In name and function.
"Milk Quantity" affects how fast a cow can produce milk. IMHO should be capacity not quantity.
Quantity affects the crop:milk ratio whereas changing Quantity to Capacity would mean that the crop:milk ratio would be a globally fixed figure but having a higher milk quantity rating on your cow would mean you could harvest the milk less often without losing any efficiency for only logging in once a day or less.
Imho if someone wants more milk then they can just make more cows. That's how i see the situation. I Look for quantity when breeding cows but it's the last thing on the list (Milk -> Qual, breed, quantity, everything else is worthless get ur bones from chickens b/c they're faster)
Even if one got a Q40 Milk Quantity cow there's no way they'de keep up with it. All around this stat would be significantly more valuable as a function of Capacity versus production rate because rate is useless with a holding cap.
The only thing rate would be good for is feeding calves and even then the Quantity function just confuses shit because everyone overdoes it anyway. If you don't slaughter a calf on initial inspection then you damn well don't want that bitch to starve. Fixed rate is more practical and all around just easier to manage and do the math on.
Also, chaning the rate so that a given cow produces 20L of milk per RL day would be *IDEAL*.