Nurse cows are a special creature, akin to preschool teachers and pioneer mothers in that they can deal with, in a happy way, multiple youngsters all at once...many of which aren't their own.
In layman's terms, a nurse cow is a cow, typically of a diary breed, that is used to raise either orphan beef calves or dairy bull calves. A good nurse cow is worth her weight in gold because she not only provides the calf or calves in her care a 24/7 milk bar (thereby relieving a farmer of bottle feeding), she also gives them the physical "mothering" that help calves thrive.
As noted above, nurse cows are typically dairy cows, often retired from commercial dairies due to age, decline in total production or high somatic cell count. While these issues keep them from being able to cut it in the thin-margin dairy world, it makes them perfect home dairy cows or nurse cows. If you are seeking a nurse cow, try to get one whose bag (udder) is still good and milking in all four quarters. I typically ask what a dairy has for older cows because an "old" dairy cow isn't old anywhere except a commercial dairy--sometimes they get churned out (no pun intended!) out of the system as young as 4 or 5 just because they can't produce the extremely high amount of milk needed to be profitable. An "underperforming" Jersey cow is still probably producing 4 gallons a day, more than enough for several calves!
We have had the best luck with Jerseys vs. Holsteins. Jerseys are a nice size to handle on the ground and feed (full grown Holsteins are HUGE and eat a lot) and their high butterfat content milk makes calves ZOOM once they get used to it. Folks tell me that Jerseys have a better mothering instinct than Holsteins too. Other dairy breeds like MIlking Shorthorn or Brown Swiss may be used if you are able to find them.
A good nurse cow will take any calf you give her, any time during her lactation. Some cows are great about this, some need to be tied if they don't have a newborn of their own. Mid-lactation is always harder for calf introduction that directly following delivery. Most dairy breeds with good udders can handle 2 calves at a time and if their milk production allows, you can sometimes do 3 or 4. During a typical 9 month lactation, you should be able to rotate at least 2 sets of calves from birth to weaning, meaning that a top notch nurse cow can raise up to 8 calves a year--that's a lot of milk replacer saved!
Training of your prospective nurse cow and introduction of new calves is a whole other topic that I will get to in another post!