First off I am so glad you rescued her and you obviously care for her and she looks lovely
Secondly THE FEED...
Personally I would stay away from any 'manufactured feed 'scientifically' done or not - they are full of mollasses (and, correspondingly high in potassium) which is why horses like it and is incredibly bad for them in such large quantity.
Also you can't actually regulate the exact amount a mare (or any horse) needs in terms of nutrient as if the horse gets too fat you have to cut it down - too thin, you feed more.
I believe the only way to feed a horse and particularly a brood mare is to use simple feed - Speedy beet, crushed barley, pollard etc and ADD a HIGH quality mineral mix. (I use the ProvideIt Premium NZ Horse Mins) which I found and find brilliant. Also they are designed for OUR conditions - grass and soil type.
Also check your grass for clover. Clover is incredibly bad for mares along with rye. Remove her from either.
If you want to know more about this the website to go to -and it is NZ based is ProvideIt Vitamin Mineral mixes for horses.
It is a brilliant resource for everything you need to know about feedin a horse.
I attribute Zephyr's muscle tone and straight legs etc at birth to the feed and nutrients Merlot got right through pregnancy -it is SO important!
Sorry to rant but the woman (Jenny Paterson) who did this website has spent years researching why people are are having so many problems over the last 15 - 20 years with their horses and found nearly all of it due to diet and the changes in the way we graze the horses etc - she knows more than anyone about what horses need in terms of nutrients and helped me enormously with major problems I was having with all of my horses!