What happens to omega-3 when grass becomes hay

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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 fatty acids called polyunsaturated fatty acids. The word polyunsaturated refers to the chemical structure: these fatty acids contain multiple points in their molecular chain that are reactive, and that reactivity is part of what makes them useful inside the body. It also makes them unstable outside it, so exposure to oxygen, heat or light causes them 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 at once — open air, sunlight and the heat of drying — so the process that preserves the grass as fodder is the same process that degrades its omega-3 content. It also happens quickly, with a significant proportion of the ALA lost within days of cutting.

By the time hay reaches the stable, the omega-3 content is a fraction of what the original grass contained, which means 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 rather than a sign that something has gone wrong.

What about haylage and compound feeds?

Haylage fares better than hay, and the reason is straightforward: instead of being dried in the open air it is wrapped and fermented in sealed bales, and less exposure to oxygen means less degradation. This shows up in the horse rather than just in the forage — a randomised trial in Thoroughbred racehorses compared dry hay, steamed hay and haylage, and measured plasma omega-3 concentrations alongside airway cytology.² The forage a horse eats is reflected in what circulates in its blood.

There is a catch, though. The ALA content of any given batch of haylage is almost never measured or reported, and variation between batches can be wide, because quality 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.

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 omega-6. This matters because omega-6 and omega-3 compete for the same processing pathways in the body, so 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 as though it contains omega-3 while the conditions for using it are being undermined at the same time. The series returns to this 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, and 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.