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Note: See text for discussion of net efficiency.

Note: See text for discussion of net efficiency.

has the world's largest population of swine (33% of the total number of pigs), which is fed largely on forages, byproducts, and wastes. Of the total land in the world, 11% is arable (suitable for crop production); 22% is range, pasture, and meadow; and 66% is not suitable for agricultural use. If animal efficiency is based on the calories that can be utilized directly by humans, their net efficiencies are nearly equal (6). Considering only the feed protein consumed by animals that could be utilized directly by humans, edible protein from animal products is produced with an efficiency of 60 to over 100% (7). Until all available land that can be used for photosynthesis yields food completely consumed directly by humans without any loss of plant material, including the supporting portions of the plant and by-products from the processing of plant materials for human consumption, animals will make a positive contribution to the supply of human food (8). Because the relationships between ruminants (cattle, sheep, goats) and human needs are complex and interdependent (Fig. 1), quantitative efficiency measurements are difficult (9).

The use of grains for food animals varies greatly throughout the world. In advanced countries, approximately 70% is used for animal feed and 30% for human food, whereas in developing countries, only about 8% is used as animal feed. Except for wheat, the principal end use of grains in the United States is for livestock feed. The current interest in oats as a source of dietary fiber in the human diet and corn for the production of fructose sweeteners and fuel alcohol has changed somewhat the usage for these two grains as well as the quantity of by-products available from these food industries.

Animals constitute a tremendous reservoir of human food, thereby reducing humankind's vulnerability to periods of crop shortages due to droughts, disease, and political events (2). Animals are found throughout the world, whereas grain reserves are found only in certain areas.

Animals also provide a host of other human needs. Wool is used for clothing and hides are used for shoes and other leather products. Many pharmaceutical products are isolated from animal glands. Fats and other lipids are used to manufacture soap and other products. Animals are used in many parts of the world for transportation and draft

Table 4. Consumption of Feed from Various Sources by Different Classes of Livestock in the United States

Grains and high-protein concentrates Forage

Class of livestock x1000 Metric tons % of Diet x 1,000 Metric tons % of Diet

Dairy cattle 25,602 37 43,739 63

Beef cattle 56,963 27 152,168 73

Sheep and goats 974 11 7,610 89

Hogs 51,396 86 8,178 14

Hens and pullets 20,499 97 734 3

Broilers 12,104 100 0

Turkeys 5,091 95 272 5

All livestock 185,105 46 218,879 54

Grain and Forage crops* Pasture, range* oilseed crops (hay, silage)

Stalks, leaves*

Industrial by-products* (sugar beet pulp, cannery wastes) _I

Nonprotein nitrogen* (urea, ammonia)

: Ruminants-

(cattle, sheep, goats, deer)

Wastes, manure-

Humans

Meat, milk and by-products

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