Trends in the Potential for Nutrient Loading
from Confined Livestock Operations

Robert L. Kellogg and Charles H. Lander, NRCS/USDA

Poster Presentation for "The State of North America's Private Land,"

A Conference Held January 19-21, 1999, Chicago, Illinois

PURPOSE OF STUDY

Lander, Moffitt, and Alt conducted a study published by the National Resources Conservation Service in 1998 titled "Nutrients Available from Livestock Manure Relative to Crop Growth Requirements." Using farm-level census data from the 1992 Agriculture Census, the authors presented county estimates of pounds of manure nitrogen and phosphorus potentially generated from confined livestock, and compared these estimates to the potential for nitrogen and phosphorus uptake/removal by crops and application on pastureland. The results of the study were widely used by policy makers in deliberations on the growing animal waste problems throughout the Nation.

The purpose of the present study is to make similar estimates for earlier years to show the spatial and temporal trends in the potential for problems associated with animal waste.

ANALYSIS

Published Agricultural Census data for 1949, 1954, 1959, 1964, 1969, 1974, 1978, 1982, 1987, and 1992 were used to make county estimates of livestock populations and crop and pastureland acreage. These data were combined with estimates of recoverable manure (mostly from confinement), nutrient content of manure, nutrient content of the harvested portion of crops, and manure application rates on pastureland to calculate the nutrient availability from manure relative to potential plant uptake and removal. All calculations were made using county data; thus, each county was treated as a single large farm. Because of confidentiality restrictions, data for some counties were not available. The analysis presented here differs slightly from that reported by Lander, Moffitt, and Alt, largely because published county data were used rather than farm-level census data. Results in this paper correspond most closely to Scenario 3 in Lander, Moffitt, and Alt.

Nutrient Availability from Manure

Sixteen livestock types were derived from the data:

• Beef animals
  • Young calves up to about 425 pounds.
  • Young heifers and steers from about 425 pounds to 650 pounds.
  • Heifers for herd replacement (650 pounds to full weight)
  • Breeding herd
  • Fattened cattle (650 pounds and up)
• Dairy animals
  • Young calves up to about 425 pounds.
  • Young heifers and steers from about 425 pounds to 650 pounds.
  • Heifers for herd replacement (650 pounds to full weight)
  • Milk cows
• Hogs and pigs
  • Hogs for breeding
  • Hogs on feed
• Poultry
  • Chicken layers
  • Chicken pullets
  • Chicken broilers
  • Turkeys for breeding
  • Turkeys for slaughter

Tons of recoverable manure was calculated for each livestock type by converting population numbers to standard animal units, multiplying the animal units by tons of manure produced, and then multiplying by the recoverable manure percentage, which was derived to reflect confined populations and included an adjustment for typical nutrient loss that occurs during storage and handling. Tons of manure nitrogen and manure phosphorus were calculated by multiplying tons of manure by nutrient content for each livestock type.

Crop Uptake and Removal

Uptake and removal of nitrogen and phosphorus for 21 crops was estimated. The total production of each harvested crop was multiplied by the nitrogen and phosphorus content of the harvested biomass. A utilization efficiency factor of 70 percent was applied to nitrogen to account for the consumption of nitrogen during plant growth. The 21 crops are:

• corn for grain
• corn for silage
• sorghum for grain
• sorghum for silage
• winter wheat
• durum wheat
• other spring wheat
• soybeans
• oats
• barley
• cotton
• tobacco
• Irish potatoes
• sweet potatoes
• rye for grain
• sugar beets for sugar
• grass silage
• alfalfa hay
• small grain hay
• sorghum hay
• other tame hay

Application of Manure on Pastureland

There were two categories of pastureland enumerated in the Agriculture Census: 1) cropland used for pasture, and 2) pastureland and rangeland. Data from other sources were used to exclude rangeland acreage. It was assumed that 100 percent of the cropland used for pasture would be accessible for application of manure, and 50 percent of the pastureland would be accessible. In the simulation, nutrients from manure were applied at a rate considered appropriate for plant growth assuming the land was being grazed--75 pounds per acre for manure nitrogen and 30 pounds per acre for manure phosphorus.

Manure Nutrient Availability Relative to Crop Uptake/Removal and Pastureland Application

The manure nutrient availability and the plant nutrient uptake and removal in pounds were aggregated over livestock types and crops by county, and the ratio of availability to uptake/removal was calculated. A ratio greater than one suggests that the quantity of nutrients in manure exceeds the potential for crop uptake and pastureland application in the county, assuming all the acreage included in the calculation could be used for disposal. Ratios were converted to percentages, and the following four categories were used to make the accompanying maps:

• Less than 25 percent of nutrients taken up and removed by crops (or applied to pasture) that could be supplied from manure. For the county as a whole, these quantities of manure nutrients would not be considered to cause livestock waste problems since the capacity of the cropland and pastureland in these counties to use nutrients exceeds the production of manure nutrients by a factor greater than 4.
• 25 to 50 percent of nutrients taken up and removed by crops (or applied to pasture) that could be supplied from manure. The capacity of the cropland and pastureland in these counties to use nutrients exceeds the production of manure nutrients by a factor of 2-4. This is generally greater than the percentage of cropland reported in farmer surveys that is used for disposal of animal wastes. For example, surveys report that in major corn producing states, only 15-20 percent of corn acreage has manure applied to it; percentages are much lower for other crops. Some problems related to livestock waste could occur in these counties.
• 50 to 100 percent of nutrients taken up and removed by crops (or applied to pasture) that could be supplied from manure. The capacity of the cropland and pastureland in these counties to use nutrients exceeds the production of manure nutrients by only a factor of 1-2. Since it is not economical to haul waste long distances, it is unlikely that all the available capacity can actually be used, and thus it is likely that these counties have significant animal waste related problems.
• 100 percent or greater of nutrients taken up and removed by crops (or applied to pasture) that could be supplied from manure. In these counties, the capacity of cropland and pastureland to absorb nutrients without buildup in the soil is exceeded by the quantity of manure nutrients produced by confined livestock. Significant livestock waste related problems are expected in these counties unless producers are treating a substantial portion of the animal manure.

Counties with less than 1000 animal units were excluded from the analysis.

Caveats

• Not all land that could be used for animal waste disposal was incorporated in the calculations. A few counties will have additional acreage of fallow land, idle cropland, minor crops, fruits, nuts, and vegetables that could be used for animal waste disposal.
• The analysis assumes that animal waste cannot be transported to adjacent counties. In some cases, it may be possible to economically transport some of the waste to counties with higher cropland and pastureland acreage.
• Application of commercial fertilizers, which is not included in the analysis, greatly increases the potential for water quality problems related to nutrients.
• The simulation cannot be used to conclude that counties with excess manure production are causing water quality or other problems, only that the potential exists.

SUMMARY OF RESULTS

Animal Units

Map 1 shows the spatial distribution of estimated total animal units from which manure is recoverable (predominately confined livestock) for 1992. The spatial distribution of animal units for each of four livestock types comprising the total (beef, dairy, swine, and poultry) are shown in maps 2, 3, 4, and 5. Beef animal units were the highest, followed by dairy, swine, and poultry. Changes in animal units over time are shown in figure 1.

(Note on the map legends: In the process of converting maps to the World Wide Web the legends on certain maps became unreadable due to their small size. As a temporary solution the legends are presented in a separate file until we can update the maps. Simply look up the map number to find the appropriate legend.)

Manure Nutrient Production

Estimated manure phosphorus and nitrogen production for 1992 are shown in maps 6 and 7, respectively. Changes in manure phosphorus and nitrogen production are shown in figures 2 and 3. In the earlier years, dairy was the predominant source of manure nutrients from confined livestock. In recent years, however, poultry was the predominant source.

Potential for Manure Nutrients to Exceed Plant Uptake and Removal

Maps showing the four categories of the ratios of manure nutrient availability to plant uptake and removal, including pastureland application, are presented for each of the 10 years. Phosphorus maps for 1949 through 1992 are shown in maps 8-17, and nitrogen maps are shown in maps 18-27. Heaviest concentrations of counties with a potential for livestock waste related problems were in the northern part of the midwest, along the coast in the northwest, counties in the southwest, and most counties in the northeast in the earlier years. In the later years, concentrations moved into the southern states and the potential decreased in the northern midwest and the northeast and intensified in the northwest and the southwest. The midwest, where a large amount of manure nutrients is produced, was largely devoid of potential problem counties because of the abundance of cropland available for disposal.

There were more counties with potential phosphorus excess than potential nitrogen excess in all years (figures 4 and 5). The total number of counties where manure phosphorus production was high relative to plant uptake and removal decreased from 1949 to 1992, but the number of counties where manure phosphorus exceeded the capacity of the crops and pastureland to assimilate the phosphorus increased, and became more concentrated in the southern states. A similar trend occurred for nitrogen, but not as pronounced.

The number of counties where manure nutrients exceed potential plant uptake and removal is a useful indicator of trends in the potential for livestock waste related problems. This indicator is shown in 6figure 6. To reverse these trends, livestock production would need to move back to areas of the country that have sufficient cropland available for application at rates that will not result in excessive build-up of nutrients in the soil.

Watersheds with High Potential for Soil and Water Degradation

County data for 1992 were converted to a watershed basis to identify watersheds that are most at risk from manure nitrogen or phosphorus leaching and runoff. Conversion weights derived from the Natural Resources Inventory for 1992 were used to obtain a weighted total of the ratio of manure nutrient availability to nutrient uptake and removal for 8-digit hydrologic units (cataloging units). Watersheds with a weighted total ratio greater than 0.5, which are the watersheds most likely to have animal waste related problems, are shown in map 28 for manure phosphorus and map 29 for manure nitrogen.

NEXT STEPS

• The time series will be extended to 1997 when the 1997 Agriculture Census is released later this year.
• Crop production statistics incorporate yield variability that is due to weather. Rather than base the calculation on reported production, the calculation will be based on average yields to provide more consistent estimates over time and space of the potential capacity within a county to assimilate livestock wastes.
• Data for some crops and livestock types were not available for all years of the Agriculture Census. For the present study, data gaps were filled using interpolations or data from other years. Future work will incorporate data from a variety of other sources, including NASS county and state data on livestock inventories, crop acreage, and crop yields.
• Using other data sources in addition to the Agriculture Census, an attempt will be made to construct an annual series.

APPENDIX: List of Figures and Maps