Feedstuff Analysis

Proximate analysis analyzes for the following:

These are determined according to the following methods:

1. Water
1. Weigh an original sample of the feed
2. Place in an over at 90-95 degrees C
3. Remove, cool and weigh the sample. Repeat 1, 2, and 3 until constant weight is obtained, or follow a previously established protocol that has determined the proper length of drying time.
4. Calculate water lost by difference between the final sample weight and the original sample weight.
5. Calculate % water by dividing by the original sample weight X 100.

 

2. Crude Protein - Kjeldahl method
1. Weigh an original sample of the feed
2. Place in a kjeldahl flash and boil with sulfuric acid, converting the N to (NH4)2SO4.
3. Cool, dilute, neutralize with sodium hydroxide, distill and titrate the distillate with a known concentration of acid.
4. Multiply the amount of Nitrogen determined by this procedure by 6.25 to calculate Crude Protein.
1. Other, newer methods, such as spectrophotometric ones, sometimes replace the wet chemistry methods of kjeldahl, but the principle of N X 6.25 remains the same.
5. Determine the % CP by dividing the quantity of CP by the original sample weight X 100.

 

3. Ether extract
1. Weigh an original sample of the feed.
2. Place in an oven to remove water, remove and weigh again.
3. Reflux the sample with ether to remove the fat.
4. Dry and weigh the remaining, moisture-free and fat-free sample.
5. Calculate the amount of fat lost by subtracting the last weight from the weight after water removal.
6. Calculate the % fat in the feed by dividing the fat lost by the original sample weight X 100.

 

4. Crude fiber
1. Weigh an original sample of the feed.
2. Boil the sample in acid, then rinse the residue to remove everything soluble.
3. Boil the sample in alkali, then rinse the residue to remove everything soluble.
4. Dry the sample and weigh it. The remaining material is the crude fiber + some ash.
5. Determine the amount of ash in the sample by placing what remains in an ashing oven at 600 degrees C. What remains is ash. Weigh this and subtract it from the crude fiber + ash weight.
6. Determine the % C. F. by dividing the crude fiber weight by the original sample weight X 100.

 

5. Ash (This needs to be done even though ash was determined in the previous step, because that value will not be accurate for the feedstuff because of the steps in which that sample was boiled in acid and alkali).
1. Weigh an original sample of the feed.
2. Place the sample in an ashing oven at 600 degrees C.
3. Remove, cool and weigh the sample. What remains is ash.
4. Determine the % ash by dividing the quantity of ash by the original sample weight X 100.

 

6. NFE
1. NFE is determined by difference.
2. !00 - Water - CP - EE - CF - Ash = NFE
3. The value will already be a % figure if all the other values were expressed as %.

Determining TDN

TDN is an index of ENERGY, on a CARBOHYDRATE basis.

TDN stands for Total Digestible Nutrients, but that is a misnomer. Learn the previous definition instead.

The starting place for TDN is Proximate Analysis. On a carbohydrate basis

• Carbohydrate (both NFE and CF) has an energy value of 1
• Protein has an energy value of 1
• Fat has an energy value of 2.25
• Water and Ash have no energy, so their value is 0

To determine TDN, the digestibility of the components must be known, so run a digestion trial and conduct proximate analysis on the feed and on the feces to determine the digestibility of CP, CF, EE and NFE. Then:

TDN = Digestible CP + Digestible CF + Digestible NFE + (Digestible EE X 2.25)

Example:

ENERGETICS

Besides TDN, which is just an index of energy on a carbohydrate basis, energy is determined in feeds by finding the amount of heat that can be produced when the feed is completely oxidized. This is measured in calories.

• When the total amount of heat from a feed is determined, it is called the GROSS ENERGY.
• Some of that energy passes through the animal (as undigested feed), so if the energy in the feces is subtracted from the GROSS ENERGY, it is called DIGESTIBLE ENERGY.
• Some energy from the feed is lost via the urine and gas (especially in ruminants, where methane is lost upon eructation), so DIGESTIBLE ENERGY - URINE & GAS energy is called METABOLIZABLE ENERGY.
• Some energy is lost because not all reactions are completely efficient; it includes heats of fermentation, digestions and absorption, product formation, and waste formation and excretion. METABOLIZABLE ENERGY - HEAT INCREMENT yields NET ENERGY. Net energy is the portion of energy the animal can really use for useful purposes. Not all net energy is used with the same efficiency for every purpose, however, so feedstuffs are assigned values of net energy according to whether it will be used for maintenance (NEm), gain (NEg), milk production-lactation (NE_l), etc.