The moral of every diet change story for horses is... go slow.
This is only partly for the needs of the horse and is mostly about the microbes that reside in the horse's hindgut. The microbial population is adapted to whatever the horse has been eating - some microbes like starch, some like fiber, some like low pH, etc. And while yes your horse might make a fuss if their diet changes too quickly, microbes have a tendency to either die (leading to decaying bacteria, rapid pH changes, and toxin release) or the microbes have a population explosion (leading to spikes in their numbers, gas creation, and pH changes... and which may also be followed by their death and resulting issues, especially if the diet change was temporary). These types of changes are strongly associated with colic and can lead to laminitis (Al Jassim and Andrews, 2009), so take care of the microbes during diet changes and you are taking care of your horse's heath.
While changes in fat and protein intake has less impact for the microbes, rapid addition of fat doesn't allow time for the horse's fat digesting compounds to adapt. Sudden addition of fat to the horse’s diet leads to fat getting through the digestive track undigested and absorbed, which leads to loose, greasy stools – but adjusting fat content over a few weeks prevents these effects (Kronfeld et al, 2004). Rapid changes in feed content (the relative amount of starch, fiber, protein, and fat) also affects the function of the stomach, as digestion begins with the stomach and the stomach then regulates the passage of feed to the rest of the GI tract based partly on content of what has been consumed (Metayer et al., 2014). In fact, just changing the type of feed processing even with the same type of feed (like pelleted oats vs. rolled oats vs. whole oats) impacts how much time feed spends in the stomach and resulting digestion potential (Julliand et al., 2006). Thus, changes in feed type/content/processing can affect how quickly the stomach empties, so slowly adjusting to new feeds helps minimize drastic changes in transit time through the upper GI tract.
Likewise, simply sudden increases in the size of a meal affects how much potential digestion occurs in the stomach, as meal size is another way the stomach regulates movement through the GI tract. Sudden larger meals can lead to less time in the stomach, which can limit time for normal gastric digestion, releasing insufficiently prepared feed into the small intestine (Metayer et al., 2014). This can reduce the potential digestion and absorption of the feed before the hindgut and may lead to nutrients reaching the microbes of the hindgut and again affecting their population dynamics rapidly (Al Jassim and Andrews, 2009).
Methods of Safely Making Changes
Therefore, whenever you change the amount fed, the carbohydrate, fat, or protein content, and certainly when adding or taking away a new type of feed, do so very gradually. A good protocol would be to employ a max 20% per day rule for any feed changes, meaning that you increase or decrease by no more than 20% of a given feed type per day.
-Changing over (but giving the same amount) to a new feed or forage by 20% per day so that this schedule is used:
Day 1: 80% old feed, 20% new feed
Day 2: 60% old feed, 40% new feed
Day 3: 40% old feed, 60% new feed
Day 4: 20% old feed, 80% new feed
Day 5: 100% new feed
-Add or decrease amount fed by only 20% of a feed total per day. For example, if you were adding in 5 lb of a new feed, take 5 days so that this schedule is used:
Day 1: 1 lb of new feed
Day 2: 2 lb of new feed
Day 3: 3 lb of new feed
Day 4: 4 lb of new feed
Day 5: 5 lb of new feed
-If you need to change feed types AND increase or decrease the amount, it would be wise to change both factor even more gradually, like over 8-10 days, with incremental exchange of the new and old feed for small adjustments in the amount of feed given. For instance, if you were going from 10 lbs of your old feed to 6 lbs of your new feed…
Day 0: 100% old feed – 10 lbs
Day 1: 90% of old feed – 9 lbs, 10% of new feed – 0.6 lbs 9.6 lbs total
Day 2: 80% of old feed – 8 lbs, 20% of new feed – 1.2 lbs 9.2 lbs total
Day 3: 70% of old feed – 7 lbs, 30% of new feed – 1.8 lbs 8.8 lbs total
Day 4: 60% of old feed – 6 lbs, 40% of new feed – 2.4 lbs 8.4 lbs total
Day 5: 50% of old feed – 5 lbs, 50% of new feed – 3.0 lbs 8.0 lbs total
Day 6: 40% of old feed – 4 lbs, 60% of new feed – 3.6 lbs 7.6 lbs total
Day 7: 30% of old feed – 3 lbs, 70% of new feed – 4.2 lbs 7.2 lbs total
Day 8: 20% of old feed – 2 lbs, 80% of new feed – 4.8 lbs 6.8 lbs total
Day 9: 10% of old feed – 1 lbs, 90% of new feed – 5.4 lbs 6.4 lbs total
Day 10: 100% of new feed – 6 lbs
Additional Tips for a Smooth Transition
If you know well in advance that you're making a diet change, a little insurance against the risks we've discussed would be to have your horse on a probiotic/prebiotic blend that helps stabilize the GI tract, provide some protection against shifts in the microbial population, and activates the ability of the GI tract to respond to disruptions from feed changes (Garber et al., 2019; Heaton et al., 2019; Moore-Colyer, 2019; Phillips et al., 2018; Respondek et al., 2008). Ideally, getting a probiotic/prebiotic blend on board a few weeks before diet changes would be the most advantageous, but even adding it in simultaneous to the start of feed changes can help attenuate some of the risks we've discussed. See more on these topics in the second part of this post addressing hindgut disturbances.
Whenever possible, avoid feed changes simultaneous to other stressors – like a new barn, vaccines, deworming, heavy training, or weather changes. Either make the feed changes well in advance of these factors, or hold off until the stressor has passed and then make the diet adjustment. This not only reduces risk of digestive problems but also helps you know more accurately what caused the issue. For example, if you move your horse and simultaneously feed AND pasture change at the same time and your horse colics, you won’t know what the actual cause was and it may prolong treatment and recovery.
Final tip – when implementing changes, make up a schedule for anyone who feeds your horse with the dates and amounts of each feed clearly laid out. This is especially important if you only have a limited amount of the old feed available – like when you have the last of a supply of hay or when the previous owner only gives you a few days’ supply of the old feed. Taking time to make those calculations and precisely listing out the appropriate amounts to feed each day makes it easier for everyone involved and may help you avoid falling short of being able to transition slowly.
If you have any questions or concerns, and especially if you are overhauling the entire diet, please reach out to your veterinarian and Science Supplements nutritionists to help reduce risks for your horse.
Al Jassim RAM, Andrews FM. The bacterial community of the horse gastrointestinal tract and its relation to fermentative acidosis, laminitis, colic, and stomach ulcers. Veterinary Clinic of North American Equine Practitioners. 2009;25:199-215
Garber A, Hastie P, Murray J-A. The effect of yeast supplementation on equine fecal microbial population dynamics following abrupt dietary changes. Journal of Equine Veterinary Science. 2019;76:79
Heaton CP, Cavinder CA, Paz H, Rude BJ, Smith T, Memili E, et al. Are prebiotics beneficial for digestion in mature and senior horses? Journal of Equine Veterinary Science. 2019;76:87-88
Julliand V, De Fombelle A, Varloud M. Starch digestion in horses: The impact of feed processing. Livestock Science. 2006;100(1):44-52
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Moore-Colyer, MJS. Gut Health and Dietary Manipulation for Performance Horses: an Overview of (New) Studies Regarding Digestive Aids Available for Horses. 9th European Equine Health and Nutrition Congress. 2019:45-52
Phillips CA, Cavinder CA, Memili E, Rude BJ, Smith T. The effect of direct fed microbials on apparent nutrient digestibility, fecal microbial population, and blood metabolites in the Moderately exercised horse. Journal of Animal Science. 2018;96:85
Respondek R, Goachet AG, Julliand V. Effects of dietary short-chain fructooligosaccharides on the intestinal microflora of horses subjected to a sudden change in diet. Journal of Animal Science. 2008;86:316-323