Take him off the pasture in the morning when the grasses are in the rapid growth stage. As the day warms the sugar increases then decreases later in the day. Much safer to let him out around dark. You'll have to feed him hay during the day but he'll likely be glad to be in as that's usually when the bugs are out.
Actually saddle, you're partly right - sugars do increase during the day. It's photosynthesis which produces sugar, so depending on the weather, the longer the day, the more sugars. Plants use up that sugar over night to grow(also weather dependant - frosts will shut down the growing). So early morning is when the sugars are generally lowest & afternoon/early evening is when they're generally highest. Type is one other big consideration too & 'improved' cattle fattening varieties such as rye grass are very high sugar, whereas native grasses are generally lower.
Also re feeding hay, sugars are only used during growing & aren't depleted with processing/storing. So depending on type, how grass was grown & what time of day/weather it was cut in, may be just as high as the highest fresh grass.
OP if you're feeding a good 'ration balancer' that balances his diet well, you shouldn't really need another 'hoof specific' supplement. If however, the rest of his diet leaves him deficient in the nutrients that are in that hoof supp, then it may be the most appropriate for him. Also consider magnesium levels & also that lucerne is quite high in calcium, protein & potassium, which may or may not be a problem depending how much & what he gets in the rest of his diet.