EVALUATION OF PROTEIN QUALITY

A. FOR NON-RUMINANTS:

1. Protein Efficiency Ratio (PER):

• It is a measure of weight gain of a growing animal divided by protein intake.

PER = (wt. gain in gm/ Protein intake in gm)

• It has most often been applied with growing rats.
• It is simple method requiring simple instruments like rat jars, food jar, water bottles.
• Disadvantage: It cannot distinguish between weight gained as fat or lean body mass.

2. Biological Values (BV):

• It is a direct measure of the proportion of food protein that can be utilized by animal for synthesizing body tissues and compound.
• It is defined as the proportion of absorbed nitrogen that is retained in the body.
• A balance trial is conducted in which nitrogen intake and urinary and fecal excretions are measured and results are used to calculate BV as follows:

BV =  (N intake- (faecal N +Urinary N)/(N intake- Faecal N)x 100

BV=  (Retained nitrogen /Absorbed nitrogen)x 100

• This measures the BV of protein for growth purpose only.

BV values of protein of different food:

3. Net Protein Utilization (NPU):

• This is the percentage of dietary protein which is converted into body protein.
• The product of BV and digestibility is termed net protein utilization (NPU).
• NPU = (retained nitrogen/Intake of Nitrogen)x 100
• NPU = BV x Digestibility
• It is less laborious but time consuming method of feed analysis.

B. FOR RUMINANTS:

1. Estimation of DCP:

• It is the most common way of expressing protein value of feed for ruminants.
• Crude protein involves both true protein as well as NPN compounds such as amides, glucosides, amino acids, ammonium salt, and others present in feedstuff.
• However, quantity of CP does not provide enough information on how it is utilized in body.
• To find out digestibility of crude protein (DCP), digestibility coefficient of that protein is multiplied with crude protein content of foodstuff.
• A typical equation used to find DCP for both grass, hays and silages is given below:

% DCP = (% CP x 0.9115) – 3.67

2. Nitrogen Balance Experiment:

• This method is applicable to determine the protein quality in ruminants as well as non-ruminants.
• This method considers accurate account of amount of nitrogen consumed through feed and excretion through feces, urine and brushings (hair, feathers, etc.)
• It involves determination of amount of endogenous urinary as well as that of metabolic fecal nitrogen. This are obtained by conducting trials or through existing formulae:

EUN mg/day = 146 W^0.75 kg

• In trials, feed are withheld in animals so that their bodies enters catabolic condition. Nitrogen excreted in feces and urine during this period represents nitrogen lost through metabolic process, which is called EUN and MFN.
• These values are then subtracted from total nitrogen values obtained in collection of urine and feces during collection period of balance trial.
• Exogenous urinary nitrogen and fecal nitrogen from feed are calculated as follows:

Exogenous urinary Nitrogen (EUN) = Total Urinary N – EUN

Fecal Nitrogen from Feed = Total fecal nitrogen – MFN

• When daily N intake < total loss from body, animal is in negative nitrogen balance
• When daily N intake = Total loss, nitrogen is in equilibrium
• When daily N intake > total loss, positive nitrogen balance. It involves deposition and storage of protein in the body