At the UL there is some truth in that horses won’t get enough caloric intake for XC day from hay alone, but I don’t think this is really a concern for functionally/metabolically healthy horses below Intermediate or Advanced.
They (UL) horses wouldn’t have enough time in the day to source all their caloric/energy needs from forage alone, which is where things like oats and barley and beep came into play as a ready source of VFAs/convertible energy for the horse. Yes, starch has its use for extreme anaerobic work. The old farmer’s adage was hard working horses needed COB – that’s Corn, Oats, Barley. Now, we know tons more about the equine digestive system and their needs and there are better sources of energy out there than COB.
Keep in mind just about no horse works as hard as a racehorse or the old farmer’s plowhorse, not even UL eventers. Don’t let a feed company rep sweet talk you into high-octane feed for your low-octane endeavors. Prelim is Fischer-Price work compared to things like racing and UL eventing.
If you find your horse is lacking oomph after outings, it may be time to investigate why. It’s really anyone’s guess - could be fitness related, could be age related, could be heart related, could be feed related. Get the horse in front of a timer and start to record their respiratory rate and recovery after interval training. If you are certain the lack of oomph isn’t fitness related, it might be time to look at diet or their body - there are simple things you can try, like the addition of beep or oats, to see if the horse improves. Or you can increase what you’re feeding. How much is your horse getting a day?
Most of the Prelim+ horses I’ve cared for were on some sort of high fat (12%+) high protein (10-12+%) concentrate that typically had wheat middlings and beet pulp as its primary ingredient, supplemented by some sort of extruded forage like alfalfa/timothy pellets, a fat or oil supplement, and as much hay in front of them as they could eat. Generally speaking none of them got more than 10lb of grain a day. Most were in the 2-4qt range.
In the summer/fall one of my horses does the athletic/anaerobic equivalent of Prelim between long distance rides, hunts, and paces, if not more – he is fed 2qts Poulin FibreMax / 1qt Alfalfa pellets 2x a day, with 24/7 roundbale access. This gives him plenty of energy and keeps his weight in good condition going into the winter.