Along with fermentation, digestion, and absorption of some nutrients, another important function of the equine large colon is to regulate the amount of water in the ingesta (a heavy, wet sludge of bacteria, protozoa, feed and water within the colon), usually by taking water back into the blood circulation from the this material.

The function of the small colon (the most downstream portion of the intestinal tract, and downstream from the large colon) is to dehydrate it further, making it into relatively dry fecal balls, and in so doing to minimize water loss.

It is normal for there to be some variation in the consistency of a horse's manure. Just because a horse has soft manure does not mean they are sick. Importantly, horses pass soft or watery manure during times when they are anxious or stressed. This is one reason horses tend to pass wet manure when loaded in the trailer, the stress of being loaded. A horse's manure may also soften as a result of a feed change, but in most cases should return to normal within a day or two.

A horse may also pass water after or alongside solid manure. Rather than the water being contained within the fecal balls, it is outside of it. One potential cause for this is long fiber length in the manure. This can result from poor grinding or the use of excessively fibrous, stalky forage like straw.

But soft manure can also be a result of poor function of the lower intestine (the colon), which can be caused by a variety of conditions. If this observation is coupled with other signs of illness (especially loss of appetite, depression or colic) or if your horse suddenly develops watery diarrhea, they may be suffering from a serious underlying problem and should be promptly examined by a vet.

Many owners put their horses on a probiotic to try to "fix" this. It's important to know that mildy soft manure is not necessarily bad in itself. The question is what is causing it.