Digestibility Trials

Digestibility trials are a research and evaluation methodology in veterinary sciences where difficult-to-digest foods are fed to animals in controlled doses, and the amount of waste they produce is monitored. Often, such trials are done across a wide variety of animals because animals of the same species and gender have slightly different digestive capacities. Also, repeated measurements allow for the detection of measurement mistakes. 

To ensure a consistent composition, the food used in the experiment should be well combined before the start of the experiment. The food is then fed to trial animals for at least a week before starting trials of digestibility, in order to acclimate them and remove residues of previous meals from their digestive tracts.

Special methods for Digestibility Trials

Here are a few different methods that are used in digestibility trials:

Nylon bag technique 

• In this method, an animal with a rumen cannula is fitted with small, square nylon mesh bags (6.5 x 14 cm) (30–50 mm). 
• The fistulated animals are incubated with feed samples at intervals between six and 120 hours, after which the components of the feed are determined. 
• After a certain period of time, the sample is washed, dried, and weighed. The primary goal is to measure the loss of dry matter and other nutrients. 
• Using this technique, dry matter, organic matter, nitrogen, and other nutritional indicators can rank meals.

Tilley and Terry IVDMD

• The Tilley and Terry two-stage laboratory test is extensively used on animals that feed on plants. 
• This technique is used for predicting in vitro digestibility in ruminants. 
• In its first stage, plant material is cultured with rumen bacteria for 48 hours at 39 °C. Then there is a 24-hour acid-pepsin digesting phase at 39 °C under anaerobic conditions.
• An ungraded feed sample is weighed in a 50-litre centrifuge tube (about 0.5g). 

The tube is incubated at 39 °C for 48 hours with McDougall’s buffer and donor ruminal fluid. 

• Buffers like Ohio, Kansas, and Van Soest are used to keep pH stable throughout fermentation. The non-fermentable liquid is then centrifuged off. 
• After adding acidified pepsin, the tube needs another 48 hours at 39 °C. Drying takes 12 hours at 105 °C. 
• There is a fire (550 °C for two hours), and the ashes are used to correct the plant sample weight for soil contamination.

Gas production technique

• It predicts in vivo digestibility by modelling in vivo fermentation of foods. Diverse gas generation techniques and their variants provide more information about fodder fermentation dynamics in live animals and the laboratory than digestibility and degradability tests. 
• The total gas output depends on the sample size, shape, and composition of the fermentation’s end products, rendering it vulnerable to use as a nutritional value metric. 
• The ratio of fermented carbohydrates to degraded carbohydrates can vary significantly between diets. 
• Accounting for the fermentation’s end products is an essential precaution when interpreting gas production profiles. 

• The profile must be defined using an “appropriate” model to estimate ruminal fermentation using gas production data. 
• There is no point in using buffers when the pH comes down within 12–24 hours.

Limitations of the different methods of Digestibility Trials

The methods of digestibility trials have the following limitations:

• Food intake: The nutritional aspects of a feed affect its digestibility. Experiments have demonstrated that limiting animal feed intake promotes nutritional absorption. Most research suggests that when food consumption increases, digestibility increases too. Faster food passage through the digestive tract likely means less time for digestion and absorption.
• Chemical composition: The feed’s nutritional content affects digestibility. The amount of various nutritional components in each feed can affect digestion because some nutrients shorten digestive enzymes’ time interacting with their substrates. However, small doses of specific nutrients such as protein or soluble carbs can increase overall digestibility.
• Feed processing: Feeds are processed to make them more digestible. Changes in physical form impact inorganic material digestion. All of these processes affect trials of digestibility. Chemical, biological, and cutting therapies help digest fibrous foods better.
• Climate: Since hotter foods retain more heat in the digestive tract than cold foods, cold-exposed sheep show worse results in digestibility trials than animals in warmer settings. Colder temperatures may also promote reticule-rumen motility. A faster passage rate could be used to meet the energy needs of a cold climate by consuming more dry materials.

Conclusion

There are many trustworthy techniques for determining digestibility, meaning it is possible to anticipate the digestibility of a feedstock based on its chemical composition during the digestibility trial. Multiple regression formulas connecting chemical components to in vivo digestibility are being developed as part of this endeavour. In vitro digestibility techniques can quickly, inexpensively, and accurately predict in vivo or conventionally established digestibility in ruminants. In the case of ruminants, nylon bag techniques are perfect for studying the kinematics of digestion.