Cow math

90lbs of hay = 2.5 cow days
18 cows =  540 cow days per month
= 216 bales a month

6 months of no forage (have to keep the cows off the wet grass so their hooves don't cut up the sod) = 1296 bales, or 58 tons of hay.

figure 3,000 lbs/acre for hay.  38.6 acres of grass to produce 58 tons of hay

Figure 40lbs of corn silage per cow per day = 1200lbs /month
for 20 cows = 24,000lbs/month or 12 tons/month
20 cows for six months = 72 tons of silage
roughly 20 tons of silage per acre of corn
4 acres or corn is the maintenance diet of the cows.

hmmm.. but don't want to run out of food.  Figure 10% for spoilage, and maybe another 20% to have some extra.  Can always sell hay to the horse people....

so I'm using roughly 50 acres to support my 20 cows.