Calorific value of 1 ha crops vs herbivores raised on 1 ha pasture

Question

I came across the following paper on social media:

Davis, S. L. (2003). The least harm principle may require that humans consume a diet containing large herbivores, not a vegan diet. Journal of Agricultural and Environmental Ethics, 16(4), 387-394.

The paper considers the the fact that animals are killed in the process of agriculture (15 / ha), whereas if one would use the same area as pasture for foragers, fewer animals would be killed (7.5 / ha). Using these numbers, the paper considers the land area of the US and counts the animal deaths using each approach (using the same land area for both) and suggests that having a diet with meat might actually lead to fewer animal deaths.

However, by using the same area for both the calculations, the paper seems to be implicitly assuming that the calorific value (i.e. number of people you can feed, which is probably the quantity that we want to keep fixed) of 1 ha crops is the same as the calorific value of foragers raised on 1 ha pasture, which seems like a sketchy assumption to make.

Are there studies giving approximate numbers for filling in the missing piece in the calculation?

Answer 1

There is a response to that paper:

Matheny, G. (2003). Least harm: A defense of vegetarianism from Steven Davis's omnivorous proposal. Journal of Agricultural and Environmental Ethics, 16(5), 505-511.

which gives some numbers in terms of kg of protein (emphasis mine):

Davis suggests the number of wild animals killed per hectare in crop production (15) is twice that killed in ruminant-pasture (7.5). If this is true, then as long as crop production uses less than half as many hectares as ruminant-pasture to deliver the same amount of food, a vegetarian will kill fewer animals than an omnivore. In fact, crop production uses less than half as many hectares as grass-fed dairy and one-tenth as many hectares as grass-fed beef to deliver the same amount of protein. In one year, 1,000 kilograms of protein can be produced on as few as 1.0 hectares planted with soy and corn, 2.6 hectares used as pasture for grass-fed dairy cows, or 10 hectares used as pasture for grass-fed beef cattle (Vandehaar, 1998; UNFAO, 1996). As such, to obtain the 20 kilograms of protein per year recommended for adults, a vegan-vegetarian would kill 0.3 wild animals annually, a lacto-vegetarian would kill 0.39 wild animals, while a Davis- style omnivore would kill 1.5 wild animals. Thus, correcting Davis’s math, we see that a vegan-vegetarian population would kill the fewest number of wild animals, followed closely by a lacto-vegetarian population.

Unfortunately, this doesn't give the full picture as a large portion of vegan/vegetarian diets comes from other grains (rice, wheat etc.), vegetables and fruits. It is also not clear why only protein content is being considered, instead of calorific content.