I used to get around 100 bales an acre. Found haymaking too stressful worrying about the weather and hard work stacking bales. I now make big bale haylege which is quicker and all the hard work is done by tractor.
you could also try making small bale haylage, this is what I did, does not depend so much on weather, is not dusty, and is much easier to handle...of course depends on how many horses you have...I had two at that stage, and felt big bale haylage would maybe go off, as obviously you do not use it as quickly. That said, (this is me arguing with myself!!) I do know several people who make the big bales with just one or two horses and they tell me they do not have a problem, they all take covering off completely when they open them. Good luck. Do'nt forget to book your contractor good and early.
Yes, Marble its fab to make small bale haylage - you dont need so long with good weather as you can make it quicker. The trouble is you have to find someone with a wrapper. When I lived in Herts the farm I was on had one but I didnt think they were that common.
Not easy to be definate as to how many as it depends on the density of the sward, the strength of growth and the length of the grasses.
I gave up doing my own as we only had 15 acres in hay and were always the last in the line when bad weather threatened - after 2 seasons where the contractor stuffed up our hay I now make baleage in standard size.
We have I think 4.5 acres and we used to do some of it for hay, probably about 3 acres of it. Think we used to get about 250 normally (thats with the horses on it all winter as well). We buy in hay nowadays as it was far too stressful worrying about the weather and trying to get the contractor down to turn it etc.! Now I dont have to stack it up either and all that malarky, it gets delivered and some strong men stack it up in the barn for me