How much protein could ruminants produce without crops

Ruminants include cows, buffalo, goats and sheep.

If ruminants were fed only grass (and not crops), how much protein could they produce globally?

Answer

About 7 grams of protein per day, per person globally, in total from all milk & meat from all animals.

Side note: If animals were also fed crop residues (i.e. the parts that humans can't eat), this number would go up to 12 grams/day protein.
For comparison: Status quo: Currently 16 grams/day protein (10g from milk + 6g from meat). Much of this comes from cows that are fed grains (and other crops) that humans could have eaten.

See calculationsSee calculations

Click on parts of the equations for details.

pasture

2.8 billion tonnes/year

Dry mass of all grass & leaves, grazed from all pasture land

Source:

Breewood, H. & Garnett, T. (2020). What is feed-food competition? (Foodsource: building blocks). Food Climate Research Network, University of Oxford.
Page 10
References primary source:
Mottet, A., de Haan, C., Falcucci, A., Tempio, G., Opio, C., & Gerber, P. (2017). Livestock: On our plates or eating at our table? A new analysis of the feed/food debate. Global Food Security.

residues

1.7 billion tonnes/year

Dry mass of all crop residues, byproducts, and oilseed cakes except for soybean

This should be, in principle, all the human-inedible parts of food crops (inedible due to being too fibrous; ruminants can digest the fiber and get calories from it).
Soybean meal is not counted here, because it can be turned into human food (soy flour).

Source:
Breewood, H. & Garnett, T. (2020). What is feed-food competition? (Foodsource: building blocks). Food Climate Research Network, University of Oxford.
Page 10
References primary source:
Mottet, A., de Haan, C., Falcucci, A., Tempio, G., Opio, C., & Gerber, P. (2017). Livestock: On our plates or eating at our table? A new analysis of the feed/food debate. Global Food Security.

conversion_ratio

133

Ruminants produce 1 gram of human-edible protein for every 133 grams of dry matter they eat.

Dry matter includes all materials eaten by ruminants (both human-edible and human-inedible).

Source:
Mottet, A., de Haan, C., Falcucci, A., Tempio, G., Opio, C., & Gerber, P. (2017). Livestock: On our plates or eating at our table? A new analysis of the feed/food debate. Global Food Security.
The number is mentioned in the Abstract: https://www.tabledebates.org/research-library/livestock-our-plates-or-eating-our-table

world.population

8 billion

If ruminants were fed only grass pasture / conversion_ratio (calculation loading)

If ruminants were fed grass and crop residues (calculation loading)

Status quo for comparison:
Run Code:food2.sql with query:

SELECT SUM(protein_grams_per_day_per_capita) FROM summary
 WHERE Item="Meat, buffalo"
    OR Item="Meat, cattle"
    OR Item="Meat, goat"
    OR Item="Meat, sheep"
    OR Item LIKE "Milk%";

Result: 15.8043445906667

Transition

In the above scenario, there would be fewer cows than the status quo. (Populations of other ruminants might be lower too.) To reach this, there is no need for mass culling; farmers would simply not breed as many animals(...)( for example, lower rates of artificial insemination until the population stabilizes. Note: The farmers who own grain-fed cattle would be the ones to reduce breeding rates, more specifically ). For this to happen, the demand for milk & beef would need to be lower (i.e. people would eat more plant-based).

Answer

Transition

See also