Alpacas are ruminants, they eat almost any green in front of their face, although you should still provide pellets for protein and hay over the winter or summer when grass may be hidden or dead.
Hay
From my experience, "You must provide the best quality hay to your alpacas or they will get sick" is overrated. I feed my alpacas just basic grass hay that I feed to my horse, goats and sheep. I was talking to one alpaca farm and in person, when I mentioned that I would be feeding them regular grass hay and if it was okay, she got very uptight and got somewhat of an attitude and told me that it was not okay for me to feed that type of hay to my alpacas and they wont make it because it does not have enough nutrition. Well, later after than incident I took a flake to my livestock vet and she said that she didnt see anything wrong with my hay and it should do perfectly fine for the alpacas. Now not having experience with pregnant females, I would myself go out and buy a couple bales of very nice high quality bales for my pregnant alpaca because she needs extra protein and has to feed basically two things, herself and her cria.
Pellets
I use Mazuri llama/alpaca pellets that I get from a small family owned feed store, I feed my alpacas about 1 cup each a day at around 4 in the afternoon. I have a plastic jar that had powdered cocoa in it that has labeled with 5 cups total so I scoop and measure out how ever many cups I want to feed. I sometimes increase the amount of pellets by a little bit over the winter. A good way to gain trust with your animals is to put pellets into your hand and get them to eat out of it. Took me 4 days of sticking my hand out there with food in it for one of them to finally reach out and nibble it. It was very exciting.