Back in the mid-1980's when Tennaco formed Case/IH by merging IH ag with their Case division, JD was the largest ag co. by percentage of market share in the 70 to 200 HP segment, probably had about 32-35% of the market. IH was second with around 30%. The remaining 35% was divided up between Case, Ford, Allis, White Farm, Duetz, And Massey Ferguson, Belaris, Leyland, SAME, LONG, etc. And some of those last co's piece of the pie were pretty darn small. David Brown was already a Tennaco company since back in 1972 and sorta combined with Case. They still had their own plant in England, but supplied parts to dealers through the Case distibution network and sold tractors through them too.
You have to keep in mind how volatile the large ag tractor market was back from 1979 till 1985. '79 was the largest sales year since the early 1960's. 1980 was a down year, and 1982 was about a QUARTER the size of '80. I remember my Boss at FARMALL getting off the phone with somebody up at IH Corp. Marketing in 1981 and saying that the week before the market had actually turned up in sales, 250 tractors between 60 and 150 HP were sold. My Boss's response was, "Well, IF they all bought Farmall's, that would keep this place running for about TWO days, we can easily make 120 to 150 tractors a day." But that would have required Waterloo,IA (JD), Racine,WI (Case), Charles City, Ia. (White Farm) Birmingham, MI (Ford), West Allis, WI (Allis), to all close their doors.
Oddly enough, M-F was the largest mfg of ag tractors world-wide by number of tractors built & sold until sometime in the 1990's because they were so popular in Europe. Their home office was in France at the time.
Several other European tractor co's too, like Fendt, Fiat, etc. but at the time M-F was the only one actively importing into the US. The Japanese Invasion in the larger size tractors hadn't really started yet with brands like Kubota.
There was SO much merging going on, for instance, Minneapolis-Moline and Oliver merging into White Farm Equipment along with the Canadian division, Cockshutt, in 1969.
http://en.wikipedia.org/wiki/White_Farm_Equipment
Much the same thing happened when Case/IH was merged with New Holland in late 1999.
http://en.wikipedia.org/wiki/CNH_(company)
It's really hard to compare one time segment to another because the number and names of the players changed so much.