I am just doing some of my wider reading for my course and I came across a paper comparing the differences between different types of equids and how they digest food. The paper was by Cuddeford et al (1995) and was called Digestibility and gastro-intestinal transit tie of diets containing different proportions of alfalfa and oat straw given to Thorougbreds Shetland ponies, Highland ponies and donkeys.
What interested me the most about this paper was that it included donkeys. While spending two weeks working with donkeys in the summer I saw how different they were to horses regarding their diet. They were in the field during the day but over night they were in their barns with straw to eat. They also put weight on very differently to horses as it was collecting in patches and not evenly spread around the body. I was told tat they have evolved differently than horses as they are from hot countries where there is not much grass so eat more roughage that is not as full of nutrients as grass.
In the paper above, they found that all animals digested the low fibre diets better than the high fibre diets. This is probably because in a high fibre diet there is more cellulose from the plant cell walls and this is difficult to break down. It was also found that donkeys digested the fibre more effectively than other equids.The transit time it took from the food entering the mouth to leaving as faeces was also longer for donkeys. As they are retaining the food for longer it will give them more chance to digest the food. The transit time was also longer for the low fibre diet than the high fibre diet in all equids.
The donkeys were also found to have the lowest water intake followed by Shetlands. Both of these had significantly lower intakes than the Thoroughbreds and Highland ponies.
Where the food intake is limited, the donkey would have the advantage as it can can get more of the nutrients out of what it eats. However, where there are unlimited food supplies, the pony or horse can compensate for this by consuming and processing more food as they do not retain the food as long.
Just a quick little post I thought you might find interesting!