Gain per season is different from Plant Divines, otherwise they function the same. I personally use the notebook on each horses' page to remind me when to pick them so I don't go too far over their prize point as you lose whatever extra skills you may have given them (e.g. you can pick at 1300 skills but you actually pick at 1500 skills, you lose all 1500 skill points, it drops to zero, you don't start at 200).

+30 in Spring
+18 in Summer
+60 in Fall
+48 in Winter

If you dont grow straw then its best to only feed them in fall. You would need to buy the straw to feed them and a lot of them cost more than what their gift is worth

It’s worth growing 2 large meadows of straw, that keeps feeding a mushroom horse for quite awhile. It makes them much cheaper to feed and collect gifts.