Soft vegetables, such as we often find in salads, aren't good for horses usually. Their gut evolved to deal with pretty rough forage. Even wild horses will colic on new spring grass, so horses that are very hungry will eat it, but the drive for them isn't grass, it's forage - with lots of course stuff like brush and trees much to their liking and better health.
The worst offender in hay, that Josepha's question remind me, is hay harvested either after being stressed, or with unripe seedheads attached.
The starch and sugar, especially sugar content is way too high for many horses. Many are just not built for their endocrine system to handle it.
Contrary to even what some equine nutritionists will say (remember many of those get a lot of income from the feed companies) the rougher you feed your horse the better off he or she will be. You can always add small amounts of fat for health and weight gain if needed.
If you are feeding beet pulp it needs rinsing ... a good solid hard stream of water in a net bag with small holes (our Walmart sells a laundry bag like this) for about 5 minutes. The excess Iron (common all over the N. American continent except in limestone and southwest desert country) will be partially diminished, and the sugar product on the surface will tend to wash off.
Sugar product? Yes, my BIL was a scientist working in a major sugar company lab (you might even recognize the name) and explained the process that in the end leaves beet pulp for sales to various users, marketing firms that aren't all that cautious about foreign substances etc.
Just after all the sugars are stripped out down to about one tenth of one percent the product is do dry that it has a taste like paper. In fact, I bet you could make paper or cardboard out of it.
The take the liquid that they extracted all the sugar from profitably and they add back a bit to the dried pulp...about one percent by volume. Now this shouldn't hurt the sugar sensitive horse, and it's great for racehorses (where a huge amount of beet pulp pellets are sold) who need the shot of sugar along with the high usable energy of beet pulp. In face the Digestible Energy is quite high and safe. I feed it to my insulin resistant mare - like a type II diabetic. But I not only wash it, I soak it for about 6 hours then rinse it again.
It tested at 17% sugar/starch which is dangerous to an IR horse.
The beet pulp packagers are non to careful either about clean machinery. I find corn, wheat, barley and oats in very small amounts so I pick out the corn, and I put the pellets in a metal garbage can and shake it to shake down the smaller stuff to the bottom - where I gather it for Bonnie who has no such dietary/endocrine problem as her mom.
I notice that if Altea gets into Bonnie's hay she will get softer stools but it is corrected by more strictly denying her access.
I'm what breed is your horse again? Some, especially those derived with Spanish and Arabian blood, are more prone to insulin resistance. Those, as you can imagine considering the feed sources in the wild, need the very coarsest of diets, with small controlled supplementation matched closely to age, health, and usage.
Coarse does not mean old dried out grasses ... those can be quite as dangerous as newly sprouted grass. It means grass hay that was harvested just before seed heads form. There is more if you have an IR horse, but that's not really a problem once you know and compensate for it.
My bet? Too much soft vegetable matter in his diet. Carrots are not roughage. Carrots are sugar and starch. For a horse, soft vegetable matter. No protein. And little energy that sustains, really. Work horses in harness can do well on root crops, but not your average riding horse who often doesn't burn off the starch and sugar each day.
Beet pulp is my favorite safe supplement. It's hard to get a horse fat on it, but it's healthy otherwise. For weight stick to stabilized flax, or grind it fresh yourself, and feed no more than 2 oz day and watch her horse .. don't get him too fat. Flax has close to the ideal ratio of Omega 6s to 3s, 3s being the better form of fat.
Many vegetable fat sources are bad for horses, and humans too. Sunflower seeds, one of my favorites is overloaded with Omega 6. Bad fat when out of balance with 3s.
If you are interested in the information resources I draw this knowledge from let me know. Those who follow the feeding/nutrition principles have very healthy horses. Bonnie is growing strong and powerful, and very playful, on such feeding. And Altea, once a huge blob of fat, still retains her baroque old style Andalusian form but can kick up and play with Bonnie, or drive her off the hay piles, very nicely still. LOL
Love is Trust, trust is All
So say Don, Altea, and Bonnie the Wonder Filly.