Alpaca eat between 2-3 pounds of hay each day. 500 alpaca would eat 288 tons of hay a year. Normal land in a decent climate with irrigation produces 2.5 tons per acre; let's assume this "grassland" is producing a GENEROUS and OPTIMISTIC 2 tons per acre. That's 240 tons per cutting, and while you CAN get 2-3 cuttings per year, that's on normal land in a decent climate with irrigation and also fences to keep hungry alpaca out.
So 240 tons when they would need at least 288 tons (more like 300, since they waste so much of it due to laziness).
End result? Alpaca are still starving.
Edit: To say nothing of the fact that they have NONE OF THE MACHINERY needed to plant and harvest hay, nor the knowledge and/or experience of how to do such a thing.