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OCR for page 66
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
OCR for page 67
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.
OCR for page 68
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,
OCR for page 69
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
OCR for page 70
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
OCR for page 71
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
OCR for page 72
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
OCR for page 73
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,
OCR for page 74
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.
Representative terms from entire chapter:
amino acids
