Determining feed digestibility

When discussing animal feeds, the terms solubility and digestibility may easily be mixed or misunderstood. Solubility means the degree to which the food breaks down to smaller particles, for example how much of protein breaks into amino acids. The digestibility of a feed determines the amount that is actually absorbed by an animal and therefore the availability of nutrients for growth, reproduction etc. For example, wood, grass silage and straws have the same amount of gross energy, but due to the differences in their digestibility, only  grass silage is useful as a feed. See more terms explained from FAO's site on animal nutrition.

Digestibility of a feed must be originally determined in animal tests. When the base values from solubility and digestibility are thus gained, more tests can be done in laboratories. Lab tests are never equivalent to tests done on living animals, but on the basis of animal welfare, speed, repeatability and cost-effectiveness, they are widely used. This text will cover some basic methods of determining feed digestibility in vivo or in sacco, "in a living animal".

Before any in vivo feed studies, each animal goes through an adaptation phase. For cattle the phase may last 1-2 months, but only 3-5 days for pigs and 1-2 for poultry. During the adaptation, the animal gets used to the new feed, and estimates can be made about its palatability. If the animal will be in a collection pen or crate during the test, it will be there during the adaptation phase too, getting used to the new environment. The actual testing phase lasts from a day to a week or two, depending on the size of the animal.

Total collection

Total collection is a quantitative research method, where the animal is kept in a collection pen or cage. Collection cages used to be very tight, preventing the animal from moving more than a few inches backwards or forwards, but they have now been banned due to obvious animal welfare issues. The crates are designed so that all feces can be collected, and some percentage of it will be analysed. If the balance of nitrogen is studied, then urine will also be gathered and analysed. The animals are fed approximately 90 % of their normal feeding levels, so that there's no leftover feed. All necessary vitamin and mineral supplements are given during the test as usual.

(c) University of Helsinki

Total collection tests can also be done for animals in pasture or in group pens. To get individual results from each animal, the animals are fitted with harnesses or other means of carrying collection bags, which are emptied at least once a day. The harnesses cannot be too heavy or uncomfortable, because stress and pain would immediately affect the animal's behavior and metabolism. A typical collection method for pigs is to glue a plastic bag to its rear end, but it raises concerns of animal welfare.

Total collection method can be used to study the effect of one feed (direct method), of basic feed + test feed (differential method), or of feeds in different amounts, for example 10, 20 and 30% of wheat (regression method). Knowing the chemical composition and dry matter (DM) content of the tested feeds is crucial.


When choosing animals for a total collection test, make sure that

• the animals are of the same age and size
• the feed given is processed identically every day
• the amount of feed eaten is recorded for each individual animal
• feces (and urine) are collected at the same intervals, often enough and without contaminating the samples
• consider the impact of proteins and fat to the digestibility of the feed
• use identical and accurate analyzing methods

Calculating the results

All values are g/kg DM if not stated otherwise. The formulas do not apply for regression method.

Digestibility of the feed:  

(amount of feed eaten - amount of  feces ) / amount of feed eaten

or

1 - (amount of  feces / amount of feed eaten) * 100

Digestibility of different nutrients (multiply with 100 to get the results in percentages):

(Nutrient content in the eaten amount of the feed - Nutrient content in the feces)

 / Nutrient content in the eaten amount of the feed

.
Digestibility of the feed and nutrients when using the differential method:

(amount of test feed eaten - (amount of  feces - undigested basic, non-test feed))

 / amount of test feed eaten

Marker method

Marker method can be used when total collection is not possible or feasible. Marking method includes adding a marker substance to the feed, and then analysing the amount of marker feed in the feces. This method may be conducted with the animal in a collection cage, but is also usable for grazing animal.

The marker substances used in marker method trials must be safe for the animal, travel at the same speed than the feed in the digestive tract, they can not impact the digestibility, mix well with the feed and be stabile. The marker must be detectable and measurable even in very small amounts. Markers can be either liquid or solid, and either added to the feed or insoluble parts of the feed (such as insoluble ash). Common markers used are

• titanium oxide TiO₂
• chromium oxide Cr₂O₃
• ytterbium acetate
• litium
• strontium
• cobalt EDTA
• chromium-mordanted straw (straw covered with chrome, fixed in place with a mordant)

Calculating the results
All values are g/kg DM if not stated otherwise.

Measuring the digestibility in a certain spot in the digestive tract

Fistulated horse (c) Todd Huffman

In some cases, it is important to measure how nutrients are digested in different parts of the digestive tract. This is especially true with ruminants and amino acids. Amino acids which are absorbed in the rumen are used by the rumen microbes, and the animal itself gets only the amino acids which absorb from the small intestine. Even the monogastric pigs can use only the amino acids which absorb from the small intestine, and some trials are done on horses as well.


These trials (animal tests) always need animals with a fistula and/or a cannula. The fistula is usually installed to the rumen or to the beginning/and or end of the small intestine. Sometimes the rumen fistula may be connected to the abomasum with a long tube, enabling collecting samples from the abomasum.

The idea of an animal having a hole in it, and people digging the innards of the animal through the hole is (and should be) very distressing. People who work with fistulated animals may not be the most reliable source, but they claim the fistula causes no discomfort for the animal. Permits for conducting and animal test are needed before the operation. Fistulas are always  made by experienced vets.  Sedation and local anesthetics are always used, and left to heal for two months under direct supervision and care. After healing, the fistula is painless even when used. If the animal is used to being treated (brushed, milked, petted etc), it shows no signs of pain, discomfort or fear when the fistula is opened and a sample is taken. At least fistulated cattle live their lives normally, milking, eating, calving and socializing without problems. However, health and welfare risks exist and must be minimized or removed before proceeding.

In sacco -method (ruminants only)

Fistulated cows (c) David Wilson

In an in sacco -method small nylon bags are filled with the test feed, and inserted into the rumen.  Like in the previous method, in sacco also requires fistulated cows. Being small and light, the nylon bags do not cause discomfort, or affect the rumen functions. It is an reliable way to measure the solubility of certain feeds. Insoluble fibres can be most reliably measured in sacco. The nylon bags are so tight, that no feed particles can escape from the bag or enter it. Only the rumen microbes are able to affect the feed in the bag.

After the bag is taken away from the rumen, it is washed with clean water, and then dried. Single bags are removed after a certain time, and the weight and contents are then analyzed to study the solubility of the feed. Most often bags are removed after 2,4,6,8,10,12 and 48 hours, but for insolubility tests, last samples may be taken as late as after 72 and 96 hours, or even 12 days. Determining the solubility of the tested feed is a matter of comparing the composition and weight of the feed samples before and after the in sacco test.

Apparent vs true digestibility

The digestive tract secretes several substances, which get mixed with the feed and eventually the feces. Most commonly tthey are proteins or other nitrogenous compounds. These animal-based substances, known as metabolical or endogenous secretions, must be considered when analysing digestibility. Apparent digestibility does not consider endogenous secretions, unlike true digestibility. True digestibility for proteins, amino acids and fats is always higher than apparent.

Endogenous excretions include bacteria, enzymes, endogenous peptides, amines, urea, mucus and dead cells of the digestive tract. Measuring the levels of endogenous secretion is difficult, so common factors are used to estimate them. Cattle is estimated to secrete 5 g of endogenous matter per a kilogram of eaten dry matter, while pigs and rats secrete 1 g. 

More information:

FAO: Animal nutrition
University of Florida, dept of Animal Sciences:  Digestibility
E R Ørskov: Methods of estimating nutritive value of fibrous residues