Very much dependent on your soil type and what you're plowing down. A 3X16 plow (both our old semi-mount IH 720 and our newer mounted Kverneland/Overums) will bring my 180 to its knees in our soil. First gear all the time. That's in sod on our hills. You'll hear lots of folks talking about pulling 4 furrows with their 50-70 HP tractors. But then you'll see a video of it and they'll be plowing clean stubble on perfectly flat land. Nothing wrong with that, but that doesn't really count as a metric of plow capacity in my opinion: I always felt the measure of how much plow you can pull should be on sod. Because if you still plow regularly, at some point you're likely to want to plow sod (For us, plowing sod/hayfields is the only time you'd ever actually plow: For stubble/row-crops, a heavy offset disc is usually sufficient without plowing). And of course, alfalfa plowing will be tougher than most, as the roots are so thick and extensive.
I'd suggest 3X16 or smaller, depending on your soil type. I'm a big fan of the Scandinavian plows: In our rough & rocky ground, they're the only ones built heavy enough to last, and they pull a lot easier than many other types (the Scandanavian ones seem to be built along the lines of the Oliver plows, with longer, sweeping boards. Way better for turning trash over and easier pulling than, say, the JD plows with their blunt, aggressive board angles). A vari-width is also handy: Our Kverneland and Overum three-furrows are vari-width: The Kverneland is 14" to 18" the Overum is 12" to 16". Very handy to be able to narrow up the width if you find it's pulling too hard, or widen it up if it's pulling easy and you want increased trash clearance and to cover a little more ground.
I will say that our ground is tougher than most. Not necessarily because of the clay content (of which there's very little), but how excessively rocky it is. Not helped by the fact we break up our hay fields so seldom - giving the fields lots of time to develop an extensive and tough root system. We break them up as little as we can because of how rocky it is: Every day of plowing means a week of picking stones, so we go as long as we can without plowing.
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