ecological footprint of an animal per unit of body mass or useful produce.
Sure, and if you look at unit of bioavailable protein, ruminants win. https://www.aleph2020.org/
Natural decomposition of plants in the soil by soil bacteria is, in turn, less methane-emitting than either the processes happening in the rumen, or even in the human digestive system.
except in high water environments (swamps and bogs). You need to also account for wild ruminants who eat the grass even if cattle are not around.
All animals have more of such footprint compared to plant-soil bacteria cycle,
You also need to account for the bioavailability disparity between a whole food plant based diet and a whole food animal based diet. 3kg vs 0.5kg in daily food. That is about a 6x food volume difference, lots of people get focused looking on calories, but if we seriously are trying to remove all ruminants from the food supply then we need to look at all essential nutrition, factor in the cost of medical care for people who have issues with plant based nutrition (such as type 2 diabetics who suffer under carbohydrate load, of which there are a billion).