You are right that you would need many animal teams to harvest 80 quarters (20 square miles).
I found the following account of a 600 acre (~1 square mile) farm harvested by a 33 horse team near Walla Walla. If gives some interesting comparisons between then and now. I found it enjoyable to read.
Walla Walla Union Bulletin (Walla Walla, Washington) > 1973 > June > 11, page 11The Burokers Recall, Miss Days When Horses Ran Farms
By Vance Orchard U-B Staff Writer
"I always felt sorry for the horses . . . when they had to work in the heat and dust . . . and was glad when we didn't have to use them any more when the tractors came into use.
That was the reply of Forrest Buroker, 86, when asked his reaction to the end of the Horse Era of farming in Walla Walla County. he and his two sisters, Mrs. Frank (Ina) McInroe and Mrs. Zenna Stimmel were interviewed and the conversation taped as part of the continuing series sponsored by Walla Walla Community College.
The taping took place at the home place just east of the home of Mrs. Ray Young (daughter of Mrs. Stimmel) where their grandfather Gallaher had settled when he came West.
The brother and two sisters all had roles in the Horse Era of farming here, and each was an accomplished horseman. "It was the only way we had to go. . . either rode the horse or hitched it to a buggy or a sleigh in the winter," said Buroker.
Buroker went on to explain that he "got into" handling horses at a very early age. . . when he was 5-6 years old.
Mrs. Stimmel, who was 90 on her last birthday, got a start almost as young. "On the farm, we had to work hard, all of us, and my share started at 7 years old when father gave me a horse."
She is still quite proud of the fact she could handle even a 6-horse hitch in her youth. "I could still drive a 6-horse today," she declared. "And, I could still harness up one too!"
Mrs. Stimmel also recalled she milked cows each day as her share of life-on-the-farm until she was 16. "Then, since I was enrolled to attend Whitman Academy in Walla Walla." she said, with a smile. "it seemed like a good time for our younger brother (Forrest) to do something. . . so he got my job of milking cows."
It's also far different today when harvest time cooking rolls around.
"We don't have to cook for the men anymore." she said. "we don't have as many men either, and the 8 or 10 hired bring their own lunches, then go home at night. We hire them to cut our crop: they board themselves.:
In the Horse Era, through, a harvest hand was paid on a "board-and-room" basis.
Even if the "room" was his own blankets, the ground beneath him and stars overhead.
Buroker broke in to recall some differences in his end of the farming on the 600-acre enterprise six miles east of Walla Walla.
"We used to handle the sacked wheat several times before we got it to market, " he said. "Now, we don't handle a kernel of it, and, with a lot fewer men, too, of course.
He said other equipment (besides horses) was about the same, but built bigger. In the Horse Era, a plow might be a two-bottom job pulled by eight horses. Today it would be a 6-bottom plow and one track-type tractor.
Seed drills in the Horse Era were made of wood; today they are steel and roll on steel or rubber tires.
Thinking of repairing farm equipment. Buroker observed that "I repaired here what I could or took it to a blacksmith in Walla Walla."
"Frank LeRoux had a shop near where the Liberty Theater is now." Buroker said. His son, Jack, had his shop later on. Ben Hunt had one too."
"We repaired and cleaned all our harness during the winter months. . . so it was ready to go in the spring." Buroker said. "A set of harness would last 20 years if you took good care of it. It cost about $15 new. Collars for the horses had to be bought according to the horse; a near perfect fit was required or we'd have a horse with sore shoulders.
"We bought our harness usually form Charlie Nye on Main Street. The Quinns also had a harness shop.
"We did a lot of our buying in town (Walla Walla) at the McFadden & Gorham's store on Alder St. and also at Drumheller's. We didn't do much trading in. . . when it came time to buy something new, the old was worn out."
Wheat was taken to town to one of the mills and the year's supply of flour was ground.
"But, I haven't baked bread for several years." Mrs. McInroe said. Mrs. Stimmel "keeps her hand in" better . . . she baked bread just a month ago.
Staple groceries like sugar, cereals, coffee, salt, were purchased in town, but the Horse Era farmer usually raised his own beef, pork and poultry products at home. A smokehouse produced a year's supply of hams and bacons.
"Butchering bees were a big affair in the community." Mrs. Stimmel said. "All the farmers for miles around came and spent the day . . . the men pitched in and got the job done and the wives prepared a big meal."
Mrs. Stimmel admits to a deep love for the horses on her father's farm. She learned to ride at an early age.
"When father traded off our 33-horse team for a tractor and drove them off. I cried." she said, "We kept a couple of horses to ride, but that one tractor took the place of 33 horses."
Source: http://www.museum.bmi.net/Picnic%20People%20A.L/Kees,%20Jacob.htm
Combines are powered at between 40-400HP.
One horse (fresh might pull upwards of 15hp, but less than 1HP over time).
Production still favors mechanical production over livestock.