Like the fleece of an alpaca there can be considerable variation between hay in different parts of the hay field. What this means is that you never buy hay without having it tested and that you take samples from several bales.
Take these two rows of hay in my own field no more than 10 feet apart.
Out of curiosity I will take samples from bales in both rows then have them analysed, it will be really interesting to see it makes any difference and if so how much.
Adrian Stewart