Frosty mornings and sunny days make for beautiful wintry days – but not for happy healthy hooves! 

 Green plants create sugar (photosynthesis): carbon dioxide + water + sunlight = sugar + oxygen
And use sugar for growth (respiration): sugar (glucose) + oxygen = carbon dioxide + water + energy for growth
When growth is prevented either by colder weather (below 6oC) and no rainfall for example, sugar continues to be produced in the day but is not being used to grow at night and so the sugars accumulate in the grass. When sugar produced from photosynthesis is greater than the sugar required for growth, the excess sugar is stored as fructan or starch (carbohydrates) for later use. Consuming excessive amount of fructans has been considered a trigger for laminitis. More recently, there is some belief that it is not strictly just the level of fructans in the grass that is responsible for causing laminitis, but rather the general calorific value of the grass and it is this calorie intake that needs to be limited. Either way you look at it sugar = calories so until science tells us more, we’re best to limit sugars from grass or other sources and in this way limit calorie intake in order to avoid obesity.
The key factor in this pathway is insulin. Pasture laminitis is now known to involve a degree of insulin resistance and shares the same final disease pathway as Cushings Syndrome and Equine Metabolic Syndrome (EMS). High levels of insulin results in pro-inflammatory changes and vasoconstriction (narrowing of blood vessels), leading to reduced blood flow to the laminae in the hoof and increases the risk of developing laminitis.
This frosty grass is therefore particularly unsuitable for horses and ponies who are prone to laminitis or those with EMS or insulin dysregulation alone. Horses grazing at this time will experience a rise in blood glucose and a consequent surge in blood insulin. Obese horses and ponies already have high insulin levels (now known to be an important trigger for laminitis) and this further insulin surge may push them over the edge into clinical laminitis.
It is important to note that it is not just when the grass is frosted that it presents an added danger, the thawed grass available later in the day is just as deadly. The presence of frost merely tells you it was a cold night and the grass sugar levels will be high and that there a risk to your horse or pony. To avoid the risk period, they shouldn’t graze the grass until the weather changes to milder night and overcast days (ideally a return to night time temperatures above 5oC). This will allow the grass to return to normal growth and use up the excess sugar before your horse grazes it. We should be feeding analysed hay with sugar and starch levels ideally below 10% instead and cut back on feed (calories, not fibre) if exercise/turnout is reduced.
Article by Swanspool Farm & Equine Vets