It is pretty well accepted and shown from experience that a dry cow or a steer will eat about 2.5% of its weight in hay per day over the winter if there is no other feed provided.

I use 3% to account for spoiled and wasted hay. There are several university studies that have focused on this and hay loss from feeders etc in the past few years. 

In my calculation above I am assuming no net weight change on the cow between now and May 1 which is also not far off if she has good hay. Mine are about a BCS 6.5 now and should be around a 6 or higher at May 1.

Jim at Dawn