Digestibility Trials

1. Convention type of digestion trail:

• It involves an experiment by which amount of nutrients actually digested and absorbed from measured amount of feed consumed by animal is determined.
• Animals are fed accurately fixed amount of mixed rations or individual feed and excreta of farm animals are collected without any loss.

Stages:

I. Pre-collection feeding period:

• In order to remove the effect of previous feeding and also to adapt animal on feed to be evaluated, animals are fed test feed for a period of 2-3 weeks.

ii. Collection period:

• After allowing test feed for number of days, collection of feces is begun and continued throughout period.
• In ruminants, this period may be of 6-10 days and in case of non-ruminant, it will be of 4 days.
• Feed that have been chemically analyzed previously are fed to animal.

iii. Preparation of fecal sample:

• At the end of each 24-hour collection period, feces from individual animals are first weighed and then sampled for analysis.
• These collected feces are preserved in cans after labelling in air tight condition in quick freeze at -16 to -20°C for entire collection period.
• After end of collection period, these samples are taken out from freezer and temperature of sample is raised to normal room temperature. Sample are then weighed and weight of sample are recorded.

iv. Calculation of digestibility:

D.C of Nutrient=  (kg nutrient eaten – kg in feaces)/ kg nutrient eaten x 100

If there is residual feed, then DC is calculated as,

D.C of Nutrient = ( kg nutrient offered -kg feed refused)-(kg nutrient in feaces)/ ( kg nutrient offered -kg feed refused) x 100

Digestibility by difference:

• Method described above is direct method.
• If concentrates are added in diet, digestibility by above method cannot be calculated
• In this method, three continuous digestibility trials in a sequence are conducted.
• In 1^st trial, good quality roughage is fed to determine digestibility of nutrients in it.
• In 2^nd trial, same roughage is fed with known quantity of concentrate.
• Digestibility of concentrate is then found by subtracting figures obtained from roughage alone with figure obtained in combined ration.

Indicator Method:

• In this method, indicator is used alone with feed to be tested.
• By determining ratio of concentration of indicator to that of a given nutrient in feed and sample ratio in feces resulting from feed, digestibility of nutrient can be obtained without measuring either the feed intake or feces output.
• It is done as follows:

Fecal dry matter (DM)=

Digestibility of DM in diet can be computed as:

Digestibility of DM (%) = 100- (100 x )

2. In-vivo digestibility Method:

I. Nylon or Dacron Bag Techniques:

• In this method, digestibility of feed in 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 of nylon, Dacron or silk cloth which is indigestible and should be very fine mesh so that feed donot pass out of bag undigested but at same time allow rumen microbes to enter into bag and act on test feed.
• The bags are removed at different time intervals. They are washed till water is clear and dried at 60°C for 48 hours.
• The percent of disappearance of dry matter, nitrogenous/crude protein, different fiber fractions, etc. are determined.

Application of this techniques:

• It is useful in screening, rapidly large number of samples developed in forage breeding experiments.
• It helps to understand the rumen processes. It is possible to vary the factors within bag or within rumen.
• Rumen environment can be varied and standard material incubated in bag can be studied to observe the effect of rumen environment on degradation rate.

Limitations of this techniques:

• Test feed in bag is not subjected to total ruminal experience, i.e. mastication, rumination and passage.
• Actual measurement is that of breakdown of materials to size small enough to leave bag. It donot necessarily indicate a complete degradation to simple chemical compounds.

ii. In-Vitro Artificial Rumen (Vivar) Techniques:

• This technique is used for studying nutrient utilization by rumen microorganism under controlled conditions in rumen.
• It consists of porcelain test tube or stainless steel or glass jars with bacteriological membranes to provide controlled interchange of the rumen contents.
• The rumen microflora pass through semi-permeable membranes and degrade feed samples present inside VIVAR tube, but sample particles cannot move outside. After complete fermentation, VIVAR tube is removed.
• The dry matter disappearance will be recorded by difference in weight of sample and residues left in VIVAR tube.