If they can have some concentrates of some kind then, as shep53 says, it would be doable.
On the upland farm in North Cumbria, we had a very wet, marshy 90-acre area just below the moorland line. We kept 6-8 Blue Greys (extremely thrifty cows, Whitebred Shorthorn bull on a Galloway cow) up there year round with an Aberdeen Angus bull. (Joined by some other cattle in summer, but just these over winter.)
The farm was on a Higher Level Stewardship scheme. This area of nearly-moorland had to be kept sheep-free, the maximum cattle headage over winter was capped at 8, and we weren't allowed to feed concentrates there. We could scatter hay or we could feed silage on the one bit of rocky ground within the area.
First year we put a ring up there, on the spot allowed, and took them 2 days ration of loose silage every other day by tractor. We didn't like that, the tractor going up there made too much mess of the ground.
Second year we made up packages of silage and took them up daily on the quad. The quad damaged the ground less than the tractor, but with that ground and the weather up there, you really didn't want to have to travel the same bit of ground every day. It could become impassable, even to an all-terrain quad, and then you couldn't feed your cattle.
So we switched to scattering hay, which we could do in different places each day. We liked that a lot better; the cattle moved about more and we could pick and vary our paths, and always get to give them their hay somewhere, no matter what the conditions.
Our other cattle were all sucklers, mainly dairy x Angus. The dry cows were fed 1/2 small bay of hay a day each, or 22 cows would get one big bale of hay or silage a day. Cows which were still lactating got half as much again.
The Blue Greys on the "fell", as this ground was called, got about 1/3 as much as the housed, larger dry cows - and they were outside, having to keep themselves warm.
As an experiment one year, we housed one of the Blue Greys to see how she did. Sweated and got fat, is what she did, lol. We reckoned we would have had to feed her about half what the other dry cows were getting to stop her getting over-fat.
So my sums tell me that the outwintered cows were deriving enough food value from the moorland grasses etc to keep themselves warm and power their travels (work Doris wasn't doing inside) plus approx 1/6 of a ration.
They may well, at one small, thrifty, dry cow per 10 acres, have been able to get through a winter without any hay at all, and we did know one or two farms up above the moorland line who did outwinter Galloways and not feed them at all, but it was not how we would have wanted our livestock to be treated.
But... that's extreme farming up there. If you had that sort of stocking density on culm grassland in Devon, it may well be more feasible (and not a welfare issue) with small, thrifty cattle.