For horses · Jul 31, 2013
What you feed your horse is directly shaping how it behaves
High-starch diets, restricted roughage, and blood-sugar swings all drive reactivity, and the research backs that up.
Yes, diet directly influences behaviour in horses. High-starch diets increase behavioural reactivity, while fibre-based diets produce calmer behaviour and better welfare outcomes. Fat-supplemented diets are also associated with reduced activity and reactivity, though whether this is due to fat itself or the accompanying reduction in starch remains unclear. Protein is not a driver of excitability, it contributes very little to energy production and does not cause excitable behaviour.
High protein content in feed is what makes horses “hot”
I hear this constantly, and it drives me up the wall, owners switching to a “low protein” feed because their horse is sharp, believing the protein percentage on the bag is the culprit. I understand where it comes from: protein percentage is the most prominent number on every feed label, so it becomes the default explanation for everything, including excitability.
But the biochemistry doesn’t support it. Protein is not a meaningful energy source for horses. The Merck Veterinary Manual is unambiguous on this point: dietary protein contributes very little to overall energy production in horses and does not cause excitable behaviour [2]. The energy that actually drives a “fizzy” horse comes primarily from carbohydrates, specifically starch and simple sugars.
The research is consistent here. High-starch diets are associated with increased behavioural reactivity in horses; ponies fed high-starch diets show measurably more alert and reactive behaviour compared to those on high-fibre diets, with changes linked to the gut-brain axis [4]. Horses on fibre-based diets spend more time in calm feeding behaviour and show fewer excitable or “fizzy” episodes [5]. High intakes of starch and simple sugars have also been directly associated with behavioural modifications alongside physical health problems [6].
So if your horse is hot, the first question isn’t “how much protein is in the feed?”, it’s “how much starch and sugar?” Those are the dietary levers that actually move behaviour.
How diet shapes behaviour: The mechanisms
Dietary starch and sugar are the clearest dietary levers on equine behaviour. Research shows that horses and ponies fed high-starch diets display measurably more alert and reactive behaviour compared with those on high-fibre diets, an effect that may be mediated through the gut-brain axis, with high-starch feeding altering the faecal microbiota in ways that appear to influence behavioural reactivity.
The digestive consequences of large starch loads compound this. Feeding large, infrequent concentrate meals causes significant gastrointestinal fermentation changes and fluid imbalances that raise the risk of impaction colic; shifting to small, frequent meals attenuates these stress responses. High starch and simple sugar intakes are also associated with gastric, intestinal, and muscle disease alongside behavioural changes, and feeding more than 50% of the ration as high-starch/sugar concentrates increases the risk of laminitis, colic, and equine gastric ulcer syndrome.
Fat- and fibre-based diets, by contrast, are associated with calmer behaviour patterns, though whether the calming effect of fat is attributable to the fat itself or to the corresponding reduction in starch intake remains unclear. Horses on fibre-based rations spend more time in normal feeding behaviour and less energy in excitable or “fizzy” activity.
Protein is widely misunderstood. It does not cause excitable behaviour; it contributes very little to overall energy production in healthy horses. Dietary protein content is not a useful proxy for how “heating” a feed will be.
L-tryptophan supplementation does raise plasma tryptophan concentrations by roughly three-fold. Tryptophan is the direct precursor to Serotonin, the ‘calmness’ neurotransmitter.
Finally, feeding management itself matters independently of feed composition. Horses on restricted, rationed diets show more signs of stress and more frequent arousal behaviours, particularly in the period approaching feeding time, compared with horses fed ad libitum. Any gap in roughage intake beyond four hours is associated with colic, ulcers, and behavioural stress.
How to put the diet-behaviour connection into practice
Keep roughage available continuously, or as close to it as possible
Horses are continuous grazers, and the research is unambiguous on this: any gap in forage intake should not exceed four hours [10]. Restricted diets increase stress behaviours, particularly in the period approaching feeding time [7]. If you have a fizzy or anxious horse, sorting the forage situation is the first practical step. Grazing or haynet all day. Enough hay at night to have leftovers in the morning.
Switch from high-starch concentrates to fibre- and fat-based feeds
High-starch diets are directly associated with increased alertness and reactivity [4], and high intakes of starch and simple sugars are linked to gastric, intestinal, and muscle problems alongside the behavioural changes [6]. Fibre-based diets, by contrast, are associated with calmer behaviour and reduced excitable energy expenditure [1][5]. The evidence for fat-based feeding as a calmer alternative to starch points in the same direction [1], though whether the calming effect comes from the fat itself or simply from the reduction in starch is not yet resolved.
If you are managing a horse with Equine Metabolic Syndrome, the threshold is clear: non-structural carbohydrates should not exceed 0.1 g/kg per meal, and high-starch/sugar concentrates should not make up more than 50% of the ration dry matter [8].
Feed small, frequent meals rather than large episodic ones
Large concentrated meals drive gastrointestinal fermentation changes and fluid imbalances that increase colic risk and physiological stress [9]. Smaller, more frequent meals support a steadier digestive and metabolic pattern, which matters for behaviour as well as gut health.
Supplements
Choose a well rounded supplement with Tryptophan (yes, when vitamin D3 is also supplemented, Tryptophan is converted to Serotonin), Magnesium and Inositol (a B vitamin that acts as an antidepressant).
What we’d recommend
Tryptophan, inositol, magnesium, vitamin D3 to reduce behavioural reactivity and support steady energy.
See the product
Frequently asked questions
Does high protein feed cause excitable or ‘heating’ behaviour in horses?
No, this is one of the most persistent myths in horse feeding. Protein is not a meaningful energy source; dietary protein contributes very little to overall energy production. The percentage on the feed bag tells you nothing about how ‘heating’ that feed will be. Energy comes from carbohydrates and fat, not protein, so blaming the protein content for a fizzy horse is looking in the wrong place.
Can a high-starch or high-sugar diet make my horse more reactive?
Yes, and there is good research to support this. Horses and ponies fed high-starch diets show significantly more alert and reactive behaviour compared to those on high-fibre diets. High starch intakes are also associated with gastric, intestinal, and muscle problems, and the changes in behaviour may be mediated through gut microbiota changes via the gut-brain axis. Swapping starch for fibre is one of the most evidence-backed steps you can take for a fizzy horse.
Will feeding L-tryptophan calm my horse down?
The evidence is less clear-cut than many supplement labels suggest. A commercial dose of 6.3 g L-tryptophan does raise plasma tryptophan concentrations roughly three-fold in horses, but that rise was not matched by measurable behavioural changes in the study. In our Calm mix, however, we include Vitamin D3 which drives the conversion of Tryptophan to Serotonin, which has a direct effect on mood and excitability.
How does a fat-based diet affect horse behaviour compared to a starch-based diet?
Fat- and fibre-based diets are associated with calmer behavioural patterns compared to starch-heavy approaches. One likely mechanism is simple displacement: adding fat to the ration means less starch, reducing the blood-sugar spikes that drive excitable behaviour. Fibre-based feeding also improves welfare scores and allows horses to spend more time in natural feeding behaviour rather than expressing excitable or stereotypic responses.
How long can a horse safely go without forage before it becomes a welfare and health issue?
Any gap in forage intake should not exceed four hours. Extended breaks are linked to colic, gastric ulcers, and measurable behavioural stress. Horses on restricted, rationed diets also display more repetitive, arousal behaviours, particularly in the period leading up to feeding time, compared to horses with continuous forage access. The horse’s digestive system is designed for near-continuous roughage intake, and managing against that has real health consequences.
What are the health risks of feeding large amounts of starchy concentrate?
Feeding more than 50% of the ration dry matter as high-starch or high-sugar concentrate increases the documented risk of laminitis, colic, and equine gastric ulcer syndrome. For horses with equine metabolic syndrome, non-structural carbohydrates should not exceed 0.1 g/kg per meal. Even in otherwise healthy horses, large starch meals create significant digestive stress, feeding small, frequent meals rather than one or two large ones substantially reduces these risks.
Sources
- Role of diet and feeding in normal and stereotypic behaviors in horses
- Nutritional Requirements of Horses and Other Equids – Management and Nutrition – Merck Veterinary Manual
- Effects of a commercial dose of L-tryptophan on plasma tryptophan concentrations and behaviour in horses
- High-starch diets alter equine faecal microbiota and increase behavioural reactivity
- Does Feeding Management Make a Difference to Behavioural Activities and Welfare of Horses Reared for Meat Production?
- Effect of high-starch or high-fibre diets on the energy metabolism and physical performance of horses during an 8-week training period
- Anticipatory Behaviour During the Approach to Feeding Times as a Measure of Horse Welfare
- Equine Metabolic Syndrome – Metabolic Disorders – Merck Veterinary Manual
- Feeding and digestive problems in horses. Physiologic responses to a concentrated meal
- Common Feeding Practices Pose A Risk to the Welfare of Horses When Kept on Non-Edible Bedding