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-dages tilbageblik i stedet for et øjebliksbillede

Fruktan i græs er ikke en øjebliksværdi – det er resultatet af de seneste dage. En kold, klar nat eller en solrig tør periode virker stadig flere dage efter. De fleste vejr-apps viser kun et øjebliksbillede. Grassrisk simulerer fruktan time for time over de seks foregående dage frem til den valgte dag.

Sådan opstår et sammenhængende, fysisk plausibelt forløb: sukkeret, der blev lagret i går, indgår i dagens vurdering. På den måde opdager vi risikosituationer, som en ren dagsvisning overser – for eksempel en periode med fint vejr over flere dage med kølige nætter.

Græsvækstindeks – unikt i fruktanvurderingen

Fruktan falder, når græsset vokser aktivt og forbruger det lagrede sukker. Derfor beregner Grassrisk desuden et græsvækstindeks (lavt/middel/højt). Det viser, hvor kraftigt græsset nedbryder sukkeret lige nu.

Et højt vækstindeks under milde, fugtige forhold betyder, at græsset forbruger fruktan hurtigere, end det produceres – risikoen falder. Udebliver væksten (kulde, tørke), hober sukkeret sig op i stedet. Ingen anden frit tilgængelig folde-app tilbyder denne vækstinformation.

Simuleringsmodellen: fotosyntese mod forbrug

Bag trafiklyset ligger en timevis simuleringsmodel, der afbalancerer to modsatrettede processer: fruktanproduktion gennem fotosyntese og fruktanforbrug gennem vækst og respiration.

Produktionen stiger med solindstråling og er højest omkring 15 °C, optimum for C3-græsser. Forbruget følger græsvæksten – kraftigt ved mild varme og tilstrækkelig jordfugt, tæt på nul ved frost, varmestress eller tørke. Timedifferencen giver fruktanforløbet.

Om natten hviler fotosyntesen, mens respiration og transport fortsætter – derfor falder fruktan som regel lidt i løbet af natten. Alle vejrdata kommer fra Open-Meteo og opdateres flere gange om dagen.

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