As a Horticulturalist and horse owner I have done my fair share of research on how and why when it comes to horses, pasture and laminitis.
During photosynthesis plants produce sugar, this happens at best when the plant is happy and under no stress, on a sunny day when it’s not to hot and not too cold, when the soil is nice and hydrated and the plants have a good amount of nutrition available to them in a soil that drains freely. This could be the day after rain when the sun is shining all day with a temperature around 22celcius. When the sun shines on your pasture (say about 6 am in October) the sugar levels start going up and the plant uses a small amount of the sugar in order to grow. As soon as the sun goes of your pasture (say 7pm) the sugar levels start going down as the plant uses this as energy to grow but it’s not being replaced as the suns not shinning, therefore just before the sun comes up, sugar levels will be at their lowest. Studies have been done on sugar levels in pasture which indicate that the highest levels of sugar are in grasses between 4 and 5pm. Hay can also have different levels of sugar as the sugars become trapped as soon as it is cut. Straight grass hay with no clover or Lucerne is best for lower sugar levels.
Try and keep horses that are susceptible to laminitis of Pasture either side of 5pm and on pasture around sun rise if you would like to let them out.