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OCR for page 43
Protein and
Amino Acids
Dietary protein generally refers to crude protein (CP),
which is defined for foodstuffs as the nitrogen (N) content
x 6.25. The definition is based on the assumption that
the average N content of foodstuffs is 16 g per 100 g of
protein. The calculated CP content includes both protein
and nonprotein N (NPN). Feedstuffs vary widely in their
relative proportions of protein and NPN, in the rate and
extent of ruminal degradation of protein, and in the intesti-
nal digestibility and amino acid (AA) composition of rumi-
nally undegraded feed protein. The NPN in feed and sup-
plements such as urea and ammonium salts are considered
to be degraded completely in the rumen.
IMPORTANCE AND GOALS OF PROTEIN
AND AMINO ACID NUTRITION
Ruminally synthesized microbial CP (MCP), ruminally
undegraded feed CP (RUP), and to a much lesser extent,
endogenous CP (ECP) contribute to passage of metaboliz-
able protein (MP) to the small intestine. Metabolizable
protein is defined as the true protein that is digested postru-
minally and the component AA absorbed by the intestine.
Amino acids, and not protein per se, are the required
nutrients. Absorbed AA, used principally as building blocks
for the synthesis of proteins, are vital to the maintenance,
growth, reproduction, and lactation of dairy cattle. Presum-
ably, an ideal pattern of absorbed AA exists for each of
these physiologic functions. The Nutrient Requirements of
Poultry (National Research Council, 1994) and the Nutri-
ent Requirements of Swine (National Research Council,
1998) indicate that an optimum AA profile exists in MP
for each physiologic state of the animal and this is assumed
to be true for dairy animals.
The goals of ruminant protein nutrition are to provide
adequate amounts of rumen-degradable protein (RDP) for
optimal ruminal efficiency and to obtain the desired animal
productivity with a minimum amount of dietary CP. Opti-
mizing the efficiency of use of dietary CP requires selection
of complementary feed proteins and NPN supplements
that will provide the types and amounts of RDP that will
meet, but not exceed, the N needs of ruminal microorgan-
isms for maximal synthesis of MCP, and the types and
amounts of digestible RUP that will optimize, in so far
as possible, the profile and amounts of absorbed AA. As
discussed later, research indicates that the nutritive value
of MP for dairy cattle is determined by its profile of essen-
tial AA (EAA) and probably also by the contribution of
total EAA to MP. Improving the efficiency of protein and
N usage while striving for optimal productivity is a matter
of practical concern. Incentives include reduced feed costs
per unit of lean tissue gain or milk protein produced, a
desire for greater and more efficient yields of milk protein,
creation of space in the diet for other nutrients that will
enhance production, and concerns of waste N disposal.
Regarding milk protein production, research indicates that
content (and thus yield) of milk protein can be increased
by improving the profile of AA in MP, by reducing the
amount of "surplus" protein in the diet, and by increasing
the amount of fermentable carbohydrate in the diet.
Major Differences from Previous Edition
In 1985, the Subcommittee on Nitrogen Usage in Rumi-
nants (National Research Council, 1985) expressed protein
requirements in units of absorbed protein. Absorbed pro-
tein was defined as the digestible true protein (i.e., digest-
ible total AA) that is provided to the animal by ruminally
synthesized MCP and feed protein that escaped ruminal
degradation. This approach was adopted for the previous
edition of this publication (National Research Council,
19891. The absorbed protein method introduced the con-
cept of degraded intake CP (DIP) and undegraded intake
CP (UIP). Mean values of ruminal undegradability for
common feeds, derived from in viva and in situ studies
using sheep and cattle, were reported. This factorial
approach for estimating protein requirements recognized
the three fates of dietary protein (fermentative digestion
43
OCR for page 44
44 Nutrient Requirements of Dairy Cattle
in the reticulo-rumen, hydrolytic/enzymatic digestion in
the intestine, end passage of indigestible protein with feces)
and separated the requirements of ruminal microorganisms
from those of the host animal. However, a fixed intestinal
digestibility of 80 percent for UIP was used, no consider-
ation was given to the contribution of endogenous CP to
MP, and no consideration was given to the AA composition
of UIP or of absorbed protein.
Some differences exist in terminology. To be consistent
with the current edition of Nutrient Requirements of Beef
Cattle (National Research Council, 1996), and to avoid
implications that proteins are absorbed, the term MP
replaces absorbed protein. To be consistent with the ~our-
nal of Dairy Science, the terms DIP and UIP are replaced
with RDP and RUP, respectively.
The primary differences between the protein system of
this publication and that used in the previous edition relate
to predicting nutrient supply. Microbial CP flows are pre-
dicted from intake of total tract digestible organic matter
(OM) instead of net energy intake. The regression equation
considers the variability in efficiency of MCP production
associated with apparent adequacy of RDP. A mechanistic
system developed from in situ data is used for calculating
the RUP content of feedstuffs. Insofar as regression equa-
tions allow, the system considers some ofthe factors (DMI,
percentage of concentrate feeds in diet DM, and percent-
age NDF in diet DM) that affect rates of passage of undi-
gested feed and thus the RUP content of a feedstuff. The
system is considered to be applicable to all dairy animals
with body weights greater than 100 kg and that are fed for
early rumen development. To increase the accuracy of
estimating the contribution of the RUP fraction of individ-
ual foodstuffs to MP, estimates of intestinal digestibility
have been assigned to the RUP fraction of each foodstuff
(range = 50 to 1001. Endogenous protein and NPN also
are considered to contribute to passage of CP to the small
intestine. Endogenous CP flows are calculated from intake
of DM. And finally, regression equations are included that
predict directly the content of each EAA in total EAA of
duodenal protein and flows of total EAA. Flows of digest-
ible EAA and their contribution to MP are calculated.
Dose-response curves that relate measured milk protein
content and yield responses to changes of predicted per-
centages of digestible Lys and Met in MP are presented.
The dose-response relationships provide estimates of
model-determined amounts of Lys and Met required in
MP for optimal utilization of absorbed AA for milk protein
production. The inclusion of equations for predicting pas-
sage of EAA to the small intestine along with assignment
of RUP digestibility values that are unique to individual
foodstuffs brings awareness to differences in nutritive value
of RUP from different foodstuffs and should improve the
prediction of animal responses to substitution of protein
sources.
P RO TE I N
Chemistry of Feed Crude Protein
Feedstuffs contain numerous different proteins and sev-
eral types of NPN compounds. Proteins are large molecules
that differ in size, shape, function, solubility, and AA com-
position. Proteins have been classified on the basis of their
3-dimensional structure and solubility characteristics.
Examples of classifications based on solubility would
include globular proteins Falbumins (soluble in water and
alkali solutions and insoluble in salt and alcohol), globulins
(soluble in salt and alkali solutions and sparingly soluble
or insoluble in water and insoluble in alcohol), glutelins
(soluble only in alkali), prolamines (soluble in 70 to 80
percent ethanol and alkali and insoluble in water, salt,
and absolute alcohol), histories (soluble in water and salt
solutions and insoluble in ammonium hydroxide)] and
fibrous proteins Le.g., collagens, elastins, and keratins
(insoluble in water or salt solutions and resistant to diges-
tive enzymes)] (Orten and Neuhaus, 1975; Rodwell, 1985;
Van Soest, 19941. Globular proteins are common to all
foodstuffs whereas fibrous proteins are limited to feeds of
animal and marine origin. Albumins and globular proteins
are low molecular weight proteins. Prolamines and glutel-
ins are higher molecular weight proteins and contain more
disulfide bonds. Generally, feeds of plant origin contain
all of the globular proteins but in differing amounts. For
example, cereal grains and by-product feeds derived from
cereal grains contain more glutelins and prolamines
whereas leaves and stems are rich in albumins (Blethen et
al., 1990; Sniffen, 1974; Van Soest, 19941. A sequential
extraction of 38 different feeds with water, dilute salt (0.5
percent NaCl), aqueous alcohol (80 percent ethanol), and
dilute alkali (0.2 percent NaOH) indicated that the classic
protein fractions (albumins, globulins, prolamines, and glu-
telins) plus NPN accounted for an average of 65 percent
of total N (Blethen et al., 19901. The unaccounted for,
insoluble N would include protein bound in intact aleurone
granules of cereal grains, most of the cell-wall associated
proteins, and some of the chloroplasmic and heat-dena-
tured proteins that are associated with NDF (Van Soest,
19941. Among the feeds that were evaluated, those with
the highest percentage of insoluble protein (> 40 percent
of CP) were forages, beet pulp, soy hulls, sorghum, dried
brewers grains, dried distillers grains, fish meal, and meat
and bone meal (Blethen et al., 19901.
Feedstuffs also contain variable amounts of low molecu-
lar weight NPN compounds. These compounds include
peptides, free AA, nucleic acids, amides, amines, and
ammonia. Nonprotein N compounds generally are deter-
mined as the N remaining in the filtrate after precipitation
of the true protein with either tungstic or trichloroacetic
acid (Licitra et al., 19961. Grasses and legume forages
contain the highest and most variable concentrations of
OCR for page 45
Protein and Amino Acids 45
NPN. Most of the reported concentrations of NPN in CP
of grasses and legume forages are within the following
ranges: fresh material (lOB15%), hay (15B25%), and silage
(30B65%) (Fairbairn et al., 1988; Garcia et al., 1989; Grum
et al., 1991; Hughes, 1970; Krishnamoorthy et al., 1982;
Messman et al., 1994; Van Soest, 1994; Xu et al., 19961.
Hays and especially silages contain higher amounts of NPN
than the same feed when fresh because of the proteolysis
that occurs during wilting and fermentation. The proteoly-
sis that occurs in forages during wilting and ensiling is a
result of plant and microbial proteases and peptidases.
Plant proteases and peptidases are active in cut forage and
are considered to be the principal enzymes responsible for
the conversion of true protein to NPN in hays and ensiled
feeds (Fairbairn et al., 1988; Van Soest, 19941. Rapid wilt-
ing of cut forages and conditions that promote rapid reduc-
tions in pH of ensiled feeds slow proteolysis and reduce
the conversion of true protein to NPN (Garcia et al., 1989;
Van Soest, 19941. The NPN content of fresh forage is
composed largely of peptides, free AA, and nitrates (Van
Soest, 19941. Fermented forages have a different composi-
tion of NPN than fresh forages. Fermented forages have
higher proportional concentrations of free AA, ammonia,
and amines and lower concentrations of peptides and
nitrate (Fairbairn et al., 1988; Van Soest, 19941. The NPN
content of most non-forage feeds is 12 percent or less of
CP (Krishnamoorthy et al., 1982; Licitra et al., 1996; Van
Soest, 1994; Xu et al., 19961.
Mechanism of Ruminal Protein Degradation
The potentially fermentable pool of protein includes
feed proteins plus the endogenous proteins of saliva,
sloughed epithelial cells, and the remains of lysed ruminal
microorganisms. The mechanism of ruminal degradation
has been reviewed (Broderick et al.,1991; Broderick, 1998;
Cotta and Hespell, 1984; ;[ouany, 1996; ;[ouany and Ushida,
1999; Wallace, 1996; Wallace et al., 19991. In brief, all of
the enzymatic activity of ruminal protein degradation is
of microbial origin. Many strains and species of bacteria,
protozoa, and anaerobic fungi participate by elaborating a
variety of proteases, peptidases, and deaminases (Wallace,
19961. The liberated peptides, AA, and ammonia are nutri-
ents for the growth of ruminal microorganisms. Peptide
breakdown to AA must occur before AA are incorporated
into microbial protein (Wallace, 19961. When protein deg-
radation exceeds the rate of AA and ammonia assimilation
into microbial protein, peptide and AA catabolism leads
to excessive ruminal ammonia concentrations. Some of the
peptides and AA not incorporated into microbial protein
may escape ruminal degradation to ammonia and become
sources of absorbed AA to the host animal.
Bacteria are the principal microorganisms involved in
protein degradation. Bacteria are the most abundant micro-
organisms in the rumen (10~°-~/ml) and 40 percent or more
of isolated species exhibit proteolytic activity (Broderick
et al., 1991; Cotta and Hespell, 1984; Wallace, 19961. Most
bacterial proteases are associated with the cell surface
(Kopecny and Wallace, 19821; only about 10 percent of
the total proteolytic activity is cell free (Broderick, 19981.
Therefore, the initial step in protein degradation by rumi-
nal bacteria is adsorption of soluble proteins to bacteria
(Nugent and Mangan, 1981; Wallace, 1985) or adsorption
of bacteria to insoluble proteins (Broderick et al., 19911.
Extracellular proteolysis gives rise to oligopeptides which
are degraded further to small peptides and some free AA.
Following bacterial uptake of small peptides and free AA,
there are f~ve distinct intracellular events: (1) cleavage of
peptides to free AA, (2) utilization of free AA for protein
synthesis, (3) catabolism of free AA to ammonia and carbon
skeletons (i.e., deamination), (4) utilization of ammonia for
resynthesis of AA, and (5) diffusion of ammonia out of the
cell (Broderick, 19981.
The bacterial population that is responsible for AA
deamination has been of considerable interest. Amino acid
catabolism and ammonia production in excess of bacterial
need wastes dietary CP and reduces efficiency of use of
RDP for ruminant production. For many years it was
assumed that deamination was limited to the large number
of species of bacteria that had been identif~ed to produce
ammonia from protein or protein hydrolyzates (Wallace,
1996~. However, this assumption was challenged by Russell
and co-workers (Chen and Russell, 1988,1989; Russell et
al., 1988) who concluded that the deaminative activity of
these bacteria was too low to account for rates of ammonia
production usually observed in vivo or in vitro with mixed
cultures. Their efforts led to the eventual isolation of a small
group of bacteria that had exceptionally high deaminative
activity and that used AA as their main source of carbon
and energy (Russell et al., 1988; Paster et al., 1993~. As a
result of these and other studies, it is now accepted that
AA deamination by bacteria is carried out by a combination
of numerous bacteria with low deaminative activity and a
much smaller number of bacteria with high activity (Wal-
lace, 1996~. Of particular interest has been the observation
that the growth of some of these bacteria with high deami-
nating activity is suppressed by the ionophore, monensin
(Chen and Russell, 1988,1989; Russell et al., 1988~.
Protozoa also are active and significant participants in
ruminal protein degradation. Protozoa are less numerous
than bacteria in ruminal contents (105-6/ml) but because
of their large size, they comprise a signif~cant portion of
the total microbial biomass in the rumen (generally less
than 10 percent but sometimes as high as 50 percent)
(;Jouany, 1996;;Jouany and Ushida, 19991. Several differ-
ences exist between protozoa and bacteria in their metabo-
lism of protein. First, they differ in feeding behavior.
Instead of forming a complex with feeds, protozoa ingest
OCR for page 46
46 Nutrient Requirements of Dairy CattIe
particulate matter (bacteria, fungi, and small feed parti-
cles). Bacteria are their principal source of ingested protein
(;[ouany and Ushida, 19991. As a result of this feeding
behavior (i.e., ingestion of food), protozoa are more active
in degrading insoluble feed proteins (e.g., soybean meal
or fish meal) than more soluble feed proteins (e.g., casein)
(Hino and Russell, 1987; ;[ouany, 1996; ;[ouany and Ushida,
19991. Ingested proteins are degraded within the cell to
yield a mixture of peptides and free AA; the AA are incorpo-
rated into protozoa! protein. Proteolytic specific activity of
protozoa is higher than that of bacteria (Nolan, 19931. A
second difference between protozoa and bacteria is that
while both actively deaminate AA, protozoa are not able
to synthesize AA from ammonia (;[ouany and Ushida, 19991.
Thus, protozoa are net exporters of ammonia and because
of this, defaunation decreases ruminal ammonia concentra-
tions (;[ouany and Ushida, 19991. And lastly, protozoa
release large amounts of peptides and AA as well as pepti-
dases into ruminal fluid. This is the result of significant
secretory processes and significant autolysis and death
(Coleman, 1985; DiJkstra, 19941. ;Jouany and Ushida (1999)
suggest that excreted small peptides and AA can represent
50 percent of total protein ingested by protozoa. Other
studies indicate that 65 percent or more of protozoa! pro-
tein recycles within the rumen (Ffoulkes and Leng, 1988;
Punia et al., 19921.
Much less is known about the involvement of fungi in
ruminal protein catabolism. Currently, anaerobic fungi are
considered to have negligible effects on ruminal protein
digestion because of their low concentrations in ruminal
digesta (103-4/ml) (;[ouany and Ushida, 1999; Wallace and
Monroe, 19861.
Kinetics of Ruminal Protein Degradation
Ruminal degradation of dietary feed CP is an important
factor influencing ruminal fermentation and AA supply to
dairy cattle. RDP and RUP are two components of dietary
feed CP that have separate and distinct functions. Rumina-
lly degraded feed CP provides a mixture of peptides, free
AA, and ammonia for microbial growth and synthesis of
microbial protein. Ruminally synthesized microbial protein
typically supplies most of the AA passing to the small intes-
tine. Ruminally undegraded protein is the second most
important source of absorbable AA to the animal. Knowl-
edge of the kinetics of ruminal degradation of feed proteins
is fundamental to formulating diets for adequate amounts
of RDP for rumen microorganisms and adequate amounts
of RUP for the host animal.
Ruminal protein degradation is described most often by
first order mass action models. An important feature of
these models is that they consider that the CP fraction of
foodstuffs consists of multiple fractions that differ widely
in rates of degradation, and that ruminal disappearance of
protein is the result of two simultaneous activities, degrada-
tion and passage. One of the more complex of these models
is the Cornell Net Carbohydrate Protein System (CNCPS)
(Sniffer et al., 19921. In this model, feed CP is divided
into five fractions (A, Be, B2, B3, and C) which sum to
unity. The five fractions have different rates of ruminal
degradation. Fraction A (NPN) is the percentage of CP
that is instantaneously solubilized at time zero, which is
assumed to have a degradation rate (k,) of infinity; it is
determined chemically as that proportion of CP that is
soluble in borate-phosphate buffer but not precipitated
with the protein denaturant, trichloroacetic acetic (TCA)
(Figure 5-11. Fraction C is determined chemically as the
percentage of total CP recovered with ADF (i.e., ADIN)
and is considered to be undegradable. Fraction C contains
proteins associated with lignin and tannins and heat-dam-
aged proteins such as the Maillard reaction products (Snif-
fen et al., 19921. The remaining B fractions represent
potentially degradable true protein. The amounts of each
of these 3 fractions that are degraded in the rumen are
determined by their fractional rates of degradation Ski ~ and
passage (kp); a single kp value is used for all fractions.
Fraction Be is that percentage of total CP that is soluble
in borate-phosphate buffer and precipitated with TCA.
Fraction B3 is calculated as the difference between the
portions of total CP recovered with NDF (i.e., NDIN) and
ADF (i.e., fraction C). Fraction B2 is the remaining CP
and is calculated as total CP minus the sum of fractions
A, Be, B3, and C. Reported ranges for the fractional rates
of degradation for the three B fractions are: BE (120-400
%/h), B2 (3-16 %/h), and B3 (0.06-0.55 %/h). The RDP
and RUP values (percent of CP) for a foodstuff using this
model are computed using the equations
RDP = A + BE Ek, BE / (k, BE + kp)]
+ B2 Ek,B2 / (k,B2 + kp)]
+ B3 Ek,B3 / (k,B3 + kp)]
and
RUP = BE Ekp / (k, BE + kp)]
+B2Lkp/(k,B2+kp)]
+B3Lkp/(k,B3+kp)]+C.
This model is used in Level II of the Nutrient Requirements
of Beef Cattle (National Research Council, 1996) report.
The most used model to describe in situ ruminal protein
degradation divides feed CP into three fractions (A, B. and
C). Fraction A is the percentage of total CP that is NPN
(i.e., assumed to be instantly degraded) and a small amount
of true protein that rapidly escapes from the in situ bag
because of high solubility or very small particle size. Frac-
tion C is the percentage of CP that is completely undegrad-
able; this fraction generally is determined as the feed CP
remaining in the bag at a defined end-point of degradation.
Fraction B is the rest of the CP and includes the proteins
OCR for page 47
Protein and Amino Acids 47
TOTAL
1 -1 1
| BORATE l l NEUTRAL l
| BUFFER | | DETERGENT |
1
A
INSOL
B2
B3
. C
1
A _
SZ
. I NSOL
B3
C
ACID
DETERGENT
A . INSOL
B2
B3
FIGURE 5-1 Analyses of crude protein fractions using borate-
phosphate buffer and acid detergent and neutral detergent solu-
tions (Roe et al., 1990; Sniffen et al, 1992~.
that are potentially degradable. Only the B fraction is con-
sidered to be affected by relative rates of passage; all of
fraction A is considered to be degraded and all of fraction
C is considered to pass to the small intestine. The amount
of fraction B that is degraded in the rumen is determined
by the fractional rate of degradation that is determined in
the study for fraction B and an estimate of fractional rates
of passage. The RDP and RUP values for a feedstuff (per-
cent of CP) using this model are computed using the equa-
tions RDP = A + B Ek, / (k, + kp)] and RUP = B Ekp /
Ski + kp)] + C. This simple model has been the most
widely used model for describing degradation and ruminal
escape of feed proteins (e.g., AFRC, 1984; National
Research Council, 1985; 0rskov and McDonald, 19791. It
is noted that data obtained from in situ, in vitro, and enzy-
matic digestions generally fit a model that divides feed CP
into these fractions (Broderick et al., 1991) and that most
of the in situ data used to validate results obtained with
cell-free proteases have been obtained using this model
(Broderick, 19981. As discussed later, it is this model in
conjunction with in situ derived data that is used for pre-
dicting ruminal protein degradability in this edition.
Numerous factors affect the amount of CP in feeds that
will be degraded in the rumen. The chemistry of feed CP
is the single most important factor. The two most important
considerations of feed CP chemistry are: (1) the propor-
tional concentrations of NPN and true protein, and (2) the
physical and chemical characteristics of the proteins that
comprise the true protein fraction ofthe feedstuff. Nonpro-
tein N compounds are degraded so quickly in the rumen
(~300%/h) that degradation is assumed to be 100 percent
(Sniffer et al., 19921. However, this is not an entirely
correct assumption because degradability is truly related
to rate of passage. For example, assuming a kp of 2.0%/h
andak~ of 300%/h,then degradation = 3.00/~3.00 + 0.02)
= 0.993 or 99.3 percent, and not 1.00 or 100 percent.
Feedstuffs that contain high concentrations of NPN in CP
contribute little RUP to the host animal. When dairy cattle
are fed all-forage diets, measurements of passage of non-
ammonia, non-microbial N (i.e., RUP-N plus endogenous
N) often are less than 30 percent of N intake (Beever et
al., 1976, 1987; Holden et al., 1994a; Van Vuuren et al.,
19921. In contrast to NPN, which is assumed to be com-
pletely degraded, the rates of degradation of proteins are
highly variable and result in variable amounts of protein
being degraded in the rumen. For example, the range in
k~ given in Tables 15-2a,b are 1.4 for Menhaden f~sh meal
to 29.2 for sunflower meal. Assuming a kp for each feed
of 7.0 percent, the range in degradabilites of the B fraction
would be 16.7 to 80.7 percent. Some characteristics of
proteins shown to contribute to differences in rates of
degradation are differences in 3-dimensional structure, dif-
ferences in intra- and inter-molecular bonding, inert barri-
ers such as cell walls, and antinutritional factors.
Differences in 3-dimensional structure and chemical
bonding (i.e., cross-links) that occur both within and
between protein molecules and between proteins and car-
bohydrates are functions of source as well as processing.
These aspects of structure affect microbial access to the
proteins, which apparently is the most important factor
affecting the rate and extent of degradation of proteins in
the rumen. Proteins that possess extensive cross-linking,
such as the disulfide bonding in albumins and immunoglob-
ulins or cross-links caused by chemical or heat treatment,
are less accessible to proteolytic enzymes and are degraded
more slowly (Ferguson, 1975; Hurrell and Finot, 1985;
Mahadevan et al., 1980; Mangan, 1972; Nugent and Man-
gan, 1978; Nugent et al., 1983; Wallace, 19831. Proteins in
feathers and hair are extensively cross-linked with disulf~de
bonds and largely for that reason, a considerable amount
of the protein in feather meal is in fraction C (Tables 15-
2a,b). Similarly, a considerable portion of the protein in meat
meal and meat and bone meal is in fraction C. Proteins in
meat meal and meat and bone meal may contain considerable
amounts of collagen that has both intramolecular and inter-
molecular cross-links (Orten and Neuhaus, 19751. In contrast,
a majority of the protein in menhaden fish meal is in fraction
B but the fractional rate of degradation of fraction B is slower
than in other protein supplements (Tables 15-2a,b). Heat
used in the drying of f~sh protein was shown to induce the
formation of disulf~de bonds (Opstvedt et al., 19841. Heat
processing also coagulates protein in meat products which
makes it insoluble (Bendall, 1964; Boehme, 1982), and cool-
ing of the products causes a random relinkage of chemical
bonds which shrinks the protein molecules (Bendall, 19641.
Collectively, these effects of heating and cooling of proteins
decrease microbial access and make the proteins more resis-
tant to ruminal degradation.
Other factors affecting the ruminal degradability of feed
protein include ruminal retention time of the protein,
microbial proteolytic activity, and ruminal pH. The effect
OCR for page 48
48 Nutrient Requirements of Dairy CattIe
of these factors on the kinetics of ruminal protein degrada-
tion have been reviewed (Broderick et al., 1991; National
Research Council, 19851.
Nitrogen Solubility vs. Protein Degradation
Several commercial feed testing laboratories in the
United States provide at least one measurement of N solu-
bility for feedstuffs. Although recognized that N solubility
in a single solvent is not synonymous with CP degradation
in the rumen, the general absence of alternatives other
than using "book values" for RUP (e.g., National Research
Council, 1985) left little else to help nutritionists ensure
that adequate but not excessive amounts of RDP were
fed. Solubility measurements have been useful for ranking
feeds of similar types for ruminal CP degradability. This
is because of the positive relationship that exists between
N solubility and degradation within similar foodstuffs (e.g.,
Beever et al., 1976; Laycock and Miller, 1981; Madsen and
Hvelplund, 1990; Stutts et al., 19881. Many studies have
indicated that changing N solubility by adding or removing
NPN supplements, by changing method of forage preserva-
tion, or processing conditions of protein supplements
affects animal response (e.g., Aitchison et al., 1976; Crish et
al., 1986; Lundquist et al., 19861. Several different solvents
have been used. At present, the most common procedure
is incubation in borate-phosphate buffer (Roe et al., 19901.
This method has gained in popularity because it is used
for determining the A and Be nitrogen fractions in the
CNCPS (Sniffer et al., 19921.
Although a high correlation exists between N solubility
in a single solvent and protein degradability for similar
feedstuffs, the same does not exist across classes of feed-
stuffs. For example, Stern and Satter (1984) reported a
correlation of 0.26 between N solubility and in viva protein
degradation in the rumen of 34 diets that contained a
variety of N sources. Madsen and Hvelplund (1990) also
reported a poor relationship between N solubility and in
viva degradation of CP when used over a range of feed-
stuffs. There appear to be several reasons for these poor
relationships. First, as indicated in the section "Chemistry
of Feed Crude Protein", the proteins that are extracted
by a solvent depend not only on the chemistry of the
proteins but also on the composition of the solvent. For
that reason, different solvents provide different estimates
of CP solubility (Cherney et al., 1992; Crawford et al.,
1978; Crooker et al., 1978; Lundquist et al., 1986; Stutts
et al., 19881. Second, soluble proteins are not equally sus-
ceptible to degradation by rumen enzymes. Among the
pure soluble proteins, casein is degraded rapidly whereas
serum albumin, ovalbumin, and ribonuclease A are
degraded much slower (Annison, 1956; Mahadevan et al.,
1980; Mangan, 19721. Mahadevan et al. (1980) also
observed that soluble proteins from soybean meal, rape-
seed meal, and fish meal were degraded at different rates
with rates of degradation for all three supplements being
intermediate between those for albumins and casein.
Therefore, structure as well as solubility determines degra-
dability. Third, as indicated in the section "Mechanism of
Ruminal Protein Degradation", solubility is not a prerequi-
site to degradation. As an example, Mahadevan et al. (1980)
observed that soluble and insoluble proteins of soybean
meal were hydrolyzed in vitro at almost identical rates.
Because bacteria attach to insoluble proteins and because
protozoa engulf feed particles, insoluble proteins need not
enter the soluble protein pool before attack by microbial
proteases. And last, soluble proteins that are not yet
degraded may leave the rumen faster than insoluble pro-
teins. This is because of a more likely association of soluble
protein with the liquid fraction of ruminal contents. For
example, Hristov and Broderick (1996) observed that
although feed NAN in the liquid phase of ruminal contents
was only 12 percent of total ruminal feed NAN, 30 percent
of the feed NAN that escaped the rumen flowed with the
liquids. This indicates a disproportional escape of solu-
ble proteins.
In conclusion, a change in N solubility in a single solvent
appears to be a more useful indicator of a change in protein
degradation when applied to different samples of the same
foodstuff than when used to compare different foodstuffs
that differ in chemical and physical properties. Clearly,
the relationship between solubility and degradability is the
highest when most of the soluble N is NPN (Sniffer
et al., 19921.
Microbial Requirements for N Substrates
Peptides, AA, and ammonia are nutrients for the growth
of ruminal bacteria; protozoa cannot use ammonia. Esti-
mates of the contribution of ammonia versus preformed
AA to microbial protein synthesis by the mixed rumen
population have been highly variable (Wallace, 19971.
Studies using Nit ammonia or urea infused into the rumen
or added as a single dose demonstrated that values for
microbial N derived from ammonia ranged from 18 to 100
percent (Salter et al., 19791. The Nit studies of Nolan
(1975) and Leng and Nolan (1984) indicated that 50 per-
cent or more of the microbial N was derived from ammonia
and the rest from peptides and AA. The mixed ruminal
microbial population has essentially no absolute require-
ment for AA (Virtanen, 1966) as cross-feeding among bac-
teria can meet individual requirements. However,
researchers have observed improved microbial growth or
efficiency when peptides or AA replaced ammonia or urea
as the sole or major source of N (Cotta and Russell, 1982;
Russell and Sniffen, 1984; Griswold et al., 19961. Maeng
and Baldwin (1976) reported increased microbial yield and
growth rate on 75% urea + 25% AA-N as compared to
OCR for page 49
Protein and Amino Acids 49
100% urea. Microbial requirements for N substrates of
ammonia-N, AA, and peptides can also be affected by the
basal diet and may explain some of the variability in the
above experiments.
There is evidence that AA and especially peptides are
stimulatory in terms of both growth rate and growth yield
for ruminal microorganisms growing on rapidly degraded
energy sources (Argyle and Baldwin, 1989; Chen et al.,
1987; Cruz Soto et al., 1994; Russell et al., 19831. However,
when energy substrates are fermented slowly, stimulation
by peptides and AA does not always occur. Chikunya et
al. (1996) demonstrated that when peptides were supplied
with rapidly or slowly degraded fiber, microbial growth was
enhanced only if the fiber was degraded rapidly. Russell et
al. (1992) indicated that microorganisms fermenting struc-
tural carbohydrates require only ammonia as their N source
while species degrading nonstructural carbohydrate
sources will benefit from preformed AA.
Recent experiments (Wallace, 1997) have confirmed the
earlier results of Salter et al. (1979) showing that the pro-
portion of microbial N derived from ammonia varies
according to the availability of N sources. The minimum
contribution to microbial N from ammonia was 26 percent
when high concentrations of peptides and AA were present,
with a potential maximum of 100 percent when ammonia
was the sole N source. Griswold et al. (1996) examined
the effect of isolated soy protein, soy peptides, individual
AA blended to profile soy protein, and urea on growth
of microorganisms in continuous culture. Griswold et al.
(1996) demonstrated that N forms other than ammonia
are needed not only for maximum microbial growth but
also as NPN for adequate ruminal fiber digestion.
Many reports of the uptake of Ci4-AA and peptides have
indicated that mixed microbial populations preferentially
took up peptides rather than free AA (Cooper and Ling,
1985; Prins et al., 19791. However, Ling and Armstead
(1995) found that free AA were the preferred form of
AA incorporated by S. Louis, Selenomonas ruminantium,
Fibrobacter succinogenes and Anaerovibrio lipolytica,
whereas peptides were preferred only by P. ruminicola. P.
ruminicola can comprise greater than 60 percent of the
total flora in sheep fed grass silage (Van Gylswyk, 19901.
In other studies where an AA preference was exhibited,
the preference may have been the result of specific dietary
conditions where P. ruminicola numbers were lower. Wal-
lace (1996) demonstrated that AA deamination is carried
out by two distinct bacterial populations, one with low
activity and high numbers and the other with high activity
and low numbers. P. ruminicola occurs in high numbers
but has low deaminase activity.
Jones et al. (1998) investigated the effects of peptide
concentrations in microbial metabolism in continuous cul-
ture fermenters. The basal diet contained 17.S percent
CP, 46.2 percent NSC, and 32.9 percent NDF. Peptides
replaced urea as a N source at levels of O. TO, 20 and 30
percent of total N. a urea-molasses mixture represented
8.6, 7.O, 4.9, and 2.9 percent of DM with increasing peptide
and glucose replacement. Digestion of DM and CP and
microbial CP production were affected quadratically by
peptide addition; the highest values for each variable occur-
red at 10 percent peptide addition. Fiber digestion
decreased linearly with increasing peptide addition.
Reduced ammonia-N concentrations appeared to be the
cause of reduced microbial CP production and reduced
fiber digestion at levels of peptides greater than 10 percent
of total N. The efficiency of conversion of peptide N to
microbial CP increased with increasing peptides; however,
there was no change in grams of microbial N produced
per kilogram of OM digested. [ones et al. (1998) suggested
that with diets containing high levels of NSC, excessive
peptide concentrations relative to that of ammonia can
depress protein digestion and ammonia concentrations,
limit the growth of fiber-digesting microorganisms, and
reduce ruminal fiber digestion and microbial protein pro-
duction. Microorganisms that ferment NSC produce and
utilize peptides at the expense of ammonia production
from protein and other N sources (Russell et al., 19921. It
should be noted that in continuous culture systems, proto-
zoa can be washed out in the first few days of operation.
Animal Responses to CP, RDP, and RUP
EACTATION RESPONSES
Crude protein. A data set of 393 means from 82 protein
studies was used to evaluate the milk and milk protein
yield responses to changes in the concentration of dietary
CP (Table 5-11. The descriptive statistics for the data set
are presented in Table 5-2. When CP content of diets
change, the relative contribution of protein from different
sources also change so this evaluation is confounded with
source of protein and concentrations of RDP and RUP.
Overall, milk yield increased quadratically as diet CP con-
centrations increased. The regression equation obtained
was:
Milk yield = 0.8 x DMI + 2.3 x CP
— 0.05 x cp2 - 9.8 (r2 = 0.29)
where milk yield and dry matter intake (DMI) are kilo-
grams/d and CP is percent of diet DM.
Dry matter intake was included in the regression to
account indirectly for some of the differences among stud-
ies such as basal milk production and BW. Dry matter
intake accounted for about 60 percent and CP about 40
percent of non-random variation. Assuming a fixed DMI
(there was no correlation between intake and CP percent
in this data set), the maximum milk production was
obtained at 23 percent CP. The marginal response to
OCR for page 50
50 Nutrient Requirements of Dairy Cattle
TABLE 5-1 Studies Used to Evaluate Milk and Milk Protein Yield Responses to Changes in the Concentration of
Dietary Crude Protein
Annexstad et al. (1987)
Aharoni et al. (1993)
Armentano et al. (1993)
Atwal et al. (1995)
Baker et al. (1995)
Bertrand et al. (1998)
Blauw~ekel and Kinca~d (1986)
Blauw~ekel et al. (1990)
Bowman et al. (1988)
Broder~ck (1992)
Broder~ck et al. (1990)
Bruckental et al. (1989)
Canf~eld et al. (1990)
Casper et al. (1990)
Chen et al. (1993)
Christensen et al. (1993a, b)
Crawley and Kilmer (1995)
Cunningham et al. (1996)
De Gracia et al. (1989)
DePeters and Bath (1986)
Dhiman and Satter (1993)
Garcia-Bojalil et al. (1998a)
Grant and Haddad (1998)
Grinds et al. (1991)
Grinds et al. (1992a)
Grummer et al. (1996)
Hadsell and Sommerfeldt (1988)
Henderson et al. (1985)
Henson et al. (1997)
Higginbotham et al. (1989)
Hoffman and Armentano (1988)
Hoffman et al. (1991)
Holter et al. (1992)
Hongerholt and Muller (1998)
Howard et al. (1987)
Huyler et al. (1999)
Jaquette et al. (1986)
Jaquette et al. (1987)
Maim et al. (1983)
Maim et al. (1987)
Kalscheur et al. (1999a,b)
Kerry and Amos (1993)
Khorasani et al. (1996a)
Kim et al. (1991)
King et al. (1990)
Klusmeyer et al. (1990)
Komaragir~ and Erdman (1997)
Lees et al. (1990)
Leonard and Block (1988)
Lundquist et al. (1986)
Macleod and Cahill (1987)
Manson and Leaver (1988)
Mantysaari et al. (1989)
McCarthy et al. (1989)
McCormick et al. (1999)
McGuffey et al. (1990)
Nakamura et al. (1992)
Owen and Larson (1991)
Palmquist and Weiss (1994)
Palmquist et al. (1993)
Polan et al. (1997)
Polan et al. (1985)
Powers et al. (1995)
Robinson and Kennelly (1988b)
Robinson et al. (199lb)
Roseler et al. (1993)
Santos et al. (1998a,b)
Sloan et al. (1988)
Spain et al. (1995)
Voss et al. (1988)
Wattiaux et al. (1994)
Weigel et al. (1997)
Wheeler et al. (1995)
Windschitl (1991)
Wohlt et al. (1991)
Wright (1996)
Wu et al. (1997)
Wu and Satter (2000)
Zimmerman et al. (1992)
Zimmerman et al. (1991)
TABLE 5-2 Descriptive Statistics for Data Set Used
to Evaluate Animal Responses to CP and RDP
Variable
N
Mean
Std. Dev.
Milk, kg/d
Milk protein yield, g/d
Day matter intake, kg/d
CP, % of day matter
RDP, % of day matter
RUP, % of day matter
393
360
393
393
172
172
31.4
972
20.2
17.1
10.7
6.2
6.1
153
3.4
2.6
1.8
1.4
increased dietary CP (first derivative ofthe CP components
of the regression equation) is: 2.3 - 0.1 x CP. Therefore,
increasing dietary CP one percentage unit from 15 to 16
percent would be expected to increase milk yield an aver-
age of 0.75 kg/d and increasing CP one percentage unit
from 19 to 20 percent would be expected to increase milk
yield by 0.35 kg/d. Although milk production may be
increased by feeding diets with extremely high concentra-
tions of CP, the economic and environmental costs must
be compared with lower CP diets. The marginal response
obtained from this data set was similar to that obtained by
Roffler et al. (19861. With their equation, increasing dietary
CP from 14 to 18 percent would result in an increase of
2.1 kg/d of milk and with the equation above the expected
increase is 2.8 kg/d.
Dietary CP was not correlated (P>0.25) with milk pro-
tein percent, but was correlated weakly (r = 0.14; P<0.01)
with milk protein yield (because of the relationship of
dietary CP with milk yield). The regression equation was:
milk protein yield (g/d) = 17.7 x DMI + 55.6 x CP
1.26 x cp2 + 31.8 (r2 = 0.19) where DMI is kilograms/
day and CP is percent of diet DM. Maximum yield of milk
protein was obtained at 22 percent CP (essentially the
same as for milk yield) and the marginal response is equal
to 55.63 - 2.52 x CP where CP is a percent of diet DM.
Rumen degradable and undegradable protein. A regres-
sion approach also was used to evaluate lactation responses
to concentrations of RDP and RUP in the dietary DM. To
evaluate lactation responses to RDP in diet DM, 38 studies
with 206 treatment means were selected in which diets
varied in content of RDP (Table 5-31. All diets were
entered into this edition's model for predicted concentra-
tions of RDP and RUP in diet DM. As expected, concentra-
tions of RDP and RUP (as percentages of diet DM) were
correlated with concentrations of dietary CP (RDP; r =
0.78, P<0.001; RUP, r = 0.53, P<0.001), therefore it is
not possible to separate effects of total CP from those of
RDP or RUP. A regression equation for milk yield with
RDP and RUP (both as percent of DM) was derived to
overcome the problems associated with the correlation
between CP and RDP and RUP (the correlation between
RDP and RUP was not significant (r = - 0.11, P>0.051.
Dietary RDP and RUP were calculated using the model
described in this publication based on values in the data
set described above. The regression equation also included
DMI for the reasons explained above. The regression equa-
tion (Figure 5-2) was:
Milk = - 55.61 + 1.15 x DMI + 8.79 x RDP—0.36
x RDP2 + 1.85 x RUP (r2 = 0.52)
OCR for page 51
Protein and Amino Acids 51
TABLE 5-3 Studies Used to Evaluate Milk Yield Responses to Changes in the Concentration of Dietary Ruminally
Degraded Protein
Annexstad et al. (1987)
Armentano et al. (1993)
Baker et al. (1995)
Barney et al. (1981)
Bertrand et al. (1998)
Blauwiekel et al. (1990)
Casper et al. (1990)
Christensen et al. (1993a,b)
Cunningham et al. (1996)
Dhiman and Satter (1993)
Garcia-Bojalil et al. (1998a)
Grant and Haddad (1998)
Grings et al. (1991)
45 ~
,
4o
Grings et al. (1992)
Grummer et al. (1996)
Ha and Kennelly (1984)
Harris et al. (1992)
Henson et al. (1997)
Higginbotham et al. (1989)
Hoffman et al. (1991)
Holter et al. (1992)
Hongerholt and Muller (1998)
Kalscheur et al. (1999a)
Khorasani et al. (1996b)
Kim et al. (1991)
lJP~ % aft 8 ~ 16 g92 ~
FIGURE 5-2 Response surface for data set described in "Ani-
mal Responses to CP, RDP, and RUP" section. Maximum milk
yield occurred at 12.2 percent RDP (percent of diet DM). Dry
matter intake was held constant at 20.6 kg/day.
where DMI and milk are kilograms/day, and RDP and
RUP are percent of diet DM. Based on that equation,
maximum milk yield occurred (DMI and RUP held con-
stant) when RDP equaled 12.2 percent of diet DM, and
the marginal change in milk to increasing RDP was 8.79
— 0.72 x RDP. The quadratic term for RUP was not
significant and was removed from the model. Milk yield
increase linearly to RUP at the rate of 1.85 kg for each
percentage unit increase in RUP.
In comparison this edition's model estimates an average
RDP requirement of 10.2 percent for this data set. Pre-
dicted milk yield (using the above regression equation) at
10.2 percent RDP (DMI and RUP held constant mean
values of the data set of 20.6 kg/d DMI and 6.2 percent,
respectively) is 31.7 kg/d and 33.2 kg/d when RDP is 12.2
percent. A portion of the discrepancy between model pre-
dicted requirement for RDP and regression predicted max-
imal milk production may be caused by the positive correla-
tion between RDP and DM intake (DMI = 14.4 + 0.58
King et al. (1990)
Komaragiri and Erdman (1997)
Leonard and Block (1988)
Mantysaari et al. (1989)
McGuffey et al. (1990)
Palmquist and Weiss (1994)
Roseler et al. (1993)
Santos et al. (1998a,b)
Wattiaux et al. (1994)
Weigel et al. (1997)
Windschitl (1991)
Wu and Satter (2000)
x RDP; r = 0.35, P<0.0011. Based on that regression,
an increase in 2 percentage units of RDP (i.e., 10.2 to 12.2
percent) would increase DMI by about 1.1 kg/d. Based on
this edition's requirements (assumed 72 percent TDN), an
increase of about 2 kg/d of milk is expected from that
change in DMI. Increasing dietary RDP above model pre-
dicted requirements may result in increased DM intake.
A similar shaped function (data not shown) was obtained
when milk protein yield was regressed on dietary RDP
and RUP:
Milk protein = - 1.57 + 0.0275 x DMI + 0.223
x RDP — 0.0091 x RDP2 + 0.041
x RUP (r2 = 0.51)
where milk protein and DMI are kilograms per day and
RDP and RUP are percentages of dietary DM. Maximum
milk protein yield occurred at 12.2 percent RDP (the same
as milk yield). Milk protein yield increased linearly with
increasing dietary RUP.
Santos et al. (1998b) published a comprehensive review
of the effects of replacing soybean meal with various
sources of RUP on protein metabolism (29 published com-
parisons) and production (127 published comparisons).
Santos et al. (1998b) reported that in 76 percent of the
metabolism studies, higher RUP decreased MCP flows to
the small intestine. Supplementation with RUP usually did
not affect flow of total EAA, and RUP supplementation
usually did not increase or actually decreased flow of lysine
to the duodenum. Supplementation of RUP increased milk
production in only 17 percent of the studies and heat-
treated or chemically-treated soybean meal or fish meal
were the most likely RUP supplements to cause increased
milk production (Santos et al., 1998b). When studies were
combined, cows fed diets with treated soybean meal
(P<0.03) or fish meal (P<0.01) produced statistically more
milk than cows fed soybean meal. Cows fed other animal
proteins (blood, feather, meat meals) or corn gluten meal
produced similar or numerically less milk than cows fed
soybean meal (Santos et al., 1998b). See additional discus-
sion in Chapter 16.
OCR for page 52
52 Nutrient Requirements of Dairy Cattle
The regression equations derived above for milk and
milk protein yield responses to dietary CP, RDP, and RUP
should be interpreted and used cautiously in view of low
r2 values. A more sophisticated statistical analysis (e.g.,
controlling for trial effects, adjusting for variances within
trials, etc.) would probably yield different and more accu-
rate coefficients.
EFFECTS ON REPRODUCTION
Protein in excess of lactation requirements has been
shown to have negative effects on reproduction. Several
workers have reported that feeding diets containing 19
percent or more CP in diet DM lowered conception rates
(Bruckental et al., 1989; Canf~eld et al., 1990; Jordan and
Swanson, 1979; McCormick et al., 19991. Others have
observed that cows fed 20-23 percent CP diets (as com-
pared to 12-15 percent CP) had decreased uterine pH,
increased blood urea, and altered uterine fluid composition
(Jordan et al., 1983; Elrod and Butler, 19931. In a majority
of the studies reviewed by Butler (1998), plasma progester-
one concentrations in early lactation cows were lower when
diets contained 19-20 percent CP vs. lower concentrations
of CP.
In a review of protein effects on reproduction, Butler
(1998) concluded that excessive amounts of either RDP
or RUP could be responsible for lowered reproductive
performance. However, intakes of"digestible" RUP in
amounts required to adversely affect reproduction without
a coinciding surplus of RDP would be uncommon. In most
of the studies reviewed by Butler (1998), excessive RDP
rather than excessive RUP was associated with decreased
conception rates. Canf~eld et al. (1990) showed that feeding
diets containing RUP to meet requirements while feeding
RDP in excess of requirements resulted in decreased con-
ception rates. Garcia-Bojalil et al. (1998b) reported that
RDP fed in excess (15.7 percent of DM) of recommenda-
tions decreased the amount of luteal tissue in ovaries of
early lactation cows.
Although most studies have indicated an adverse effect
on reproductive performance of feeding high CP diets,
others indicate no effect of diet CP on reproduction. Car-
roll et al. (1988) observed no differences in pregnancy rate
or first service conception rates of dairy cows fed 20 percent
CP and 13 percent CP diets. Howard et al. (1987) reported
no difference in fertility between cows in second and
greater lactation fed 15 percent CP or 20 percent CP diets.
There are many theories as to why excess dietary CP
decreases reproductive performance (Barton, 1996a,
1996b; Butler, 1998; Ferguson and Chalupa, 19891. The
first theory relates to the energy costs associated with meta-
bolic disposal of excess N. To the extent that additional
energy may be required for this purpose, this energy may
be taken from body reserves in early lactation to support
milk production. Delayed ovulation (e.g., Beam and Butler,
1997; Staples et al., 1990) and reduced fertility (Butler,
1998) have been associated with negative energy status.
Another effect of negative energy status is decreased
plasma progesterone concentrations (Butler, 19981.
Another theory is that excessive blood urea N (BUN)
concentrations could have a toxic effect on sperm, ova, or
embryos, resulting in a decrease in fertility (Canf~eld et
al., 19901. High BUN concentrations have also been shown
to decrease uterine pH and prostaglandin production (But-
ler, 19981. High BUN may also reduce the binding of
leutinizing hormone to ovarian receptors, leading to
decreases in serum progesterone concentration and fertil-
ity (Barton, 1996a). Ferguson and Chalupa (1989) reported
that by-products of N metabolism may alter the function of
the hypophysealpituitary-ovarian axis, therefore decreasing
reproductive performance. And last, high levels of circulat-
ing ammonia may depress the immune system and, there-
fore, may result in a decline in reproductive performance
(Anderson and Barton, 19881.
Milk urea nitrogen (MUN) and blood urea nitrogen
(BUN) are both indicators of urea production by the liver.
Milk urea N concentrations greater than 19 mg/dl have
been associated with decreased fertility (Butler et al.,
19951. Likewise, BUN concentrations greater than 20 ma/
dl have been linked with reduced conception rates in lactat-
ing cows (Ferguson et al., 19881. Bruckental et al. (1989)
found that BUN levels increased when diet CP was
increased from 17 to 21.6 percent and pregnancy rate
decreased by 13 percentage units. In a case study, Ferguson
et al. (1988) observed that cows with BUN levels higher
than 20 mg/dl were three times less likely to conceive than
cows with lower BUN concentrations. Although high BUN
concentrations have been associated with decreased repro-
ductive performance, others have reported no adverse
effects on pregnancy rate, services per conception, or days
open with BUN levels above 20 m~/dl (Oldick and Fir-
kins, 19961.
Studies by Carroll et al. (1987) and Howard et al. (1987)
indicate that maintaining a strict reproductive management
protocol can reduce the negative effects of excess protein
intake on reproduction. Barton (1996a) demonstrated that
an intense reproductive program could be used to reach
reproductive success regardless of diet CP level or plasma
urea N concentrations. These studies highlight the idea
that dietary protein is just one of many things that have
an effect on reproductive performance. Protein intake,
along with other factors such as reproductive management,
energy status, milk yield, and health status all have an
effect on reproductive performance in dairy cattle.
in.
Synchronizing Ruminal Protein and Carbohydrate
Digestion: Effects on Microbial Protein Synthesis
Microbial protein synthesis in the rumen depends largely
on the availability of carbohydrates and N in the rumen.
OCR for page 53
Protein and Amino Acids 53
Bacteria are capable generally of capturing the majority of
ammonia that is released in the rumen from AA deamina-
tion and the hydrolysis of NPN compounds. However,
dietary conditions often occur in which the rate of ammonia
release in the rumen exceeds the rate of uptake by ruminal
bacteria. Examples of such conditions would include a
surplus of RDP or a lack of available energy (Maeng et al.,
19971. This asynchronous release of ammonia and energy in
the rumen results in inefficient utilization of fermentable
substrates and reduced synthesis of MCP. A variety of
studies have focused on increasing the efficiency of micro-
bial protein synthesis by manipulating dietary components
(Aldrich et al., 1993a; Hoover and Stokes, 1991; Herrera-
Saldana et al., 1990; Maeng et al., 19761. Excellent reviews
describe the relationship between ruminal protein and car-
bohydrate availability and its impact on MCP synthesis in
the rumen (Hoover and Stokes, 1991; Clark et al., 1992;
Stern et al., 1994; Dewhurst et al., 20001.
Several studies indicate that synchronizing for rapid fer-
mentation with fast degradable starch and protein sources
stimulates greater synthesis or efficiency of synthesis of
MCP. Herrera-Saldana et al. (1990) reported that MCP
passage to the duodenum of lactating cows was highest
(3.00 kg/d) when starch and protein degradability were
synchronized for fast rates of digestion (barley and cotton-
seed meal). Flows of MCP were lower when the primary
fermentable carbohydrate and protein sources were either
synchronized for slow degradability (milo and brewer's
dried grains; 2.14 kg/d) or asynchronized (barley and brew-
er's dried grains or milo and cottonseed meal; 2.64 and 2.36
kg/d, respectively). Efficiency of MCP synthesis (MCP/kg
of truly digested OM) followed similar trends as MCP
passage to the duodenum. Aldrich et al. (1993b) formulated
diets to contain high and low concentrations of rumen-
available nonstructural carbohydrates (HRANSC and
LRANSC) and high and low concentrations of rumen-
available protein (HRAP and LRAP) using high moisture
shelled corn vs. coarse ground, dry ear corn and canola
meal vs. blood meal, respectively. Flow of MCP to the
duodenum was highest (1.64 kg/d) with HRANSC/HRAP
and lowest (1.34 kg/d) with HRANSC/LRAP, flows were
intermediate (1.46 and 1.48 kg/d) for the two LRANSC
diets. Similar to the findings of Herrera-Saldana et al.
(1990), efficiencies of synthesis of MCP were highest with
the HRANSC/HRAP diet. Stokes et al. (1991a) reported
that diets formulated to contain 31 or 39 percent NSC and
11.8 or 13.7 percent RDP in diet DM supported greater
MCP synthesis than a diet containing 25 percent NSC and
9 percent RDP. Diets formulated to be synchronous vs.
asynchronous in ruminal digestion rates of carbohydrate
and protein have also increased flows and efficiency of
synthesis of MCP in sheep (Sinclair et al., 1993, 19951. In
the study by Sinclair et al. (1995), diets were similar in
carbohydrate source (barley) and were either synchronous
with rapeseed meal (diet A) or asynchronous with urea
(diet B). The efficiency of MCP synthesis was 11-20 per-
cent greater in sheep given diet A vs. diet B.
Numerous other studies have reported higher MCP pas-
sage (in viva or in continuous culture) when either the
NSC level was increased or more degradable carbohydrates
were substituted for those less degradable (McCarthy et
al., 1989; Spicer et al., 1986; Stokes et al., 1991a; Stern et
al., 1978) or when RDP in diet DM was increased (Cecava
et al., 1991; Hussein et al., 1991; McCarthy et al., 1989;
Stokes et al., l991b). A review of 16 studies indicated that
MCP flow to the duodenum was increased by an average
of 10 percent when slowly degradable sources of starch
(e.g., corn grain) were replaced by more rapidly degraded
starch (e.g., barley) (Sauvant and van Milgen, 19951. How-
ever, there was no effect of differences in rate of starch
degradation on the efficiency of conversion of ruminally
digested OM to MCP. LyLos et al. (1997) evaluated diets
formulated to have similar rates of RDP with three rates
(6.04,6.98, and 7.94%/h) of NSC degradation in the rumen.
Concentrations of RDP and NSC in diet DM were held
constant across treatments. Rates of NSC degradation were
achieved primarily by replacing cracked corn with ground
high moisture corn. Flow of MCP to the duodenum tended
to be the highest with the highest rate of NSC degradation.
Efficiency of conversion of ruminally digested OM to MCP
was increased as ruminal NSC availability increased, dem-
onstrating the importance of timing of available energy to
the ruminal microorganisms.
Studies evaluating the importance of providing a gradual
or even supply (vs. an uneven supply) of energy and N
substrates to ruminal microorganisms are limited. Henning
et al. (1993) investigated this issue in cannulated sheep
fed both at maintenance and at a higher level of nutrition.
Treatments consisted of a soluble carbohydrate mixture
(maltose, dextrose and maltotriose) and a soluble N mixture
(urea and sodium caseinate). Providing an even supply of
energy increased passage of MCP and efficiency of MCP
synthesis when the maintenance diet was fed but only
tended to increase efficiency of MCP synthesis when the
more adequate diet was fed. In contrast, the even supply
of N increased passage of MCP only when the more ade-
quate diet was fed. The results indicate that merely improv-
ing the degree of synchronization between energy and N
release rates in the rumen does not necessarily increase
microbial cell yield and that a gradual or even release of
energy and possibly N as well are also important.
Synchronizing rates of ruminal degradation of carbohy-
drates and protein may have a more pronounced effect in
animals having high rates of ruminal passage (e.g., high
DMI). Newbold and Rust (1992) observed in batch culture
that a temporary restriction of supplies of either N or
carbohydrate reduced subsequent bacterial growth rate.
However, given the same total supply of nutrients, bacterial
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Representative terms from entire chapter:
amino acids
