Fructan in Grass: When Is the Risk Highest?

For many horses and ponies, pasture grass is the biggest threat to their hooves. The culprit is often fructan, a storage sugar that grasses produce and accumulate. When fructan levels rise sharply, they can trigger a laminitis flare in susceptible horses. The good news: you can predict high-risk days surprisingly well from the weather.

What is fructan?

During daylight, grasses photosynthesise and produce sugar, which they use for growth. When more sugar is produced than consumed, C3 grasses (the typical pasture grasses of temperate regions) store the surplus as fructan. Fructan is not a toxin – it is an ordinary plant sugar. Laminitis-prone horses simply react sensitively to large amounts.

The key rule: production vs. growth

Fructan rises whenever grass produces a lot of sugar but grows little – the sugar cannot flow into growth and gets stored instead. This combination occurs in specific weather conditions:

1. Sunny and cold (classic in spring and autumn)

Sun drives sugar production, while cold strongly slows growth – below roughly 5 °C soil temperature, grass barely grows. The result: lots of sugar, little consumption, high fructan. Think sunny days with clear, cold nights or night frost in spring and autumn.

2. Drought

When water is scarce, grass stops growing to conserve reserves – but photosynthesis continues, so sugar piles up. An innocent-looking, short-grazed dry lawn can therefore contain surprisingly high fructan.

3. Frost with sun

The most dangerous combination: night frost stops growth completely, and morning sun immediately ramps up sugar production. On such days fructan can spike sharply during the morning.

When is the risk low?

The risk eases when grass grows well and continuously uses up its sugar:

• Mild and cloudy – little production, normal growth.
• Warm and moist – grass grows vigorously and pulls sugar into growth.
• At night – no sun means no production, and levels fall overnight.

That is why early mornings after a mild night are comparatively safe – but after a cold, clear night they are not.

Spring and autumn are especially tricky

In these transitional seasons, strong sun meets cold soil. Soil warms slowly: in spring it lags behind the air, so growth starts late while the sun already produces plenty of sugar. In autumn the soil stays warm longer, but falling temperatures and shorter days increasingly slow growth. Both phases are classic high-risk periods for laminitis.

How to spot dangerous days

You do not have to calculate this yourself. The traffic-light tool evaluates exactly these factors – sun, air and soil temperature, soil moisture – hourly for your location and shows the risk as a simple light: green, yellow or red. At a glance you see whether today is a good or a critical grazing day.

Important: Fructan is only one factor. Every horse is different, and with known laminitis susceptibility, EMS or Cushing's you should always coordinate grazing management with your vet.