The diet most horses ate for thousands of years — and what changed

April 23, 2026

In this article

How the horse’s nutritional biology developed around a diet of fresh grass
What the shift from pasture to hay does to the omega-3 available in the diet
Why stable-based management creates a nutritional gap that didn’t exist on open pasture
Why understanding this baseline matters before considering what supplementation can do

Part of a series

Article 2 of 6 in a series on omega-3 in horses. The series is intended to be read oldest to newest — each article builds on the previous.

Previous: Why the form of omega-3 you feed determines whether it works

Next: What happens to omega-3 when grass becomes hay

The modern horse's digestive system, metabolism, and nutritional biology were not built for a stable. They were built for a landscape.

Picture a horse in the way the species developed. Not stabled, not rugged, not waiting for the 7am feed. Moving almost continuously across open grassland, grazing for fourteen, fifteen, sixteen hours a day, covering ground as it goes. Selecting grasses, herbs, and plants as they come. Eating fresh, living material from before dawn until after dark.

This was the default long before domestication. And the fundamental nutritional architecture of the horse's digestive system — the way it processes forage, the fatty acids it depends on, the rhythms it was built around — reflects that history. Domestication and selective breeding have changed horses in many ways. The underlying biology of how they extract and use nutrients from forage has changed considerably less.

That image matters, because we are still working with that same underlying biology. The horse standing in your stable tonight carries the same fundamental nutritional architecture as its grazing predecessors. The expectations built into its systems have not changed. What has changed is almost everything about how it lives.

From grassland to hay barn

Fresh grass is nutritionally quite different from the preserved forage most horses eat for much of the year. It contains water, live plant compounds, antioxidants, and a fatty acid profile that includes a meaningful amount of alpha-linolenic acid — ALA, the plant-based form of omega-3, which the horse must convert into the long-chain forms that are actually used in the body. When horses graze freely on good pasture, omega-3 is not a supplement. It is just part of what they eat, every day, in consistent amounts, alongside everything else fresh grass provides.

The shift to hay changes this more than most owners realise. Cutting, drying, and storing grass degrades ALA substantially. Good quality hay still has nutritional value — this is not an argument against it — but it is not fresh grass, and it does not deliver omega-3 in anything like the same quantities. Haylage preserves more, but the variation between batches is wide and almost never measured. Compound feeds vary enormously. The result is that a horse spending the winter on hay, on restricted turnout, or in a predominantly stable-based system is likely receiving considerably less omega-3 than the biology was shaped around.

The shift from fresh grass to hay is not a nutritional failure — it is a practical necessity. But cutting and drying grass degrades ALA substantially. Even well-made hay delivers a fraction of the omega-3 that fresh pasture provides.

None of this is anyone's fault. Horses are kept the way they are because it works — because it allows them to be managed safely, trained consistently, and used in ways their predecessors never were. Modern management has genuine advantages. It has also created some nutritional gaps that did not exist when horses lived on the diet their systems were built around.

For many horses, this isn't obvious, because nothing visibly breaks when omega-3 intake drops. The diet still works. The horse still functions. The change is in what is no longer being supplied, not in what immediately fails.

When omega-3 intake drops, the horse still functions. The diet still works. The change is in what is no longer being supplied — and because nothing immediately fails, the gap can go unnoticed.

Why this is the right place to start

It would be easy to skip straight to supplementation — to ask which product, at what dose, and whether it seems to be working. Many owners do, reasonably enough. But the question of whether omega-3 supplementation is useful, and what kind is likely to make a difference, only makes sense once the baseline is understood.

If a horse is on good summer pasture with long daily turnout, the picture is different from a horse on hay and limited grazing through a long winter. If the diet already provides a reasonable omega-3 intake, what you are trying to achieve changes. If it does not — and for many horses, it does not — then supplementation is not an addition to an already sufficient system. It is an attempt to restore something the diet is no longer providing.

That framing changes how you think about what you are feeding, what you are looking for, and what counts as a result.

Omega-3 is one of those gaps. And understanding it begins not with the supplement, but with what the biology was built on before supplementation was ever necessary.