Now, I fully realize that as horses add weight they can become proverbial fruit loops, ....This horse was brought in off-track and was very thin..I saw the pics.
Firstly I'd consider, if this was a horse recently off the track, he was likely in racing fitness, which would indeed look thin to many people but is not necessarily underweight. He also may still be a growing baby & going through a lanky phase. Also coming off the track into a more normal life(I assume), you may find that that is all he needs to settle & gain some weight.
No, I wouldn't say that adding feed/weight to a horse turns them into fruit loops, but feeding the wrong foods can. Of course, the simple equation of energy in=energy out also applies, so they also need to get enough exercise, or they can go 'stir crazy'. Traditionally what we tend to give to horses to put on weight is grainy, high starch/sugar diets. For energy levels, same sort of effect as people eating junk food, they tend to give you 'spikes' of energy. Also for horses who's system is not well designed to cope with high starch & digest large, rich meals, this sort of feed can cause digestive & other health issues. Feeding large &/or infrequent meals can really worsen the problems.
I'd be opting for healthy, natural feed & feeding practices, sticking to free choice hay/grazing and low starch, high fibre hard feeds fed in small meals. If for some reason you do want to feed a high grain/sugar diet, it's especially important to feed little & often, which will help reduce the problems.
As the horse, having been a racehorse was likely fed and kept quite unnaturally & on a 'high octane' diet, it is quite possible he has ulcers or other digestive problems, so I agree that a pro biotic is a good idea and particularly if he continues to have problems on a healthy diet, I'd talk to your vet about ulcer treatments.
He is eating Horsemen's Edge which I gather is a pellet only. My barn feeds a sweet feed mix or senior pellet depending on the horse in question.
Can't comment on Horseman's Edge(being a Purina feed, they don't tell ingredients
) or the senior pellets, not knowing what's in them, but sweet feed is sugar coated junk generally. Someone likened it to McDonalds - nice as a treat, but you wouldn't be too healthy living on it regularly! I'd stick to feeds that are high fibre, low starch, as per info above. I'd also opt generally for basic feeds over pelleted, to give the horse more to chew, for one.
The horse is vetted out regularly and watching him eat his dinner last night he didn't outwardly show signs of teeth (tooth?) issues. :) Obviously a PPE would tell for sure.
I've seen the beet pulp and rice bran mentioned before. What is the basis of those feeds?
Not sure what you mean 'vetted out regularly'. You can't necessarily see tooth problems just from watching them eat, but horses generally need their teeth 'floated' yearly. Some, especially the young or very old may need it more frequently.
Beet pulp(unsweetened) & ricebran are both high fibre, low starch feeds. Soy hulls are another popular option. All of these feeds are high in energy, for putting on weight without the problems associated with high sugar/starch.
Now, the next question is how to portion. The horse is currently on Horseman's Edge as I indicated. Due to schedules I haven't been able to talk to my barn manager about the feed. Would there or should there be a grain/pellet portion change in order to accomodate the Rice Bran?
Not entirely sure what you're asking there. How to portion? I'd be portioning the daily ration into at least a few small meals. I'd feed over a minimum of a couple of feeds daily, but if that makes the meal too large(I wouldn't feed more than app 8 litres in a feed) & especially if you are going to be feeding anything starchy/grainy, I'd be feeding it over at least 3-4 meals daily.
Grain/pellet portion change? Do you mean would you feed so much Horseman's if you're feeding ricebran or such as well? No, if he's doing OK on the amount he's on, you want to maintain that, not add to it. It's very important, the way a horse's hind gut works, to change feed gradually, over at least a couple of weeks, starting by adding a small amount of the new. That includes weaning them off different feed too.
I actually spoke with an equine nutritionist :)...didn't even know there WERE equine nutritionists out there. Anyhow, she indicated that the ingredients in pellets are almost the exact same thing as what is in the textured, or sweet feed.
Oh, so I imagine the pellets aren't great then either. If this is a nutritionist from a feed co, you may want to consider looking at independent advice & info, through feedxl.com or such.
Triple Crown Mature with roughly the same mixture (Edge is 10:6; the Legend equivalent is 11:6 and I think the Triple Crown was the same as the Legend) and start adding that to the sweet feed being used at our barn..doing a gradual substitution until he is completely on the sweet feed.
Don't get why you'd want to do that. TC is a sweetfeed anyway. TC, Legend & all other sweets that I know of are also grain based & otherwise high in starch. Legend also has a high percentage of corn in it, which is one of the starchiest & hardest to digest grains for horses.