Fructan Risk in Grass – What Horse Owners Need to Know

What Is Fructan?

Fructan is a storage sugar produced by grasses through photosynthesis. When a plant produces more sugar than it can use for growth, the excess is stored as fructan in stems and leaves. Perennial ryegrass (Lolium perenne), which dominates most horse pastures, can accumulate particularly high fructan levels.

For healthy horses, fructan in normal amounts is harmless. It becomes problematic when concentrations in the grass spike – because even short grazing periods can then deliver a dangerous amount of fructan.

Why Is Fructan Dangerous for Horses?

Horses cannot digest fructan in the small intestine. It passes undigested into the hindgut, where bacteria rapidly ferment it. This causes acidosis in the gut, killing beneficial bacteria and releasing toxins (endotoxins) into the bloodstream.

These endotoxins can trigger laminitis – an extremely painful inflammation of the sensitive laminae in the hoof, which in severe cases leads to rotation of the coffin bone. Horses with Equine Metabolic Syndrome (EMS), Cushing's disease (PPID), obesity, or a history of laminitis are especially at risk.

Which Weather Factors Influence Fructan Risk?

Fructan concentration in grass is heavily influenced by weather conditions. The key factors are:

• Sunlight – More sun means more photosynthesis and therefore more fructan production.
• Low temperatures – Below about 5°C (41°F), grass barely grows but continues to store fructan. Night frost followed by a sunny day is particularly critical.
• Drought – Stressed grass grows slower and accumulates fructan.
• Cloud cover and rain – Overcast, mild days with precipitation promote growth and tend to lower fructan levels.
• Wind – Strong wind can stress grass and increase evaporation, indirectly affecting fructan content.

The most dangerous conditions typically occur in spring and autumn: cold nights followed by sunny days, when grass is full of energy but barely growing.

How Does the Grassrisk Traffic Light Work?

Grassrisk analyses hourly weather data for your location and calculates a risk score. This is displayed as a simple traffic light system:

6-Day Lookback Instead of a Snapshot

Fructan in grass is not a momentary value – it's the result of the past few days. A cold, clear night or a sunny dry spell still has an effect days later. Most weather apps only show a snapshot. Grassrisk simulates fructan hour by hour across the previous six days up to the selected day.

This produces a continuous, physically plausible trajectory: the sugar stored yesterday feeds into today's assessment. That reveals risk situations a single-day view misses – for example a multi-day spell of fine weather with cool nights.

Grass Growth Index – Unique in Fructan Assessment

Fructan drops when grass actively grows and consumes its stored sugar. That's why Grassrisk also calculates a grass growth index (low/medium/high). It shows how strongly the grass is currently breaking down sugar.

A high growth index under mild, moist conditions means the grass uses fructan faster than it is produced – risk falls. If growth stalls (cold, drought), sugar accumulates instead. No other freely available pasture app offers this growth information.

The Simulation Model: Photosynthesis vs. Consumption

Behind the traffic light sits an hourly simulation model that balances two opposing processes: fructan production through photosynthesis and fructan consumption through growth and respiration.

Production rises with sunlight and peaks around 15°C, the optimum for C3 grasses. Consumption follows grass growth – strong in mild warmth with adequate soil moisture, close to zero under frost, heat stress or drought. The hourly difference yields the fructan trajectory.

At night photosynthesis pauses while respiration and translocation continue – which is why fructan usually dips slightly overnight. All weather data comes from Open-Meteo and is updated several times a day.

How to Use Grassrisk

Using the app is simple and free:

1. Enter your location or pasture – or use GPS.
2. Check the current traffic light colour and risk score.
3. Plan your horse's grazing time according to the recommendation.

You can also view the forecast for the coming days to plan the week ahead. With an account, you can save locations and receive push notifications when risk is high.

Practical Tips for Laminitis Prevention

• Keep morning grazing short – fructan builds up overnight in cold temperatures and is only reduced through growth during the day.
• After frost with sunshine the following day: avoid or severely limit grazing.
• Overweight horses and those with EMS/Cushing's should always be grazed in a controlled manner.
• Don't let them graze too short – fructan concentration is highest in the bottom 5cm of stems.
• Increase spring grazing time very gradually and check risk daily.

Frequently Asked Questions