Nutrients Available from Livestock Manure Relative to Crop Growth Requirements
Note: Research from this paper has been updated in Manure Nutrients Relative to the Capacity of Cropland and Pastureland to Assimilate Nutrients.
Charles H. Lander, David Moffitt, and Klaus Alt (retired)
U.S. Department of Agriculture, Natural Resources Conservation Service
Resource Assessment and Strategic Planning Working Paper 98-1
OVERVIEW AND SUMMARY OF RESULTS
"Livestock and poultry manure is a valuable resource which may be used to supplement or replace inorganic fertilizer or livestock feed. It may also be used for energy production or other purposes. Ineffective use may result in waste of manure and nutrients or pollution from improper storage and disposal."
Van Dyne and Gilbertson, 1978.
These statements made by authors Donald L. Van Dyne and Conrad B. Gilbertson (1978) in a U.S. Department of Agriculture report, Estimating U.S. Livestock and Poultry Manure and Nutrient Production, are still true in 1998. Nutrients in animal manure remain a valuable resource, and ineffective use or storage of manure results in a waste of this nutrient resource and a potential source of pollution.
The years since 1978 have seen changes in the complexion of America's animal production industry. The changes have included shifts in geographic locations where specific types of animals are produced, expansion of confinement production techniques, integration and concentration of some of the industry, and the trend toward geographic separation of animal production and feed production operations.
The U.S. Department of Agriculture (USDA), under the leadership of the Natural Resources Conservation Service (NRCS), assessed recent characteristics of the U.S. cropland agriculture and animal production activities. A 1996 report, Nutrient Use in Cropland Agriculture, documents information about crop production, the sale and use of commercial fertilizers, growth and shifts in animal production, trends in the use of nitrogen and phosphorus on selected crops, and the status in 1992 of the use of nitrogen and phosphorus on 36 crops for which detailed information was available (Lander and Moffitt, 1996).
In 1996, NRCS working with the Economic Research Service (ERS) and the Environmental Protection Agency (EPA), initiated a study to determine the degree to which the nutrients in manure from confined livestock operations could potentially satisfy crop nutrient requirements. For the purposes of this report, "confined livestock" includes livestock produced in confinement, or under any other type of operation where manure typically is recoverable, i.e., collected, stored, and available for use as a resource.1 Manure, as it is used in this report, refers only to the manure produced under operations where the resource is recoverable.
Nutrient availability from manure and crop nutrient uptake and removal were estimated for each county in the 48 states. Sixteen livestock population types and 78 harvested crops (including hay crops) and pasture were included in the analysis. From these estimates, nitrogen and phosphorus availability relative to crop need was calculated for each county. Three animal manure application scenarios were simulated:2
Non-legume crop and hay production.
Non-legume crop and hay production and pasture.
Non-legume and legume crop and hay production and pasture.
In the first scenario, it was assumed that only non-legume crops and hayland were available in the county for manure utilization. In the second scenario, the land base was expanded to include pasture. In the third scenario, the land base was expanded further to include legume crops.
In the non-legume crops and hay simulation, manure phosphorus availability exceeds the plant uptake and removal potential in 485 counties (Table 1). As would be expected, the potential for phosphorus or nitrogen to exceed crop uptake and removal decreases as the breadth of the evaluated land base increases. When the crop system of non-legume and legume crops and hay, and pasture is evaluated, only 107 counties fall into the category of having manure phosphorus in excess of plant uptake and removal potential. This translates to a nearly 78 percent decline in the number of counties where phosphorus from manure exceeds uptake and removal potential of all crops, hay, and pasture produced in the county.
Table 1: Number of Counties Where Nutrient Availability from Manure Exceeds 100 Percent of Crop System Need
Non-legume crops and hay
Non-legume crops and hay, and pasture
Non-legume and legume crops and hay, and pasture
The same relationship holds true for nitrogen. However, manure nitrogen availability exceeded crop system uptake and removal potential in fewer counties than did phosphorus. In the narrowest simulation, manure nitrogen availability exceeded the uptake and removal potential of non-legume crops and hay in 266 counties. In the broadest simulation, only 35 counties show manure nitrogen availability exceeding the uptake and removal potential of crops, hay, and pasture. This translates to a nearly 87 percent decline in the number of counties where nitrogen from manure exceeds uptake and removal potential of all crops, hay, and pasture produced in the county.
Where counties appear with a nitrogen excess, they also have a phosphorus excess; while the reverse is not true. For the narrowest simulation, 54 percent of the counties ranking in excess for phosphorus also had a nitrogen excess; for the broadest simulation, only one-third of the counties ranking in excess for phosphorus also had a nitrogen excess.
These relationships demonstrate that even when simulating the broadest range of potential crop uptake and removal, some counties still have excess nutrients from manure. Yet, it also reveals how substantially the increase or decrease of the land area evaluated can affect the percentage of the nutrient resource that can be utilized at agronomic rates.
These simulations reflect potential rather than actual nutrient use since producers may manage manure in ways other than to meet crop nutrient needs. Further, it does not imply that in areas where nutrient availability does not exceed crop nutrient needs that the nutrient resource is being managed properly. Improved nutrient management approaches, alternatives to land application (e.g., production of energy or other products, treatment), and conservation practices will be particularly important to reduce the potential for pollution where nutrient availability from manure exceeds crop needs. The primary value of simulating manure nutrient source and crop uptake and removal capacity by county is as a decision-making tool to help identify potential problem areas, to target resources to them, and to work toward solutions when problems are identified.
Data from the 1992 Agricultural Census were used to estimate livestock populations, manure production, and crop nutrient utilization.3 The Census of Agriculture provides the best source of consistent, county-level statistics on agricultural operations throughout the United States. Estimates were not made for Alaska, Hawaii, or U.S. commonwealths, territories, or freely associated governments. (Detailed documentation, including parameter estimates and example calculations, is provided in the Appendix.)
Nutrient Availability from Manure
The amount of recoverable nitrogen and phosphorus associated with confined livestock manure was calculated for each farm based on tons of manure produced and the nutrient content of the manure. Livestock populations were adjusted to reflect the number held in confinement using confinement factors originally published in the Van Dyne and Gilbertson report (1978) and updated by NRCS. The 16 livestock population types for which calculations were done include: COWS- young calves (beef and dairy), grass-fed (beef and dairy), heifers (beef and dairy), feedlot animals, breeding herd (beef and dairy); HOGS- breeding hogs, feeder pigs; CHICKENS- layers (hens and pullets), broilers; and TURKEYS- breeding turkeys, turkeys for slaughter.4
The Agricultural Census numbers do not provide for an average inventory of livestock, rather the inventory variable in the Census files reflects the inventory on December 31 of the census year. Thus, for this analysis, the average livestock numbers were derived from a combination of sales over the year and end-of-year inventory (if both numbers were available) or from either sales or inventory if only one value was available. This allowed for manure credit estimates on farms that raise livestock sporadically during the year but have no inventory on December 31, or farms that ceased production during the census year.
Total nutrient availability was estimated for each of the 16 livestock types by multiplying the average confined livestock population (in animal units)5 by the number of tons of manure produced by each type of livestock by the recovery factor. This result - tons of recoverable manure - was multiplied by the number of pounds of nitrogen or phosphorus contained in one ton of manure to compute the total nutrient pounds available. The resulting value was adjusted for typical nutrient losses that occur during storage and handling to generate an estimate of total available nitrogen and phosphorus from confined livestock manure, by farm. Estimates for individual farms were aggregated to a final value for the county.
Crop Nutrient Uptake and Removal
Total nutrient uptake and removal was estimated for each of the crop production categories analyzed (Table A-1 in the Appendix). On cropland and hayland, where production and yield data were available, nutrient uptake and removal was estimated based on the total yield and the nitrogen and phosphorus content of the harvested biomass. The total yield of each harvested crop was obtained from the 1992 Agricultural Census. Plant nutrient content values were calculated for each agronomic and forage crop based on a literature search of published sources of plant nutrient composition. It was assumed that plant residue was not removed from the field unless it was routinely considered part of the harvested material. For pasture, the nutrient uptake and removal was based on a supplemental nutrient application rate that was adjusted to reflect the nutrients returned by defecation of grazing animals.
Nutrient Availability Relative to Crop Need
A ratio of nutrient availability from manure to crop nutrient uptake was used to estimate the proportion of crop nutrient utilization that could be met or exceeded by the nutrients available from livestock manure produced in that county. Where ratios were greater than 1 (equivalent to 100 percent), the nutrients available from manure produced in the county exceed typical plant uptake and removal potential.
Nutrient use efficiency factors were established for nitrogen and phosphorus. These factors were based on the estimate of the amount of nutrient that would be available for plant uptake and utilization after manure was applied to the land. For nitrogen, an efficiency factor 0.7 pounds was used. For phosphorus, an efficiency factor of 1.0 was used.
The estimated application rates do not account for potential nitrogen volatilization losses after land application. Application rates did not account for variability of soils and their ability to hold and release nutrients, or the background levels of nitrogen or phosphorus that might have been present in the soils. Vegetable, citrus, and nut crops were not included in the evaluation, thus, this study could underestimate nutrient use potential in counties with large amounts of such production acreage (e.g., counties in lower California show excess nutrient availability although they contain large areas of cropland in fruit and vegetable production). Other caveats include:
These simulations can not be used to conclude that: 1) all manure is currently being land applied, 2) counties with simulated nitrogen and phosphorus excesses (i.e., above 100 percent values) either already experience or will experience water quality impairment from nitrogen or phosphorus moving from farm fields to the broader environment, 3) counties with no simulated nitrogen or phosphorus excesses (i.e., less than 100 percent values) have no potential for water quality impairment from nitrogen or phosphorus moving from farm fields, or 4) all land in a county is at risk of nutrient build-up. These simulations can be used to conclude that in counties with simulated nitrogen or phosphorus excesses, some land in the county is susceptible to nutrient overapplication, nutrient buildup, and potential release of nutrients to the broader environment if land application of manure is the primary way manures are used.
The analysis assumes that all of the land identified for each simulation is available for land application, and that manure cannot be exported from one county to another.
These estimates account only for nutrients available from manure produced in the county and do not include estimates of the amount of nitrogen or phosphorus from other sources such as application of commercial fertilizers, atmospheric deposition, or mineralization of soil organic matter.
The simulations do not account for site vulnerability or other factors that influence the environmental fate and transport of nitrogen or phosphorus, such as nutrient application rates and timing, soil type, climate, tillage and pasture management practices, and other conservation practices.
This analysis does not reflect the changes in the crop or livestock sector since 1992.
This analysis accounted for 78 harvested crops (including hay crops) and grazed pasture acres. Idle or failed acres were excluded, as were acres in a conserving use or in cultivated fallow. Inclusion of these acres would increase the estimated land base on which manure nutrients could be applied and reduce some of the "percent crop nutrient need exceeded by manure application" estimates.
Estimates are provided only for 1992, and may not be representative of other years. Yields vary substantially from year to year because of weather. Since the calculation of excess nutrients depends critically on yield, estimates may vary substantially from year to year.
Each of our maps is available as a zipped postscript file. These files can be used to produce high quality copies of our maps. For more infomation see our postscript help page.
Maps 1 [GIF | Postscript] and 5 [GIF | Postscript] present the estimates, by county, of nitrogen and phosphorus content from confined livestock manure. Maps 2 [GIF | Postscript], 3 [GIF | Postscript], and 4 [GIF | Postscript] present the simulation results on the potential for manure nitrogen to meet or exceed the crop utilization for the three different cropping systems, and maps 6 [GIF | Postscript], 7 [GIF | Postscript], and 8 [GIF | Postscript] present the simulation results on the potential for manure phosphorus to meet or exceed the crop utilization for the three different cropping systems.
The eight graphics provide a snapshot of livestock production and associated nutrient availability in 1992; and where the levels of production could pose potential production, management, or environmental concerns from excess nutrient supplies. The strong assumptions embedded in the analytical model (see caveats) mean that this information provides only a general tool for identifying areas where nutrient availability from manure may exceed crop uptake and removal.
In 1992, the supply of nutrients from manure alone exceeded crop nutrient requirements in several counties. In such counties, the land application of manure at rates that exceed plant nutrient requirements can result in the accumulation of nutrients (particularly phosphorus) in the soil and an increased potential for these nutrients to become pollutants. This analysis shows that the relative potential for phosphorus from confined livestock manure to meet crop nutrient needs was substantially greater than that of nitrogen.
Susceptibility to problems from high manure nutrient concentrations varies by region, in part because of agricultural structure characteristics (e.g., farm size, geographic separation of animal production from feed production), but also because of other factors such as manure nutrient application rates and timing, soil type, climate, tillage and pasture management practices, and conservation practices. Nitrate nitrogen is highly mobile and has a high potential to leach below the root zone into groundwater, volatilize through denitrification into the atmosphere, or be carried overland in runoff to streams, lakes or other waters. Phosphorus is not as mobile as nitrate nitrogen and tends to be carried on soil particles that move off the field because of erosion. However, as the phosphorus content in the soil increases, the potential for the element to move off the field in solution also is increased.
As livestock production becomes more geographically concentrated, the challenge of using manure effectively, while minimizing adverse environmental effects, becomes more complex. Decision makers will have to grapple with development planning questions; agribusiness and technology developers will have to focus on improved on-farm manure management practices and alternative manure uses; and conservationists will need to dedicate effort to working with producers to reduce potential adverse impacts through the application of conservation systems.
The authors would like to thank Barry Kintzer, Robert L. Kellogg, and Patricia Durana for their analytical and editorial support; Dennis Thompson and Keith Tichnor for their technical input; Richard Huzil and Susan Wallace, for their GIS support; Pei Jin for data collection and programming support; and their many colleagues from NRCS, ARS, ERS, and EPA for their valuable review of the draft material.
Lander, C.H., and Moffitt, D., "Nutrient Use in Cropland Agriculture (Commercial Fertilizer and Manure): Nitrogen and Phosphorus," Working Paper No. 14, RCAIII (Washington, DC: U.S. Department of Agriculture, Natural Resources Conservation Service, February 1996).
U.S. Department of Commerce, Bureau of the Census, 1992 Agricultural Census data. [NOTE: this function was recently moved from DOC to USDA, National Agricultural Statistics Service]
Van Dyne, D.L., and Gilbertson, C.B., Estimating U.S. Livestock and Poultry Manure and Nutrient Production, U.S. Department of Agriculture, Economics, Statistics, and Cooperatives Service, ESCS-12 (Springfield, VA: National Technical Information Service, March 1978).