In my opinion there are two causes for the recent shortage in grass. First, last year during the late summer and fall we had very little rainfall and very high summer temperatures. These conditions greatly reduced the growth of both warm and cool season grass species. Second, the addition of three more bison about a year ago probably created too much grazing pressure. With the recent availability of hay this grazing pressure should now be greatly reduced.
Finding hay to purchase wasn't too difficult. Craigslist had a wide assortment of different types of hay listed in my area and I soon found a source that fit my needs about 30 minutes away. I started feeding hay last week with a single test bale. This allowed me to confirm they liked the hay and also gauge their consumption rate. They enjoyed it very much and consumed most of the bale within the week.
Today I hooked the trailer to the truck and bought eleven more big bales. This should hold them for quite a while and allow the pastures to recover.