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> That same cow produces 3-4 gallons of milk a day.

Does that only include the periods when the cow is lactating or is it an average that takes the cow's entire lifespan into account?

> On top of that, most of that water is excreted again - otherwise, your cow would gain 640lbs a day.

Wait. Is the 80 gallons per day figure referring to the water that is consumed by the cow directly, or does it include the water used to produce the cow's food?

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80 gallons is direct cow consumption. Note that OP treated energy for feed generation separately as well. (And, again, most of that water passes through the cow, it's not actually absorbed)

And 3-4 gallons/day is a rough average. Lactating cows produce 6-7 gallons of milk, for about 10 months after calving. And after 12 months, they calve again. (But really, all these numbers are very rough estimates. It doesn't matter because they're used as a ballpark sanity check for OPs post, not as accurate numbers)

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