Jeremy nibbled this resource from OK State on livestock breeds. The intro page made a couple of points that I've made before but are perhaps worth repeating.

There are many who feel that because the world population is growing at a faster rate than is the food supply, we are becoming less and less able to afford animal foods because feeding plant products to animals is an inefficient use of potential human food. It is true that it is more efficient for humans to eat plant products directly rather than to allow animals to convert them to human food. At best, animals only produce one pound or less of human food for each three pounds of plants eaten. However, this inefficiency only applies to those plants and plant products that the human can utilize. The fact is that over two-thirds of the feed fed to animals consists of substances that are either undesirable or completely unsuited for human food. Thus, by their ability to convert inedible plant materials to human food, animals not only do not compete with the human rather they aid greatly in improving both the quantity and the quality of the diets of human societies.

And, a third of all human protein comes from animals foods. You can easily get empty calories from plants, and they account for 80% of the total calories consumed by humans in the world, but that's not a healthful diet.

There are other considerations.

Only about one-third of the land area of the world is classified as agricultural. Thus, roughly two-thirds of the land area of the world is not suited for any sort of agricultural use because it is covered by cites, mountains, deserts, swamps, snow, etc. Of the 35 percent that can be devoted to agriculture, less than one-third (or about 10% of the total land area) can be cultivated and produce plant products that the human can digest. The remaining two-thirds of the world's agricultural land is covered by grass, shrubs or other plants that only ruminant animals can digest. Thus, the inefficiency of animals is not a major concern since they represent the only way these plants can be converted to human food. As the human population of the world increases, it is likely that we will be forced to depend more and more on ruminant animals to meet the increased demands for food.

It can be argued that some of the two thirds of ag land used for ruminant grazing would be better left unused, or used less. It can also be argued that some of that one third of ag land used for cropping would be better left unused, or used for ruminant grazing.

It is true that swine and poultry can be competitors with the humans for food if they are produced by the intensive confinement systems widely practiced in the developed countries. In fact the highest proportion of feed grains and other concentrates, such as oilseed meals, fed to livestock in the United States are fed to swine and poultry. Current grain prices make this profitable. This obviously could change if grain prices increase in the future. However, the high reproductive rate and favorable feed efficiency of swine and poultry would keep them as important contributors to the diets of humans.

Informed commentators reasoning in good faith draw very different conclusions about the role of animal agriculture for human food supply than those so often trumpeted by activists with eating disorders and ideological blinkers. Improving human nutrition while also caring for the environment requires improved agronomic systems based on accurate information. All livestock are not the same. All plant material is not the same. All land is not the same. Making intelligent use of these truths to produce abundant and nutritious foods for human consumption while having the least adverse impact on the environment is a requirement as human population increases.