Using Manure Characteristics to Determine Land-Based Utilization

By: David C. Moffitt and Charles Lander

INTRODUCTION

United States Department of Agriculture (USDA) in general, and the Natural Resources Conservation Service (NRCS) in specific, are required to conduct a periodic assessment of the state of the nation's agricultural resources, commonly referenced as the Resource Conservation Assessment (RCA). These assessments vary in degree of complexity and the number of resources assessed, but generally focus on the five resources; soil, water, plant, animal, and air.

During planning for the Third RCA, the leadership of NRCS requested a comprehensive look be taken at the impacts of animal agriculture on the nation's resources. With the rapid expansion of the poultry and swine industry, both at traditional locations and into new locations, there was a general impression that adequate land resources for manure utilization could be a limiting factor [ 4 ].

The discussion that follows describes the analysis process used to relate manure nutrient availability to the land resource. The focus of this paper will be on the development of the manure coefficients used in the analysis, but enough of the process will be described to give the reader the concept as to how the manure data was used. A further explanation of the development of the manure coefficients is contained in an internal NRCS document [ 5 ].

DESCRIPTION OF ANALYSIS PROCESS

The analysis consisted of two primary tasks at a county level; determining the amount of manure nutrients available for plant growth, and determining the amount of nutrients removed by plants where adequate nutrients are available to optimize yields. The amount of manure nutrients were matched with plant nutrients removed, and the results expressed on a county basis as a percentage of nitrogen, phosphorus, and potassium potentially supplied by manure. Table 1 is an example of how the results were displayed. The decision was made at the beginning of the analysis that manure application would only be considered for agricultural production that would normally receive nitrogen fertilizer applications; namely non-legume crop, pasture, and hayland.

The 1992 Agricultural Census [ 11 ] was used as the source of animal numbers and acres of non-legume crop, pasture, and hayland in each county. The expected yields and nutrients removed from agricultural production were determined in a separate effort [ 10 ] .

Manure nutrients were determined specific to each type of livestock multiplied by the average number of that specific livestock for the census year. The analysis excluded farms where livestock were raised specifically for family pleasure or consumption. These were assumed to be small operations with few water quality impacts and little opportunity for improvement.

The Census data unfortunately, are not average livestock production values for the year, but rather are the inventory on December 31. For the RCA study, the average livestock numbers for the year were derived from a combination of sales over the year and end of year inventory, if both were available, or from either sales or inventory if only one was available. This allowed manure estimates for operations that had no inventory on December 31 but produced livestock at some time during the year. The process for converting the Census data to average production numbers is documented elsewhere [ 1 ] and will not be discussed here.

MANURE NUTRIENT COEFFICIENTS

The selection or determination of appropriate manure coefficients for the analysis involved a three step process. First, the manure composition "as excreted" is determined from available data bases; second, a regional estimate of the amount of "as excreted" manure that is recoverable is determined as a percentage; and third, an estimate of the amount of nutrient lost in collection and handling is applied. The resulting coefficients are a product of the three steps. Each of the above steps will be discussed below.

MANURE CHARACTERISTICS DATA BASES

The types of livestock and poultry identified in the Agricultural Census tracked in our analysis are as follows:

Beef and Dairy
Mature beef cows
Mature dairy cows
Heifers and heifer calves
Cattle and calves (steers, steer calves, bulls and bull calves)
Fattened cattle
Hogs and Pigs
Hogs used for breeding
Other hogs and pigs
Poultry
Broilers and other meat type chickens
Layers
Pullets over three months
Pullets under three months
Turkeys for slaughter
Turkeys for breeding

Available data bases do not lend themselves directly to these livestock and poultry categories. In preparing the tables that follow, we assumed the category "hogs used for breeding" were composed of gestating and lactating sows as well as boars. "Other hogs and pigs" were assumed to be "grower" pigs listed in the typical data bases. There is some confusion as to the definition of a "pullet". The definition and use of the term varies across the country and we surmise the response to the category on the Agricultural Census form also varied. For the purpose of this analysis we assumed pullets are replacement hens for breeding as will be discussed below. "Broiler and other meat type chickens" were assumed to be broilers.

Sheep, goats, horses, and ponies had been identified as important in an earlier effort and considered for this analysis. Sheep and goats are locally important livestock, but not a major contributor to the overall confined animal manure problem in the United States due to a large number of the animals in a pasture setting. Sheep and goats are included (listed as sheep) in this documentation since the percent of recoverable manure was reported by the states; however, the category does not appear in the final tabulation sheet.

Many different data sources have been used over the years by various individuals and groups for a variety of purposes. These data sources may be from results of a single manure analysis, or more likely, the compilation of the results from numerous samples. In any event, there are several factors to be considered in selecting a data base. This will be briefly discussed to set the stage for the data bases selected. Some of the factors include; the age of the data, the number of observations used to compile the data, the range of conditions represented by the observations, and the expertise of those who collected and analyzed the samples.

Age of the data

The past few decades has seen a revolution in animal husbandry. These changes are observed not only in the selective breeding of the livestock and poultry to maximize desired results, but also in the preparation and feeding of selected rations. These more productive animals have resulted in a change in waste characteristics. Generally a 1000 pound animal of a specific category produces more manure with more nutrients than did a similar animal two decades ago. A data base that reflects manure characteristics from animals grown in the past may not be representative of current situations.

Number of observations

Waste characteristics based on a single observation or sample may or may not be representative based on some of the factors which will be discussed below. While there is no rule or standard to require a certain number of samples before the data is reliable, one can assume the larger number of samples used to compile the data base, the more precise the resulting values will be.

Range of conditions

The use of waste characteristic data should be considered when selecting a data base. Data needed for a specific waste management system plan and design should be localized; from the specific operation if possible. This data is generally not available. The NRCS has the rule; "Use local data where possible." This seems to be the best when working with specific livestock operations.

When going beyond the local situation, the range of conditions comprising the data base should also be expanded. Data from one farm will probably not be as representative of a county as observations from several farms in the county. Data from one county will not be as representative of a state as data from several counties; etc. As a general rule, the data base should be as representative as possible of the conditions to be encountered in the use of the data. Variability in livestock type, climate, and management strategies, including feed, should be considered.

Expertise of those who collected and analyzed the manure samples

There is an art to collecting representative samples of manure. Proven sampling protocols will most likely produce the most representative samples. The skill of the person doing the sampling is equally important. One who samples on a regular basis will most likely follow a set procedure that will produce consistent results. This discussion could also focus on "operator collected" samples as compared to "professionally collected" samples; however no such focus is intended. Whoever the sampler is, the one that has the skill in sampling will most likely collect the most representative sample.

Manure sample analysis is unique in comparison to the typical water and wastewater analysis. First of all, the concentrations of constituents in the manure such as nutrients, basis of daily (BOD), etc. are normally much higher than for municipal waste water. Tests designed for the much smaller concentrations of some characteristics are not appropriate for livestock manure. In addition, the heavy concentrations of some constituents can interfere with the determination of others. Only a lab technician trained in manure analysis can be expected to produce consistent results. Before manure characteristic data bases can be used, the users must assure themselves that the samples for the data base were properly collected and analyzed.

Data base selection

One of the concepts originally considered in preparing to develop regional or state manure characteristics was to solicit the states for their locally collected data and to statistically examine the data. One of the first drawbacks to this approach (beyond the time required) was how to examine or combine a great deal of data assembled or displayed with different assumptions. For example, how would data from one state which consisted of the actual or "raw" data from 200 different observations be combined with the data from another state or locality where the data from 200 observations were combined and an "average" value was submitted?

Several specialists in NRCS State Offices were contacted to determine the source of manure data used in their planning and design of animal manure management systems. Of those contacted, none had locally developed numbers (where the actual samples and analyses were limited to the locality of the preparer). NRCS specialists often referenced data from state universities, but in many cases these proved to be local adaptations of nationally available information. With the uncertainty of the proper procedures for weighting local data bases and with limited time available to prepare the material, the approach used was to stick with nationally available manure characteristic data, and to combine these with regional estimates of recoverable manure. The four data bases selected for further consideration were from the Midwest Plan Service (MWPS) [3], NRCS [8], American Society of Agricultural Engineers (ASAE) [2] and North Carolina State University (NC State) [6]. All these data bases showed manure characteristics in English units which will be used in the context of this paper.

The genesis of the NRCS data base was a data base search contract with Dr. Clyde Barth, who then was with Clemson University. The contract was in the late 1980's, which indicates the NRCS data values are at least ten years old. The ASAE data is examined every five years for "correctness" and is reaffirmed by the Society "as is" or is revised. The North Carolina (NC) State University data carries a 1990 date. One of the advantages of the NC State data is that it lists the number of observations for some animal types used to arrive at the data. The four data sets appear to be national in scope. We can't be certain of the sample collection and analysis expertise that went into the data, but we are assuming the contributors to the data were screened to assure a minimum quality of information.

In examining a tabular display of all four data bases, the values were, in most cases, remarkably similar. In one or two cases there was dissimilarity, but the weights of the animals in the sampled group explained the differences. There is the likelihood the data bases have some common data sources. With this in mind, averaging the values from all the data available seemed to be appropriate. Some of the values listed in the data sets appeared to be out of the range of similar data in the other sources. Where we could find similar values for the questionable data in other reports, we left the values intact.

Average livestock and poultry weights

One of the prerequisites in this analysis is agreeing to the appropriateness of "average" weight for an individual livestock or poultry. Where the animal is taken essentially from birth to market in the same operation, a simple average between birth weight and market weight seems appropriate and was used in this analysis for categories such as broiler and turkey. For other animal groups the decision was not as easy. Three sources of information were used. One source was, "Livestock and poultry waste management - a national overview," by Dr. John Sweeten of Texas A&M [ 7 ]. A second source of information on average weights was the NC State data base already discussed. One of the items listed in support of the manure characteristics was the average weight of the animal contributing to the data base. The final data source was the 1994 set of factors developed for the "Status and Trends" report of the NRCS RCA effort [ 10 ]. The 1994 NRCS values were determined primarily by assuming average weights from birth to market for most animal categories with allowances for mature animals for breeding, laying, milking, etc. The resulting values were coordinated among USDA agencies and discussed with industry spokespersons where available.

For this analysis the 1994 NRCS data was the starting point for determining average weights. Where the NC State data could help, such as classifying hogs and pigs as growers or breeders, average weights by sub category were established. Dr. Sweeten's work was used as a check for the categories as a whole. Knowing average weights and approximate numbers of animals from Sweeten's paper, the average weights and numbers in each sub category in this data base should correspond to the category as a whole. The results of the "weighting" was relatively close to Sweeten's values and better than the 1994 NRCS data.

Nutrient values

The nutrient values in the MWPS data and the NC State data were reported as fertilizer equivalents rather than elemental form. The combined or molecular weights were converted to elemental Nitrogen (N), Phosphorus (P), and Potassium (K) before they were tabulated. In the case of nitrogen, the reported value is total Kjedahl nitrogen (TKN). By definition TKN does not account for nitrogen in the nitrate-nitrite form. But since the data is for fresh manure, very little nitrate would be expected. The value of TKN was used as total nitrogen.

Length of production year

The length of the production year was set at 365 days for all livestock categories which tends to over estimate manure production on an annual basis for livestock groups, such as broilers, that have multiple growth cycles per year. However, since total livestock and poultry sales were used to determine average animal production in a county, use of the 365 day year is mitigated to some extent. The "down" time between production cycles is variable depending on market pressures and the availability of birds to restock. The down time is normally a matter of days, but can be a week or more. The individual producer often has little control of the down time.

Tabular values

Manure characteristics in the four data bases were expressed in terms of pounds produced per each 1000 pounds animal weight per day. To convert the "as excreted" manure production in units shown in Table 2, the "as excreted" manure value was converted to manure production as tons per year per 1000 pound animal unit (AU) and nutrient values were converted to pounds of elemental nutrients per ton of manure. These values correspond to nutrient content (as excreted) values in Table 5 for selected livestock types. The term "1000 pound animal unit" is defined as 1000 pounds of animal weight.

Table 2 average values of manure characteristics are in a format conducive to use in a spread sheet with animal numbers from the Agricultural Census. The animal numbers from the Census should be divided by the values in the column " # of animals per AU" to arrive at the number of animal units. This value is multiplied by the "Tons manure per AU" column to arrive at manure quantity. The pounds of each of the nutrients can be determined by multiplying the appropriate value from "lb nutrients per ton manure" times the tons of manure.

Animal categories listed in Table 5 but not included in Table 2 include "number of fattened cattle sold," "pullets over three months, but not laying inventory," "pullets under three months inventory," and "turkey hens for breeding inventory." The category of "Number of fattened cattle sold" is a sub-category of "Steers, calves, bulls and bull calves." From definitions and information provided in the data bases, it is not obvious where to draw the line between "steer" and "fattened cattle" except possibly by weight with the fattened cattle category at the high end of the steer category. The data bases did show some differences in the weight of manure produced between steers and fattened cattle, but the nutrient content in terms of pounds per 1000 pound animal units was essentially identical. Since arriving at total nutrients is the desired outcome of this activity, the decision was made to use the same manure characteristics used for the category of "Number of fattened cattle sold" as used for the category of "Steers, calves, bulls and bull calves."

There was not a great deal of information on pullet manure production in any of the data bases used. The NRCS Agricultural Waste Management Field Handbook (AWMFH) Table 4-14 [ 8 ] was the only data base that even listed the category. A typical "dictionary" definition is a young hen. In the common vernacular of the poultry industry, a pullet is a replacement layer, either to be used to lay eggs for food consumption or to lay eggs for hatchery production. Generally, the pullet is fed a maintenance diet, which is reflected in the manure characteristics. For the purposes of these characteristics, a pullet older than three months was assumed to have the average weight of the layer; a pullet less than three months, the average weight of the broiler. The category of turkey hens for breeding was included in the North Carolina State University data with an average weight of 20 pounds. The manure characteristics for pullets and breeding turkeys was converted as described above and included in Table 5.

RECOVERABLE MANURE

Estimates of recoverable manure shown in Tables 3 and 4 were prepared using the responses to a 1994 NRCS questionnaire [ 9 ] tempered with clarifying information from phone contacts and will be discussed below. The reference most commonly used for recoverable manure is Van Dyne and Gilbertson [ 12 ]. This twenty year old reference was prepared over a period of years with much input from others in the agricultural community. While it is desirable to update the Van Dyne and Gilbertson study in its entirety, funds nor time were available to do so.

Figure 1 -- Map of NRCS Regions (472 KB)

Van Dyne and Gilbertson used the term "recoverable" in terms of economic feasibility which could not be adequately defined in today's setting. When asking the NRCS specialists in each state to determine economic feasibility of recovery, we described feasibility in terms of "What would it be practical to recover?" Recovery was expressed in terms of solids content, not weight, to avoid having to continually account for changes in moisture content. Even so, the quality of the states' responses to the questionnaire were highly variable, and not every state responded. Only the data that appeared to us to be "reasonable" was included in the regional summary. See Figure 1 for a map of the NRCS regions. The following procedure was used to prepare the final values for the percent of recoverable manure by animal type by region shown in Table 3. Most of the states' responses were used as directly.

1. State values for recoverable manure for each animal type were estimated independently using the written responses to other questions in the questionnaire relating to waste management systems.
2. Where there appeared to be a large discrepancy between state reported recoverable values and those that would likely correspond to the systems described, members of the NRCS State Conservation Engineer's Staff were contacted to discuss appropriate values.
3. Where state contacts confirmed a large discrepancy (> 10 percent of reported value), the reported percentage of recoverable manure was adjusted in the spreadsheet with concurrence from the states.
4. The animal numbers for states not responding to the questionnaire were used to determine if animal numbers from non-reporting states could negate the regional weighted value.
5. In a few instances for specific animal types it appeared the weighting process would not be correct without data from non-reporting states. Contacts were made with these states to get information on types of waste management alternatives commonly used and an estimate of recoverable manure.
6. Percentage of recoverable manure in each region for each animal type was determined by weighting the state values in reporting states with the number of animals in each state as reported in the Agricultural Census.
7. Adjustments in the resulting regional value were made to compensate for non-reporting states and for reported values not adjusted as mentioned in step 3. above.
8. The final regional value of percentage recoverable manure by animal type was weighted by total animal numbers in the region (reporting and non-reporting states) to arrive at a value for percentage of recoverable manure for the nation.

Recoverable manure from beef animals

The categories considered as "beef" include heifers, steers and bulls, and beef cows. The beef categories to be used for waste characteristics were different in that most of the beef sub-categories spend a portion of their life cycle in a grazing situation The concept was that at any given time a certain number of animals are grazing and others are in feedlots. The challenge was to develop a "recoverable" factor that integrated these two periods.

Again the state responses to the manure management questionnaire was examined to see if the states had provided clues as to how to proportion the manure between grazing and feedlot animal numbers (time). While several pointed to the situation, none offered a percent of animals or time in confinement or grazing that could be used for weighting. As was discussed above, the only reliable source of information appeared to be the states. With a limited number of return calls to selected NRCS State Offices, a percent confinement was assigned for each region for the category of beef.

Even with the input of our NRCS field staff, we weren't comfortable with the partition between grazing and confined animals. While re-reading Van Dyne and Gilbertson, we noted a set of tables that presented separate manure quantities by grazing and confined beef. The obvious question was the validity of 1978 data in the current setting. With the lack of more recent definitive data, the division seemed appropriate. The partition between the amount of manure from grazing animals and confined animals is shown in Table 4.

The percent recoverable manure for the beef sub-categories was calculated by multiplying the beef values in Table 3 times the proportion of manure shown in Table 4 and dividing by 100. While it would be desirable to assign a different percentage recoverable value for each sub-category, the available information was not sufficient to support this level of detail.

During computations, the "recoverable" factors from Table 3 or 4 would be multiplied times the "tons of manure per animal unit" column in Table 5 to arrive at the value of recoverable manure. Nutrient value per ton would remain the same based on the assumption that the decrease in nutrients from "as excreted" to "recoverable" would mirror the loss of solids in the recovered manure. The assumption is environmentally conservative, and for phosphorus and potassium is relatively sound. For nitrogen, however, the assumption does present some problems, but when coupled with the nutrient loss factors discussed below, it appears to give reasonable estimates for nitrogen.

NUTRIENT LOSSES

The categories of nitrogen, phosphorus, and potassium content (after collection, storage, treatment, and transfer losses) were added to provide a mechanism for estimating the amount of nutrients that would be present in land applied manure and effluent. There is no "national" or even regional perspective on what these values should be. The estimates shown are based on a three part assumption that was valid at the time of our assessment, but may well change in the future:

• Nitrogen losses will exceed greatly exceed those of phosphorus and potassium primarily due to volatilization and denitification of nitrogen compounds.
• As the number of manure management systems increase with movement toward more automated components (flushing systems, etc.), the losses of nutrients, particularly nitrogen, may increase.
• Large amounts of phosphorus and potassium are present within the bottom sludge of lagoons and ponds, and even though the sludge are not removed on a regular basis, the P and K content must be considered in an application strategy; i.e. effluent nutrient composition may not reflect actual N, P, and K in the lagoon or holding pond.

Numerous individuals in USDA, universities, and industry groups were consulted to arrive at the "national" values of losses. The discussions focused on the types of manure systems typically used by the industry in different parts of the country, the losses typically associated with these systems as estimated by NRCS [8], and the portion of the nation's livestock raised in different parts of the country. Estimates of the losses of N, P, and K for each livestock type were expressed as a percentage of "as excreted" values and multiplied by the "as excreted" nutrient content to arrive at the values shown in Table 5.

APPLICATION

Figure 2 is a graphical presentation of manure nutrients (nitrogen in this case) calculated using procedures described above. The map is for NRCS's South Central Region, and the displayed values are in pounds. In this figure, the county data has been blended into a watershed format and displayed. Again, the information, while not definitive in describing a problem, indicates counties where additional investigation and management are warranted.

Figure 2 -- Available Manure Nitrogen by 8-Digit Hydrologic Units (370 KB)

Table 1 is developed using the manure nutrient coefficients described in this paper and the plant nutrient developed separately. While none of the counties listed in Table 1 show manure nutrients in excess of plant nutrient needs on the land normally receiving nutrients, nationally the analysis did show some counties with this situation. At least three counties in Table 1 show an appreciable part of the acceptable acreage available as being required for nutrient cycling. Since not all the farms have confined livestock and poultry operations, there may be reason for concern, particularly if the confined animal operations are clustered in a portion of the county. A presentation such as Table 1 identifies county situations needing further analysis and when they are validated, the analysis may identify areas for targeting resources.

MANURE MASTER is a nutrient screening tool assessable through the USDA-NRCS home page using the manure coefficients discussed above. The users describe their operation in terms of livestock numbers, acres of crops and pastures along with yields and type of manure application, and the tool indicates whether the producer has sufficient land to recycle manure nutrients produced by the livestock operation. If soil test recommendations are available, the producer enters the soil test results to calculate amount of nutrients required. This is only a screening tool, and more detailed field and manure information is needed to make final determinations of the adequacy of any land application strategy. The internet address of the NRCS Home page is http://www.nrcs.usda.gov, and MANURE MASTER is found as a part of "Technical Tools."

FUTURE NEEDS

The analysis described in this paper points to the need for a common data base of manure characteristics for common livestock and poultry types. We found enough similarities in the four national data bases listed above to raise the issue as to why the responsible groups don't coordinate their efforts and come to consensus on a national value. ASAE Committee SE-412 is trying to do just that. The proposed revision of ASAE D384, "Manure Production and Characteristics" attempts to identify a range of values for characteristics by major livestock and poultry categories and sub-categories that can be acceptable to all major groups. We commend ASAE and Committee SE-412 for their efforts.

We also see the need for on-going studies to determine the characteristics of manure produced by today's livestock and poultry. The most recent national data is approaching ten years of age, and may or may not represent current conditions. We see this being most critical when planning with producers where local data is not available, and the producer needs to make critical decisions as to the adequacy of the land base for the livestock or poultry enterprise.

Finally, we recognize our estimates of recoverable manure and nutrient losses in collection and handling of manure are just that - estimates. Detailed studies are needed to refine these factors for future assessments. We encourage state and federal agencies to partner with their university contacts in each state to develop a strategy for looking at factors such as recoverable manure and nutrient losses. Not only would the information be important during local planning, it would provide information for broad area planning that could lead to implementation dollars.

REFERENCES

1. Alt, Klaus, C Lander and DC Moffitt, Fertilizer nutrients from livestock manure-documentation of computation of manure credit estimates, Unpublished working document, USDA-Natural Resources Conservation Service, November 1996

2. American Society of Agricultural Engineers (ASAE), "D384.1 - Manure Production and Characteristics," ASAE Standards, D384.1 Revised December 1993

3. Midwest Plan Service, "Livestock Waste Facilities Handbook;" MWPS-18, Table 2-1 (1985).

4. Moffitt, David C., "Animal Manure Management," RCA Issue Brief, USDA-Natural Resources Conservation Service, Washington, DC, November 1995

5. Moffitt, David C., Documentation for manure characteristics (manure-2.xls, manure-3.xls, manure-4.xls), Unpublished working document, USDA-Natural Resources Conservation Service, Fort Worth, Texas, June 1996

6. North Carolina State University, "Livestock Manure Characterization Values From the North Carolina Database;" Selected tables from unpublished computer summaries (1990).

7. Sweeten, John, "Livestock and poultry waste management - a national overview," Texas A&M (1992).

8. USDA-Natural Resources Conservation Service, "Agricultural Waste Management Field Handbook;" Chapters 4 - 11, Washington, DC (1992).

9. USDA-Natural Resources Conservation Service, "Organic Waste: Production, Storage, Treatment, and Utilization," Questionnaire sent to NRCS State Offices (1994)

10. USDA-Natural Resources Conservation Service, "Status and Trends of Nutrient Use (Commercial Fertilizer and Manures) Nitrogen and Phosphorus," Washington, DC (1994)

11. US Department of Commerce, Bureau of Census, 1992 Census of Agriculture (data base) - State and County Data

12. Van Dyne, Donald L, and CB Gilbertson, "Estimating US Livestock and poultry manure and nutrient production;" ESCS-12, USDA Economics Statistics, and Cooperatives Service (1978).

 TABLES

The tables are available for downloading using Microsoft Excel.

Manure Characteristics Tables (31 KB)

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