Protein and Amino Acids

62 Pages·2001·0.85 MB·English

5
Protein and
Amino Acids
Dietary protein generally refers to crude protein (CP), of complementary feed proteins and NPN supplements
whichisdefinedforfeedstuffsasthenitrogen(N)content that will provide the types and amounts of RDP that will
(cid:1) 6.25. The definition is based on the assumption that meet,butnotexceed,theNneedsofruminalmicroorgan-
the average N content of feedstuffs is 16 g per 100 g of isms for maximal synthesis of MCP, and the types and
protein. The calculated CP content includes both protein amounts of digestible RUP that will optimize, in so far
and nonprotein N (NPN). Feedstuffs vary widely in their as possible, the profile and amounts of absorbed AA. As
relative proportions of protein and NPN, in the rate and discussed later, research indicates that the nutritive value
extentofruminaldegradationofprotein,andintheintesti- ofMPfordairycattleisdeterminedbyitsprofileofessen-
naldigestibilityandaminoacid(AA)compositionofrumi- tial AA (EAA) and probably also by the contribution of
nallyundegradedfeedprotein.TheNPNinfeedandsup- totalEAAtoMP.Improvingtheefficiencyofproteinand
plementssuchasureaandammoniumsaltsareconsidered Nusagewhilestrivingforoptimalproductivityisamatter
to be degraded completely in the rumen. ofpracticalconcern.Incentivesincludereducedfeedcosts
per unit of lean tissue gain or milk protein produced, a
desireforgreaterandmoreefficientyieldsofmilkprotein,
creation of space in the diet for other nutrients that will
IMPORTANCE AND GOALS OF PROTEIN
enhance production, and concerns of waste N disposal.
AND AMINO ACID NUTRITION
Regardingmilkproteinproduction,researchindicatesthat
Ruminally synthesized microbial CP (MCP), ruminally content (and thus yield) of milk protein can be increased
undegraded feed CP(RUP), and to amuch lesser extent, by improving the profile of AA in MP, by reducing the
endogenousCP(ECP)contributetopassageofmetaboliz- amountof‘‘surplus’’proteininthediet,andbyincreasing
able protein (MP) to the small intestine. Metabolizable the amount of fermentable carbohydrate in the diet.
proteinisdefinedasthetrueproteinthatisdigestedpostru-
minallyandthecomponentAAabsorbedbytheintestine.
Major Differences from Previous Edition
Amino acids, and not protein per se, are the required
nutrients.AbsorbedAA,usedprincipallyasbuildingblocks In1985,theSubcommitteeonNitrogenUsageinRumi-
forthesynthesisofproteins,arevitaltothemaintenance, nants(NationalResearchCouncil,1985)expressedprotein
growth,reproduction,andlactationofdairycattle.Presum- requirementsinunitsofabsorbedprotein.Absorbedpro-
ably, an ideal pattern of absorbed AA exists for each of teinwasdefinedasthedigestibletrueprotein(i.e.,digest-
thesephysiologicfunctions.TheNutrientRequirementsof ible total AA) that is provided to the animal by ruminally
Poultry(NationalResearchCouncil,1994)andtheNutri- synthesized MCP and feed protein that escaped ruminal
ent Requirements of Swine (National Research Council, degradation. This approach was adopted for the previous
1998) indicate that an optimum AA profile exists in MP edition of this publication (National Research Council,
foreachphysiologicstateoftheanimalandthisisassumed 1989). The absorbed proteinmethod introduced the con-
to be true for dairy animals. ceptofdegradedintakeCP(DIP)andundegradedintake
The goals of ruminant protein nutrition are to provide CP (UIP). Mean values of ruminal undegradability for
adequateamountsofrumen-degradableprotein(RDP)for common feeds, derived from in vivo and in situ studies
optimalruminalefficiencyandtoobtainthedesiredanimal using sheep and cattle, were reported. This factorial
productivitywithaminimumamountofdietaryCP.Opti- approach for estimating protein requirements recognized
mizingtheefficiencyofuseofdietaryCPrequiresselection the three fates of dietary protein (fermentative digestion
43
44 Nutrient Requirements of Dairy Cattle
in the reticulo-rumen, hydrolytic/enzymatic digestion in PROTEIN
theintestine,andpassageofindigestibleproteinwithfeces)
Chemistry of Feed Crude Protein
andseparatedtherequirementsofruminalmicroorganisms
fromthoseofthehostanimal.However,afixedintestinal Feedstuffscontainnumerousdifferentproteinsandsev-
digestibility of 80 percentfor UIP was used, no consider- eraltypesofNPNcompounds.Proteinsarelargemolecules
ation was given to the contribution of endogenous CP to thatdifferinsize,shape,function,solubility,andAAcom-
MP,andnoconsiderationwasgiventotheAAcomposition position.Proteinshavebeenclassifiedonthebasisoftheir
of UIP or of absorbed protein. 3-dimensional structure and solubility characteristics.
Somedifferencesexistinterminology.Tobeconsistent Examples of classifications based on solubility would
withthecurrenteditionofNutrientRequirementsofBeef include globular proteins [albumins (soluble in water and
Cattle (National Research Council, 1996), and to avoid alkalisolutionsandinsolubleinsaltandalcohol),globulins
implications that proteins are absorbed, the term MP (soluble in salt and alkali solutions and sparingly soluble
replacesabsorbedprotein.TobeconsistentwiththeJour- or insoluble in water and insoluble in alcohol), glutelins
nalofDairyScience,thetermsDIPandUIParereplaced (soluble only in alkali), prolamines (soluble in 70 to 80
with RDP and RUP, respectively. percent ethanol and alkali and insoluble in water, salt,
Theprimarydifferencesbetweentheproteinsystemof and absolute alcohol), histones (soluble in water and salt
thispublicationandthatusedinthepreviouseditionrelate solutions and insoluble in ammonium hydroxide)] and
topredictingnutrientsupply.MicrobialCPflowsarepre- fibrous proteins [e.g., collagens, elastins, and keratins
dicted from intake of total tract digestible organic matter (insolubleinwaterorsaltsolutionsandresistanttodiges-
(OM)insteadofnetenergyintake.Theregressionequation tiveenzymes)](OrtenandNeuhaus,1975;Rodwell,1985;
considers the variability in efficiency of MCP production Van Soest, 1994). Globular proteins are common to all
associatedwithapparentadequacyofRDP.Amechanistic feedstuffswhereasfibrousproteinsarelimitedtofeedsof
system developed fromin situ data isused for calculating animalandmarineorigin.Albuminsandglobularproteins
theRUPcontentoffeedstuffs.Insofarasregressionequa- arelowmolecularweightproteins.Prolaminesandglutel-
tionsallow,thesystemconsiderssomeofthefactors(DMI, insarehighermolecularweightproteinsandcontainmore
percentageofconcentratefeedsindietDM,andpercent- disulfide bonds. Generally, feeds of plant origin contain
ageNDFindietDM)thataffectratesofpassageofundi- all of the globular proteins but in differing amounts. For
gestedfeedandthustheRUPcontentofafeedstuff.The example,cerealgrainsandby-productfeedsderivedfrom
system is considered to be applicable to all dairy animals cereal grains contain more glutelins and prolamines
withbodyweightsgreaterthan100kgandthatarefedfor whereasleavesandstemsarerichinalbumins(Blethenet
early rumen development. To increase the accuracy of al., 1990; Sniffen, 1974; Van Soest, 1994). A sequential
estimatingthecontributionoftheRUPfractionofindivid- extractionof38differentfeedswithwater,dilutesalt(0.5
ual feedstuffs to MP, estimates of intestinal digestibility percentNaCl),aqueousalcohol(80percentethanol),and
have been assigned to the RUP fraction of each feedstuff dilutealkali(0.2percentNaOH)indicatedthattheclassic
(range (cid:2) 50 to 100). Endogenous protein and NPN also proteinfractions(albumins,globulins,prolamines,andglu-
areconsideredtocontributetopassageofCPtothesmall telins) plus NPN accounted for an average of 65 percent
intestine.EndogenousCPflowsarecalculatedfromintake of total N (Blethen et al., 1990). The unaccounted for,
ofDM.Andfinally,regressionequationsareincludedthat insolubleNwouldincludeproteinboundinintactaleurone
predict directly the content of each EAA in total EAA of granules of cereal grains, most of the cell-wall associated
duodenalproteinandflowsoftotalEAA.Flowsofdigest- proteins, and some of the chloroplasmic and heat-dena-
ible EAA and their contribution to MP are calculated. tured proteins that are associated with NDF (Van Soest,
Dose-response curves that relate measured milk protein 1994). Among the feeds that were evaluated, those with
content and yield responses to changes of predicted per- thehighestpercentageofinsolubleprotein((cid:1)40percent
centages of digestible Lys and Met in MP are presented. of CP) were forages, beet pulp, soy hulls, sorghum, dried
The dose-response relationships provide estimates of brewersgrains,drieddistillersgrains,fishmeal,andmeat
model-determined amounts of Lys and Met required in and bone meal (Blethen et al., 1990).
MPforoptimalutilizationofabsorbedAAformilkprotein Feedstuffsalsocontainvariableamountsoflowmolecu-
production.Theinclusionofequationsforpredictingpas- lar weight NPN compounds. These compounds include
sage of EAA to the small intestine along with assignment peptides, free AA, nucleic acids, amides, amines, and
of RUP digestibility values that are unique to individual ammonia. Nonprotein N compounds generally are deter-
feedstuffsbringsawarenesstodifferencesinnutritivevalue minedastheNremaininginthefiltrateafterprecipitation
of RUP from different feedstuffs and should improve the of the true protein with either tungstic or trichloroacetic
prediction of animal responses to substitution of protein acid (Licitra et al., 1996). Grasses and legume forages
sources. contain the highest and most variable concentrations of
Protein and Amino Acids 45
NPN.MostofthereportedconcentrationsofNPNinCP organismsintherumen(1010–11/ml)and40percentormore
of grasses and legume forages are within the following of isolated species exhibit proteolytic activity (Broderick
ranges:freshmaterial(10B15%),hay(15B25%),andsilage etal.,1991;CottaandHespell,1984;Wallace,1996).Most
(30B65%)(Fairbairnetal.,1988;Garciaetal.,1989;Grum bacterial proteases are associated with the cell surface
et al., 1991; Hughes, 1970; Krishnamoorthy et al., 1982; (Kopecny and Wallace, 1982); only about 10 percent of
Messman et al., 1994; Van Soest, 1994; Xu et al., 1996). the total proteolytic activity is cell free (Broderick, 1998).
HaysandespeciallysilagescontainhigheramountsofNPN Therefore,theinitialstepinproteindegradationbyrumi-
than thesame feedwhen freshbecause ofthe proteolysis nal bacteria is adsorption of soluble proteins to bacteria
thatoccursduringwiltingandfermentation.Theproteoly- (NugentandMangan,1981;Wallace,1985)oradsorption
sis that occurs in forages during wilting and ensiling is a of bacteria to insoluble proteins (Broderick et al., 1991).
result of plant and microbial proteases and peptidases. Extracellular proteolysis gives rise to oligopeptides which
Plantproteasesandpeptidasesareactiveincutforageand aredegradedfurthertosmallpeptidesandsomefreeAA.
areconsideredtobetheprincipalenzymesresponsiblefor FollowingbacterialuptakeofsmallpeptidesandfreeAA,
theconversionoftrueproteintoNPNinhaysandensiled there are five distinct intracellular events: (1) cleavage of
feeds(Fairbairnetal.,1988;VanSoest,1994).Rapidwilt- peptides to free AA, (2) utilization of free AA for protein
ingofcutforagesandconditionsthatpromoterapidreduc- synthesis,(3)catabolismoffreeAAtoammoniaandcarbon
tions in pH of ensiled feeds slow proteolysis and reduce skeletons(i.e.,deamination),(4)utilizationofammoniafor
theconversionoftrueproteintoNPN(Garciaetal.,1989; resynthesisofAA,and(5)diffusionofammoniaoutofthe
Van Soest, 1994). The NPN content of fresh forage is cell (Broderick, 1998).
composed largely of peptides, free AA, and nitrates (Van The bacterial population that is responsible for AA
Soest,1994).Fermentedforageshaveadifferentcomposi- deaminationhasbeenofconsiderableinterest.Aminoacid
tion of NPN than fresh forages. Fermented forages have catabolismandammoniaproductioninexcessofbacterial
higher proportional concentrations of free AA, ammonia, need wastes dietary CP and reduces efficiency of use of
and amines and lower concentrations of peptides and RDP for ruminant production. For many years it was
nitrate(Fairbairnetal.,1988;VanSoest,1994).TheNPN assumedthatdeaminationwaslimitedtothelargenumber
content of most non-forage feeds is 12 percent or less of of species ofbacteria that had beenidentified to produce
CP (Krishnamoorthy et al., 1982; Licitra et al., 1996; Van ammonia from protein or protein hydrolyzates (Wallace,
Soest, 1994; Xu et al., 1996). 1996).However,thisassumptionwaschallengedbyRussell
andco-workers(ChenandRussell,1988,1989;Russellet
al., 1988) who concluded that the deaminative activity of
Mechanism of Ruminal Protein Degradation
thesebacteriawastoolowtoaccountforratesofammonia
The potentially fermentable pool of protein includes productionusuallyobservedinvivoorinvitrowithmixed
feed proteins plus the endogenous proteins of saliva, cultures.Theireffortsledtotheeventualisolationofasmall
sloughedepithelialcells,andtheremainsoflysedruminal group ofbacteria that had exceptionallyhigh deaminative
microorganisms. The mechanism of ruminal degradation activity and that used AA as their main source of carbon
hasbeenreviewed(Brodericketal.,1991;Broderick,1998; and energy (Russell et al., 1988; Paster et al., 1993). As a
CottaandHespell,1984;Jouany,1996;JouanyandUshida, result of these and other studies, it is now accepted that
1999; Wallace, 1996; Wallace et al., 1999). In brief, all of AAdeaminationbybacteriaiscarriedoutbyacombination
the enzymatic activity of ruminal protein degradation is of numerous bacteria with low deaminative activity and a
of microbial origin. Many strains and species of bacteria, much smallernumber ofbacteria with highactivity (Wal-
protozoa,andanaerobicfungiparticipatebyelaboratinga lace,1996).Ofparticularinteresthasbeentheobservation
varietyofproteases,peptidases,anddeaminases(Wallace, thatthegrowthofsomeofthesebacteriawithhighdeami-
1996).Theliberatedpeptides,AA,andammoniaarenutri- nating activity is suppressed by the ionophore, monensin
ents for the growth of ruminal microorganisms. Peptide (Chen and Russell, 1988, 1989; Russell et al., 1988).
breakdownto AAmust occurbeforeAA areincorporated Protozoa also are active and significant participants in
intomicrobialprotein(Wallace,1996).Whenproteindeg- ruminal protein degradation. Protozoa are less numerous
radationexceedstherateofAAandammoniaassimilation than bacteria in ruminal contents (105–6/ml) but because
into microbial protein, peptide and AA catabolism leads of their large size, they comprise a significant portion of
toexcessiveruminalammoniaconcentrations.Someofthe the total microbial biomass in the rumen (generally less
peptides and AA not incorporated into microbial protein than 10 percent but sometimes as high as 50 percent)
mayescaperuminaldegradationtoammoniaandbecome (Jouany, 1996; Jouany and Ushida, 1999). Several differ-
sources of absorbed AA to the host animal. encesexistbetweenprotozoaandbacteriaintheirmetabo-
Bacteria are the principal microorganisms involved in lism of protein. First, they differ in feeding behavior.
proteindegradation.Bacteriaarethemostabundantmicro- Instead of forming a complex with feeds, protozoa ingest
46 Nutrient Requirements of Dairy Cattle
particulate matter (bacteria, fungi, and small feed parti- proteinistheresultoftwosimultaneousactivities,degrada-
cles).Bacteriaaretheirprincipalsourceofingestedprotein tionandpassage.Oneofthemorecomplexofthesemodels
(Jouany and Ushida, 1999). As a result of this feeding istheCornellNetCarbohydrateProteinSystem(CNCPS)
behavior(i.e.,ingestionoffood),protozoaaremoreactive (Sniffen et al., 1992). In this model, feed CP is divided
in degrading insoluble feed proteins (e.g., soybean meal into five fractions (A, B, B, B, and C) which sum to
1 2 3
orfishmeal)thanmoresolublefeedproteins(e.g.,casein) unity. The five fractions have different rates of ruminal
(HinoandRussell,1987;Jouany,1996;JouanyandUshida, degradation. Fraction A (NPN) is the percentage of CP
1999). Ingested proteins are degraded within the cell to that is instantaneously solubilized at time zero, which is
yieldamixtureofpeptidesandfreeAA;theAAareincorpo- assumed to have a degradation rate (k) of infinity; it is
d
ratedintoprotozoalprotein.Proteolyticspecificactivityof determined chemically as that proportion of CP that is
protozoa is higher than that of bacteria (Nolan, 1993). A soluble in borate-phosphate buffer but not precipitated
second difference between protozoa and bacteria is that with the protein denaturant, trichloroacetic acetic (TCA)
while both actively deaminate AA, protozoa are not able (Figure 5-1). Fraction C is determined chemically as the
tosynthesizeAAfromammonia(JouanyandUshida,1999). percentage of total CP recovered with ADF (i.e., ADIN)
Thus,protozoaarenetexportersofammoniaandbecause andisconsideredtobeundegradable.FractionCcontains
ofthis,defaunationdecreasesruminalammoniaconcentra- proteinsassociatedwithligninandtanninsandheat-dam-
tions (Jouany and Ushida, 1999). And lastly, protozoa agedproteinssuchastheMaillardreactionproducts(Snif-
releaselargeamountsofpeptidesandAAaswellaspepti- fen et al., 1992). The remaining B fractions represent
dases into ruminal fluid. This is the result of significant potentially degradable true protein. The amounts of each
secretory processes and significant autolysis and death of these 3 fractions that are degraded in the rumen are
(Coleman,1985;Dijkstra,1994).JouanyandUshida(1999) determinedbytheirfractionalratesofdegradation(k)and
d
suggestthatexcretedsmallpeptidesandAAcanrepresent passage (k); a single k value is used for all fractions.
p p
50 percent of total protein ingested by protozoa. Other Fraction B is that percentage of total CP that is soluble
1
studiesindicatethat65percentormoreofprotozoalpro- in borate-phosphate buffer and precipitated with TCA.
teinrecycleswithintherumen(FfoulkesandLeng,1988; Fraction B is calculated as the difference between the
3
Punia et al., 1992). portionsoftotalCPrecoveredwithNDF(i.e.,NDIN)and
Much less is known about the involvement of fungi in ADF (i.e., fraction C). Fraction B is the remaining CP
2
ruminalproteincatabolism.Currently,anaerobicfungiare and is calculated as total CP minus the sum of fractions
considered to have negligible effects on ruminal protein A, B, B, and C. Reported ranges for the fractional rates
1 3
digestion because of their low concentrations in ruminal of degradation for the three B fractions are: B (120–400
1
digesta (103–4/ml) (Jouany and Ushida, 1999; Wallace and %/h), B (3–16 %/h), and B (0.06–0.55 %/h). The RDP
2 3
Monroe, 1986). andRUPvalues(percentofCP) forafeedstuffusingthis
model are computed using the equations
Kinetics of Ruminal Protein Degradation RDP (cid:2)A (cid:3) B [kB / (kB (cid:3) k)]
1 d 1 d 1 p
(cid:3) B [kB / (kB (cid:3) k)]
RuminaldegradationofdietaryfeedCPisanimportant 2 d 2 d 2 p
(cid:3) B [kB / (kB (cid:3) k)]
factorinfluencingruminalfermentationandAAsupplyto 3 d 3 d 3 p
dairycattle.RDPandRUParetwocomponentsofdietary and
feedCPthathaveseparateanddistinctfunctions.Rumina- RUP (cid:2)B [k / (kB (cid:3) k)]
llydegraded feedCPprovides amixtureof peptides,free 1 p d 1 p
(cid:3) B [k / (kB (cid:3) k)]
AA, and ammonia for microbial growth and synthesis of 2 p d 2 p
(cid:3) B [k / (kB (cid:3) k)] (cid:3) C.
microbialprotein.Ruminallysynthesizedmicrobialprotein 3 p d 3 p
typicallysuppliesmostoftheAApassingtothesmallintes- ThismodelisusedinLevelIIoftheNutrientRequirements
tine. Ruminally undegraded protein is the second most of Beef Cattle (National Research Council, 1996) report.
importantsource ofabsorbable AAto theanimal. Knowl- Themostusedmodeltodescribeinsituruminalprotein
edgeofthekineticsofruminaldegradationoffeedproteins degradationdividesfeedCPintothreefractions(A,B,and
isfundamentaltoformulatingdietsforadequateamounts C). Fraction A is the percentage of total CP that is NPN
ofRDPforrumenmicroorganismsandadequateamounts (i.e.,assumedtobeinstantlydegraded)andasmallamount
of RUP for the host animal. of true protein that rapidly escapes from the in situ bag
Ruminalproteindegradationisdescribedmostoftenby becauseofhighsolubilityorverysmallparticlesize.Frac-
first order mass action models. An important feature of tionCisthepercentageofCPthatiscompletelyundegrad-
these models is that they consider that the CP fraction of able; this fraction generally is determined as the feed CP
feedstuffs consists of multiple fractions that differ widely remaininginthebagatadefinedend-pointofdegradation.
inratesofdegradation,andthatruminaldisappearanceof FractionBistherestoftheCPandincludestheproteins
Protein and Amino Acids 47
contributelittleRUPtothehostanimal.Whendairycattle
are fed all-forage diets, measurements of passage of non-
ammonia,non-microbialN(i.e.,RUP-Nplusendogenous
N) often are less than 30 percent of N intake (Beever et
al., 1976, 1987; Holden et al., 1994a; Van Vuuren et al.,
1992). 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 giveninTables15-2a,bare1.4forMenhadenfishmeal
d
to 29.2 for sunflower meal. Assuming a k for each feed
p
of7.0percent,therangeindegradabilitesoftheBfraction
would be 16.7 to 80.7 percent. Some characteristics of
proteins shown to contribute to differences in rates of
FIGURE5-1 Analysesofcrudeproteinfractionsusingborate-
phosphatebufferandaciddetergentandneutraldetergentsolu- degradationaredifferencesin3-dimensionalstructure,dif-
tions(Roeetal.,1990;Sniffenetal,1992). ferencesinintra-andinter-molecularbonding,inertbarri-
ers such as cell walls, and antinutritional factors.
Differences in 3-dimensional structure and chemical
thatarepotentiallydegradable.OnlytheBfractioniscon-
bonding (i.e., cross-links) that occur both within and
sidered to be affected by relative rates of passage; all of
betweenproteinmoleculesandbetweenproteinsandcar-
fractionAisconsideredtobedegradedandalloffraction
bohydrates are functions of source as well as processing.
Cisconsideredtopasstothesmallintestine.Theamount
These aspects of structure affect microbial access to the
offractionBthatisdegradedintherumenisdetermined
proteins, which apparently is the most important factor
bythefractionalrateofdegradationthatisdeterminedin
affectingtherate andextentofdegradation ofproteinsin
thestudyforfractionBandanestimateoffractionalrates
the rumen. Proteins that possess extensive cross-linking,
ofpassage.TheRDPandRUPvaluesforafeedstuff(per-
suchasthedisulfidebondinginalbuminsandimmunoglob-
centofCP)usingthismodelarecomputedusingtheequa-
tions RDP (cid:2) A (cid:3) B [k / (k (cid:3) k)] and RUP (cid:2) B [k / ulins or cross-links causedby chemical or heat treatment,
(k (cid:3) k)] (cid:3) C. This sdimpled modpel has been the mopst arelessaccessibletoproteolyticenzymesandaredegraded
d p more slowly (Ferguson, 1975; Hurrell and Finot, 1985;
widelyusedmodelfordescribingdegradationandruminal
Mahadevanetal.,1980;Mangan,1972;NugentandMan-
escape of feed proteins (e.g., AFRC, 1984; National
gan,1978;Nugentetal.,1983;Wallace,1983).Proteinsin
ResearchCouncil,1985;ØrskovandMcDonald,1979).It
feathersandhairareextensivelycross-linkedwithdisulfide
isnotedthatdataobtainedfrominsitu,invitro,andenzy-
bonds and largely for that reason, a considerable amount
maticdigestionsgenerallyfitamodelthatdividesfeedCP
of the protein in feather meal is in fraction C (Tables 15-
into thesefractions (Brodericket al.,1991) andthat most
2a,b).Similarly,aconsiderableportionoftheproteininmeat
of the in situ data used to validate results obtained with
meal and meat and bone meal is in fraction C. Proteins in
cell-free proteases have been obtained using this model
meatmealandmeatandbonemealmaycontainconsiderable
(Broderick, 1998). As discussed later, it is this model in
amountsofcollagenthathasbothintramolecularandinter-
conjunction with in situ derived data that is used for pre-
molecularcross-links(OrtenandNeuhaus,1975).Incontrast,
dicting ruminal protein degradability in this edition.
amajorityoftheproteininmenhadenfishmealisinfraction
NumerousfactorsaffecttheamountofCPinfeedsthat
willbedegradedintherumen.ThechemistryoffeedCP BbutthefractionalrateofdegradationoffractionBisslower
isthesinglemostimportantfactor.Thetwomostimportant than in other protein supplements (Tables 15-2a,b). Heat
considerations of feed CP chemistry are: (1) the propor- used in the drying of fish protein was shown to induce the
tionalconcentrationsofNPNandtrueprotein,and(2)the formation of disulfide bonds (Opstvedt et al., 1984). Heat
physical and chemical characteristics of the proteins that processing also coagulates protein in meat products which
comprisethetrueproteinfractionofthefeedstuff.Nonpro- makesitinsoluble(Bendall,1964;Boehme,1982),andcool-
tein N compounds are degraded so quickly in the rumen ing of the products causes a random relinkage of chemical
((cid:1)300%/h)thatdegradationisassumedtobe100percent bondswhichshrinkstheproteinmolecules(Bendall,1964).
(Sniffen et al., 1992). However, this is not an entirely Collectively,theseeffectsofheatingandcoolingofproteins
correct assumption because degradability is truly related decreasemicrobialaccessandmaketheproteinsmoreresis-
to rate of passage. For example, assuming a k of 2.0%/h tant to ruminal degradation.
p
andak of300%/h,thendegradation(cid:2)3.00/(3.00(cid:3)0.02) Otherfactorsaffectingtheruminaldegradabilityoffeed
d
(cid:2) 0.993 or 99.3 percent, and not 1.00 or 100 percent. protein include ruminal retention time of the protein,
FeedstuffsthatcontainhighconcentrationsofNPNinCP microbial proteolytic activity, and ruminal pH. The effect
48 Nutrient Requirements of Dairy Cattle
ofthesefactorsonthekineticsofruminalproteindegrada- seedmeal,andfishmealweredegradedatdifferentrates
tion have been reviewed (Broderick et al., 1991; National with rates of degradation for all three supplements being
Research Council, 1985). intermediate between those for albumins and casein.
Therefore,structureaswellassolubilitydeterminesdegra-
dability.Third,asindicatedinthesection‘‘Mechanismof
Nitrogen Solubility vs. Protein Degradation
RuminalProteinDegradation’’,solubilityisnotaprerequi-
Several commercial feed testing laboratories in the sitetodegradation.Asanexample,Mahadevanetal.(1980)
UnitedStatesprovideatleastonemeasurementofNsolu- observed that soluble and insoluble proteins of soybean
bilityforfeedstuffs.AlthoughrecognizedthatNsolubility meal were hydrolyzed in vitro at almost identical rates.
inasinglesolventisnotsynonymouswithCPdegradation Becausebacteriaattachtoinsolubleproteinsandbecause
in the rumen, the general absence of alternatives other protozoaengulffeedparticles,insolubleproteinsneednot
thanusing‘‘bookvalues’’forRUP(e.g.,NationalResearch enter the soluble protein pool before attack by microbial
Council, 1985) left little else to help nutritionists ensure proteases. And last, soluble proteins that are not yet
that adequate but not excessive amounts of RDP were degraded may leave the rumen faster than insoluble pro-
fed.Solubilitymeasurementshavebeenusefulforranking teins.Thisisbecauseofamorelikelyassociationofsoluble
feeds of similar types for ruminal CP degradability. This protein with the liquid fraction of ruminal contents. For
is because of the positive relationship that exists between example, Hristov and Broderick (1996) observed that
Nsolubilityanddegradationwithinsimilarfeedstuffs(e.g., althoughfeedNANintheliquidphaseofruminalcontents
Beeveretal.,1976;LaycockandMiller,1981;Madsenand wasonly12percentoftotalruminalfeedNAN,30percent
Hvelplund, 1990; Stutts et al., 1988). Many studies have ofthefeedNANthatescapedtherumenflowedwiththe
indicatedthatchangingNsolubilitybyaddingorremoving liquids. This indicates a disproportional escape of solu-
NPNsupplements,bychangingmethodofforagepreserva- ble proteins.
tion, or processing conditions of protein supplements Inconclusion,achangeinNsolubilityinasinglesolvent
affectsanimalresponse(e.g.,Aitchisonetal.,1976;Crishet appearstobeamoreusefulindicatorofachangeinprotein
al.,1986;Lundquistetal.,1986).Severaldifferentsolvents degradationwhenappliedtodifferentsamplesofthesame
havebeenused.Atpresent,themostcommonprocedure feedstuff than when used to compare different feedstuffs
isincubationinborate-phosphatebuffer(Roeetal.,1990). that differ in chemical and physical properties. Clearly,
This method has gained in popularity because it is used therelationshipbetweensolubilityanddegradabilityisthe
for determining the A and B nitrogen fractions in the highest when most of the soluble N is NPN (Sniffen
1
CNCPS (Sniffen et al., 1992). et al., 1992).
Although a high correlation exists between N solubility
in a single solvent and protein degradability for similar
Microbial Requirements for N Substrates
feedstuffs, the same does not exist across classes of feed-
stuffs. For example, Stern and Satter (1984) reported a Peptides,AA,andammoniaarenutrientsforthegrowth
correlationof0.26betweenNsolubilityandinvivoprotein of ruminal bacteria; protozoa cannot use ammonia. Esti-
degradation in the rumen of 34 diets that contained a mates of the contribution of ammonia versus preformed
variety of N sources. Madsen and Hvelplund (1990) also AA to microbial protein synthesis by the mixed rumen
reported a poor relationship between N solubility and in population have been highly variable (Wallace, 1997).
vivo degradation of CP when used over a range of feed- StudiesusingN15ammoniaorureainfusedintotherumen
stuffs. There appear to be several reasons for these poor or added as a single dose demonstrated that values for
relationships.First,asindicatedinthesection‘‘Chemistry microbialNderivedfromammoniarangedfrom18to100
of Feed Crude Protein’’, the proteins that are extracted percent (Salter et al., 1979). The N15 studies of Nolan
by a solvent depend not only on the chemistry of the (1975) and Leng and Nolan (1984) indicated that 50 per-
proteins but also on the composition of the solvent. For centormoreofthemicrobialNwasderivedfromammonia
that reason, different solvents provide different estimates and the rest from peptides and AA. The mixed ruminal
of CP solubility (Cherney et al., 1992; Crawford et al., microbial population has essentially no absolute require-
1978; Crooker et al., 1978; Lundquist et al., 1986; Stutts mentforAA(Virtanen,1966)ascross-feedingamongbac-
etal.,1988).Second,solubleproteinsarenotequallysus- teria can meet individual requirements. However,
ceptible to degradation by rumen enzymes. Among the researchers have observed improved microbial growth or
pure soluble proteins, casein is degraded rapidly whereas efficiencywhenpeptidesorAAreplacedammoniaorurea
serum albumin, ovalbumin, and ribonuclease A are asthesoleormajorsourceofN(CottaandRussell,1982;
degradedmuch slower(Annison,1956;Mahadevan etal., Russell and Sniffen, 1984; Griswold et al., 1996). Maeng
1980; Mangan, 1972). Mahadevan et al. (1980) also andBaldwin(1976)reportedincreasedmicrobialyieldand
observed that soluble proteins from soybean meal, rape- growth rate on 75% urea (cid:3) 25% AA-N as compared to
Protein and Amino Acids 49
100% urea. Microbial requirements for N substrates of replaced urea as a N source at levels of 0, 10, 20 and 30
ammonia-N,AA,andpeptidescanalsobeaffectedbythe percent of total N, a urea-molasses mixture represented
basal diet and may explain some of the variability in the 8.6,7.0,4.9,and2.9percentofDMwithincreasingpeptide
above experiments. and glucose replacement. Digestion of DM and CP and
There is evidence that AA and especially peptides are microbial CP production were affected quadratically by
stimulatoryintermsofbothgrowthrateandgrowthyield peptideaddition;thehighestvaluesforeachvariableoccur-
for ruminal microorganisms growing on rapidly degraded red at 10 percent peptide addition. Fiber digestion
energy sources (Argyle and Baldwin, 1989; Chen et al., decreased linearly with increasing peptide addition.
1987;CruzSotoetal.,1994;Russelletal.,1983).However, Reduced ammonia-N concentrations appeared to be the
when energy substrates are fermented slowly, stimulation cause of reduced microbial CP production and reduced
by peptides and AA does not always occur. Chikunya et fiberdigestionatlevelsofpeptidesgreaterthan10percent
al.(1996)demonstratedthatwhenpeptidesweresupplied of total N. The efficiency of conversion of peptide N to
withrapidlyorslowlydegradedfiber,microbialgrowthwas microbialCPincreasedwithincreasingpeptides;however,
enhancedonlyifthefiberwasdegradedrapidly.Russellet there was no change in grams of microbial N produced
al.(1992)indicatedthatmicroorganismsfermentingstruc- perkilogramofOMdigested.Jonesetal.(1998)suggested
turalcarbohydratesrequireonlyammoniaastheirNsource that with diets containing high levels of NSC, excessive
while species degrading nonstructural carbohydrate peptide concentrations relative to that of ammonia can
sources will benefit from preformed AA. depress protein digestion and ammonia concentrations,
Recentexperiments(Wallace,1997)haveconfirmedthe limit the growth of fiber-digesting microorganisms, and
earlierresultsofSalteretal.(1979)showingthatthepro- reduceruminalfiberdigestionandmicrobialproteinpro-
portion of microbial N derived from ammonia varies duction. Microorganisms that ferment NSC produce and
according to the availability of N sources. The minimum utilize peptides at the expense of ammonia production
contributiontomicrobialNfromammoniawas26percent fromproteinandotherNsources(Russelletal.,1992).It
whenhighconcentrationsofpeptidesandAAwerepresent, shouldbenotedthatincontinuousculturesystems,proto-
with a potential maximum of 100 percent when ammonia zoa can be washed out in the first few days of operation.
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 Animal Responses to CP, RDP, and RUP
of microorganisms in continuous culture. Griswold et al.
LACTATIONRESPONSES
(1996) demonstrated that N forms other than ammonia
are needed not only for maximum microbial growth but Crudeprotein.Adatasetof393meansfrom82protein
also as NPN for adequate ruminal fiber digestion. studies was used to evaluate the milk and milk protein
ManyreportsoftheuptakeofC14-AAandpeptideshave yieldresponses tochangesinthe concentrationofdietary
indicated that mixed microbial populations preferentially CP (Table 5-1). The descriptive statistics for the data set
took up peptides rather than free AA (Cooper and Ling, are presented in Table 5-2. When CP content of diets
1985; Prins et al., 1979). However, Ling and Armstead change,therelativecontributionofproteinfromdifferent
(1995) found that free AA were the preferred form of sources also change so this evaluation is confounded with
AA incorporated by S. bovis, Selenomonas ruminantium, source of protein and concentrations of RDP and RUP.
Fibrobacter succinogenes and Anaerovibrio lipolytica, Overall,milkyieldincreasedquadraticallyasdietCPcon-
whereaspeptideswerepreferredonlybyP.ruminicola.P. centrations increased. The regression equation obtained
ruminicola can comprise greater than 60 percent of the was:
total flora in sheep fed grass silage (Van Gylswyk, 1990). Milk yield (cid:2) 0.8 (cid:1) DMI (cid:3) 2.3 (cid:1) CP
In other studies where an AA preference was exhibited, (cid:4) 0.05 (cid:1) CP2(cid:4) 9.8 (r2 (cid:2) 0.29)
thepreferencemayhavebeentheresultofspecificdietary
conditionswhereP.ruminicolanumberswerelower.Wal- where milk yield and dry matter intake (DMI) are kilo-
lace (1996) demonstrated that AA deamination is carried grams/d and CP is percent of diet DM.
out by two distinct bacterial populations, one with low Dry matter intake was included in the regression to
activityandhighnumbersandtheotherwithhighactivity accountindirectlyforsomeofthedifferencesamongstud-
and low numbers. P. ruminicola occurs in high numbers ies such as basal milk production and BW. Dry matter
but has low deaminase activity. intake accounted for about 60 percent and CP about 40
Jones et al. (1998) investigated the effects of peptide percent of non-random variation. Assuming a fixed DMI
concentrationsinmicrobialmetabolismincontinuouscul- (there wasno correlation between intakeand CP percent
ture fermenters. The basal diet contained 17.8 percent in this data set), the maximum milk production was
CP, 46.2 percent NSC, and 32.9 percent NDF. Peptides obtained at 23 percent CP. The marginal response to
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
Annexstadetal.(1987) Hendersonetal.(1985) McCormicketal.(1999)
Aharonietal.(1993) Hensonetal.(1997) McGuffeyetal.(1990)
Armentanoetal.(1993) Higginbothametal.(1989) Nakamuraetal.(1992)
Atwaletal.(1995) HoffmanandArmentano(1988) OwenandLarson(1991)
Bakeretal.(1995) Hoffmanetal.(1991) PalmquistandWeiss(1994)
Bertrandetal.(1998) Holteretal.(1992) Palmquistetal.(1993)
BlauwiekelandKincaid(1986) HongerholtandMuller(1998) Polanetal.(1997)
Blauwiekeletal.(1990) Howardetal.(1987) Polanetal.(1985)
Bowmanetal.(1988) Huyleretal.(1999) Powersetal.(1995)
Broderick(1992) Jaquetteetal.(1986) RobinsonandKennelly(1988b)
Brodericketal.(1990) Jaquetteetal.(1987) Robinsonetal.(1991b)
Bruckentaletal.(1989) Kaimetal.(1983) Roseleretal.(1993)
Canfieldetal.(1990) Kaimetal.(1987) Santosetal.(1998a,b)
Casperetal.(1990) Kalscheuretal.(1999a,b) Sloanetal.(1988)
Chenetal.(1993) KerryandAmos(1993) Spainetal.(1995)
Christensenetal.(1993a,b) Khorasanietal.(1996a) Vossetal.(1988)
CrawleyandKilmer(1995) Kimetal.(1991) Wattiauxetal.(1994)
Cunninghametal.(1996) Kingetal.(1990) Weigeletal.(1997)
DeGraciaetal.(1989) Klusmeyeretal.(1990) Wheeleretal.(1995)
DePetersandBath(1986) KomaragiriandErdman(1997) Windschitl(1991)
DhimanandSatter(1993) Leesetal.(1990) Wohltetal.(1991)
Garcia-Bojaliletal.(1998a) LeonardandBlock(1988) Wright(1996)
GrantandHaddad(1998) Lundquistetal.(1986) Wuetal.(1997)
Gringsetal.(1991) MacleodandCahill(1987) WuandSatter(2000)
Gringsetal.(1992a) MansonandLeaver(1988) Zimmermanetal.(1992)
Grummeretal.(1996) Mantysaarietal.(1989) Zimmermanetal.(1991)
HadsellandSommerfeldt(1988) McCarthyetal.(1989)
TABLE 5-2 Descriptive Statistics for Data Set Used milkproteinyield(g/d)(cid:2)17.7(cid:1)DMI(cid:3)55.6(cid:1)CP(cid:4)
to Evaluate Animal Responses to CP and RDP 1.26(cid:1)CP2(cid:3)31.8(r2(cid:2)0.19)whereDMIiskilograms/
dayandCPispercentofdietDM.Maximumyieldofmilk
Variable N Mean Std.Dev.
protein was obtained at 22 percent CP (essentially the
Milk,kg/d 393 31.4 6.1 sameasformilkyield)andthemarginalresponseisequal
Milkproteinyield,g/d 360 972 153 to55.63(cid:4)2.52(cid:1)CPwhereCPisapercentofdietDM.
Drymatterintake,kg/d 393 20.2 3.4
CP,%ofdrymatter 393 17.1 2.6 Rumendegradableandundegradableprotein.Aregres-
RDP,%ofdrymatter 172 10.7 1.8 sionapproachalsowasusedtoevaluatelactationresponses
RUP,%ofdrymatter 172 6.2 1.4
toconcentrationsofRDPandRUPinthedietaryDM.To
evaluatelactationresponsestoRDPindietDM,38studies
with 206 treatment means were selected in which diets
increaseddietaryCP(firstderivativeoftheCPcomponents
varied in content of RDP (Table 5-3). All diets were
oftheregressionequation)is:2.3(cid:4)0.1(cid:1)CP.Therefore,
enteredintothisedition’smodelforpredictedconcentra-
increasing dietary CP one percentage unit from 15 to 16
tionsofRDPandRUPindietDM.Asexpected,concentra-
percentwouldbeexpectedtoincreasemilkyieldanaver- tionsofRDPandRUP(aspercentagesofdietDM)were
age of 0.75 kg/d and increasing CP one percentage unit correlated with concentrations of dietary CP (RDP; r (cid:2)
from19to20percentwouldbeexpectedtoincreasemilk 0.78, P(cid:2)0.001; RUP, r (cid:2) 0.53, P(cid:2)0.001), therefore it is
yield by 0.35 kg/d. Although milk production may be not possible to separate effects of total CP from those of
increasedbyfeedingdietswithextremelyhighconcentra- RDP or RUP. A regression equation for milk yield with
tions of CP, the economic and environmental costs must RDP and RUP (both as percent of DM) was derived to
becomparedwithlowerCPdiets.Themarginalresponse overcome the problems associated with the correlation
obtainedfromthisdatasetwassimilartothatobtainedby betweenCPandRDPandRUP(thecorrelationbetween
Roffleretal.(1986).Withtheirequation,increasingdietary RDP and RUP was not significant (r (cid:2) (cid:4)0.11, P(cid:1)0.05).
CP from 14 to 18 percent would result in an increase of Dietary RDP and RUP were calculated using the model
2.1kg/dofmilkandwiththeequationabovetheexpected described in this publication based on values in the data
increase is 2.8 kg/d. setdescribedabove.Theregressionequationalsoincluded
DietaryCPwasnotcorrelated(P(cid:1)0.25)withmilkpro- DMIforthereasonsexplainedabove.Theregressionequa-
teinpercent,butwascorrelatedweakly(r(cid:2)0.14;P(cid:2)0.01) tion (Figure 5-2) was:
with milk protein yield (because of the relationship of Milk(cid:2)(cid:4)55.61(cid:3)1.15(cid:1)DMI(cid:3)8.79(cid:1)RDP(cid:4)0.36
dietary CPwith milk yield). Theregression equation was: (cid:1) RDP2 (cid:3) 1.85 (cid:1) RUP (r2 (cid:2) 0.52)
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
Annexstadetal.(1987) Gringsetal.(1992) Kingetal.(1990)
Armentanoetal.(1993) Grummeretal.(1996) KomaragiriandErdman(1997)
Bakeretal.(1995) HaandKennelly(1984) LeonardandBlock(1988)
Barneyetal.(1981) Harrisetal.(1992) Mantysaarietal.(1989)
Bertrandetal.(1998) Hensonetal.(1997) McGuffeyetal.(1990)
Blauwiekeletal.(1990) Higginbothametal.(1989) PalmquistandWeiss(1994)
Casperetal.(1990) Hoffmanetal.(1991) Roseleretal.(1993)
Christensenetal.(1993a,b) Holteretal.(1992) Santosetal.(1998a,b)
Cunninghametal.(1996) HongerholtandMuller(1998) Wattiauxetal.(1994)
DhimanandSatter(1993) Kalscheuretal.(1999a) Weigeletal.(1997)
Garcia-Bojaliletal.(1998a) Khorasanietal.(1996b) Windschitl(1991)
GrantandHaddad(1998) Kimetal.(1991) WuandSatter(2000)
Gringsetal.(1991)
(cid:1) RDP; r (cid:2) 0.35, P(cid:2)0.001). Based on that regression,
anincreasein2percentageunitsofRDP(i.e.,10.2to12.2
percent)wouldincreaseDMIbyabout1.1kg/d.Basedon
thisedition’srequirements(assumed72percentTDN),an
increase of about 2 kg/d of milk is expected from that
changeinDMI.IncreasingdietaryRDPabovemodelpre-
dicted requirements may result in increased DM intake.
Asimilarshapedfunction(datanotshown)wasobtained
when milk protein yield was regressed on dietary RDP
and RUP:
Milk protein (cid:2) (cid:4)1.57 (cid:3) 0.0275 (cid:1) DMI (cid:3) 0.223
(cid:1)RDP (cid:4) 0.0091 (cid:1) RDP2 (cid:3) 0.041
(cid:1)RUP (r2(cid:2) 0.51)
where milk protein and DMI are kilograms per day and
RDPandRUParepercentagesofdietaryDM.Maximum
FIGURE5-2 Responsesurfacefordatasetdescribedin‘‘Ani-
milkproteinyieldoccurredat12.2percentRDP(thesame
malResponsestoCP,RDP,andRUP’’section.Maximummilk
as milk yield). Milk protein yield increased linearly with
yieldoccurred at12.2 percentRDP(percent ofdiet DM).Dry
matterintakewasheldconstantat20.6kg/day. increasing dietary RUP.
Santosetal.(1998b)publishedacomprehensivereview
of the effects of replacing soybean meal with various
where DMI and milk are kilograms/day, and RDP and sourcesofRUPonproteinmetabolism(29publishedcom-
RUP are percent of diet DM. Based on that equation, parisons) and production (127 published comparisons).
maximum milk yield occurred (DMI and RUP held con- Santos et al. (1998b) reported that in 76 percent of the
stant) when RDP equaled 12.2 percent of diet DM, and metabolismstudies,higherRUPdecreasedMCPflowsto
the marginal change in milk to increasing RDP was 8.79 thesmallintestine.SupplementationwithRUPusuallydid
(cid:4) 0.72 (cid:1) RDP. The quadratic term for RUP was not not affect flow of total EAA, and RUP supplementation
significant and was removed from the model. Milk yield usuallydidnotincreaseoractuallydecreasedflowoflysine
increase linearly to RUP at the rate of 1.85 kg for each totheduodenum.SupplementationofRUPincreasedmilk
percentage unit increase in RUP. production in only 17 percent of the studies and heat-
Incomparisonthisedition’smodelestimatesanaverage treated or chemically-treated soybean meal or fish meal
RDP requirement of 10.2 percent for this data set. Pre- werethemostlikelyRUPsupplementstocauseincreased
dicted milkyield (using the aboveregression equation) at milkproduction(Santosetal.,1998b).Whenstudieswere
10.2 percent RDP (DMI and RUP held constant mean combined, cows fed diets with treated soybean meal
values of the data set of 20.6 kg/d DMI and 6.2 percent, (P(cid:2)0.03)orfishmeal(P(cid:2)0.01)producedstatisticallymore
respectively)is31.7kg/dand33.2kg/dwhenRDPis12.2 milk than cows fed soybean meal. Cows fed other animal
percent.Aportionofthediscrepancybetweenmodelpre- proteins (blood,feather, meatmeals) or corngluten meal
dictedrequirementforRDPandregressionpredictedmax- produced similar or numerically less milk than cows fed
imalmilkproductionmaybecausedbythepositivecorrela- soybeanmeal(Santosetal.,1998b).Seeadditionaldiscus-
tionbetween RDPandDMintake (DMI(cid:2)14.4 (cid:3)0.58 sion in Chapter 16.
52 Nutrient Requirements of Dairy Cattle
The regression equations derived above for milk and milkproduction.Delayedovulation(e.g.,BeamandButler,
milkproteinyieldresponsestodietaryCP,RDP,andRUP 1997; Staples et al., 1990) and reduced fertility (Butler,
should be interpreted and used cautiously in view of low 1998) have been associated with negative energy status.
r2 values. A more sophisticated statistical analysis (e.g., Another effect of negative energy status is decreased
controlling for trial effects, adjusting for variances within plasma progesterone concentrations (Butler, 1998).
trials,etc.)wouldprobablyyielddifferentandmoreaccu- Another theory is that excessive blood urea N (BUN)
rate coefficients. concentrationscouldhaveatoxiceffectonsperm,ova,or
embryos, resulting in a decrease in fertility (Canfield et
al.,1990).HighBUNconcentrationshavealsobeenshown
EFFECTSONREPRODUCTION todecreaseuterinepHandprostaglandinproduction(But-
ler, 1998). High BUN may also reduce the binding of
Protein in excess of lactation requirements has been
leutinizing hormone to ovarian receptors, leading to
shown to have negative effects on reproduction. Several
decreasesinserumprogesteroneconcentrationandfertil-
workers have reported that feeding diets containing 19
ity(Barton,1996a).FergusonandChalupa(1989)reported
percentormoreCPindietDMloweredconceptionrates
thatby-productsofNmetabolismmayalterthefunctionof
(Bruckental et al., 1989; Canfield et al., 1990; Jordan and
thehypophysealpituitary-ovarianaxis,thereforedecreasing
Swanson, 1979; McCormick et al., 1999). Others have
reproductiveperformance.Andlast,highlevelsofcirculat-
observed that cows fed 20–23 percent CP diets (as com-
ingammoniamaydepresstheimmunesystemand,there-
pared to 12–15 percent CP) had decreased uterine pH,
fore, mayresult in a declinein reproductive performance
increasedbloodurea,andaltereduterinefluidcomposition
(Anderson and Barton, 1988).
(Jordanetal.,1983;ElrodandButler,1993).Inamajority
Milk urea nitrogen (MUN) and blood urea nitrogen
ofthestudiesreviewedbyButler(1998),plasmaprogester-
(BUN)arebothindicatorsofureaproductionbytheliver.
oneconcentrationsinearlylactationcowswerelowerwhen
Milk urea N concentrations greater than 19 mg/dl have
dietscontained19–20percentCPvs.lowerconcentrations been associated with decreased fertility (Butler et al.,
of CP. 1995).Likewise,BUNconcentrationsgreaterthan20mg/
In a review of protein effects on reproduction, Butler dlhavebeenlinkedwithreducedconceptionratesinlactat-
(1998) concluded that excessive amounts of either RDP ing cows (Ferguson et al., 1988). Bruckental et al. (1989)
or RUP could be responsible for lowered reproductive found that BUN levels increased when diet CP was
performance. However, intakes of ‘‘digestible’’ RUP in increased from 17 to 21.6 percent and pregnancy rate
amountsrequiredtoadverselyaffectreproductionwithout decreasedby13percentageunits.Inacasestudy,Ferguson
acoincidingsurplusofRDPwouldbeuncommon.Inmost et al. (1988) observed that cows with BUN levels higher
of the studies reviewed by Butler (1998), excessive RDP than20mg/dlwerethreetimeslesslikelytoconceivethan
rather than excessive RUP was associated with decreased cowswithlowerBUNconcentrations.AlthoughhighBUN
conceptionrates.Canfieldetal.(1990)showedthatfeeding concentrationshavebeenassociatedwithdecreasedrepro-
dietscontainingRUPtomeetrequirementswhilefeeding ductive performance, others have reported no adverse
RDPinexcessofrequirementsresultedindecreasedcon- effectsonpregnancyrate,servicesperconception,ordays
ception rates. Garcia-Bojalil et al. (1998b) reported that open with BUN levels above 20 mg/dl (Oldick and Fir-
RDPfedinexcess(15.7percentofDM)ofrecommenda- kins, 1996).
StudiesbyCarrolletal.(1987)andHowardetal.(1987)
tions decreased the amount of luteal tissue in ovaries of
indicatethatmaintainingastrictreproductivemanagement
early lactation cows.
protocolcan reducethe negativeeffects ofexcess protein
Although moststudies haveindicated anadverse effect
intakeonreproduction.Barton(1996a)demonstratedthat
on reproductive performance of feeding high CP diets,
an intense reproductive program could be used to reach
othersindicatenoeffectofdietCPonreproduction.Car-
reproductivesuccessregardlessofdietCPlevelorplasma
rolletal.(1988)observednodifferencesinpregnancyrate
urea N concentrations. These studies highlight the idea
orfirstserviceconceptionratesofdairycowsfed20percent
that dietary protein is just one of many things that have
CPand13percentCPdiets.Howardetal.(1987)reported
an effect on reproductive performance. Protein intake,
no difference in fertility between cows in second and
alongwithotherfactorssuchasreproductivemanagement,
greaterlactationfed15percentCPor20percentCPdiets.
energy status, milk yield, and health status all have an
There are many theories as to why excess dietary CP
effect on reproductive performance in dairy cattle.
decreases reproductive performance (Barton, 1996a,
1996b; Butler, 1998; Ferguson and Chalupa, 1989). The
Synchronizing Ruminal Protein and Carbohydrate
firsttheoryrelatestotheenergycostsassociatedwithmeta-
Digestion: Effects on Microbial Protein Synthesis
bolic disposal of excess N. To the extent that additional
energymayberequiredforthispurpose,thisenergymay Microbialproteinsynthesisintherumendependslargely
be taken from body reserves in early lactation to support on the availability of carbohydrates and N in the rumen.