One is a 5-10 acre lucerne block with a creek
The creek sounds good!
I wouldn't put horses on one acre of lucerne(alfalfa), let alone 5-10! Perhaps you can put her in there & rest the other paddock over winter if the lucerne is dead & gone then.
in which case I will be getting bales of lucerne hay free of charge (included in the agistment price).
Awesome! Where are you?? Can I keep my horses there for the same deal too??
But seriously, while some lucerne/legume hay in the diet is generally fine/good, it's very high in calcium, protein and also energy, which depending on the beast & the rest of the diet, may not be at all good to feed them much/any.
The other pasture is only a couple of acres right beside the lucerne one. It's rather... dead looking, but it still has plenty of feed in it.
I'd definitely choose that one then. Horses have evolved in semi arid environs to eat 'poor grade'(compared to our 'improved' cattle fattening pastures) & small amounts(but near constantly). Obviously it depends on your horse's condition, the pasture... whether we're having another drought... etc, but one horse on 2 acres, I could see you having to restrict grazing on that. Will she be having company? I think that's very important too.
My issue is that I've read recently that putting a horse on such rich feed as a lucerne pasture can cause colic, ....currently in need of a diet who is fed oaten chaff and pellets.
Colic among other probs. It's not just a matter of richness, but nutrient imbalance, not just a matter of 'weaning' her onto it or getting her used to it gradually. Oh & for another spanner in the works - people usually think of oaten chaff as about as nourishing as sawdust, but it can actually be quite high in sugar/energy... & potentially include a fair amount of oats too! So I'd personally ditch that, or soak & drain before feeding if the horse needs to lose weight. What are the pellets & why are you going to keep feeding them?