I think there is a bit of a tall tale there. In that area of michigan doing dry hay you have at most in the best part of July 8 hours to bale, and you likely need acid to do that. Most days closer to 6 tops. Lets run with 8 hours, and a baler from the late 70's like a JD 336 to get you some extra capacity, you can make 200 pretty and 32" shorty bales per hour or 350 or so rougher looking bales per hour. You won't find many fields to keep that up all day but 300 bale per hour average on that era baler can be done.
So we have 300 x 8 = 2400 bales per day. I've baled for many years with a JD 336 before I got my JD 348 to go with it and I'll tell you, 2400 bales, that would be a baling day for the record books.
I ran my 348 hard one day last year on a 100 hp tractor and we managed to get almost 1700 bales in about 5 hours one day. It has about 10-20% more capacity than my 336. I didn't stop for barely anything and the person raking did a perfect job ahead of me in good yielding fields so the pickup was full. With a few more improvements I could have made 2500 bales if I had the full 8 hours but that is the exception.
When we run the 336 and 348 together 500+ bales per hour is pretty easy but that is still only 4000 bales per perfect day.
I know of a farm running a triple hitch with 3 high capacity inline balers in the dry west were they can do 12 hour days, and they are getting 6000 bales a day.
I also know a neighbour that running a high capacity baler, running with acid to extend his baling window to maybe 10 hours, has made into the 3000's with one baler in a day maybe twice in the last 20 years.
Growing up I worked for a farm that ran an 80's era MF224 baler with 18 people on the haying crew the best day I recall was 18 wagon loads that would average about 200 bales each.
--- I have asked my Uncle Lee this question a couple of times over the last 20 years so I am fairly certain it is accurate.
--- I asked him what was the most bales of hay he had ever baled in one day. He said that he and his buddies baled, and put up, about 10,000 bales in one day. He also said that they did not quit at the normal time (if there is such a thing) but continued [I assume] well into the night.
--- This would have been in the 60's or 70's.
--- Does anybody have a similar tale where you baled until you couldn't bale anymore?
--- I've yet to ask him how many guys he had baling with him to get an accurate picture of what I am describing. But I will tomorrow.