Applied Animal Nutrition-I

LABORATORY METHODS OF ESTIMATING DIGESTIBILITY

• Since digestibility trials are laborious to perform, there have been numerous attempts made to determine the digestibility of foods by reproducing in the laboratory the reactions which take place in the alimentary tract of the animal.
• Digestion in non-ruminants is not easily simulated in its entirety, but the digestibility of food protein may be determined from its susceptibility to attack in vitro by pepsin and hydrochloric acid.
• It is also possible to collect digestive tract secretions via cannulae and to use them to digest foods  in vitro.

Tilley and Terry method

• Digestibility of feeds for ruminants can be measured quite accurately in the laboratory by treating them first with rumen liquor and then with pepsin.
• During the first stage a known weight of the finely ground sample of the feed whose organic matter composition is already determined is incubated for 48 hours with buffered rumen liquor in a tube under anaerobic conditions.
• In the second stage the bacteria are killed by acidifying with hydrochloric acid to pH 2 and are then digested by incubating them with pepsin for a further 48 hours.
• The insoluble residue is filtered off, dried and ignited and again weighed.
• The difference between the two weighing gives the organic matter present in the residue.
• The digestibility coefficient determined in vitro is generally 1-2 percentage units lower than the coefficient measured in vivo.

In sacco or In situ or Semi in vivo or Nylon or Dacron Bag Technique.

The digestibility/degradability of feeds in the rumen can be determined  by keeping the feed sample in bags which are immersed in rumen contents of rumen fistulated animals. The bags are made up of nylon, dacron or silk cloth which is indigestible and should be of very fine mesh so that the test feed particles should not pass out of the bag undegraded but at the same time it should allow the rumen microbes to enter into the bag and act on the test feed. The bags on removal at different time intervals are washed till the wash water is clear and dried at 600C for 48 hours. The percent disappearence of dry matter, nitrogen/crude protein, different fibre fractions  etc are determined.

Applications of the technique

• This technique provides a powerful tool for initial evaluation of feedstuffs and is useful in screening, rapidly, large number of samples developed in forage breeding  experiments .          
•  This technique is helpful to understand the rumen processes. It is possible to vary the factors within the bag or within the rumen. The animal can be fed a constant diet, and the effect of (treated straw over untreated straw  or hay or complete diet) manipulating the feedstuffs incubated in the bag on its degradation kinetics can be studied. Alternatively, the conditions within the rumen ie: rumen environment, can be varied  and a standard material incubated in the bag in order to study the effect of rumen environment on the rate of degradation.

Limitations 

• The technique has certain inherent limitations.
• The test feed in the bag is not subjected to the total ruminal experience, ie., mastication, rumination and passage. What is actually measured is the breakdown of material to a size small enough to leave the bag and not necessarily a complete degradation to simple chemical compounds.

In Vivo Artificial Rumen (VIVAR) Technique

An invivo artifical rumen (VIVAR) was developed for studying nutrient utilisation by rumen micro-organisms under controlled conditions in the rumen. The system consists of a porcelain test tube or stainless steel or glass jars fitted with bacteriological membranes to provide controlled interchange of the VIVAR and rumen contents. The rumen microflora pass through the semipermeable membranes and degrade feed samples present inside the VIVAR tube, but the sample particles cannot move outside. After completion of the fermentation period, VIVAR tubes is removed. The dry matter disappearence may be recorded by difference in weight of the sample and the residues left in the VIVAR tube.