It would definitely be cheaper to buy hay.
This!!!!
I tried what you want to do a few yeas back. It took two years to get it out of my system...
I had about 25 acres of level pasture around the house and no livestock on it for a few years so I had a good stand of grass hay. Had been buying about 200-300 bales of hay to feed my horses and a few cows from a local friend who cut hay for a living. But, I thought if I could get 100 bales per acre which seem doable, I could keep what I needed and sell the rest. What is hard about getting some equipment and just cut some hay... My hay friend tried to talk me out of it offering to give me 200 bales of hay. But, I was all in.
I already had a couple of tractors that would work so I shopped around for what I thought were good deals for hay equipment (which seldom are deals) and bought a disk cutter, rake and JD 336 square baler. I was all set... I thought... Well, when you get used, it is generally well used (and maybe abused), so spend a small fortune replacing wore parts, repairing broken parts and general maintenance that has been neglected for years.... And, unless you grew up working on square balers, they will chew you up and spit you out getting them to work (I did grow up on a farm and we cut a lot of hay but I did the loading and unloading which did not prepare me for working on a square bailer).
Finally, the day came to cut and I cut about 4 acres to start with as I a one man crew. Cutting was mostly trouble free, except when I found some wire that had been half buried in the ground and missed by the brush hog for years but my disk cutter had no problem finding and wrapping several feet of it around most of the disks which ended up locking it up up and burning the PTO slip clutch up... more work and money...
Fast forward a week and now I am ready to bale. I had the baler serviced and supposedly tuned by the JD dealer who replaced a lot of worn parts. Spend the better part of a week of vacation trying to get it to tie a knot. My friend was no help, he trades equipment every 4-5 years and had not seen a 336 since the early 80s. Finally, got a JD service tech to come out and help. No deal, it would not tie knots consistently. He even left me their service manual and a OM for the 336 they had in shop. Did everything the SM stated to do, nope, it just would not cooperate.
Visited my local farmers lunch stop watering hole, found an older gentleman who I knew did hay back in the day who told me the 336s were great machines no matter my experience. I talked him into coming to visit me in exchange for all the adult beverages he could drink while he watched me bale my hay. He spend about an hour wrenching and walking behind the baler. Presto, it baled like a new one. Best beer money I ever spent.
Now for the rest of the story. I baled about 550 bales the first year but lost about half to mold as it was rained on a few times before I could get it out of the field (one man crew thing was rough). But putting up around 300 bales in the June sun wore me out, so I quit for the year as I had more than enough for my needs. Afterwards, I thought, well that was not that bad, so I was ready for next year. The following year, did almost the 5 acres and got about 700 heavy bales. Paid for some help this time as I did not have time or willpower to get all the hay up (hired some of my brother in laws farmhands). Now I have some to sell, well it was a great year for hay and everybody had plenty, could not give it away.... I told my wife, she could make fun of me now as she said in the beginning it was a stupid idea (she grew up farming). Wife reminded me I was at $30 a bale with all costs counted. Hay was selling around then for $2.50-3 a bale.
Sold my equipment the following year and never looked back yet... but I retire in a few years, never know when the itch will start again... maybe a mini round baler this time
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Sorry for the long winded response.