What is digestibility?
Digestibility is just a measure of how much of the nutrients in a sample an animal can actually extract and absorb.
How is digestibility measured?
There are several ways to measure digestibility, all of which involve some placing a sample in a small permeable bag and weighing back how much of the sample is removed after being treated with some sort of real or artificial digestive fluid. For caribou, this can be done by placing the bags inside the rumen of an actual animal if it is surgically equipped with a fistula, much like cows sometimes are at agricultural schools. After allowing the fluid inside the rumen time to digest the sample, it is then removed and subjected to an artificial acid/pepsin treatment, much like our own stomachs. An alternative way to do this is to remove the fluid from the fistulated animal and incubate it with the sample back in the lab. One of the disadvantages of using these animal-based approaches (besides actually needing a set of fistulated animals) is that the results tend to be highly variable – different animals digest things differently, and as a result you can’t tell if small differences between forage samples are real or just a result of the animals digesting the sample for you.
In order to solve this problem, we’ve been working to develop a new way to measure digestibility – using the same sample bag approach, but incubating the sample in an artificial set of enzymes to simulate passage through the digestive tract of an animal. This method is a significant improvement on the old ways of measuring digestibility because the results are consistent and better for monitoring small differences in digestibility between forage plants. Unfortunately, though, this method wouldn’t be good for determining things like how the microbial ecosystem in the gut of an animal adapts to new foods because we have completely removed microbe-food interactions from the equation
Why is it important to measure digestibility?
Digestibility ultimately determines what is available to the animal – an animal can pick and choose what foods it eats, but its digestive tract determines what actually goes into the tissues of the animal. We can do things like measure the nitrogen content and stable isotope ratio of the digestibility residues (i.e., the fake poo left over from the digestibility procedure) to determine how much of these components are actually absorbed by the animals – for instance, not all of the protein in a forage sample is necessarily absorbed, and it would be beneficial to know how much actually is rather than just assuming they can absorb all of it. This would lead to more accurate predictions for things like modelling animal growth and estimating how many animals can live in an area.