HOW TO CALCULATE DIGESTION COEFFICIENTS AND TDN

DIGESTION COEFFICIENTS must be determined for CP, NFE, CF, and EE portions of the feed sample to determine TDN. In some digestion trials, we may also want to determine the digestion coefficients for dry matter and sometimes ash as well. However, the digestibility of DM and ASH is not necessary to determine TDN. Digestion coefficients are most commonly expressed on a percentage basis. The digestion coeffficient (% Digestibility) can be calculated for any nutrient by the following equation:

                (TOTAL NUTRIENT CONSUMED - TOTAL NUTRIENT EXCRETED)
% DIGESTIBILITY = ______________________________________________    X 100
                               TOTAL NUTRIENT CONSUMED

EXAMPLE CALCULATION

Say the Nitrogen content of a feed is 2%. If the animal eats 400g then the total N consumed is 400 X .02 = 8g. If the amount of N in the feces was 3% and the animal excreted 50g of feces the total N excreted in feces would be 50 X .03 = 1.5g.

The % N digestibility would be (8 - 1.5) / 8 X 100 = 81.25%

This example gave only APPARENT nitrogen digestibility. To determine TRUE nitrogen digestibility, you would have to know how much of the N in the feces was of endogenous origin. If you did know that the endogenous portion in the example above was 0.5% N (in the 50 g of feces), then for true N digestibility, correct the fecal N value:

True N Digestibility = Nfeed - (NFeces - endogenous N) 
                      --------------------------------- X 100

                                   Nfeed

or for the example above (8 - (1.5 - 0.25)) / 8 = 84.375% = 84.4%

The 0.25 comes for multiplying .005 times the 50g of feces.  So 2.5% of the total 3% N came from the diet and 0.5% came from endogenous sources.
 
 

NITROGEN BALANCE AND NITROGEN RETENTION:

These two terms are often used interchangably, but it is harder to envision negative nitrogen retention than it is negative nitrogen balance (whether the animal is losing or gaining nitrogen). Often nitrogen balance is expressed in amount (grams) whereas nitrogen retention is expressed as a percentage. To determine these values you need the total N intake (the source is feed) minus the total N excretion (the main sources are feces and urine).

Using the example above, and including a N loss in urine of 2 g, the NITROGEN BALANCE would be + 4.5g N (8g - 1.5g - 2g). This is sometimes called nitrogen retention when the value is positive.

BIOLOGICAL VALUE

B.V. is a measure of the protein quality of a feed. It is determined as the proportion of protein retained as a % of that digested.

It is calculated as: Nfeed - Nfeces - Nurine
                     ----------------------- X 100
                           Nfeed -Nfeces

Using the previous data, the value would be 8g -1.5g -2g
                                            ------------- = 69.2%
                                             (8g -1.5g)

That is APPARENT B. V.

As before, if you had correction factors for endogenous N in the feces (MFN for metabolic fecal nitrogen) and urine (EUN for endogenous urinary nitrogen), then you would make those corrections:

True B.V. = Nfeed - (Nfeces - MFN) - (Nurine - EUN)
           ----------------------------------------- X 100
                    Nfeed - (Nfeces - MFN)

EXAMPLE: If MFN = 0.5g and EUN = .4g then:

True B.V. = 8 - (1.5 - 0.5) - (2 - 0.4)      8 - 1 - 1.6
           ---------------------------   =  ------------ = 77.1%
                   8 - (1.5 - 0.5)              8 - 1
 
 

TDN

TDN represents ENERGY based on a carbohydrate equivalent basis.

TDN stands for total digestible nutrients. I suggest you ignore that as irrelevent and misleading; it represents ENERGY.

TDN is calculated as the digestible CP, Crude Fiber, NFE and EE X 2.25 added together. IE, take the proximate analysis values for the things that contain energy: crude protein, crude fiber, ether extract, and NFE. Multiply each times their digestibility, and fat times 2.25, and add the values. Example:
 
 

       %   ENERGY   Digestibility

Water 15    0

CP    18    1        .85 =      15.3

EE     9    2.25     .8 =       16.2

CF    35    1        .6 =       21

ASH    4    0

NFE   19    1        .8 =       15.2

      ---                       ----

total 100                       67.7
 
 

DE, ME, NE

Digestible energy is the gross energy minus energy in the feces. Multiply the kcal/g times the g of feed eaten to get energy consumed; multiply the kcal/g times the g of feces excreted to get energy excreted in the feces.

Metabolisable energy is the digestible energy minus energy in the urine (and gasseous loss of ruminants).

Net energy is metabolisable energy minus HEAT INCREMENT.