If your donor is THAT fat, a round bale of hay will blow her out of the water -- she will finish like a fat steer ina feed lot!
We had some cows come to us many years ago that had not calved in three years -- they were BBQ ready if ya know what I mean! hehehe
The guy said he didn't want to flush them till fall, and asked that I just take care of them for him till then, and get them in shape to flush.
For the next 30 days -- here is what we did - Salt, Mineral, Water -- and NO more than 10 pounds of grass hay a day -- period!
The first few days they act like they are going to die -- bellow all the time, then the rumen starts to shrink, and they are actually happy and chewing thier cuds. It takes several weeks to really see the difference, then it starts to come off like crazy. After about 30 days, slowly start them back up again to about 25-30 pounds, and they level out and start cycling again.
Those cows had not produced a good egg in 4 years, and the first flush thereafter they averaged over 100 good ones apiece. He wwas exstatic, and they did good several times more. FAT IS NOT YOUR FRIEND here -- trust me on that. We bred them back for spring calves and they all calved first service! End of story!
Good luck -- suck her up and get it off if she is that fat -- it makes a HUGE difference!
Terry