it is because of excess nitrogen in the grass which gets converted into urea in the liver so it can be excreted in the urine.

Consecutive days of cool, cloudy, rainy weather keeps grass green and favours clover growth. When interspersed with HOT sunny days this creates high risk conditions for all equines especially those prone to laminitis and other issues caused by such grass.

There is a BIG difference between a horse whose nervous system is disturbed by the mineral/nutrient imbalances in Spring (or Autumn) grass and one that merely has excess energy from 'sugars'.