It depends a great deal on the field and how decent the sward is. Summer before last my brother got 50 large round bales off 6 acres that hadn't been grazed for about a year. Last summer, same field, but grazed over winter, then rested for three or four months, he only got 20 something bales.
as above its variable on your soil, grass and how much you graze the land. Im struggling to remember exactly but I believe we got 64 bales off 6.5 acres which would work out at almost 10bales an acre. 5acres had been rested for around 6 months, 1.5 acres had been rested for a year and was the first cut for half of the paddock as it had been reseeded. Both fields have good soil and the 5acres had had nitrogen added the year before as it was deficient, and both had a very good growth of grass on them. Also both have never been properly grazed down. The best thing about using your own field is knowing its ragwort/ poisonous plant free and saving a lot of money! .
It depends on what type of grasses it contains, and whether its first or second cut. Other variables are the rate per acre it was seeded, what the ground is like, i.e. heavy or light soil, what time of year, the machinery used and the weather conditions at the time of bailing. But rule of thumb is that one tonne of hay is equal to approx 40/45 bales or 4 to 5 round bales and one acre would yield approx two tonnes of hay.
Other figures I've found online are 320 off a 3 acre field, and another 435 off 5 acres all of which I am guessing are normal bales of hay (approx £4 or £5 per bale at current prices).
We do large round bales of haylage, hay and or silage and would want to be averaging 10 bales to the acre to justify the costs of fertiliser, and contractors to spread it and then cut, turn, bale and in the case of haylage wrap and then of course you have the cost of drawing it in and stacking it in the yard. What with having to put up with the weather as well maybe it'd be easier to buy it? but then we need at least 600 bales a year so I guess we'll stick with doing our own! Sorry, gone off topic blame it on the christmas rush. Meant to say we only do one cut a year.