If anyone in the future is looking at this, for a highbutter fat, less milk, and easier keeping animal. Cross your Jersey you have with a Highland bull. I have done this and yes, I get less milk, but a highland has a higher butter fat then a Jersey and it is more like goats milk in that it is slightly naturally homogenized and does not separate as fast. I have pure highlands, jerseys, and crosses here.
As for keeping body weight on in early lactation with a jersey. We ran into that as well. We started timing our calving differently and it has helped more then giving different feed. We time it for prime spring grass. They no longer loose body condition as badly. Here, our pastures are prime in mid June and last til Mid August. We calve starting in mid June to mid July here. And we put them on fresh pasture, free choice minerals, and still our own grain ration that we grow. We generally put them in the fresh pasture about three weeks before calving. We found the same works for our goats.
Also in general, protein will equal milk. Calories will equal body condition. So, if milk production is good, body bad, try to up calories.