Abundance

Intuitively, high populations of specific animals would be expected to offer the greatest opportunity to serve as sources of waste biomass because waste generation is maximized. Because of the relationship of waste productivity and animal size, this is not always the case as will be shown here. Domestic farm animals and those confined to feedlots are appropriate choices. In addition, commercial poultry production systems, some of which have bird populations over 200,000, would be expected to provide large accumulations of manures in one location. The animals that produce large, localized quantities of excreta are cattle, hogs and pigs, sheep and lambs, and poultry. U. S. populations of these animals in the mid-1990s, the estimated total, annual manure production for each species, and the human population equivalents in terms of solid waste generation are shown in Table 5.2. Several observations can be derived from

TABLE 5.2 Livestock and Poultry Manures Generated in the United States and Their Human Population Equivalent”

Human

population

Manure production equivalent

aU. S. Dept, of Agriculture (1995) for population data. Populations of cattle, hogs and pigs, and sheep and lambs are for 1995; remaining populations are for 1994. With the exception of the commercial broiler population, other populations are assumed to be steady-state values because the variations are relatively small for each of the preceding 10 years. Commercial broiler production was approximately 20% higher in 1995 than in 1990, and 57% higher than in 1985. Daily manure production factors on a dry basis include ash and were calculated from Stanford Research Institute (1976). The factors for converting animal populations to human population equivalents in terms of waste generation are from Wadleigh (1968).

these data assuming that they represent reasonably steady-state conditions. With the exception of the commercial broiler population, the animal popula­tions are assumed to be steady-state values because the variations are relatively small for each of the preceding 10 years. Commercial broiler production was approximately 20% higher in 1995 than in 1990, and 57% higher than in 1985. But it is assumed that the population is relatively constant for purposes of this assessment because the population increases each year. This should tend to eliminate some of the fluctuations in manure production throughout the year because of animal growth and marketing cycles.

Commercial broilers had the highest population, about 7 billion, and they produced the second largest amount of manure; cattle, which had a population of about 100 million, produced the largest amount. No differentiation was made in this assessment between dairy and beef cattle. The daily production rate per head of dry manure solids used was the arithmetic average of dairy cattle and beef cattle, since the dairy-to-beef cattle ratio of the reported manure production rates is about 1.45 (Stanford Research Institute, 1976). Some assess­ments indicate that the ratio of productivities is 3.1 (Jaycor, 1990). The effects of daily manure production per animal are evident as shown in Table 5.2. When published waste production factors are used for conversion of the animal populations to human equivalents in terms of solid waste generation as shown in the table, the total human population equivalent of these animals is estimated at almost 3 billion people. This is approximately 12 times the U. S. population in the mid-1990s, a ratio considerably less than reported in 1968, when it was estimated to be 20 times that of the human population (American Chem­ical Society, 1969). But the latter ratio included liquid wastes as well. The ratio of total animal excreta to total municipal biosolids generation calcu­lated here, both of which exclude liquid wastes, is about 36 on a mass basis (307.1 dry t/8.6 million dry t).