Zero food-feed competition

Cows, goats and sheep can digest fibre and use it as calories.

These animals can thus eat a few things that don't compete with food production:

1. Crop residues (also known as biomass waste) which are the fibrous leftover parts of food crops we have to grow anyway.
2. Grass from pasture lands.

This feed can't be eaten by humans, chickens, or pigs. (...)( Except in very small quantities for roughage only. Humans, chickens and pigs can't extract any significant amount of calories from fiber. ) (...)( Should mention: There are also some fibrous crops grown specifically to feed ruminants; we don't count those, because those still compete with food production: The same land could have grown some other crops that humans could eat. Even when soil is poor, there are options. However, the same reasoning doesn't apply to pasture grasses, because pasture has permanent roots which store carbon. Uprooting pasture and converting it to crop land, would produce significant carbon emissions^{[QUANTIFICATION needed]}. Crops, on the other hand, already get uprooted every year (harvest), so they don't sequester any carbon long-term. )

The big question is: How much animal protein could be produced if these were the only sources of animal feed?

Here's a simple estimate assuming only cows:

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 human-inedible parts of food crops (inedible due to being too fibrous, whereas ruminants can digest the fiber and get calories from it). Soybean meal is not counted, 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

7.9 billion

(pasture + residues) / conversion_ratio (grams/day per capita)(world.population) (calculation loading) ^ Total amount of protein from both meat and dairy, combined. (...)( This is probably a slight overestimate, because some sources say that feed efficiency ratios are lower when the cows are fed less human-edible grains. )

There would be no animal protein from chickens, pigs, or any other non-ruminant animals.

There could be some animal protein from goats, sheep, buffalo or other ruminants - but it would subtract from the amount of animal protein available from cows (...)( because they both depend on the same feed supply above ). (...)( Research needed on this page: How do the feed-efficiency ratios of other ruminants compare to cows? If we switched to other ruminants, would there be more animal protein available or less? )

Also, there would be even less animal protein available if we rewilded some crop land (...)( since we don't have to grow as many crops if we aren't raising chickens, pigs, etc. There would be less crop residue available for ruminants, so we'd have to raise fewer of them. ) or some pasture land (...)( either allowing it to become forest, or allowing it to become wilder grasslands which are less optimized for feeding cows etc. Both types of rewilding are beneficial in that they sequester more carbon than conventional pasture. ).

For comparison, status quo:

Protein from cows:

milk.production

708264265 tonnes/year

Global production of milk

Using data from 2019. Other years are almost the same.

Source: Crop and livestock products - FAOSTATFood and Agriculture Organization of the United Nations [1]

beef.production

67915624 tonnes/year

Global production of beef (edible parts)

Using data from 2019. Other years are almost the same.

Source: Crop and livestock products - FAOSTATFood and Agriculture Organization of the United Nations [2]

milk.protein

3.15%

Protein as a fraction of whole milk, by weight.

Source: FoodDataUS Department of Agriculture

Using database food named "Milk, whole, 3.25% milkfat, without added vitamin A and vitamin D"

beef.protein

19.42%

Protein as a fraction of beef (edible portion), by weight.

Source: FoodDataUS Department of Agriculture

Using database food named "Beef, grass-fed, ground, raw" because (even though not all beef is grassfed) it was the only database entry that wasn't a specific cut of beef.

milk.production*milk.protein + beef.production*beef.protein (grams/day per capita)(world.population) (calculation loading)

Protein from chickens:

chicken.production

118616699 tonnes/year

Global production of chicken meat (edible parts)

Using data from 2019. Other years are almost the same.

Source: Crop and livestock products - FAOSTATFood and Agriculture Organization of the United Nations [3]

eggs.production

84363316 tonnes/year

Global production of eggs (from hens) (edible parts)

Using data from 2019. Other years are almost the same.

Source: Crop and livestock products - FAOSTATFood and Agriculture Organization of the United Nations [4]

chicken.protein

18.33%

Protein as a fraction of chicken (edible portion), by weight.

Source: FoodDataUS Department of Agriculture

Using database food named "Chicken, broilers or fryers, meat and skin and giblets and neck, raw"

eggs.protein

12.56%

Protein as a fraction of eggs (edible portion), by weight.

Source: FoodDataUS Department of Agriculture

Using database food named "Egg, whole, raw, fresh"

eggs.production*eggs.protein + chicken.production*chicken.protein (grams/day per capita)(world.population) (calculation loading)

Protein from pigs:

pork.production

109635731 tonnes/year

Global production of pig meat (edible parts)

Using data from 2019. Other years are almost the same.

Source: Crop and livestock products - FAOSTATFood and Agriculture Organization of the United Nations [5]

pork.protein

16.88%

Protein as a fraction of pork (edible portion), by weight.

Source: FoodDataUS Department of Agriculture

Using database food named "Pork, fresh, ground, raw"

pork.production*pork.protein (grams/day per capita)(world.population) (calculation loading)

In other words, globally most cows are already fed mostly non-human-edible matter, but beef and dairy production are fairly low.