We're a small, low input, commercial suckler herd. We calve year round, although do tend to have less Sept thru Jan than the rest of the year.
If you calve her August you will need to make sure she and the calf get plenty of forage from October onwards (she'll make it plain when she needs it, I would think!) but if you are okay for hay / good silage, I wouldn't worry about an August calf. It'll be three months old before the wintry weather really gets going - and they can take quite a lot of cold, it's wet that's the problem. As a purebred Dexter it'll be fairly hairy, so well weatherproofed, won't it? Do you have a shed or stable they could come in to if it gets really wet?
She'll self-wean her 2011 calf, I would think. If it's getting close to August and she hasn't weaned it, would you be able to separate them?