Most horse owners have heard that feed changes should be made gradually, usually over ten to fourteen days.
It is one of those golden rules that gets passed down through generations of riders. But many people do not realise that this simple habit protects one of the most finely balanced systems in the horse’s body: the digestive tract.
How the Horse’s Digestive System Works
The horse’s digestive system is quite different from that of most other animals. Horses are non-ruminant herbivores, meaning they live on plant material but have only a single, relatively small stomach. Feed moves quickly through the foregut, which includes the stomach and small intestine, where enzymes break down proteins, fats and some carbohydrates.
The bulk of digestion happens further along in the hindgut. This section includes the caecum and the large colon, which together hold well over one hundred litres of material. Inside this enormous fermentation vat live billions of bacteria and microorganisms. Their job is to break down the fibrous plant matter that the horse cannot digest on its own. The bacteria convert fibre into volatile fatty acids, which supply much of the horse’s energy. They also produce B-vitamins and help maintain a healthy gut environment.
These microbes are specialists. Different types flourish on different diets. A horse living mainly on hay and pasture will have a bacterial community designed to digest fibre, while a horse eating more grain will have bacteria that cope with starch. The balance between these groups is what keeps the digestive system stable.
What Happens When Feed Changes Are Made Too Quickly
If the diet changes too quickly, the balance of bacteria in the hindgut is disrupted. The new feed arrives before the microbial population has had time to adjust.
When a horse suddenly receives more grain, or is turned out onto rich green pasture, the starch and sugars that are not digested in the small intestine reach the hindgut. There they are rapidly fermented by bacteria that produce lactic acid. The acidity of the gut increases and the pH drops. Beneficial bacteria die off, releasing toxins that can enter the bloodstream. Gas builds up, fermentation slows, and the normal movement of feed through the gut is disturbed.
This chain of events can lead to colic, hindgut acidosis or laminitis. The caecum, which is roughly thirty litres in size, has both its entrance and exit at the top, so feed must mix thoroughly before it can pass through. When fermentation is disrupted, feed may sit in the caecum for too long, creating the risk of impaction. Horses that do not drink enough, or that suddenly start on a new hay or supplement, are particularly vulnerable.
Why the Gut Microbes Need Time to Adjust
It takes around two to three weeks for the microbial population in the caecum and colon to fully adapt to a new feed. During this time, the numbers of bacteria suited to the new diet gradually increase while others decline. If you allow this adjustment period, the fermentation process continues smoothly and your horse’s digestive health remains stable.
That is why the recommendation is to change feed over ten to fourteen days. By mixing a small amount of the new feed with the old and increasing the proportion every few days, you give those microbes a chance to catch up. The same principle applies to pasture changes. When moving your horse from dry paddocks to lush grass, begin with short grazing sessions and extend the time slowly over a couple of weeks.
Best Practices for Changing Your Horse’s Feed
- Plan ahead. Keep enough of your horse’s current feed on hand so you never have to make a sudden switch.
- Make gradual adjustments. Replace no more than twenty per cent of the old feed every two to three days.
- Monitor your horse. Changes in appetite, manure consistency or behaviour can be early signs of digestive upset.
- Ensure adequate water. Proper hydration helps feed move smoothly through the gut and supports microbial function.
- Be careful with lush pasture. Introduce access slowly, particularly after rain or seasonal growth.
The Take-Home Message
To the horse, consistency is everything. Their digestive system is built to process a steady supply of fibrous feed, not sudden changes in ingredients or quantity. Behind the scenes, a complex community of microbes works constantly to turn that feed into energy. When their environment is altered too abruptly, the whole system is thrown off balance.
Understanding why the process takes time helps make sense of advice that might otherwise seem routine. Changing feed slowly is not just about avoiding stomach discomfort. It protects the delicate ecosystem inside your horse’s hindgut, keeping your horse healthy, comfortable and ready to perform at its best.
Sources:
This article draws on information from the Pony Club Australia A Certificate Manual (Nutrition section, “The Horse’s Digestive System”) and Pony Club Australia’s feeding guidelines.
