What happens to omega-3 when grass becomes hay

April 24, 2026

In this article

Why ALA — the plant-based omega-3 in fresh grass — degrades during hay-making
How much omega-3 is lost between fresh grass and the hay that reaches the stable
How haylage compares to hay for preserving omega-3
What this means for horses on hay-based or restricted-turnout diets

Part of a series

Article 3 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: The diet most horses ate for thousands of years — and what changed

Next: Why feeding linseed makes sense for coat — and what it doesn’t do

The process of making hay is well understood. What it does to omega-3 is less often considered.

Hay is not simply dried grass. The process of cutting, drying, and storing it involves heat, air, and time — and each of those things affects nutrients in different ways. For omega-3, the effect is substantial.

Fresh grass contains ALA — the plant-based form of omega-3 — in amounts that represent a meaningful part of what a grazing horse takes in each day. To understand why that changes when grass becomes hay, it helps to know a little about what kind of substance ALA actually is.

ALA belongs to a group of fats called polyunsaturated fatty acids. The word polyunsaturated refers to the chemical structure — these fats contain multiple points in their molecular chain that are reactive. That reactivity is part of what makes them useful inside the body. But it also makes them unstable outside it. Expose them to oxygen, heat, or light, and they begin to break down.

This is exactly what happens during hay-making. When grass is cut and laid out to dry in the field, the ALA it contains is suddenly exposed to all three: open air, sunlight, and the heat of drying. The process that preserves the grass as fodder is the same process that degrades its omega-3 content. And it happens quickly — within days of cutting, a significant proportion of the ALA has already been lost.

By the time hay reaches the stable, the omega-3 content is a fraction of what the original grass contained. The horse eating hay is not receiving the omega-3 equivalent of the horse at grass — even when the hay is well made, well stored, and good quality. The loss is built into the process, not a sign that something has gone wrong.

The loss of ALA during hay-making is structural — not a quality issue. Oxygen, heat, and time all degrade polyunsaturated fats. Well-made hay still loses most of the omega-3 content the original grass contained.

What about haylage and compound feeds?

Haylage fares better than hay, and the reason is straightforward. Instead of being dried in the open air, haylage is wrapped and fermented in sealed bales. Less exposure to oxygen means less degradation. Studies comparing horses fed haylage versus dry hay have found meaningfully different omega-3 profiles in their blood as a result — the haylage-fed horses showed better omega-3 to omega-6 ratios, which reflects the more intact fatty acid content of the forage they were eating. That is a genuine nutritional advantage.

However, there is a catch. The ALA content of any given batch of haylage is almost never measured or reported, and variation between batches can be wide. The quality of haylage depends on when the grass was cut, how quickly it was wrapped, and how well it has been stored. Knowing that haylage generally preserves more omega-3 than hay does not tell you how much a specific batch contains.

Haylage generally preserves more omega-3 than hay — but its actual content is almost never measured, and variation between batches is wide. Knowing that haylage is better in principle does not tell you what a particular batch delivers.

Compound feeds present a different picture again. Some include linseed or linseed oil, which does add ALA to the diet. But many also include significant quantities of grains — and grains are high in a different type of fat called omega-6. This matters because omega-6 and omega-3 compete for the same processing pathways in the body. When omega-6 levels are high, it becomes harder for the horse to make good use of whatever ALA it does receive. The diet can look like it contains omega-3, while in practice the conditions for using it are being undermined at the same time. This is a topic the series returns to in more detail later.

What this means for most horses

A horse on good summer pasture with generous daily turnout is receiving omega-3 as a natural part of its diet — broadly in line with what its nutritional systems were built around. As turnout decreases, as the season turns, as the diet shifts towards hay and supplementary feeds, that baseline changes.

For a horse spending much of its time stabled, or on restricted turnout through winter, or in hard work on a predominantly hay-based diet, the omega-3 available from food alone is likely to be considerably lower than under grazing conditions. Not dramatically lower in ways that immediately cause obvious problems — but meaningfully lower in ways that affect what the body has to work with day to day.

That is the gap that supplementation is trying to address.

References

Boufaied et al. (2003) — fatty acid profiles of forage grasses; established omega-3 to omega-6 ratios in fresh pasture versus conserved forage.