Corporal, it depends on types of grass(eg cattle fattening 'improved' rye is very high in sugar) and weather & time of day too as to how much sugars are in the grass, but it's not just in spring, or long, lush grass.
While gorging large quantities will obviously yield them more sugar, long, lush grass is typically lower than overgrazed or otherwise stressed pasture, so the really short stuff can be more problematic. Sugar levels build in plants due to photosynthesis and are used over night in growth. So generally sugars are lowest in the wee hours/early morn & highest in early evening. Frost & such shuts down growth though, so plants may retain their sugars & still be high in the morning, only to build higher levels with more sunshine.
Another common misconception is that grass is bad but hay is fine. The only difference effectively is water content. Unfortunately while hay loses a lot of nutrients with cutting, drying, storing, sugar isn't one of them. So that hay may be just as high as your 'lush' pasture if it was grown from rich grass, cut in the late arvo or on a frosty morning or such.
'Unimproved', native grasses are often lower in sugars than 'improved' varieties, so better horse feed. Like us, horses can generally handle some 'junk food' without health probs, but like us if we develop type 2 diabetes from too much junk or just calories, horses with IR or other metabolic 'diseases' are far more sensitive to any sugars, so if it isn't low NSC may need their hay soaked & drained before feeding, to leach out sugars.
So... Stingers, you may well not need worry too much, but if there is heaps of grass & horses overfeed, if they are overweight to begin with, etc, there may indeed be a concern & just because it hasn't happened yet (that you have noticed - often 'low grade' laminitis isn't recognised) doesn't mean that it won't. Just be proactive & don't wait for a problem before addressing.