>> most the grass feed cows are raised in fields that used to be rain forests in Brazil
> Evidence for that rather large claim?
While most cows in Brazil are grass-fed, about half are raised on former rainforest:
"Nearly 50% of Brazilian livestock are raised in fields that used to be rainforest."[0,1]
"Most cows in Brazil, the world’s largest beef exporter, are grass-fed."[2]
See also:
How beef demand is accelerating the Amazon’s deforestation and climate peril[3]:
> Cattle ranchers in the Brazilian Amazon — the storied rainforest that produces oxygen for the world and modulates climate — are aggressively expanding their herds and willing to clear-cut the forest and burn what’s left to make way for pastures. As a result, they’ve become the single biggest driver of the Amazon’s deforestation, causing about 80 percent of it, according to the Yale School of Forestry & Environmental Studies.
Cattle Ranching in the Amazon Region[4]:
> Cattle ranching is the largest driver of deforestation in every Amazon country, accounting for 80% of current deforestation rates. Amazon Brazil is home to approximately 200 million head of cattle, and is the largest exporter in the world, supplying about one quarter of the global market. Low input cost and easy transportation in rural areas make ranching an attractive economic activity in the forest frontier; low yields and cheap land encourage expansion and deforestation. Approximately 450,000 square kilometers of deforested Amazon in Brazil are now in cattle pasture. Cattle ranching and soy cultivation are often linked as soy replaces cattle pasture, pushing farmers farther into the Amazon.
Please also explain how any large monoculture crop is a net gain for diversity?