First cut is far better, spring grass, second cut is normally fertiliser forced and is taken in August so not the best. that is often used as cattle haylage.
1st cut haylage is of a higher quality in agricultural terms as it is higher in feed value. There is more leaf in it than stalk.
In terms of horses 1st but high feed value haylage is the worse you can feed.
Cattle and sheep are ruminants that chew the cut and are better able to cope with the higher sugar and starch levels.
Horses have a more simple digestive system which is designed for lots of low quality roughage.
The best haylage for horses is like hay, cut late when the grass has seeded. Or second cut without having been given high levels of nitrogen fertilizer to force leaf growth.