Nutrient Utilisation

Hello :) In this post we'll compare and contrast growth and digestibility trials. I'll also explain the difference between apparent and true digestibility, why people use markers in food, the advantages of determining ileal digestibility as well as the factors that affect digestibility.  

Growth Trials

Growth trials - when the growth of an animal is measured after it has been on different feeds - is the simplest way to compare different diets or nutrient sources that we give to animals.Feeding different diets ad libitum allows us to determine rate of gain, voluntary feed intake and efficiency of gain.

Growth trials are good because they are relatively cheap and large amounts of data can be collected under realistic conditions. The measurements are also obtained non-invasively and the results represent the biological response to the diet.  

However, growth trials are really only useful for meat-producing animals as they have limited usefulness in animals in which weight change is not the desired outcome (eg. we may want wool to be produced instead). In addition, there is variability between individuals in the biological response to the diets. There are also challenges in obtaining accurate weights and the data often doesn't reflect changes in body composition.

Digestibility Trials

Digestibility trials aim to determine the digestibility of a food. That is, to work out how much of the energy in the food is actually used by the animal and how much is excreted. Digestibility represents the proportion of food that is not excreted in faeces and urine and is assumed to be absorbed by the body. To determine digestibility, animals are fed a diet of known composition for a period of time and all the faeces voided are collected and analysed. 

The results of digestibility trials are only useful if the following conditions are met:

• The same kind of animals are used
• Same gender
• Same age and bodyweight
• Same energy level of dry matter intake
• Same feeding level
• Same diet preparation or processing
• Same environmental conditions. 

 Apparent Digestibility

Digestibility is estimated by the difference between the nutrients ingested and excreted in faeces as a proportion of the amount of food ingested. This can be expressed as:

Apparent Digestibility (%) = (nutrientDIET  - nutrientFAECES) / nutrientDIET   x  100

Apparent digestibility accounts for both undigested/unabsorbed feed residues and the components of faeces that have an endogenous origin. 

True Digestibility

True digestibility refers to the proportion of dietary intake that is absorbed from the gastrointestinal tract excluding contributions from endogenous sources. This is expressed as:

True Digestibility (%) = [nutrientDIET - (nutrientFAECES - nutrientENDOGENOUS)] / nutrientDIET

Markers

Markers can be used in digestibility trials so that not all the faeces has to be collected and all intake recorded to measure digestibility. This is particularly useful when animals are in group housing.  

A good marker must be completely recoverable - it mustn't be digested or metabolised. It should also be safe, palatable and measurable and should be able to mix evenly in the feed. People can use internal markers (present in the food) like lignin or external markers (added to the food) like titanium dioxide or chromic oxide.

Illeal Digestibility

Illeal digestibility is the digestibility measured at the end of the small intestine. This is useful to know because the value will contain a significantly smaller contribution from bacteria. There are various ways to determine this:

• Slaughter method: an indigestible marker is fed to the animal. The animal is then euthanased and the samples of digesta are removed from the intestine. The samples are then freeze-dried and analysed.
• Simple T-cannula: A T-shaped cannula is surgically placed in the distal ileum to collect samples of digesta. An indigestible marker is also fed and the samples are freeze dried and analysed.
• PVTC method: similar to the T-cannula method except that the caecum is removed and replaced with a silicon cannula.
• Re-entrant cannulas: another surgical technique where the digesta is diverted outside the pig and then returned to the ilem or caecum. This is a high-cost technique which is difficult to perform and as such is rarely used. 
• Ileorectostomy: The ileum is joined to the rectum so that the large intestine and caecum are by-passed. Normal electrolyte balance is disturbed and maintenance of the animals is difficult.

Factors Affecting Digestibility

Several factors affect how much food can be used by the body:

• Food composition: the more lignin present in the cell wall of the plants fed to the animal results in lower digestibility. A deficiency of sulphur and nitrogen or an excess of lipids may result in a decrease in microbial activity and thus a decrease in digestibility. High ash content as well as the presence of tannins and phytate may also reduce digestibility.  
• Level of feeding: increased intake causes the food to spend less time in the rumen which results in lower digestibility.
• Particle size: increased particle size increases digestibility to an extent. This is because the surface area available for the microbes is increased. However, if the food particles are too small, the feed passes through the rumen too quickly and digestibility decreases.
• Individual variation: the digestibility may vary slightly in different animals within the same species.  

That's all we need to know for this topic, see you next time :)