All of that said lets take a look at some of those things: 3-6 gallons of milk per day - if you have your heart set on keeping a 100% grass fed animal then this number is not realistic I have met 1 person who has made the claim and later admitted that the cow get grain in the stanchion. (Please note that it is not impossible to get milk, we are discussing the quantity).
Since I have not had any issues with calving, I will only point out once again that the breed used to breed back is key to this.
Fairly Hardy (the ability to survive adverse growing conditions) - I use the word fairly because while Jersey's are smaller than lots of other dairy cows their condition (essentially muscle tone and fat coverage) tends to suffer depending on quantity and quality of the food they eat. There is a conditioning chart to help determine if your cow is too fat/thin the latter being the most likely. Milk production burns A LOT of calories. I had a comment on here a couple weeks ago accusing Treva of being too thin and claiming that she could die if I didn't dry her off immediately etc.
Well I called my vet and had him come out as Treva tends to stay between a 2(a little underweight) and a 3 (optimal), he found her to be in the same condition as she has always been, and offered a few suggestions if we wanted to try to bulk her up to a 3. For some jerseys this will work, so we figured it was worth a shot, I always think she is thin but I am comparing her to beef cows. The vet keeps pointing out that if she looked like a beef cow then that would be a problem. Anywho, we switched her to 1 milking a day to help her keep more of her calories. I'll let you know how it goes.
Since then I have noticed on many boards that this is a common issue for Jersey owners, I have not seen any other dairy breed have to worry as much about this, maybe it's the high butterfat? Just thinking aloud.
Hoof care is very important with dairy animals but I can only speak to Treva at this point. Recently she was moving rather slowly and I was concerned, the next day there was a noticeable limp. We called the vet and he instructed dh to check her hoof for a possible stone or nail that may have gotten stuck, but there wasn't one. I did a bit of searching and discovered that the grain can cause their hooves to grow rapidly and need to be kept trimmed. The vet told dh to get her an antibiotic from the co-op and the next day the limp was replaced by a little sluggishness and now she is back to her old self (ornery, lol).
So what was the point of all of this? The point was to remind future milk cow owners that getting a milk cow is like having another child around the house, you will have to spend a lot of time around them and take as much care or better to make sure that they are healthy and strong. They may seem tough because they're big animals but they need you to stay attentive.