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Ruminant Nitrogen Usage (1985)

Chapter: 10 Application to Ruminant Feeding

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Suggested Citation:"10 Application to Ruminant Feeding." National Research Council. 1985. Ruminant Nitrogen Usage. Washington, DC: The National Academies Press. doi: 10.17226/615.
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Suggested Citation:"10 Application to Ruminant Feeding." National Research Council. 1985. Ruminant Nitrogen Usage. Washington, DC: The National Academies Press. doi: 10.17226/615.
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Suggested Citation:"10 Application to Ruminant Feeding." National Research Council. 1985. Ruminant Nitrogen Usage. Washington, DC: The National Academies Press. doi: 10.17226/615.
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Page 68
Suggested Citation:"10 Application to Ruminant Feeding." National Research Council. 1985. Ruminant Nitrogen Usage. Washington, DC: The National Academies Press. doi: 10.17226/615.
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Page 69
Suggested Citation:"10 Application to Ruminant Feeding." National Research Council. 1985. Ruminant Nitrogen Usage. Washington, DC: The National Academies Press. doi: 10.17226/615.
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Suggested Citation:"10 Application to Ruminant Feeding." National Research Council. 1985. Ruminant Nitrogen Usage. Washington, DC: The National Academies Press. doi: 10.17226/615.
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Suggested Citation:"10 Application to Ruminant Feeding." National Research Council. 1985. Ruminant Nitrogen Usage. Washington, DC: The National Academies Press. doi: 10.17226/615.
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Suggested Citation:"10 Application to Ruminant Feeding." National Research Council. 1985. Ruminant Nitrogen Usage. Washington, DC: The National Academies Press. doi: 10.17226/615.
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Suggested Citation:"10 Application to Ruminant Feeding." National Research Council. 1985. Ruminant Nitrogen Usage. Washington, DC: The National Academies Press. doi: 10.17226/615.
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Application to Ruminant Feeding INTRODUCTI ON Metabolism of nitrogen (N) in the ruminant is defined and reviewed in the several sections preceding this one. No attempt has been made to exhaustively review the literature describing research that has led to the conclu- sions drawn, although critical ant] important new con- tributions are referenced. The ruminant is unique in its N metabolism in that the active microbial and protozoa! populations in the reticulo-rumen modify the composition of the dietary protein (IP) sources en route to the absorptive area in the intestine. In addition, the nutrient requirements of the microbial population are not the same as those of the animal. These events result in mollified microbial activ- ity and reduced efficiency of the total digestive process (applier] to IP). In acldition, these processes affect the quantity of amino acids available to the animal and the makeup of the mixture of the amino acids absorbed compared to that in the diet. Any improvement in the utilization of N by the rumi- nant ultimately starts with diet formulation, dietary composition in terms of N. energy and other nutrients, and the behavior of the diet in the digestive tract of the animal. This is an important area of research in rumi- nant nutrition. New principles can be incorporated into the description of the diet, which should encourage fur- ther development. Prior to this publication, protein allowances for rumi- nants, as reported by NRC, included only amounts of crude protein either to be fed (IP) or digested per 24 h. Although certain guidelines were implied in the use of nonprotein N (NPN), there has been no attempt to clear with other N fractions or with the metabolic dynamics that affect utilization. This report will review current knowledge of N metabolism in the ruminant, present the critical concepts associated with that knowledge, and recommend a method of implementation based on those concepts. This application is designed to be broad and flexible to respond to the ever-increasing under- standing of N metabolism by the ruminant and to allow change as needed. The application of the principles discussed here is or- ganized so that computers can be used to generate solu- tions. Transfer coefficients and variables have been named so that computer solution can be obtainer] with- out using many multiple iterative steps. NEW CONCEPTS Several new concepts have been discussed. These can be summarized as they relate to metabolism of N in the ruminant. Although N may be present in different forms in vari- ous pools, all values will be cited in protein (N x 6.25) equivalents to reduce the need for repeated mathemati- cal interconversions. Dietary protein (IP) can be described in a variety of ways. However, when related to the digestive physiol- ogy of the ruminant, three major protein fractions inter- est nutritionists and producers. Herein these fractions are de~signated"A," "B." and "C." The discussion below includes reference to the use of the in situ procedure for obtaining estimates of rate and extent of digestion of protein fractions in feeds. Com- plete discussion of the method can be found in Mehrez and 0rskov (1977) and McDonald (1981~. It must also be noted that the in situ procedure is only one of several methods for defining the extent and rate of protein deg- radation in the rumen. Enzymatic procedures and those employing various solvents or detergents may find in- creased application in the future. The in situ procedure involves the incubation in the rumen of a fistulated animal of a specific amount of 66

Application to Ruminant Feeding 67 feed, in a polyester or nylon bag of pore size (ideally) uniform at 1,500 to 2,000 p2. By removal of replicate bags at various times of incubation, the rate and extent of degradation of feed matter can be determined. Math- ematical treatment of the data can result in rate con- stants for digestion and the definition of various chemi- cal fractions of feeds based on their degradation in the rumen. Concerns offers associated with the in situ technique include: (1) loss of undegraded proteins that are soluble or become small enough to pass the bag pores with fluid ire the rumen or during washing, (2) contamination of residue with attached microbial matter, and (3) the in- fluence of the local environment of the bag on digestion (particle hydration, end product concentration' etc.~. The three protein fractions to be quantitated are: A. Rapidly degraded IP-that fraction of IP that is rapidly converted to ammonia. Included in that frac- tion is the majority of NPN, free amino acids, and small peptides. The N in this fraction is, for practical pur- poses, rapidly and almost totally converted to ammonia in the rumen, since the rate of degradation is over 10 times faster than that of passage of solids from the ru- men. If ammonia is not incorporated by rumen mi- crobes into protein (BTP) or nucleic acid (NCP), it passes from the rumen (absorbed across the rumen wall or leaves with fluid) and is subject to at least partial loss as urinary urea (UP) or other NPN forms. Whereas many different techniques for measuring this fraction have been suggested, as has been reviewed earlier, the most desirable procedures are either solubility in buffer solutions or incubation in situ for 1 to 2 h. Loss of small particles through pores in bags may limit the usefulness of the in situ procedure to evaluate this fraction with some feeds. In addition, some slowly degraded but solu- ble proteins are inappropriately classified in this frac- tion. Designating fraction A as "soluble protein" fre- quently causes confusion. Since the absolute quantity is most important, and most diets are mixtures of feerl- stuffs, it is recommended that when used to describe the diet that this fraction be expressed as a percentage of feed or ration DM, rather than as percentage of IP. B. Slowly degraded, available IP the difference be- tween total IP and the sum of rapidly degraded (A) plus unavailable IP (C, below). This fraction represents that part of the IP that can potentially escape degradation in the rumen and be available for absorption in the intes- tine. The extent of degradation of IP in the rumen de- pends on the residence time of the IP in the rumen. Dietary characteristics and level of feecling both alter the extent of ruminal degradation. Fraction B differs from fraction A in that the rate of degradation of frac- tion B is of the magnitude of the fractional rate of pas sage of solids from the rumen. In light of these variables, the expression of the slowly degraded, available IP should be as an absolute quantity, in units of percentage of ration or fee stuff dry matter. If rate constants for ruminal degradation are listed, they should be based on measurements made by incubating the feed in question in polyester bags (or other appropriate procedure) for variable lengths of time and fitting regression equa- tionts) (usually of the general form Y = A + B ~ UBX) to the relationship between X = time and Y = percentage of original slowly degraded, available IP (B) disappear- ing from the bag (Mehrez and 0rskov, 1977~. Fractions A and B must be estimated, although some of B and C will be lost through pores in bags and result in an infla- tion in the value of "A." The overall calculation of deg- radation of fraction B should be based on the formula: Degradation = B * idB , LAB + kpB where B = slowly degraded, available IP; LAB = degradation rate constant; and kpB = rate of passage from the rumen (measured by the best method available). It is possible to expand the above equation to incorpo- rate subfractions of "B" and a rate constant appropriate to each. The prediction of degradation of total IP is made according to the equation presented in an earlier chapter. Since most foodstuffs contain a variety of dif- ferent types of protein, degradation of total protein in situ need not necessarily follow first-order kinetics. C. Undegraded, unavailable IP-that fraction that, due either to natural conditions or chemical, heat, or other reactions during processing, is not available to the ruminant by any of the digestive processes and is quanti- tatively recovered in feces. It behaves as an inert compo- nent in any dynamic description of the digestive process. While this fraction is normally associated with silages and forages, many chemical processes can create un- available IP in nonforage feeds as well. One estimate of unavailable IP is the residue that remains after treat- ment with acid detergent (Goering and Van Soest, 1972~. Questions still to be resolved include the ade- quacy of acid detergent as a method for quantifying un- available IP and the impact of this concept on presently accepted protein allowances, as the unavailable IP in feeds is not presently measured. However, it is proposed that until new technology enables a better practical esti- mate, this is the method of choice. This fraction has a residence time in the rumen similar to feed particles of similar size and specific gravity.

68 Ruminant Nitrogen Usage Recycled N (RP) The role of N recycled into the rumen can be quanti- tatively important in situations where the microbial re- quirement exceeds that of the animal as shown by the quantity of N in the diet (i.e., when low-protein diets are fed). While the nonlactating, mature animal is the most common example, at high rates of turnover of ru- men contents, more BCP may leave the rumen than would have enterer] from the diet even at moderately high percentages of IP. This is most apparent when IP is fed in forms that have low "A" fractions and low LAB values or high "C" fractions. As derived earlier, the RP Is: Y = 121.7 - 12.01 X + 0.3235X2; R2 = 0.97, where Y = Urea N recycled (percent of N intake), and X - IPDM (percent of DM) . From IPOM, it is possible to predict how much RP is presented to the rumen. The latter is dependent on sa- liva flow and composition and concentration of urea N in the Hood plasma. Also, the impact of lactation and type of diet (roughage, concentrate) has not been ade- quately assessed. The quantity of RP that will be used is based on the factors that govern removal of N from the ammonia pool and is a direct function of the amount of fermented en- ergy that is available in the rumen. The definition and description of the amount of RP is not complete and needs further study. In the development of these recom- mendations, a constant percentage of IP was consid- ered, recognizing that a single constant would not fit all situations, especially where animals were fed diets very low in protein (IPDM). A value of RP = 0.15 IP fits the lactating dairy cow data reasonably well and is pro- posed as the factor to use, but it does not fit the data from beef cows fed diets with IPDM of 0.05 to 0.08. In those cases the value for RP would be higher, although precise estimates are not available. The fact that the flow of N from the rumen exceeds intake by an increas- ing amount at dietary IPDM (percent) of 10 or less sug- gests that recycling plays an important role. If one solves the above equation for several IPDM (percent) and calculates RP (percent IP), the following data emerge: IPDM (percent) Is 10 15 20 RP (percent IP) 70 34 12 11 RP (g at 10 kg DM intake) 350 340 180 220 This illustrates the sensitivity of RP to low IPDM. The user should be aware that various metabolic pools or "sinks" (lactation, etc.) can alter the RP at a given IPDM (percent), thus making any of the above useful only as estimates. In the beef cow or feedlot steer, solving the equation above for normal IPDM (percent) will suggest diets that undersupply protein needs. Clearly, more work is needed, and on the basis of the significant lack of data, the Committee has chosen the relationship RP 5 O. 15 IP to allow noniterative and di- rect solutions to ration formulation, recognizing that in many instances that this value may be in error. Ruminal Ammonia Ruminal ammonia-N concentration often serves as an indicator of N-status for microbial production. Roffler and Satter (1975a,b) have presented an equation to pre- dict ruminal ammonia from IP and dietary energy den- sity. This equation was developed for ad-libitum-fed dairy cows fed diets that consisted of commonly fed feedstuffs and may overestimate ammonia N in low- IPDM (percent) diets or other conditions outside the original data set, or those with protein sources more re- sistant to degradation than soybean meal. Ammonia concentration represents the residual bal- ance between input and extraction from the ammonia pool in the rumen. Because there is not an equation that contains enough variables to address all of these inputs and balances for all ruminants, ammonia concentration was not part of the calculations used here. Microbial N Uptake and Efficiency The quantity of N used in the rumen for microbial synthesis (BCP) is a function of the amount of energy available for microbial growth. While several expres- sions have been used to relate BCP to fermentable en- ergy in the rumen, the factors that modify the fraction of energy in a ration or feed that is available in the rumen are not well described. Currently, feed analysis reports present an estimate of the energy value of the feed based on the apparent digestibility in the entire digestive tract and when fed at the maintenance level of feeding in many cases (TDN). Until it is possible to predict the fraction of energy actually fermented in the rumen, and the dietary and physiological factors that modify it, it is recommended that BCP be predicted from the follow- ing equations, when values preceded by + are the SE of the coefficient in questions: Lactating Dairy Cow, Dairy Replacements and AZ! Cat- tle Fed Diets with 40 Percent or More Roughage: BCP(g) - 6.25 ~ - 31.86 + 10.74 + 26. 12 + 1.30 TDN); R2 = 0.77,

Application to Ruminant Feeding 69 where TDN - consumed TDN (kg), unadjusted for the influence of level of feed intake. For lactating dairy cows using NEL as the energy unit, an alternative equation is: BCP(g) = 6.25 (-30.93 + 10.69 + 11.45 + 0.57 NEL); R2 = 0.77, where NEL = consumed NEL (Meal), based on intake at three times maintenance as used by NRC (1978~. The relationship between TDN (percent) and NEL (Meal/kg) is (NRC, 1978~: NEL = 0.12 + 0.0245TDN. This equation can be used to convert feed analysis results from TDN to NEL as needed or desired. Cattle Consuming Diets with Less Than 40 Percent Roughage: BCP(g) = 6.25 TDN (8.63 + 1.67 ~ 14.60 + 2.8 FI - 5.18 + 1.37FI2 + 0.59 + 0.80CI), R2- 0.96, where Sheep TDN = consumed TDN (kg), unadjusted for the influence of level of feed intake; FI = forage intake (percent of body weight) (from NRC publications); CI = concentrate intake (percent of body weight) (from NRC publications). BCP(g) = where 6.25 (- 1.29 + 0.96 + 23.04 + 1.71 TDN); R2 = 0.73, TDN = consumed TDN (kg), unadjusted for the influence of level of feet! intake. The efficiency with which ruminally available pro- tein (RAP) is trapped by microbes is important in ade- quately describing the overall metabolism of N in the animal. While the trapping efficiency cannot be 100 percent due to passage of fluid from the rumen that con- tains RAP and direct absorption of RAP across the ru- men wall, there are few data that adequately describe this relationship. It is recognized that as the amount of RAP increases, relative to the energy available in the rumen, the efficiency goes down. However, we cannot define that efficiency at the optimum balance at this time. As a starting point, a maximum trapping effi- ciency of RAP of 0.90 is used here, although BCP synthe- sis is normally driven by energy availability, not RAP. Future research may allow that constant to be converted to an equation or other variable relationship, especially under conditions of very low IPDM as is found in many rations fed to mature, noniactating cows. Intestinal Absorption of N The various allowances for N by ruminants stated by previous NRC subcommittees have been criticized for presenting apparent N absorption (as digestible protein) data that are not precise due to a variety of modifiers. As a result, the NRC Subcommittee on Dairy Cattle (1978) reported only crude protein. This was done to allow time for refinement of more precise estimates of allow- ance. The concepts introduced here should better de- scribe the allowances when adequate data become available to validate these concepts. The review of work published previously and presented earlier in this report produces a reasonably consistent value of 0.65 percent as the apparent absorption and 0.75 as true absorption of nonammonia N. The apparent absorption of amino acid N is 0.7 and true absorption is 0.8. It is more useful to partition the components of N into fractions that can be evaluated than to treat N as a single entity, although digestibilities for microbial and un(legraded dietary protein (UIP) appear similar. Variable amounts of frac- tion C will be found in UIP, and thus more variation in digestibility of UIP would be expected. Fecal N of Nondietary Origin (Metabolic) The quantity of fecal N that does not result directly from uncligested feed or microbial N (FPN) has not been adequately quantitated. Metabolic fecal N represents a major loss of a portion of the dietary N in many feeding instances, particularly the mature ruminant fed near maintenance. It has been common to plot the relation- ship between N in the diet dry matter (g/kg) and ab- sorbed (apparent) N/diet dry matter (g/kg) to enable an estimate of fecal N at zero IF. Such a plot also produces a slope that has been used to estimate true absorption of N. Reexamination of existing data suggests that there are some deviations from the assumed constancy of the fecal N content from nondietary origin. However, these deviations cannot be expressed as a specific function. If fecal N is plotted against dietary N. both in g/kg DM, diet and physiological status cause marked differences that cannot be related to specific variables at this time. A function based on the quantity of fecal DM necessi- tates an accurate prediction of that quantity. That can

70 Ruminant Nitrogen Usage be done if digestion of DM is known. We are recom- mending that as an average, fecal protein of metabolic origin (FPN) be computed from indigestible DM (IDM), which is calculated from TDN. Since TI)N percentage declines from the maintenance value (BTDN) as intake increases, and since this decline reflects IDM, we fee! that BTDN should be adjusted to an actual value (ATDN) for animals fed diets with more than 40 percent roughage. The NRC (1978) adjusts BTDN downward by 8 percent under the assumption that the dairy cow consumes at three times the maintenance level of intake and the decline in BTON is 4 percent per multiple of intake equal to maintenance. We recommend this ad- justment for computing IDM ant] FEN for dairy cows. Thus: B. OBLIGATORYMETABOLICFECALPROTEIN: b.1. Metabolic fecal protein (FPNj (g/day) = 90 IDM IDM = daily indigestible dry matter excretion (kg), calculated from: DM (1 ATDN) where: ATDN - 0.92 BTDN BTDN = TON at maintenance, as nor- mally reported from feed analysis laboratories. C. PRODUCTION: c. 1. Growth requirement (g/day) - RPN O.SO(g/dayJ 0.50 = amount of gained tissue protein pro- duced by 1.0 g absorbed protein (RPNRPA) ATDN = 0.92BTDN,RPN = gain in tissue protein, (g/day), from Tables 16 or 17, or estimated from and gain in empty body (rligesta free) (EB) IDM = (1 - ATDN),by: Cattle where ATDN and BTDN are fractional values. It is further assumed that IDM contains 14.4 g N of metabolic origin/kg, or 90 g FPN/kg. The total requirement of the animal will include the needs for maintenance protein (SPN + UPN), meta- bolic fecal protein (FPN), and production (RPN + YEN + LPN). CALCULATION OF DAILY ABSORBED TRUE PROTEIN NEEDED BY ANIMAL As indicated above, the protein requirement of the animal can be estimated as the sum of three functions: (a) maintenance, (b) obligatory metabolic fecal protein, and (c) production. In a factorial approach, the follow- ing relationships can be used to establish the protein needs of the animal, in units of absorbed N x 6.25 (AP): A. MAINTENANCE: Maintenance protein Steers: = [scarf protein (SPN) + endogenous urinary Sheep: protein (UPNJ] 0. 67) a.1. Scurf protein (g/day) = 0.2 W0 6 Males: RPN (g/day) a.2. Endogenous urinary protein (g/day) = 2.75 We 5 (cattle) = 1 125 W0 55 (sheep) W = body weight (kg) Females: RPN (g/day) 0.67 = amount of tissue (maintenance) protein pro duced from 1.0 g absorbed protein (MPNMPA) . RPN (g/day) = LWG (268 - 29.4 Energy/kg EBWG) where: LWG = live weight gair1 (kg) Energy/kg EBWG = Meal retained energy (RE)/kg gain in empty body EBWG = 0.956 LWG EBW = 0.891 LW (live weight) and: RE(Mcal/day) = 0.0635 EBW075*EBWG~097 Heifers: RE(Mcal/day) = 0.0783 EBW0 75*EBWGi ii9 (both of above with medium frame and implanted with hor- monal adjuvants) Modifications to the above: (1.) Cattle without hormonal adjuvants contain 5 percent more energy per unit gain; (2.) Medium-frame bulls are equivalent to me- dium-frame steers weighing 15 percent less. (3. ~ Large-frame animals are equivalent to me- dium-frame animals of the same sex at 15 percent lighter weight. = ~ EBWe~ 4ss4 3 EBWG (kg/day3 = ) 8~4e 3032 3*EBWG ~kg/~ay

c.2. Reproduction requirement (g/day) = gain in protein in fetus and uterus during second half of gestation (days 141-281, cattle; 63-147, sheep) = [YPN (g/day) . 0.50] where: 0.5 = amount of uterine and fetal protein pro ducec3 from 1.0 g absorbed protein (YP NYPA) YPN = gain in protein (g/day), as uterine and fetal tissue, from Tables 18 or 19, or estimated from: Cattle: YPN (g/day) = <34 37S' te(S 5357 - 13.120le ~ 0.00262X _ 0.00262X)] X = days from conception between 141 and 281. Sheep: YPN (g/day) = `0 0674) [e(1 1.3472 - 1 1.2206e ~ 0.00601X _ 0.00601X) ] X = days from conception between 63 and 147. c.3. Woo! growth requirement (glciay) =~3.0+0.10RPN) . 0.50 RPN = estimated gain from growth equations for sheep 0.S0 = amount of wool protein produced from 1.0 g absorbed protein (SPNSPA) c.4. Lactation requirement~glday) = LPN . 0.6S b 0.65 = amount of milk protein produced from 1.0 g absorbed protein (LPNLPA) D. PROTEINLOSS: d.1. Tissue proteir! mobilization (g/day) - 160 EBWL 160 = amount of absorbed protein (g) in 1.0 c. kg mobilized body tissue EBWL = empty body weight loss (kg/day). Total Amount of Absorbed True Protein Needed = (a.1. + a.2. + b.1. + c.1. + c.2. + c.3. + c.4. - d.1.) CALCULATION OF DAILY NEED OF TRUE PROTEIN IN THE SMALL INTESTINE OF THE ANIMAL The difference between the amount of absorbed true protein needed by the animal and the amount to be de livered to the small intestine is flue to indigestibility and the inefficiency of absorption. As noted in an earlier sec tion, the total disappearance of amino acids from the small intestine and presumed absorption of amino acids is, on the average, 0. 80. Thus, in order to provide 0.80 g of absorbed amino acids (protein), 1.00 g of material must be provided to the small intestine: Protein to Small Intestine (glday) - Absorbed True Protein Need (g/day): 0.80. Application to Ruminant Feeding 71 CALCULATION OF FLOW OF TRUE PROTEIN TO SMALL INTESTINE The protein flow to the small intestine is the com- bined sum of microbial protein and the protein in feed- stuffs that escapes degradation in the rumen. Certain corrections must be made to equate the protein flow with that needed in the small intestine. First, it is as- sumed that 80 percent of the microbial crude protein (BCP) is true protein (BTP), and thus 20 percent (nu- cleic acids, etc.) will not contribute to the absorbed amino acid pool (unless recycled to the rumen, since a large percentage of this N is absorbed). Second, in- cluded in the escaped feed protein is the unavailable fraction, C, which passes through the animal undi- gested. The flow of protein to the small intestine must be corrected for both of these components before they are compared with the amount needed by the animal. Microbial Protein fBCP) (g/day) a. Lactating Cows and Other Cattle Consuming Diet with More than 40 Percent Roughage BCP = 6.25(-31.86 + 26.12 TDN), or BCP= 6.25(-30.93 + 11.45NEL). Cattle Consuming Diets with Less than 40 Percent Roughage BCP = 6.25TDN(8.63 + 14.60FI - 5.18FI2 + 0.S9 CI). BTP = 0.80BCP. . Sheep BCP = 6.25 ( - 1.29 ~ 23.04 TDN) The variables in the above equations are defined ear- lier. Microbial True Protein (BTP) (g/dayJ = 0.80 BCP. Feed Protein Escape (g/day) =IP(B* kpB +C) kdB + kpB The variables in this equation are defined earlier. The quantity of fraction B (g/day) that escapes is de- penclent on the rate of passage (kpB) and digestion (kdB) of fraction B. The kdB is variable and depends on the chemical and physical properties of IP and level of feed- ing and KPB, rate of passage, is variable also. Thus, even though the equation suggests that one can easily com- pute the IF escape, the variation in the components of the equation makes estimation imprecise. Some estimates of the amount of protein escaping ru- minal degradation can be found. In most cases, the ta- bles of values are more useful for ranking of feeds than in

72 Ruminant Nitrogen Usage actual quantitation, because of the variation noted above ant] the presence of fraction C. For now, the user is faced with the need to choose a value for IP escape based on limited current data. CALCULATION OF AMOUNT OF NITROGEN AVAILABLE IN THE RUMEN FOR MICROBIAL SYNTHESIS The amount of N available for BCP in the rumen is the sum of the N from DIP and that recycled into the rumen as urea or other soluble sources in saliva (RP). Whether this N is incorporated into BCP is a function of energy supply, as noted above. A further set of conse quences of the microbial growth process are: (a) that only 80 percent of the N trapped in BCP is amino acid N (BTP) (thus, the overall process is no more than 80 per cent efficient) ant] (b) that the efficiency of trapping N (ammonia) from rumen flail] is less than 100 percent (assumed to be 90 percent here), due to flux of ammonia with fluids to the omasum. Efficiency probably ap proaches 100 percent at very low concentrations of am monia and drops below 90 at higher concentrations. B. Requirements: Hence, no more than 72 percent of the nitrogen from a protein degraded in the rumen can be expecter] to be recoverer! as BTP. Hence, RP (primarily as urea) be comes important in the nitrogen economy of the animal. Recycled nitrogen (RP) (percent of intakeJ can be pre dicted from dietary crude protein percentage by: RP = 121.7 - 12.01 IPDM + 0.323S IPDM2; R2 = 0.97. This is an iterative process. The alterna tive is to use O. IS IP in a direct solution, which we recommend. DegradedJeed protein (DIP) (g/day) IP, A, B. LAB, kpB are defined above. An alternative would be to estimate the quantity of degraded protein from values in tables comparing feeds. Estimates of degradation are subject to the errors of es cape protein, discussed above. The user should be aware that the conversion of avail- able N in the rumen to microbial protein is here assumed to have a maximum efficiency of 0.90. The above represents a set of approximations, mean- ing that once the need is calculated, and a sample diet is balanced, it must be checked and modified to ensure that the inputs meet needs of the animal. The material on the following page represents an ex- ample of a form that can be used to set up and complete the calculation of the protein needs of an animal and the dietary characteristics which best meet those needs, based on the information presented herein. Further ex- amples and tables can be found in the Appendix tables. EXAMPLE AND FORM FOR CALCULATING PROTEIN NEED AND DIETARY PROTEIN CHARACTERISTICS A. Example: 600-kg BW dairy cow, 30 kg 3.S percent fat milk, 3.3 percent protein, lSO days pregnant, + 0.10 kg/day body weight change. 1. Maintenance= tSPN + UPN] . 0.67 a. SPN = 0.2BWO6=(9.3g) b. UPN = 2.75BW°5=(67.4g) c. LSPN + UPN] . 0.67 =115 g 2. Metabolic Fecal Protein = FEN = 90 IDM a. BTDN = BTDNM + BTDNL BTDNM - 0.0352 BW0 75 = (4.27 kg) BTDNL = (Milk, kg) (NRC TDN/kg milk) = (30) (0.302) = BTDN = (4.27) + (9.06) = b. ATDN = 0.92 BTDN = c. DM = BTDN/NRC BTDNDM = 13.33/0.75= v ~ (9.06 kg) (13.33 kg) (12.26 kg) (17. 77 kg) d. ATDN . DM (0. 69) e. IDM = DM (1 - ATDN . DM) = (5.51 kg) -IP//A~ B * kdB ~I. ~N=90 (5.51)= 496 g \ kdB + kpBJ 3. Prod action = (EN . 0. 50) + (YPN 0. 50) + (LPN . 0.65) RPN: a. Use large frame, no hormonal adjuvants b. Adjustment for frame = 600 x (1 - 0.15) = (SlO kg) c. EBW = 0.891 (510) = (454 kg) cI. EBWG = 0.956(0.10) = (0.096 kg/day) e. RE (Meal/day) = 0.0783(454)° 75 (0 096) ~ in = (O. 56 Mcal/day) if. RE adjustment for no hormones = RE l.OS - (0.59 Mcal/day) g. RE (Meal/kg EBWG) = 0.59 0.096 = (6.12 Meal) Thus: Protein (available in rumen (RAPJ g/day) = (RP * IP) + DIP When comparing protein available in the rumen with microbial protein: Maximum microbial protein (BCP) < 0.9 RAP

h. RPN (g/day) = 0.10(268 - 29.4(6.12)) - (8.8g) i. RPNx0.50= 17.6g YPN: a. YPN (g/day) = (34.375) [e(8.5357~ 13.1201e° °°°262(15°)-0.00262(150)] - (34.375) (0.4895) = (16.8 g) (Note: extrapolation from Table 18 = 16.9 g b. YPN . O. 50 = 33. 6 g LPN: a. Milk protein = (30) (0.033) (1000) = (990 g) b. LPN 0.65 - 1523 g Total Requirement for Absorbed Protein (AP): AP= (115) ~ (496) + (17.6) + (33.6) + (1523) = C. Production of Bacterial Protein (BCP): (Assume that diet more than 40 percent roughage) BCP (g) = 6.25 ( - 31.86 + 26.12 (13.33~) = 1977 g D. Bacterial True Protein (BTP): BTP(g) = 0.80BCP= = 0.80 (1977) - lS81 E. Ruminally Available Protein (RAP): RAP (g)-BCP . 0.90 2 (1977) 0.90 - 2196g F. Digested Bacterial True Protein (DBP): DBP= 0.80BTP= = 0.80 (1581) - 1265 g G Digestible Undegraded Intake Protein (D UP): DUP = AP- DBP - (2185.2) - (1265) = 920.2 g H. Undegraded Intake Protein (UIP): UIP = DUP . 0.80= = (920.2) . 0.80= 1150 g 1. Smalilutestine True Protein Flo~v (STP): STP - BTP + UIP = (1581) + (llSO) 1. Intake Protein (IP): (Use 1S percent of [P as RP) IP = (RAP + UIP) 1.15 - (2196) + (llSO) 1.15 = 2910g K. Intake Protein in Diet Dry Matter (IPDM): IPDM= (IP) . (1OOODM) = (2910) . (17770) = 0.163 g = 16.38 percent L. Undegraded Protein Needed in Diet (UIPIP): UIPIP = UIP x IP = (1150) . (2910) = M. Degraded Protein Needed in Diet (DIPIP): DIPIP = DIP . IP = (2913- 1150) Application to Ruminant Feedix~g 73 Utilization of Nonprotein Nitrogen (NPN) Originally, interest in defining many of the parame ters associated with ruminant nitrogen usage dealt with ways to predict the usefulness of NPN. Many publica tions have been written on that subject. This subcommittee feels that the system that has been presented, complex as it may seem to be, represents a quantitative evaluation of the entire set of conditions under which NPN can be used, and how much. By de fining the quantity of the dietary protein that must be degraded in the rumen to meet the need for microbial growth, the potential for reduced intake and digestion should be avoided. On the other hand, by defining the total amount of protein that must leave the rumen to 2185.2 g meet the animals' needs, the user is in a position to pre dict when NPN can be usec] to help achieve those needs. Based on the equation and relationships developed in this publication, a set of tables (Appendix Tables 4 to 6) are presented as guidelines for determining those die tary and production conditions under which additional NPN would not be expected to be utilize(1 by the rumen microbial population. In addition, Appendix Tables 7 and 8 present clata, computed from these same concepts, on the concentration of clietary protein needed for a va riety of conditions for beef cattIe as well as the percent age of that protein that should escape ruminal degrada tion to result in the optimum feeding program for that animal. These latter tables can also be used to evaluate the potential for using NPN and to aid in selecting sup plemental protein sources. Computer Programs 0~395 = 39.5 percent (2910~= 0.605 = 60.5 percent It is recognized that many users of this publication will not be in a position to use a computer program at 2731 g this time. The number of opportunities for computer application will certainly increase in the future, how ever. In addition, many advisors, extension specialists, and industry personnel, plus those in teaching and re search, use computers routinely and increasingly in the formulation and evaluation of rations and feeding pro grams. In order to anticipate the increased dependence on the computer, ancl to present a rigorous mode! to evalu ate the concept presented here, the subcommittee has chosen to provide Fortran IV programs for the calcula tion of the dairy (Appendix 93 and beef (Appendix 10) applications. These programs are presentefd with appro priate commentary and explanation to allow one to use them with little difficulty. In acldition, there is an in creasing number of published microcomputer pro grams, spreadsheet applications, etc. (Lane and Cross,

74 Ruminant Nitrogen Usage 1985) that will enable the user to apply these concepts easily. It is anticipated the microcomputer application will be the common mechanism of use, and the reader is thereby encouraged to pursue that avenue. Unresolved Problems and Some Areas Needing More Research During the course of the cleliberations of this subcom- mittee, many areas of ruminant N metabolism were found to be poorly defined or were defined in specific narrow conditions that did not allow application to all classes. We feel that these are some of the areas that need research attention. A. Recycled N. The data here are both meagre and questionable in their application to normal or practical diets. Whereas we recognize that at low IPDM, RP is of great importance, application of the relationship pre- sented in which RP is a function of IPDM results in un- reasonably low IPDM for animals at low production levels. As a result, we present the ratio approach (RP - 0.15 IPDM) as an estimate. B. Efficiency of Microbial Uptake of RAN. We know that this cannot be 100 percent as long as RAN can leave the rumen on a continual basis with fluids, etc. We also know that when RAN is in excess of that which can be converted to BCP, the efficiency is low. However, when RAN is suppliecl in amounts intended to minimize waste and maximize BCP yield at prevailing dietary non-N circumstances, the biological efficiency is not clear. We have chosen 90 percent as an estimate and hope that more quantitative data will emerge from fu ture research. C. Prediction of Microbial Yield (BCP). There are many data on this subject, gathered by a variety of tech- niques. In the process of developing a set of predictors that can be driven from dietary information that is available for practical use, the picture is less clear. While we have resorted to a whole-gut measure of en- ergy, knowing that this is subject to many animal and dietary factors, the alternatives are not clear. A review of the variation around some of the coefficients in the prediction equations for BCP will point out the lack of precision. In order to construct a system that is driven from commonly measured (or predicted) energy mea- surements at the Ieve! of the rumen, much work is needed on the appropriate relationships. D. Transfer Coefficients. In addition to the BCPRAP relationship noted above, there is a need for more data on the other N transfers that take place in ruminants. While some term describing "Biological Value" is desirable in defining the N metabolism of all organisms, it is not possible to make such a jump with ruminants. For example, the assumer] values for LPNLPA (0.65), RPNRPA (0.50), and MPNMPA (0.65) are based on some data and "best estimates." It is recog- nized that the balance in available amino acids (AP) is going to have an impact on the transfer coefficients and that the sensitivity of these impacts will depend on the number of metabolic options available to the animal. As more emphasis is given to IF that escapes rumen degra- dation (UIP), the amino acid balance of the UIP be- comes important in evaluating the transfer coefficients. Formulation of diets only on the basis of how much AP is presented to the animal will in some cases be inappro- priate because of poor distribution of essential amino acids in the AP. Particular attention needs to be given to lysine and methionine. Until specific data are available, further refinement is not possible. This subcommittee presents the above document as an attempt to improve the mechanisms for rationing of nitrogen and protein for ruminants, based on the cur- rent knowledge. It is hoped that subsequent revisions will be able to build on and advance these concepts.

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This book brings together the latest research on protein absorption by ruminants and takes a look at the calculation of optimum nutrient requirements, including bacterial digestion, in the calculations. It also describes the parameters of nitrogen conversion in the ruminant and examines the different kinds of protein found in animal feedstuffs. "Animal Feed Science and Technology" calls it "essential for all scientists and teachers actively working in ruminant nutrition research and instruction."

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