Essential nutrition for a yearling is pretty much the same as for an adult, except that they often benefit from slightly higher protein. I'd speak to a nutritionist if you want to get more specifics than you'd find in a book, and I'd be looking at the most current research, as a lot of nutritional info is recent & older books outdated.
FeedXL.com is a really good program for working out exactly what your horse needs & what feeds/supps available in your area will supply it. You might find some good articles on Dr Elanor Kellon's site too & you can be sure they're up to date.
Afraid I have to disagree with you Ashley, that grain & other starchy feed is generally not a great option for horses, and Purina feeds aren't the best quality. You don't say exactly which 'growth food' it is though. I've also found that you tend to get what you pay for, so a cheap feed is not usually that great. Eg. KER supps & feeds are generally quite expensive on outlay, but they're such low dose and are very well balanced, so you can feed very little, be assured you don't generally have to add anything & it comes out very economical per dose.
Also with regard to feeding tailor made feeds, *assuming* they do have well balanced nutrition anyway(also depends on what else is in their diet), you do need to feed the amount on the bag for them to get adequate levels, which is generally more than the horse needs in calories. It's also important, especially if you are going to feed grainy/starchy feeds that you feed it over a few small meals daily, rather than only once.