That's a good idea, Butch.
Maybe upsize this...
https://www.chewy.com/van-ness-auto...fWYao6HCAUfCQ60yIEzljRoCL6bw_wcB&gclsrc=aw.ds
How does this work? Does negative pressure keep the water from pouring out?
That's it.
The jug is sturdy enough to sustain a differential pressure inside/outside. A plastic bag doesn't work for that (actually it does but with a very very little amount).
It's a matter of balance between the air pressure plus the water height of both the outside and inside of the jug. When the pet drinks the water, the outside level gets to zero and there's only the atmospheric air pressure on the outside at the communication point. Inside, it's the same global pressure, but divided between the air and the water's height. The water's weight drags it toward the bottom. So the inside air pressure is less than the outside, sucking the water up.
If the pet drink enough to uncover the gap, the air can find its way toward the inside while the water tries to flow out. A bubble appears inside and goes up, rising the (negative) pressure. That allows some water to go out, decreasing the water's weigh/heigh to recover the balance point.
If there's only a tiny hole as the entrance point, all stays put because the air doesn't have enough room to push the water aside. No air can come in and no water can come out as is.
Then, one way to get water is to decrease the external pressure by sucking at the hole. The pressure inside becomes higher relatively and pushes the water out. You just got the principle of the feeding bottle.
There are some other ways, like blowing in the hole, shacking or squeezing the bottle, rising the temperature ...
You can get as big a jug as you want, many cubic meters, it doesn't change the principle. The onliest limitation is the water's height : it isn't allowed to go over 10 meters. But that would give you some margin, even for the cows.
Don't forget the sturdy thing to make an overseized model. Even a small pressure can create some massive forces.
Beside that, some trials are required to find the right size for the gap and get enough flow for a bunch of thirsty cows.