Does McDonald's use rainforest for grazing land?

No. McDonald's does not own any grazing land to graze their cattle in--as a matter of fact, this multi-million dollar franchise doesn't own any cattle at all.

The reality about rainforests being used for grazing is that they are not used for grazing at all. Rainforests are primarily destroyed for the production of crops and for exotic wood products, not grazing land for livestock. Livestock come in and graze after the crops have essentialled "mined" the nutrients from the soil, eating the sparse grasses that have been sown or grew after the farmers have left. There are exceptions, but this is the most common practice that occurs.