP digestibility may vary with the source; however, it is assumed that UIP is 80 percent digestible. National Research Council (1985) used 0.8 BCP=BTP because BCP contains approximately 20 percent nucleic acids. This value has been challenged by other MP systems (Agricultural and Food Research Council, 1992). Logically, the important measure is amino acid content (true protein) of BCP. These measures (Agricultural and Food Research Council, 1992) suggest a value of 0.75 rather than 0.8. However, the net absorption of amino acids is the important coefficient. Systems using lower BCP to BTP values used higher (0.85) digestibility values for BTP; therefore, these values compensate. Until more definitive data are available in the United States on digestible amino acid content of rumen bacteria, use of the value of 0.64, calculated as 0.8 BCP=BTP * 0.8 digestibility of BTP is suggested.

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Nutrient Requirements of Beef Cattle: Seventh Revised Edition, 1996 MP REQUIREMENTS NRC requirements (1984, 1985) for MP were based on the factorial method. Factors included were metabolic fecal losses, urinary losses, scurf losses, growth, fetal growth, and milk. Metabolic fecal, urinary, and scurf losses represent the requirement needed for maintenance. It is difficult, however, to measure fecal and urinary losses independent of each other. It also is difficult to separate microbial (complete cells or cell walls) losses in the feces from true metabolic fecal losses. In the preceding edition of this report (National Research Council, 1984) metabolic fecal loss was calculated as a percentage of dry matter intake; in Ruminant Nitrogen Usage (National Research Council, 1985) metabolic fecal loss was calculated as a percentage of indigestible dry matter intake. Diet digestibility obviously affects the resulting calculated metabolic fecal losses. Most beef cows are fed diets containing 45 to 55 percent TDN during gestation. Consequently, for most beef cows, MP and CP requirements, using the calculation based on indigestible dry matter intake (National Research Council, 1985), are unrealistically high. The high requirement can be attributed to the fact that nitrogen is being excreted in the feces as microbial protein rather than as urea in the urine (National Research Council, 1985) as a result of microbial growth in the postruminal digestive tract. The Institute National de la Recherche Agronomique (INRA) (1988), using nitrogen balance studies that included scurf, urinary, and metabolic fecal losses, determined that the maintenance requirement was 3.25 g MP/kg SBW0.75. This system simplifies calculations and is based on metabolic body weight (BW0.75), as are maintenance energy requirements, and is similar to the concept and value proposed by Smuts (1935). Assuming CP * 0.64 (CP converted to BCP: 80 percent true protein * 80 percent digestibility)=MP, Smuts (1935) calculated the requirement to be 3.52 g MP/kg BW0.75. Wilkerson et al. (1993) estimated the maintenance requirement of 253 kg growing calves was 3.8 g MP/kg BW0.75 using growth as the criteria. Their diets were high in roughage and were based on the assumption that 0.13 TDN=BCP. If actual BCP synthesis efficiency was less than 0.13, the estimate of the maintenance would be less than 3.8 g MP/kg BW0.75. In this publication 3.8 g MP/kg BW0.75 is used because the maintenance requirement estimated was based on animal growth rather than on nitrogen balance. However, recent nitrogen balance data reported by Susmel et al. (1993) do support the 3.8 g MP/kg BW0.75 value. CONVERSION OF MP TO NP Studies by Armstrong and Hutton (1975) and Zinn and Owens (1983) reported that the average biological value of absorbed amino acids was reported to be 66 percent (National Research Council, 1984). A constant conversion of MP to net protein (NP) for gain of 0.5 and to NP for milk of 0.65 was assumed (National Research Council, 1985). These efficiency values are based on two components—the biological value of the protein and the efficiency of use of an “ideal mixture of amino acids” (Oldham, 1987). Oldham (1987) suggests that the efficiency value is 0.85 for all physiological functions. Biological values will vary with the source(s) of UIP in the diet. Biological value is defined herein and by Oldham (1987) as the relative amino acid balance. The biological value of microbial protein is quite high and strongly influences the biological value of the MP in many diets. Biological value will vary for different functions (Oldham, 1987)—for example, it is likely that the overall efficiency value for pregnancy and lactation are higher than for gain. Based on data for lactation and pregnancy (National Research Council, 1985), this subcommittee has chosen to use 0.65 (0.85 * 0.76; efficiency * biological value). Efficiency of use for gain is not likely to be constant across body weights (maturity) and rates of gain. The INRA (1988) system assumes a decreasing efficiency as body weight increases. This was confirmed by Ainslie et al. (1993) and Wilkerson et al. (1993). Based on these data, the following equation is used: where EQSBW is equivalent shrunk body weight in kilograms. This is the overall efficiency value (biological value * efficiency of use of ideal protein). This equation was developed by Ainslie et al. (1993) from data presented by INRA (Institut National de la Recherche Agronomique, 1988). The equation predicts a conversion efficiency of MP to NP of 66.3 percent for a 150-kg calf. A 300-kg steer has an efficiency of only 49.2 percent. The data of Ainslie et al. (1993) and Wilkerson et al. (1993) only cover the weight range from 150 to 300 kg. Therefore, these bounds have been placed on the conversion efficiency equation. Thus, for cattle weighing more than 300 kg, this maintains similar protein requirements to previous NRC publications (National Research Council, 1984, 1985) and recognizes the low CP requirements of cattle weighing more than 400 kg (Preston, 1982). Validation Few studies have been conducted that were designed either to validate protein requirement systems or to meet the requirements for validation. Most difficult to interpret are data where energy intake increases with protein supple-

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Nutrient Requirements of Beef Cattle: Seventh Revised Edition, 1996 mentation because one does not know whether the increased gain is the result of increased MP or NEg. Also, it is often difficult to determine whether the effect was caused by DIP or UIP. Karges (1990) maintained equal intakes in gestating cows and supplemented low-quality prairie hay with rumen degraded protein. He obtained a requirement of 608 g CP/day. Hollingsworth-Jenkins (1994) estimated a requirement of 605 g CP/day for gestating cows grazing winter range. The system proposed herein estimates the requirement to be 684 g CP/day. Based on predicted intake, the requirement is 725 g CP/day (National Research Council, 1984); at actual intake, the requirement was calculated to be 658 g CP/day. The calculation of 828 g CP/day (National Research Council, 1985) seems unreasonably high as a result of the high metabolic fecal protein value based on indigestible dry matter intake. Validation data sets were developed for growing-finishing cattle (Wilkerson et al., 1993; Ainslie et al., 1993). Rates of gain varied from 0 to 1.5 kg/day. Diets ranged from 90 percent low-quality roughage to 90 percent concentrate. Generally, the cattle used were young because a deficiency in MP is difficult to demonstrate at heavier weights (Zinn, 1988; Ainslie et al, 1993; Sindt et al., 1993; Zinn and Owens, 1993). The data sets included 70 observations. Prediction model level 1 had an r2 of 0.80 and a bias of +20 percent, and level 2 had an r2 of 0.67 and +18 percent bias. By comparison, gain predicted by ME intake had, in level 1, an r2 of 0.90 and a +19 percent bias; level 2 had an r2 of 0.95 and a bias of +13 percent. Gain limited by the first-limiting amino acid in level 2 had an r2 of 0.74 and a +16 percent bias. Gain limited by the first-limiting nutrient (ME, MP, first-limiting amino acid) gave an r2 of 0.81 and a bias of +12 in level 1 and an r2 of 0.92 with 0 bias in level 2. Validation is more difficult with cattle on high-grain finishing diets. Corn is the most common feed grain in the United States. It contains 8 to 10 percent protein, but approximately 60 percent of the protein escapes ruminal digestion. In diets that are 85 percent corn, this results in 4.0 to 5.3 percent of the diet being UIP. Shain et al. (1994) and Sindt et al. (1994) found that 4.6 percent UIP in addition to the BCP was sufficient to meet the needs of yearling cattle. In addition Shain et al. (1994), Milton and Brandt (1994) estimated the requirement for DIP for yearling cattle by feeding graded amounts of urea. Both found a response to urea that is consistent with the DIP requirement calculated herein (6.8 percent of dry matter). In the work of Shain et al. (1994), the UIP supplied was higher than the requirement (5.3 vs 3.6), and the CP required was 12 percent of dry matter because UIP was overfed. Presumably, the DIP requirement is needed to maximize microbial activity in the rumen because MP was in excess. REFERENCES Agricultural and Food Research Council. 1992. Nutritive requirements of ruminant animals: Protein. Nutr. Abstr. Rev. Ser. B 62:787–835. Ainslie, S.J., D.G.Fox, T.C.Perry, D.J.Ketchen, and M.C.Barry. 1993. Predicting amino acid adequacy of diets fed to Holstein steers. J. Anim. Sci. 71:1312–1319. Armstrong, D.G., and K.Hutton. 1975. Fate of nitrogenous compounds entering the small intestine. P. 432 in Digestion and Metabolism in the Ruminant, I.W.McDonald and A.C.I.Warner, eds. Armidale, NSW, Australia: The University of New England Publishing Unit. Beaty, J.L., R.C.Cochran, B.A.Lintzenick, E.S.Vanzant, J.L.Morrill, R.T.Brandt, Jr., and D.E.Johnson. 1994. Effect of frequency of supplementation and protein concentration in supplements on performance and digestion characteristics of beef cows consuming low quality forages. J. Anim. Sci. 72:2475–2486. Burroughs, W., A.H.Trenkle, and R.L.Vetter. 1974. A system of protein evaluation for cattle and sheep involving metabolizable protein (amino acids) and urea fermentation potential of feedstuffs. Vet. Med. Small Anim. Clin. 69:713–722. Clanton, D.C. 1979. Nonprotein nitrogen in range supplements. J. Anim. Sci. 47:765–779. Hannah, S.M., R.C.Cochran, E.S.Vanzant, and D.L.Harmon. 1991. Influence of protein supplementation on site and extent of digestion, forage intake, and nutrient flow characteristics in steers consuming dormant bluestem-range forage. J. Anim. Sci. 69:2624–2633. Henning, P.H., D.G.Steyn, and H.H.Meissner. 1993. Effect of synchronization of energy and nitrogen supply on ruminal characteristics and microbial growth. J. Anim. Sci. 71:2516–2528. Hollingsworth-Jenkins, K.J. 1994. Escape Protein, Rumen Degradable Protein, or Energy as the First Limiting Nutrient of Nursing Calves Grazing Native Sandhills Range. Ph.D. dissertation. University of Nebraska, Lincoln, Nebraska. Institut National de la Recherche Agronomique. 1988. Alimentation des Bovins, Ovins, et Caprins. R.Jarrige, ed. Paris: Institut National de la Recherche Agronomique. Karges, K.K. 1990. Effects of Rumen Degradable and Escape Protein on Cattle Response to Supplemental Protein on Native Pasture. M.S. thesis. University of Nebraska, Lincoln, Nebraska. Krysl, J.J., M.E.Branine, A.U.Cheema, M.A.Funk, and M.L.Galyean. 1989. Influence of soybean meal and sorghum grain supplementation on intake, digesta kinetics, ruminal fermentation, site and extent of digestion and microbial protein synthesis in beef steers grazing blue gramma rangeland. J. Anim. Sci. 67:3040–3051. Lintzenick, B.A., R.C.Cochran, E.S.Vanzant, J.L.Beaty, R.T.Brandt, Jr., G.St. Jean, and T.G.Nagaraja. 1993. Influence of method of processing supplemental alfalfa on intake and utilization of dormant, bluestem-range forage by beef steers. J. Anim. Sci. 71(Suppl. 1):186. Milton, C.T., and R.T.Brandt, Jr. 1994. Level of urea in high grain diets: Finishing steer performance. J. Anim. Sci. 72(Suppl. 1):231 (abstr.). National Research Council. 1984. Nutrient Requirements of Beef Cattle, Sixth Revised Ed. Washington, D.C.: National Academy Press. National Research Council. 1985. Ruminant Nitrogen Usage. Washington, D.C.: National Academy Press. National Research Council. 1989. Nutrient Requirements of Dairy Cattle, Sixth Rev. Ed. Washington, D.C.: National Academy Press. Nocek, J., and J.B.Russell. 1988. Protein and carbohydrate as an integrated system. Relationship of ruminal availability to microbial contribution and milk production. J. Dairy Sci. 71:2070–2107. Oldham, J.D. 1987. Efficiencies of amino acid utilization. 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