You are correct, but its rather irresponsible to break it down to that level.

If you're really wanting to break it down to the daily dry matter intake of a 2300 lb cow(which is actually really heavy) eating an average quality hay is 24 pounds per head and the hay that they are generally consuming is 88% dry matter, these cows would consume about 27 (24 pounds/.88) pounds per head on a daily basis.

If you took the same 2300 lb. cow and fed it a ration where part of the ration called for corn silage to be fed at 10 pounds per head per day on a strict dry matter basis and the corn silage were 35% dry matter and 65% moisture, the pounds of corn silage in the daily diet would be 28.5 (10 pounds/.35) lbs. per head per day on an as fed basis. Remember that of the 28.5 pounds of silage, 18.5 pounds is water and 10 pounds is silage, which only adds another 2.3 gallons (8.6 lbs per gallon) of raw consumption. Thinking of all of the variables is actually maddening.

Calculating the total irrigation needed for a field of grain is about as pointless as saying it took 23,400,000 gallons of water to grow a single morel mushroom consumed from the Mt Hood National Forest or calculating the computational fluid dynamics of pissing into a pile of sand.