Untitled Quiz

Measures of Protein Quality in Ruminants and Non-Ruminants

• Digestible protein figures are not entirely satisfactory for measuring protein value to animals because absorbed protein use efficiency differs between sources.

• Protein efficiency ratio (PER), net protein retention (NPR), and gross protein value (GPV) evaluate protein use by measuring growth response in experimental animals consuming the protein

• Gross protein value (GPV) is calculated as the ratio of extra liveweight gain from test protein to the extra liveweight gain in supplementary casein per unit.

• Endogenous urinary nitrogen comes from protein breakdown and replacement by various secretions and structures within the body.

• Total fecal and urinary endogenous fractions represent nitrogen absorbed and utilized, rather than unusable nitrogen.

• Net protein utilization (NPU) measures the proportion of intake nitrogen retained by the animal.

• Net protein utilization (NPU) is the product of biological value (BV) and digestibility.

• Net protein value (NPV) measures protein available for animal metabolism and is the product of NPU and % crude protein (CP).

• Protein quality depends on the amino acid in greatest deficit compared to a standard.

• Egg protein was historically used as the standard; now, the FAO Recommended Reference Amino Acid Pattern is frequently used.

• The chemical score expresses the proportion of each essential amino acid in a protein compared to the standard protein.

• The lowest proportion determines the chemical score.

• Lysine is often the limiting amino acid in wheat protein.

• The essential amino acid index (EAAI) considers all ten essential amino acids.

• EAAI is the geometric mean of the egg ratio of essential amino acids to food amino acids.

• Proteins with different composition can have similar EAAI values.

• Biological assays assess available amino acid content by measuring live weight gain, food conversion efficiency, or nitrogen retention in animals consuming a protein supplement in a deficient diet.

• Chick is the usual experimental animal in biological assays and pure amino acid responses are compared for assessment.

• Microbiological assays use microorganisms with similar amino acid requirements to higher animals to evaluate food proteins.

• Growth is measured in microorganisms in culture media containing the tested protein.

• Streptococcus zymogenes and Tetrahymena pyriformis are common microorganisms used, often requiring predigestion.

• Tetrahymena pyriformis has intrinsic proteolytic activity and is used for soluble proteins without predigestion.

• An improved method uses predigestion by pronase, with tetrahymanol content of culture medium measured.

• Ileal digestibility measures amino acid availability at the end of the small intestine (ileum) as there is no absorption in the large intestine.

• Microflora in the large intestine prevents undigested amino acids from appearing in feces.

• Ileal digestibility is measured by total recovery of ileum flow with an indigestible marker (chromic oxide) from ileal digesta in pigs with cannulae.

Measures of Protein Quality for Ruminant Animals

• Crude protein

• Digestibility crude protein (DCP)

• True protein (TP)

• Protein equivalent (PE)

• Metabolizable protein

• RDP & RUP

Crude Protein

• Crude protein content in food is estimated by nitrogen content determined by Kjeldahl or Dumas method.

• Assumptions in converting nitrogen to crude protein:

• All food nitrogen exists as protein

• All food protein contains 160 g N/kg.

• CP (g/kg) = g N/kg * 1000/160 or g N/kg * 6.25

Digestible Crude Protein (DCP)

• Determined via digestibility trials, multiplying CP content by digestibility coefficient of fodder protein.

• Most common way to express protein values and requirement of ruminants.

• DCP is a commonly used measure for protein values of feed for ruminants in India.

True Protein (TP)

• Denotes the proteins only.

• Separated from NPN by precipitation with cupric hydroxide.

Protein Equivalent (PE)

• NPN fraction is given half the nutritive value of true protein.

• Calculated as ( % DCP + % DTP) / 2

Metabolizable Protein

• Microbial demand for protein expressed in terms of rumen-degradable protein (ERDP)

• ERDP calculation: ERDP = Cp × [0.8a + bc/(c + r)]

• a, b, c are fitted parameters for degradability determination

• 0.8 is efficiency of capture for readily degradable Fraction

• r is outflow rate

RDP & RUP

• Rumen degradable protein (RDP) is the fraction of crude protein broken down by rumen microbes.

• Rumen undegradable protein (RUP), also known as bypass protein, escapes rumen degradation and is digested in small intestine.

• New NRC (2001) suggests that maximum milk and milk protein yields occur when RDP is 12.2% of diet dry matter.

Expressions of Protein

• Digestible Crude Protein (DCP).

• True Protein (TP)

• Protein Efficiency Ratio (PER)

• Net Protein Retention (NPR)

• Gross Protein Value (GPV)

• Protein Replacement Value (PRV)

• Biological Value (BV)

• Net Protein Utilization (NPU)

-Net Protein Value (NPV)

-Biological assays

• Protein Equivalent (PE)

• Degradibility

Protein Efficiency Ratio (PER)

• Calculate PER as weight gain (grams) / protein intake (grams) for monogastric animals only.

• Limitations: Applies to monogastric animals only, protein is used in producing energy if no other source is available, dietary caloric value should be adequate, excessive protein intake is not optimal, protein intake profile needs to be adequate in quantity and proportion.

Biological Value (BV)

• Measures %age of absorbed nitrogen retained in the body for maintenance, growth, production, reproduction.

• BV calculation:

• Apparent BV = (nitrogen intake − (fecal N2 + urinary N2)) × 100 / nitrogen intake.

• True BV = 100 × (N2 intake − (fecal N2 − MFN)) − (urinary N2 − EUN) / (N2 intake − (fecal N2 − MFN))

• MFN = Metabolic Fecal Nitrogen

• EUN = Endogenous Urinary Nitrogen

• BV depends on the quality, quantity, and proportion of essential amino acids. Animal protein typically has higher BV than vegetable protein.

• Higher the proportion of adequate amino acids, higher the BV.

Disadvantages/Limitations of BV

• Doesn't apply well to ruminants.
• Difficulty measuring endogenous nitrogen.
• Specific protein intake level might not apply to feeds with different protein levels.
• Sufficient non-nitrogenous sources (energy, minerals, etc.) are needed for accurate results.

Other relevant indices

• Net Protein Utilization (NPU)

• Percentage of dietary protein converted into body protein

• Calculated as (Retained N2 / Intake N2) x 100

• Nitrogen Balance Index (NBI)

• The difference between the Nitrogen intake and the nitrogen output.

• If the Nitrogen intake is zero, the NBI is zero.

• Net Protein Ratio (NPR)

• Compares weight gain of animals fed a protein-containing diet to a protein-free diet.

• Calculated as (weight gain + weight loss) / protein intake

• Essential amino acid index (EAAI)

• Represents the geometric mean for essential amino acid ratios compared to a reference protein (like egg protein).