Evaluation of feed protein

1. Evaluation of feed protein
K.GURU MOHAN REDDY
TVM/2016-13
DEPARTMENT OF ANIMAL NUTRTION
COLLEGE OF VETERINARY SCIENCE, TIRUPATI
SRI VENKATESWARA VETERINARY UNIVERSITY

2. MEASURES OF PROTEIN QUALITY FOR
MONOGASTRIC ANIMALS
• Digestible protein figures are not entirely satisfactory measures of the
value of a protein to an animal, because the efficiency with which the
absorbed protein is used differs considerably from one source to
another.
• In order to allow for such differences, methods for evaluating
proteins, such as the protein efficiency ratio (PER), the net protein
retention (NPR) and the gross protein value (GPV), which are based
on the growth response of experimental animals to the protein under
consideration, have been devised

3. • Protein efficiency ratio
• This is defined as follows:
• The rat is the usual experimental animal.

4. • Net protein retention
• This is calculated as follows:
• Where TPG= group of given the test protein
NPG= group of protein free-diet

5. • Gross Protein Value
• The extra liveweight gain per unit of supplementary test protein
stated as a proportion of the extra liveweight gain per unit of
supplementary casein is the GPV of the test protein,
• i.e.: GPV = A/A°
• where A= g increased weight gain/g test protein
A°= g increased weight gain/g casein.

6. • The live weight gains of chicks receiving a basal diet containing 80 g
crude protein/kg are compared with those of chicks receiving the
basal diet plus 30 g/kg of a test protein, and of yet others receiving
the basal diet plus 30 g/kg of casein

7. Nitrogen balance
• Liveweight gains may not be related to protein stored, and a more
accurate evaluation of a protein may be obtained by using the results
of nitrogen balance experiments.
• In such experiments, the nitrogen consume in the food is measured
together with that voided in the faces, urine and any other nitrogen-
containing products such as milk, wool and eggs.

8. • When the nitrogen intake is equal to the output, the animal is said to
be in nitrogen equilibrium;
• when intake exceeds output , it is in positive balance;
• when output exceeds intake, it is in negative balance.

9. Biological value
• This is a direct measure of the proportion of the food protein that can
be utilized by the animal for synthesizing body tissues and
compounds.
• May be defined as the proportion of the absorbed nitrogen that is
retained by the body.
• A balance trial is conducted in which nitrogen intake and urinary and
fecal excretions of nitrogen are measured, along with the endogenous
fractions.

10. • The biological value is then calculated as follows:
• where MFN = metabolic (endogenous) fecal nitrogen and
EUN = endogenous urinary nitrogen

11. • The endogenous urinary nitrogen results from irreversible reactions
involved in the breakdown and replacement of various protein
secretions and structures within the body.
• Thus, both the fecal and urinary endogenous fractions represent
nitrogen that has been absorbed and utilized by the animal rather
than nitrogen that cannot be so utilized.
• Their exclusion from the fecal and urinary values in the above formula
gives a measure of the true biological value.

12. • In determining biological value, as much as possible of the dietary
protein should be provided by that under test.
• Protein intake must be sufficient to allow adequate nitrogen
retention, but it must not be in excess of that required for maximum
retention;
• if the latter level were exceeded, then the general amino acid
catabolism resulting would depress the estimate of biological value.
• For the same reason, sufficient non protein nitrogenous nutrients
must be given to prevent protein being catabolized to provide energy.

13. • Such biological values are for the combined functions of
maintenance, meaning the replacement of existing proteins, and
growth (i.e. the formation of new tissues).
• Biological values for maintenance alone may be calculated from
balance data.
• A linear relationship exists between nitrogen intake and balance
below equilibrium, which may be represented by the following
equation:
• where y = N balance, x = N absorbed, a = N loss at zero intake (all
expressed as gN per basal kJ), and b = nitrogen balance index, i.e. that
fraction of the absorbed nitrogen retained by the body and is equal to
the BV for maintenance.

14. • The BV are dependent on amino acid composition .
• If all the essential amino acid present in right amount and proportion
of BV will be higher since the protein will be utilized for body tissues
rather than being diverted for energy supply.
• In the latter case the amino acid will be deaminated and there will be
more excretion urea.
• Animal proteins have higher BV since the essential amino acids
present in them are very near to the proportion in which they are
needed by body.
• Deficiency or excess of any one of the amino acids lowers the BV

15. • Net Protein Utilization (NPU)
• Is the proportion of the nitrogen intake retained by the animal.
• The product of BV and digestibility is termed the net protein
utilization (NPU)
• Net Protein Values (NPV)
• Is a measure of protein actually available for metabolism by animals
• The product of NPU and % CP

16. Chemical score
• the quality of a protein is decided by the amino acid that is in greatest
deficit when compared with a standard.
• The standard generally used has been egg protein, but many workers
now use a defined amino acid mixture, the FAO Recommended
Reference Amino Acid Pattern.
• The content of each of the essential amino acids of a protein is
expressed as a proportion of that in the standard(the standard
pattern ratio) and the lowest proportion taken as the score.

17. • In wheat protein, for example, the essential amino acid in greatest
deficit is lysine.
• The contents of lysine in egg and wheat proteins are 72 g/kg and 27
g/kg, respectively,
• and the chemical score for wheat protein is therefore 27/72 = 0.37

18. • The essential amino acid index (EAAI)
• In this case all the ten essential amino acids are considered.
• EAAI is the geometric mean of the egg ratios of the essential amino
acids to the food amino acids
• It has the advantage of predicting the effect of supplementation in
combinations of proteins.
• On the other hand, it has the disadvantage that proteins of very
different amino acid composition may have the same or a very similar
index.
• Both the chemical score and the essential amino acid index are based
upon gross amino acid composition.

19. • Biological assay of available amino acids
• The available amino acid content of a food protein may be assayed by
measuring the live weight gain, food conversion efficiency or nitrogen
retention of animals given the intact protein as a supplement to a diet
deficient in the particular amino acid under investigation.
• The chick is the usual experimental animal and the response to the
test material is compared with the response to supplements of pure
amino acids.

20. • The method has been used successfully for lysine, methionine and cysteine
• Disadvantages associated with biological methods – time, technical
expertise and supply of suitable animals
• There is the major problem of constructing diets deficient in specific amino
acids.

21. Microbiological assay of essential amino acids
• Certain microorganisms have amino acid requirements similar to
those of higher animals and have been used for the evaluation of
food proteins.
• The methods are based on measuring the growth of the
microorganisms in culture media that include the protein under test.
• Best results have been obtained with Streptococcus zymogenes and
Tetrahymena pyriformis.
• The former is used after an acid or enzymic predigestion of the food
protein; estimates of the availability of lysine and methionine have
agreed well with chick assays and measurements of NPU.

22. • T. pyriformis has intrinsic proteolytic activity and is used, for soluble
proteins, without predigestion.
• An improved method, using predigestion with the enzyme pronase
• and measuring response in terms of the tetrahymanol content of the
culture medium,
• has given results for available lysine, methionine and tryptophan that
correlate well with those of biological assays.
• Tetrahymanol, the characteristic pentacyclic terpene synthesized by T.
pyriformis, is determined by gas–liquid chromatography.

23. Ileal digestibility
• The availability of amino acids (AA) is primarily determined by their
digestibility measured at the end of the small intestine, i.e., at the
ileum level,
• since it has been well established that there is no AA absorption from
the large intestine.
• Furthermore, the micro flora metabolize some of the undigested AA
in the large intestine thus preventing them from appearing in the
faces.
• For this reason, the concept of “ileal digestibility” has been
established.

24. • Measurement of ileal digestibility
• Ileal digestibility may be measured from either total recovery of
ileum flow from pigs fitted with ileo-rectal anastomosis (IRA), or
• From determination of the concentration of an indigestible marker,
generally chromic oxide, in a sample of ileal digesta from pigs fitted
with cannulae.
• Apparent ileal digestibility:
• DApp =
[(AADietDM x DMI) - (Digesta x DME x 100)] x 100 / (AADietDM x DMI)
• DApp =
[(AADietFM x FMI) - (Digesta x DME x 100)] x 100 / (AADietFM x FMI)

25. • Corrected apparent ileal digestibility
• The corrected apparent ileal digestibility can be calculated either from
the apparent digestibility or from the standardised digestibility
• DAppCorr = DApp + EndoDMI x 10 x (1/AADietDM - 1/ AARMDM)
• DAppCorr = DStd - EndoDMI x 10 / AARMDM
• DAppCorr = DApp + EndoFMI x 10 x (1/AADietFM - 1/ AARMFM)
• DAppCorr = DStd – EndoFMI x 10 / AARMFM

26. • Standardised ileal digestibility is now used for evaluating proteins for
pigs.
• This approach has the advantage of using ileal digestibility and
therefore avoiding microbial amino acid production in the hind gut
inherent in using whole-tract digestibility, and also accounting for
basal endogenous amino acid loss.
• Values are also additive, simplifying diet formulation.

27. • For poultry, evaluation of protein sources is also based on
standardised ileal digestibility of the three major amino acids: lysine,
methionine and tryptophan
• For horses, evaluation of protein sources is based on crude protein or
digestible crude protein.

28. • MEASURES OF PROTEIN QUALITY FOR RUMINANT ANIMALS
• Crude Protein
• Digestibility Crude Protein
• True Protein
• Protein Equivalent
• Metabolisable protein
• RDP & RUP

29. • CRUDE PROTEIN
• The protein content of a food is calculated from its nitrogen content
determined by a modification of the classical Kjeldahl technique or the
Dumas Method.
• Two assumptions are made in calculating the protein content from that of
nitrogen:
• first, that all the nitrogen of the food is present as protein;
• second, that all food protein contains 160 g N/kg.

30. • The nitrogen content of the food is then expressed in terms of crude
protein (CP):
• CP (g/kg) = g N/kg * 1000/160
or
• CP (g/kg) = g N/kg * 6.25

31. • DIGESTIBLE CRUDE PROTEIN:
• It is determined by digestibility trials
• When the CP content of feed stuffs is multiplied by digestibility coefficient
of protein in the fodder gives the DCP
• It is the most common way of expressing the protein values and
requirements of ruminants
• In India, DCP taken as measure for the expressing the proteins values of
feeds for ruminants

32. • True protein:
• The term TRUE PROTEIN is used to denote the proteins only.
• It can be separated from NPN compounds by precipitation with cupric
hydroxide. Then nitrogen estimation by kjeldal method.
• Protein equivalent:
• In this, NPN fraction is given half the nutritive value of true protein.
• It is calculated as PE= (%DCP+%DTP)/2

33. Metabolizabile protein
• The microbial demand for protein is stated in terms of effective
rumen-degradable protein (ERDP) and foods have to be evaluated in
the same terms.
• The ERDP of a food is calculated as follows:
• where a, b and c are the fitted parameters derived from the
determination in sacco of the degradability of the food,
• 0.8 is the efficiency of capture of the nitrogen of the readily
degradable fraction, and r is the outflow rate

35. • Account is thus taken of the differential capture of rapidly and slowly
degradable proteins and the rate of passage through the rumen.
• The demand for amino acids at tissue level is quantified in terms of truly
digestible protein required to be absorbed from the small intestine and
designated ‘metabolisable protein’ (MP),
• Microbial protein contributes towards satisfying this demand.
• The yield of microbial crude protein is related to the energy available to the
rumen microorganisms in terms of fermentable metabolisable energy:
• MEferm is assumed to be 0.1 ME for silages and 0.05 ME for brewery and
distillery by-products, and MEfat is 35 MJ/kg.

36. • The assumption made here for silage must be suspect.
• The major fermentation product in well-made silages is lactate, and
there is evidence that several rumen bacteria, notably Megasphaera
elsdenii are able to utilise lactate with the production of propionate.
• The microbial crude protein yield (g) is calculated as follows:
FME (MJ)y
• where y 9 at maintenance, 10 for growth and 11 for lactation; or
alternatively: y = 7 + 6(1 - e-0.35L)

37. • The proportion of the microbial crude protein present as true protein
is assumed to be 0.75 and the true digestibility to be 0.85, and
• the contribution of microbial protein (DMP) to the truly absorbed
amino acids is:
• DMP (g>kg DM) = FME (y * 0.75 * 0.85) = 0.6375(FMEy)
• When this contribution is taken into account, there remains a residual
metabolisable protein requirement, which may be calculated as MP -
DMP and which has to be satisfied by the truly digestible undegraded
protein of the diet.

38. • The true digestibility of dietary undegraded protein is calculated on
the assumption that the ADIN content is indigestible and that the
remainder has a true digestibility of 0.9.The truly digestible
undegraded true protein (DUP) is then:
• DUP = 0.95(CP(1 - a – bc/(c + r)) - 6.25(ADIN)
• where a, b and c are the usual in sacco constants and DUP, CP and
ADIN are g/kg DM.
• The metabolisable protein supplied by the food may be calculated as
MP (g/kg DM) = DMP + DUP.

39. • The metabolisable protein content of a food is of no use as a guide to
the food’s ability to satisfy the residual demand for metabolisable
protein, since it includes a contribution from ERDP, which has already
been taken into account in the form of DMP.
• The protein content of foods is thus stated in terms of ERDP and DUP.

40. • RDP & RUP
• Rumen Degradable Protein (RDP) is the fraction of Crude Protein (CP)
consumed which is broken down by rumen microbes.
• And remaining protein which reaches the small intestine without
degradation called as Rumen undegradable protein (UDN).
• The new NRC (2001) suggests that maximum milk and milk protein yields
occur when RDP is 12.2% of diet dry matter.

41. • Rumen undegradable protein (UDN) is also called bypass protein or
escaped protein or rumen undegradable protein (RUP).
• It is the portion of intake protein that escapes rumen degradation and
is digested directly in the small intestine.
• About 80 to 85 per cent of the microbial bacterial protein and UIP or
true protein that flows out of the rumen is digested in the small
intestine and it is expressed as a percentage of crude protein (CP).