800 pounds of hay is being delivered...

Right into Bourbon's pen tomorrow morning. He has been getting an orchard grass and alfalfa mix and is still quite underweight. I want him to have free choice, and my only options for free choice were coastal (I watched a horse die from colic caused by coastal... not in a million years would I feed it to Bourbon.) or straight alfalfa. I am nervous about all that extra protein... should I be worried? The barn owner and manager seem to think it wont be an issue because most of the hay he is already on is alfalfa....

ETA: Thanks PeggySue for helping to point me in the right direction!

If he's already on alfalfa, adding a bit more wouldn't be an issue, but I would be very hesitant at feeding him straight alfalfa as free choice hay. IMO it would be much better to up his alfalfa a bit, and then add free choice bermuda, or something similar. I do want to say that obviously what you decide to feed is your choice, but just because one horse you know colicked because of a certain feed, doesn't mean that your horse will have problems. I have known a couple of horses that have died from colicked caused by eating too much alfalfa, and a horse that died of colic from bermuda grass, which got twisted up in his intestines, and yet I still feed my mare a mix of alfalfa and bermuda, and aside from a chocking episode, she's had no problems. Just something to think about. Again though, it is obviously your choice based on what you feel comfortable with, and what your horse does good on.

Unless he already has kidney problems the excess will be passed just fine just make sure he has plenty of water at all times

Yep, for a horse with healthy kidneys, the added protein should be no problem at all. When we can get it, many of ours get free choice alfalfa and we've never had a problem with colic or founder or any other "feed" issues. Alfalfa is a great way to put weight on a horse in a way that is so much healthier than filling them up on sweet feeds and the like.

When you go to the feed store, you read the labels intensly. When you buy your supplements, you read the labels intensly. When you buy your hay - YOU NEED TO LOOK AT THE LABEL OR FIND SOMEONE WHO CAN PROVIDE YOU A LABEL!

All of these different species of hay can have identical feed quality and can have drastically different feed quality. Bermuda can be a wonderful hay - it can also have a very high ndf causing a very slow fermetation in the hind gut causing impaction. Alfalfa orchardgrass can be wonderful and nearly rocket fuel, it can also be woody and nearly cardboard in value - same with straight alfalfa. You can founder every horse in the pasture feeding freechoice 225 rfv alfalfa to them. You can also have a wonderfully good hay freechoice feeding a 110rfv alfalfa.
"I am nervous about all that extra protein" who cares about protein if it is not digestible. You need to know digestible energy, or calories, or rfv, or rfq - these are important.