Thank you loosie for your input.
Yes...my gelding is a chunk. I swear he's an air fern. All he gets is the round bale, honestly!
I know how it is with 'air ferns'!
But free access to good quality(usually meaning cattle fattening, high sugar, improved pasture) hay is not at all an 'all he gets' kind of a thing & I'd suggest you restrict his access, feed in a slow feeder or some such. You may want to do some reading at safergrass.org & ecirhorse.com for more info on diet/pasture considerations.
who was truly emaciated when I got him
Hard to imagine!
As with people who diet, if his metabolism was accustomed to very little, when he suddenly found himself in a 'good paddock'(oh, don't need those inverted commas when talking horses!), his body would have stored as much as it could, which would account for sudden, great weight gain too.
What do you recommend I do while she is lactating as far as food and supplements go?
Personally if I had a mare in foal(or foal at foot) I'd be wanting to do a pasture/hay test, analyse the diet & speak to an equine nutritionist(good idea anyway but not always practical), to give her & the baby the best I could. As with at any other time, I'd basically want to ensure she was getting a high fibre, low carb diet & well balanced nutrition. As for how much/what type, what you're feeding(minus at least half the oil & the other half fed over a few meals daily at least) her should be fine & I'd see how she goes on that. If she starts to lose too much weight(won't hurt her to lose a little) then you can feed a bit more alfalfa(alfalfa/timothy means it's half alfalfa, half grass), beet pulp, ricebran, soy hulls or other low NSC options.