[krrrh]

Bison have never roamed outside of North America, so let’s look at local cattle numbers instead of worldwide populations if we want to compare.

There are 94.4 million cattle in the United States and around 4.5 million in Canada according to a quick web search.

A full grown Bison weighs 1600kg, and the average weight of a steer at slaughter is 600kg or so.

There was likely a dip between the virtual wiping out of Bison in the 19th century and the re-establishment of large herds of ruminants across the Great Plains via ranching, but in terms of biomass I think we can at least say that they are in the same ballpark.

(this puts aside the extirpation of antelope and deer species from large areas of their previous ranges. There were for instance 10 million elk in North America prior to European contact compared to 1 million today at 400-500kg fully grown).

(edit: according to statista your headline number of 1.5 billion is also off: “ The global cattle population amounted to about 989.03 million head in 2019, down from over one billion cattle in 2014.” — https://www.statista.com/statistics/263979/global-cattle-pop... )