You have to be careful with BOSS as it throws off the Calcium Phosphorus ratio.
@OneFineAcre did a great job of explaining that yes, corn is the main ingredient in the feeds. The feed is blended and balanced for proper nutrition for goats. With corn, there are some that just throw corn out to their goats and that is a disaster waiting to happen.
We have never fed any sweet feed and our goats won't touch it.
Our feed is locally milled "Bartlett" and is 17% or 18%. They get very little feed and some goats don't get any feed if it is spring/summer /early fall and they still have forage available and if they are not lactating. Our Standard Breed goats generally get something but the amount varies.
As far as GMO non GMO and Organics... well I have many thoughts on the matter and am not a fan of GMO. Having said that, we searched high and lo for a good organic feed supplier and we have not found a good feed that is balanced. Some the CA/Phos ratio is so off it would be foolish to spend that kind of money for a feed that ultimately will do more harm than good. We have tried different brands for our chickens and it seemed the inconsistency was just too great from batch to batch.
Occassionally we will mix up a special batch when we want to see coat improvement- if their coats are dull-about 200lbs of feed at a time and we mix the bartlett, alfalfa pellets and throw in some whole oats with oils and a handful here and there of BOSS. We do not keep them on the special mix for long.
Feeding programs vary from breeder to breeder and region makes a difference too. Natural forage is great, some regions have a longer season of forage than others. Doing what works for you is key. A few things that are a really important are loose minerals, a 2to1 Calcium phosphorus ratio when feeding feed, hay (they must have long fiber).
My goats like pumpkin. They also like Doritoes and root beer.
And congratulations on your new goat! Hope kidding goes well, poor girl she is so young to have been bred. and last but not least
Changing feed much could cause for too much weight gain in the kids and lead to harsh labor. Did that with our very first pregnancy and the baby died before he could get out and the dam was a mess. We had changed the feed from hay, water, minerals to offer oats, alfalfa, and something else (can't recall---been awhile). The babies were TOOO big.
As long as you have hay, minerals and water, you are fine. If you are going to add anything else, take it easy on quantity and just be consistent with giving it daily so the digestive system is stable and not upset.
And as always (anyone who knows me know this is my constant disclaimer), don't give molasses in a feed and do not use sweet feed. Fine to do it after kidding for an energy boost, or on other rare occasion when fast energy is needed but don't give it regularly.