Horses were designed to eat grass!
Horses normally eat pasture constantly. As such their digestive systems are designed to constantly receive the chewed pasture and digest it.
There are two aspects of the horse gut that reflect this:
1. Horses produce very low amounts of amylase in their saliva - this is because the normal diet of horses is not heavy in starches.
2. The gut microbiome of the horse plays a crucial role in ensuring that food is properly digested. For this reason it is important that the microbiome is healthy. An abnormal balance in the microbiome has been associated with a number of adverse health issues.
Modern horses' diets are high in starches.
When we consider the very small amount of amylases produced by horses, it is no surprise that most starches produced by the horse end up passing through the stomach undigested, and into the small intestine and hindgut, where they accumulate.
Undigested starches cause malfermentation.
This accumulation of undigested starches in the hindgut ends up being metabolised by bacteria, producing D-lactate and other endotoxins that lead to an increase in the acidity of the hindgut. This process is known as malfermentation.
Malfermentation alters the gut environment unfavourably.
This in turn changes the gut environment, affecting the microbiome - normal bacteria and microorganisms that live in the gut, such that it encourages an increase in the population of bacteria that are less beneficial, and a decrease in more beneficial bacteria..
Other effects noted are that feed passes through the gut more quickly, giving the gut less time to absorb nutrients, as well as an increase in gut permeability which lessens the control of chemicals that go through the gut wall.
So how do we correct this today?
This problem has been identified and there are many people who are changing the way they feed their horses. They do this by increasing the proportion of forage and pasture that they eat, while at the same time reducing the starch in their feed.
Alternatively, pro-biotics and pre-biotics are often used to sort out gut issues. These work by introducing live organisms (pro-biotics) or nutrients for these organisms (pre-biotics).