How much meat and milk could grassland alone provide?

Not much, compared with what we eat today. A report from the Food Climate Research Network at the University of Oxford estimates that animals raised only on grassland could supply 7 to 18 grams of animal protein per person a day. If food waste and by-products are also fed to other livestock, that rises to about 11 to 32 grams, against about 50 to 60 grams in rich regions today1. A systematic review of eight studies from Wageningen University arrives at 9 to 23 grams from animals fed on grass and leftovers2. For comparison, the WHO puts an adult's need at 50 to 60 grams of protein in total a day, and Europeans get on average 51 grams of animal protein from land animals, figures quoted in the review3. The review finds that some animal food from such livestock is the most efficient use of cropland, but that grazing may rule out other uses of grassland, such as rewilding4.