Applied Animal Nutrition-I

INDICATOR METHOD OF DETERMINING DIGESTIBILITY

• In some circumstances the lack of suitable equipment of the particular nature of the trial may make it impracticable to measure directly either food intake or faeces out put, or both. For instance, when animals are fed as a group it is impossible to measure the intake of each individual.
• Digestibility can still be measured, however, if the food contains some substance which is known to be completely indigestible.
• If the concentrations of this indicator substance in the food and in small samples of the faeces of each animal are then determined, the ratio between these concentrations gives an estimate of digestibility.
• For example, if the concentrations of the indicator increased from 1% dry matter to 2% in the faeces, this would mean that 50% of the dry matter had been digested  and absorbed.

• The indicator may be a natural constituent of the food or be a chemical mixed into it. It is difficult to mix chemicals with foods like hay, but an indigestible constitutent such as lignin may be used.
• Other indicators in use today are fractions of the food known as indigestible acid-detergent fibre and acid insoluble ash (mainly silica) and also some naturally occuring n-alkanes of long chain length (C₂₅- C₃₅).
• The indicator most commonly added to foods is chromium in the form of chromic oxide, Cr₂O_3.Chromic oxide is very insoluble and hence indigestible; moreover, chromium is unlikely to be present as a major natural constituent of foods.
• For non ruminants ,titanium dioxide may be added to foods as an indicator.
• Chromic oxide may be used as an indicator in a different way, to estimate faeces output rather than digestibility.
• In this application the marker is given for 10 -15 days in fixed amounts ( eg. administered in a gelatin capsule) and once its excretion is assumed to have stabilised its concentration in faeces samples is determined.Faeces dry matter output (kg/day) is calculated as follows:
• Marker dose (g per day)/ Marker concentration in faeces DM (g/kg). For example, if an animal was given 10 g of chromic oxide per day and the concentration of the marker was found to be 4 g/kg faecesDM, faeces output would be calculated as 10/4 =2.5 kg DM/day. If food intake was known, dry matter digestibility could be calculated in the usual way.

The ideal specification of an indicator/marker are :

• It should be totally indigestible.
• It should not have any pharmacological action on the digestive tract. It should be inert to the digestive system.
• It must mix intimately and remain uniformly distributed in the digesta.
• It should pass through the tract at a uniform rate and should be voided entirely.
• It can readily be determined chemically, and
• Preferably be a natural constituent of the feed under test.

Indicators may be used to measure digestibility of feed under the following circumstances :

• If metabolism cages and other facilities for direct collection of feces and urine voided are not available
• If animals are fed in groups, then it is impossible to record the feed consumed and feces voided by each animal in the group but still it is possible to measure digestibility of feed by the indicator method.
• To know intake of herbage from cultivated or natural pastures and digestibility of nutrients in the pasture consumed by the animal.