Dairy Nutrition

Protein in Dairy Cows Minimize

Protein in Dairy Rations

James D. Ferguson, VMD, MS
Section Animal Production Systems

Crude protein is defined as total N times 6.25
  1) Includes true protein, ammonia, nitrates, amines, amides, amino acids and peptides, (nonprotein N), and nucleic acid nitrogen
Protein has three fates in ruminants:
  1) Fermentation (degradation) in the rumen
    a) Proteolytic enzymes digest protein to amino acids and peptides
      i) Proteases, peptidases, and deaminases are elaborated into rumen liquid         
    b) Amino acids and peptides are fermented to ammonia and carbon skeletons (isoacids)
    c) Rumen bacteria utilize ammonia, amino acids, and peptides for growth
  2) Hydrolytic/enzymatic digestion in small intestine
    a) Feed protein and bacterial protein which escapes from the rumen
  3) Passage to feces of indigestible protein
  1) MP - metabolizeable protein
    a. True protein digested postruminally and the component amino acids absorbed from the intestine
  2) RDP - rumen degradable protein
  3) RUP - rumen undegradable protein
  4) BCP - bacterial crude protein (MCP: microbial crude protein)
    a) Flow of protein contained in rumen bacteria which is produced in the rumen and is passed to the intestine for digestion
    b) 80% true protein and 80% digestible
  1) Provide sufficient RDP in the ration to support microbial growth and synthesis
  2) Provide sufficient RUP to support production
  3) Provide sufficient quality of amino acids from RUP and digested BCP
  4) Minimize losses of excess rumen ammonia from degradation of over supply of RDP
  Nitrogen efficiency – minimize N losses in feces and urine
  1) RDP/RUP of protein in feeds
    a. How to predict these fractions
  2) Rates of passage of feed material from the rumen, which influence proportions of RDP and RUP in feeds
    a. Intestinal digestibility of RUP of feeds needs to be characterized
  3) Factors which determine BCP synthesis in the rumen
  4) Rumen production of BCP is dependent on a functioning rumen
    a) Animals over 100 kg body weight (220 lb)
Rumen degradation of feed protein
  1) Rumen organisms
    a) Bacteria 1010-11/ml, 40% of species have proteolytic activity
    b) Protozoa 105-6/ml, ingest particulate matter therefore are importan in degrading insoluble feed proteins
      i) cannot utilize ammonia but are net exporters of ammonia
      ii) release large amounts of peptides and amino acids and peptidases into rumen fluid
    c) Fungi 103-4/ml, less is known about their protein degradation
  2) Enzymatic activity primarily of bacterial origin associated with cell surface
    a) Adsorption of soluble feed proteins to cell surface is important step in degradation
    b) Bacteria uptake small peptides, free amino acids, and ammonia
  3) N in the form of peptides, ammonia or amino acids are the nitrogenous nutrients for ruminal microorganisms
  4) If protein degradation exceeds rate of N incorporation into microbial protein, ammonia builds up to excessive concentrations
    a) Can waste dietary N and reduce efficiency of CP utilization
    b) Small group of bacteria that use amino acids as primary source of carbon and energy
    c) Combination of bacteria with low deaminative activity and a small group of bacteria with high activity
  5) Rumen degradation of feed proteins is most often described as a first order mass action model
    a) Feeds are composed of CP fractions that differ in rates of degradation
    b) Rumen disappearance of protein is the sum of two processes
      i) digestion
      ii) passage
  6) Protein fractions in feeds
    a) A fraction (NPN)
      i) instantaneously soluble at time zero
      ii) rate of degradation of infinity
      iii) that portion of feed CP soluble in borate-phosphate buffer but not precipated by trichloroacetic acid (TCA)
      iv) mean 9849 sd 1610 range 0 to 18,540 %/h CPMDairy
    b) B fractions
      i) potentially degradable true protein
      ii) 3 potential fractions B1, B2, B3
      iii) The degradation of these fractions are influenced by the kd and kp of these fractions. kd is a function of feed fraction. kp is a value used for all fractions.
      iv) B1
        (a) Percentage of total CP soluble in borate-phosphate buffer and precipitated with TCA – soluble true
        Protein; very rapidly degradable
        (b) Mean 185.54 sd 95.28 range 0 to 500 %/h CPMDairy
      v) B2
        (a) CP - A - B1 - B3 – C
        (b) Degraded at rates similar to passage rates
        (c) Mean 8.67 sd 4.69 range 0 to 20 %/h CPMDairy
      vi) B3
        (a) Difference in protein recovered in NDF-CP and ADF-CP. (NDF-CP, NDF-N, NDFIN).       
        (b) (NDF-CP) - (ADF-CP)
        (c) very slowly degraded in the rumen
        (d) mean 0.44 sd 0.73 range 0 to 7 %/h %/h CPMDairy
    c) C fraction
      i) chemically determined as the CP in the ADF residue (ADIN, ADF-CP, ADF-N)
      ii) considered to be undegradable and indigestible
      iii) proteins associated with lignin and tannins and heat-damaged proteins
      iv) 0 %/h
  7) RDP for a feedstuff may be estimated as:      
    RDP = A + B1[(kdB1/(kdB1 +kp)] + B2[(kdB2/(kdB2 +kp)]   
    + B3[(kdB3/(kdB3 +kp)]
  8) Worldwide, the most used model to describe RDP uses only three
fractions based on dacron bags suspended in the rumen
    A = NPN fraction of CP and a small amount of true protein
    B = 100 - A - C
    C = undegraded feed protein remaining in the bag after a defined end point of degradation
    a) Only B fraction is influenced by passage, all A is degraded, all C is passed to small intestine

b) In this model then RDP and RUP are as follows:

      RDP = A     + B[(kd/(kd + kp)] 
      RUP  =  B[(kp/(kd + kp)] + C
    c) This model is used in NRC
  9) Rumen degradations of feeds are influenced by two most important factors
    a) The proportion of NPN  and true protein
    b) The physical and chemical characteristics of the proteins that comprise the true proteins
    c) Range of degradation of B fraction is 1.4 - 29.2 %/h
    d) Three dimensional protein structure, processing, heat treatment, rumen pH, ruminal retention time, microbial proteolytic activity all influence rumen degradation of feed protein
  10) Rumen microbial requirements for N substances
    a) Peptides, amino acids, and ammonia are nutrients for the growth of ruminal bacteria. Protozoa cannot use ammonia.
    b) Amino acids and peptides are stimulatory in terms of both growth rate and growth yield for ruminal microorganisms growing on rapidly degraded energy sources.
      i) Peptides stimulate growth of bacteria which ferment NSC

ii) Ammonia is important for SC bacteria (and NSC bacteria)

Animal Responses to CP, RDP, and RUP
  1) Milk yield increases with increasing dietary CP at a decreasing rate
    a) MY = .8 * DMI + 2.3 * CP - .05 * CP2 - 9.8 (r2 = .29)   
  2) MY = kg of milk yield
    DMI = kg of DMI
    CP = % of diet
    a) As CP increases, MY increases at a decreasing amount
    b) Efficiency of N utilization decreases
    c) Protein yield, but not protein percentage in milk, was weakly correlated with dietary CP (not surprising)
  3) RDP and RUP
    a) MY = -55.61 + 1.15 * DMI + 8.79*RDP - 0.36*RDP2 + 1.85*RUP (r2 = .52)        
      MY = kg of milk
      DMI = kg of DMI
      RDP and RUP = % of dietary DM
    b) Maximum milk yield at RDP of 12.2 % of dietary DM
    c) DMI increased with increasing RDP
    d) Similar effects on milk protein yield
Predicting Microbial Protein
  1) 1989 NRC
    a) BCP = 6.25 * (-30.93 + 11.45 * Nel) lactating cows
    b) BCP = 6.25 * (-31.86 + 26.112 TDN) growing animals
  2) 2001 NRC
    a) BCP = .130 * TDN (discounted)
    If RDP exceeds 1.18 * MCP yield
    Need sufficient RDP to support BCP growth
    b) If RDP is less than 1.18 * MCP, then BCP = .85*RDP intake
    c) Same value for growing and lactating animals
    d) MP from bacteria = 64% of BCP production
Predicting passage, %/h
  1) kp of wet forages = 3.054 + .614*DMI (as percentage of BW)
  2) kp of dry forages = 3.362 + .479*DMI (as percentage of BW)
    - 0.007*concentrate as percentage of diet DM
    - 0.017*NDF of feedstuff, percentage of DM
  3) kp of concentrates = 2.904 + 1.375*DMI(as percentage of BW)
    - 0.020*concentrate as a percentage of diet DM
Digestibility of rumen undegraded feed protein
  1) 80% was used for RUP in 1989 NRC
  2) UK used ADF-CP to predict digestibility

g/kg DM = .90 * (RUP N-ADIN)/RUP N

  3) 2001 NRC used  values from 54 studies using mobile bag technique in total tract and range from 50 to 100%
Endogenous Protein
  1) Sources that flow to duodenum
    Mucoproteins in saliva
    Epithelial cells from respiratory tract
    Cellular debris from mouth, GI tract
    Enzyme secretions
  2) Endogenous N (g/d) = 1.9 * DMI (kg/d)

A large component is undigested bacterial CP              


MP requirement

  1) Maintenance
    Urinary and scurf protein losses
      4.1 * BW0.50 (conversion of MP to net protein, 67% efficient)
      0.3 * BW.60
    Metabolic fecal protein
      MFP (g/d) = 30 * DMI (kg)
      MP requirements for MFP adjusted for endogenous losses
      MFP in MP units = [(DMI (kg) * 30 - 0.50((MPBCP/0.80)-MPBCP)
      Endogenous MP/0.67
  2) Lactation
    MPlact = Yprotein/0.67
  3) Pregnancy and growth
    MPpreg = (((0.69*DaysPreg) - 69.2) * (CBW/45))/EffMPpreg (EffMPpreg = 0.33)
Amino Acids
  1) Essential amino acids (EAA) versus nonessential amino acids (NEAA)
     Arginine (Arg), histidine (His), isoleucine (Ile), leucine (Leu), lysine(Lys), methionine (Met), phenylalanine (Phe), threonine (Thr), tryptophan (Trp), valine (Val).
  2) EAA cannot be synthesized rapidly enough to meet needs
  3) EAA supplied in ratio to the MP relative to the ratio in the tissue of interest appears to improve efficiency of use
    Ratio in milk, muscle, ...
  4) Methionine and lysine appear to be most limiting for milk yield


    a) LYS most limiting on corn based diets, Met most limiting on SBM based diets
    b) Six observations from NRC:
      i.) Content of protein in milk is more responsive than milk yield to supplemental Met and Lys
      ii) Increases of milk protein percentage are independent of milk yield
      iii) Casein is the most influenced milk protein fraction
      iv) Increases in milk protein production to increased supplies of either Met or Lys in MP are the most predictable when the resulting predicted supply of the other AA in MP is near or at estimated requirements.
      v) Milk yield responses to Lys and Met are more common in cows during early lactation than in mid or late lactation cows

vi) Production responses to increased supplies of Met and Lys in MP are typically are greater when CP in the diet DM approximates normal levels (14 to 18 %) than when it is lower or higher concentrations.

  1) Fecal N is largely undigested feed and bacterial proteins
  2) Urinary N is largely in the form of urea (70 to 80%), creatinine, ammonia
  3) Conversion of feed N to milk N is only about 20% efficient across a herd
    a) efficiency increases as milk production increases
    b) 30% to 40% efficient when production is over 70 lbs
    c) High milk production improves efficiency
  4) Urea is quickly degraded to ammonia due to urease activity from bacteria in feces
    a) ammonia may be lost rapidly in volatilization from the barn or collection unit
    b) urease activity is temperature and pH dependent
      i) Neutral pH highest rates
      ii) Acidification slows rate and converts ammonia to
ammonium ion, which can slow volatilization   
      iii) Alkalization slows rate (liming alleys)
  5) fecal N is slowly degraded to ammonia
    a) more stable form of N
  6) Excess rumen ammonia is primary source of urinary nitrogen
    a) some tissue degradation of amino acids for gluconeogenesis contributes to urinary urea
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