Mass and Nutrient Losses during Composting of Dairy Manure with Sawdust versus Straw Amendment

Mass and Nutrient Losses during Composting of Dairy Manure with Sawdust versus Straw Amendment

Frederick C Michel Jr.#, John A Pecchia1, Jerome Rigot, Harold M Keener

Department of Food, Agricultural, and Biological Engineering, Ohio Agricultural Research and Development Center, The Ohio State University, Wooster, Ohio 44691

1- current address, Iowa Department of Natural Resources, Mason City, IA 50402 #-corresponding author

Manuscript Submitted to Compost Science and Utilization journal, 8/5/2003

Composting has become an increasingly popular manure management method for dairy farmers. However, the design of composting systems for farmers has been hindered by the limited amount of information on the quantities and volumes of compost produced relative to farm size and manure generated, and the impact of amendments on water, dry matter, volume and nitrogen losses during the composting process. Amendment type can affect the free air space, decomposition rate, temperature, C:N ratio and oxygen levels during composting. Amendments also initially increase the amount of material that must be handled. A better understanding of amendment effects should help farmers optimize, and potentially reduce costs associated with composting. In this study, freestall dairy manure (83% moisture) was amended with either hardwood sawdust or straw and composted for 110-155 days in turned windrows in four replicated trials that began on different dates. Initial C:N ratios of the windrows ranged from 25:1 to 50:1 due to variations in the source and N-content of the manure. Results showed that starting windrow volume for straw amended composts was 2.1 to 2.6 times greater than for sawdust amendment. Straw amended composts had low initial bulk densities with high free air space values of 75-93%. This led to lower temperatures and near ambient interstitial oxygen concentrations during composting. While all sawdust-amended composts self-heated to temperatures >55°C within 10 days, maintained these levels for more than 60 days and met EPA and USDA pathogen reduction guidelines, only two of the four straw amended windrows reached 55°C and none met the guidelines. In addition, sawdust amendment resulted in much lower windrow oxygen concentrations (< 5%) during the first 60 days. Both types of compost were stable after 100 days as indicated by CO2 evolution rates <0.5 mg CO2-C/g VS/dy. Both types of amendments also led to extensive manure volume and weight reductions even after the weight of the added amendments were considered. Straw amendment resulted in greater volume decreases than sawdust amendment due to greater changes in bulk density and free air space. Through composting, farmers can reduce the volume and weights of material to be hauled by 50 to 80% based on equivalent nitrogen values of the stabilized compost as compared to unamended, uncomposted dairy manure. Moisture management proved critical in attaining reductions in manure weight during composting. The initial total manure nitrogen lost during composting ranged from 7% to 38%. P and K losses were from 14 to 39% and from 1 to 38%, respectively. There was a significant negative correlation between C:N ratio and nitrogen loss (R2=0.78) and carbon loss (R2=0.86) during composting. An initial C:N ratio of greater than 40 is recommended to minimize nitrogen loss during dairy manure composting with sawdust or straw amendments.

Introduction

Composting is becoming a popular alternative manure management method for dairy

farms that results in manure stabilization, mass and moisture reduction, and the reduction of

pathogen levels (Willson and Hummel, 1975; Hong et al, 1983; Rynk et al., 1992; Haug, 1993;

Lufkin et al, 1995; Lopez Real and Baptista, 1996; Keener et al., 2000; Wright and Inglis, 2002;

Michel et al., 2002; Changa et al., 2003). The costs of the process can be offset by the value

added nature of composts. For example, composts enhance soil fertility, increase crop yields

(Dick and McCoy, 1993) and reduce diseases caused by soilborne plant pathogens (Hoitink and

Fahy, 1986; Bollen, 1993; Hoitink and Boehm, 1999). Furthermore, as compared to raw manure

and synthetic fertilizers, composted animal manures can reduce nutrient leaching when applied

to agricultural fields (Pecchia, 1996; Leclerc et al., 1995). They can also be stored easily until

value-added residential, organic or nursery markets become available (Rynk et al., 1992; Hoitink

et al., 1997; USDA, 2002; Michel et al., 2002). Raw or liquid stored manures, on the other hand,

have limited uses, can be applied to land just a few times during the year and are expensive to

transport (Jongbloed and Lenis, 1998; Veenhuizen et al., 1992). Recently, dairy manure compost

quality parameters were identified that allow the consistent preparation of high quality products

with value-added marketing potentials (Changa et al., 2002; Wang et al., 2003).

Dairy cows in freestall barns (1 Animal Unit=1000 lb) produce approximately 1720 kg

manure/AU/yr on a dry weight basis with a moisture content of 80-87% (Keener et al., 1999;

Veenhuizen et al., 1992). Amendments must be added to compost this manure to reduce its

moisture content within the optimum range (60-65%) for composting (Rynk et al., 1991; Keener

et al., 1999). Dairies use a variety of organic materials as bedding or compost amendments which

principally include sawdust and straw (Ashfield, 1978; Rynk et al., 1992). Many dairies use stall

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mattresses to reduce the amount of bedding required (Stowell et al., 1998). In addition to

moisture content, compost amendments impact compost free air space, decomposition rate,

temperature, C:N ratio and oxygen concentrations (Fraser and Lau, 2000; McCartney et al,

2002). Addition of amendments also increases the amount of material that must be managed.

The design of composting systems for dairy farms has been hindered by the limited

amount of knowledge about the mass and volume of compost produced relative to manure

generated, the amount and properties of product (compost) remaining and available for sale at

the completion of the process, and the impact of bedding/amendment types on composting rate,

moisture loss, dry matter conversion, and nutrient loss, particularly that of ammonia (Barrington

et al., 2002; Dewes, 1999; Bicudo et al., 2002; Gibbs et al, 2002).

Few full-scale mass balance studies have been conducted on the manure composting

process relative to the amount of manure generated on farms to determine the overall effect of

composting on manure properties, N-losses and application costs (Tiquia et al., 2002; Tiquia et

al., 2000). A better understanding of amendment effects should help farmers design more

rational systems for composting manures and optimize costs associated with operating

composting systems.

The objectives of this study were to compare the effects of the two most commonly used

organic amendments (hardwood sawdust and wheat straw) on the decomposition rate and overall

mass, volume, carbon and nutrient balances during full-scale windrow composting of dairy

manure.

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Materials and Methods

Feedstocks

The compost feedstocks were dairy manure, wheat straw and hardwood sawdust (Table 1).

To fully characterize the properties of the dairy manure, 34 different samples were collected over

the course of an entire year (Table 1). The dairy manure was obtained from the The Ohio State

University/Ohio Agricultural Research and Development Center (OARDC) dairy barn

immediately after the manure was scraped from alleyways. This dairy has 100 milking cows and

21 dry cows in a free stall facility that uses mattresses with separated manure solids bedding

(75%) and sawdust (25%). Thus the manure also included 0.006 m3/cow/dy (0.2 ft3/cow/dy) of

bedding material. Hardwood sawdust and wheat straw used as amendments were sampled 22 and

10 times respectively (Table 1).

Windrow Composting

Compost windrows were prepared on four different occasions (Table 2) by mixing dairy

manure with either sawdust (DM/SD1 to DM/SD4) or straw (DM/ST1 to DM/ST4) amendments.

The windrows were formed on a ½ acre concrete composting pad with a 2% slope and a leachate

collection system. The site was located on the Wooster campus of The Ohio State University. A

feed mixing wagon with a load cell (accuracy within 4 kg) was used to weigh and blend the

manure and amendments and determine the total initial and final windrow weights. Amendments

were added to the manure to yield a mixture with moisture content of approximately 65% (Table

3). The sizes of the windrows were typical of those used on farms that utilize a tractor-pulled

windrow turner. The cross sectional dimensions of the sawdust amended windrows averaged

2.9m x 1.2m (w x h) while those of the straw dairy manure windrows averaged 3.5m x 1.2m

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(wxh). The windrows ranged from 11 m (DM/SD3) to approximately 28 m in length (DM/SD1

and DM/ST1). Because of the time required to accumulate enough manure for each windrow, the

manure was collected over four to seven days for each windrow. The final weights of the

windrows were adjusted to account for the total weight of samples removed during the

experiment.

Windrows were turned with a tractor-assisted, Aeromaster 120 windrow turner on days

1 and 4 during the first week and weekly thereafter through week 10. Thereafter, the windrows

were turned once every two weeks for an additional six weeks (3 additional turns). Windrow

DM/ST1 was turned again on day 135 and Windrow DM/SD1 on day 142 when final samples

were removed. Samples were collected from windrows DM/SD2, DM/SD3, DM/SD4, DM/ST2,

DM/ST3 and DM/ST4 after turning on day 116.

Water was added to all windrows to readjust the moisture content to 50-65%.

Approximately 900 gallons of water were added to the DM/SD1 windrow on day 81 and 900

gallons were added to the DM/ST1 windrow on day 66 by spraying the water into the compost

during turning. Some runoff was observed during application so the exact quantity of water

remaining in the windrows was unknown. The windrows were not covered so additional water

was also introduced by rain. Water was added to windrows DM/SD2, DM/SD3, DM/SD4,

DM/ST2, DM/ST3 and DM/ST4 on days 80-90. Heavy rains late in the composting cycle

unfortunately increased the compost moisture contents of these windrows to levels of 56-81%

(Table 3).

Sampling

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Six replicate composite samples were collected on days 0, 61, 88, 122, and 157 from

windrow DM/SD1 and on days 0, 54, 81, 115 and 150 from windrow DM/ST1. Triplicate

composite samples were collected on days 0, 30, 60, 95 and 116 from windrows DM/SD2,

DM/SD3, DM/SD4, DM/ST2, DM/ST3 and DM/ST4. Each sample consisted of approximately

20 liters of compost collected from a cross section of the windrow that was mixed thoroughly in

a 120 liter (32 gal) container. Sub-samples of the composites were used for analyses.

Chemical and Physical Properties

Changes in chemical properties of the composts were monitored according to standard

protocols specified by the US Composting Council (TMECC, 2002). Sub-samples were ground

to the particle size specified by the analytical method for each chemical property. Moisture

content (w/w) was determined after oven drying (60-80 °C) to a constant weight. The pH was

determined on a slurry according to TMECC method 04.11-A1:5. Electrical conductivity also

was determined on this slurry with a solu-bridge conductivity meter (Beckman Instruments,

Cedar Grove, NJ.) according to TMECC method 04.10-A 1:5. Ash content was determined after

heating for 4 h in a muffle furnace at 550 °C (TMECC method 03.02-A). Percent volatile solids

(VS) was determined by subtracting percent ash from 100. Total nitrogen (N) and total carbon

(C) analyses were performed with the Dumas combustion method (VarioMax N analyzer,

Elementar Americas) (TMECC methods 04.02-D and 04.01-A). The detection limit for this

instrument was 200 mg N kg-1. Total C was determined using coulometry. This instrument

converts C in the sample to CO2 by oxidation at 1100°C. The detection limit was 1 mg C kg-1.

Total nitrate–N and NH4+-N were determined by ion chromatography (TMECC methods 04.02-

B) and micro-kjeldahl distillation-titration methods, respectively. Windrow length, width and

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height were measured using a tape measure and cross sectional area was estimated by

observation of cross sectional geometry. Bulk density and free air space were measured by

weighing a 21 l (5 gallon) volume of each composite sample. Water was added to replace the

free air space and the sample was reweighed. Free air space was calculated assuming a water

density of 1 g/cm3. Stability testing was performed by CO2 respirometry at 25º C following

TMECC method 05.08-B on days 15, 29, 43, 57, 70, 84, 98, 105 and 112 for DM/SD1 and on

days 18,33,48,60,74,88,95 and 102 for DM/ST1 (Changa et al, 2003).

Temperature and oxygen readings were recorded at 3 locations along the length of each

windrow prior to windrow turning at six points per location. Measurements were made at 1/3 and

2/3 depths on either side and from the top giving 18 data points per sampling time per windrow

(Michel et al., 1996). Temperature data was collected 3 times per week using a hand-held 0.6m

temperature probe. Oxygen concentrations were measured once per week (DM/ST1 and

DM/SD1) or three times per week (DM/ST2-4 and DM/SD2-4) using a hand-held Teledyne

Series 320 portable oxygen analyzer (City of Industry, CA).

Results

Feedstock and Initial Compost Properties

The moisture content of the free stall dairy manure varied little (83 ±3%) during the course

of one year and was somewhat less than the value of 89% previously reported for dairy manure

(Veenhuizen et al, 1992). The C:N ratio of of the dairy manure was 15 ±3 (Table 1). However,

total nitrogen and ammonia varied substantially at 2.9 ±0.6% and 7329 ±4119 ppm, respectively.

Nitrate was not detectable (<0.5 ppm) in the manure samples. Some of this variation may have

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been due to the relative quantities of manure from lactating cow versus heifers in the collected

samples.

The moisture contents of the straw and sawdust amendments were 9-10% and the C:N

ratios were 54 ±12 and 254 ±103, respectively. The straw and sawdust properties complimented

the high moisture content and low C:N ratio of the manure. For composting, C:N ratios of 25:1

to 30:1 and moisture contents of 60-65% are generally thought to be optimal (Haug, 1993; Rynk

et al., 1992). Both amendments had substantially lower P and K concentrations than the manure

(Table 1). Neither ammonia nor nitrate was detected in the straw or sawdust amendments (Table

1). The manure, straw and sawdust had volatile solids contents of 83%, 91% and 99% with

similar carbon contents of 44%, 46% and 47% respectively. The ratios of carbon to volatile

solids in the three feedstocks (53% for manure, 51% for straw and 47% for sawdust) indicated

that the carbon in the sawdust was the most oxidized while the manure was the most reduced.

The dairy manure was mixed with the straw or sawdust amendments (Table 2) to give a

moisture content of approximately 65% (Table 3). Because of variations in N-content of the

manure (Table 1), initial C:N ratios of the windrows ranged from 25:1 to 50:1 (Table 3). Enough

compost was prepared to form windrows of approximately 28 m in length. Due to differences in

bulk densities of the amendments, the total weight of material in the sawdust amended windrows

was nearly twice that in the straw amended windrows (Table 2). The quantity of manure added to

the sawdust windrows was more than twice that in straw windrows on a length basis (Table 2).

Weight and Volume Changes during Composting

After composting, the total weight of the compost decreased dramatically in 7 of the 8

windrows despite the addition of water late in the process (Table 2). For example, windrow

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DM/SD1 decreased from 30,241 to 5,273 kg and windrow DM/ST1 decreased from 23,696 to

4,000 kg (Table 2). The percent mass loss ranged from 41-83% for the sawdust amended

windrows and from 2 to 83% for the straw-amended windrows (Tables 2 and 4). Because of

heavy rainfall, moisture content increased in one of the straw amended windrows (DM/ST4). In

all four of the sawdust amended composts and 2 of the 4 straw amended composts, final compost

weight was less than the weight of the original manure (Table 2). This contrasts with liquid

manure handling systems where the weight of manure removed from the barn often increases

substantially due to the addition of water to improve manure flow and settling properties. A

complete mass balance on water could not be calculated because of water addition due to

rainfall, generation through decomposition, and leaching and runoff during moisture adjustment.

Substantial changes in volume occurred in all of the windrows (Fig. 3, Table 3). The

sawdust-amended windrows lost 33-79% of their initial volumes, while the straw-amended

windrows lost even more (65-93%) of their initial volumes (Table 3). Dry weight losses ranged

from 44 to 72% for sawdust amended and from 54-76% for straw amended windrows (Table 4).

Bulk density and free air space changed considerably in the straw amended windrows (DM/ST1

to DM/ST4) but remained relatively constant in the sawdust amended windrows (DM/SD1 to

DM/SD4; Fig. 3). This change contributed to the much greater volume loss observed in the straw

amended as compared to comparable sawdust amended windrows made on the same dates (Fig.

4).

Temperature and Oxygen Concentration

Straw and sawdust amendments had very different effects on windrow temperatures and

oxygen concentrations during composting (Fig. 1). All of the sawdust-amended manure

composts reached temperatures greater than 55°C (131 F) after 3 to 10 days and maintained

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temperatures in a narrow range between 50º and 70°C past day 50 (Fig. 1). By contrast, only two

of the straw-amended manure composts (DM/ST1 and DM/ST3) exceeded temperatures of 55º C

and this did not occur until after 40 and 70 days of composting (Fig. 1). In addition, in straw

amended windrows, temperatures fluctuated greatly (10º - 60°C) throughout the composting

period (Fig. 1). The straw-amended composts cooled down rapidly after day 80, and sooner than

the sawdust amended composts which exhibited temperatures greater than 50º C. through day

100 (Fig. 1).

Oxygen concentrations in all of the sawdust-amended windrows (DM/SD1 to DM/SD4)

remained very low (<5%) through day 60. Between days 60 and 80, the concentration of oxygen

in these windrows increased toward ambient levels (Fig. 2). However, even after 100 days,

oxygen concentrations in the sawdust amended windrows were well below ambient

concentrations. In contrast, in the straw-amended composts, oxygen levels remained relatively

high (>15%) through the first 60 days of composting. Thereafter, oxygen concentrations

decreased in two of the windrows (DM/ST1 and DM/ST3) to low levels (~5%) and this

coincided with a decrease in compost stability in windrow DM/ST1 (Fig. 4). After 110 days,

oxygen concentrations in all of the straw amended windrows returned to near the ambient

concentration (Fig. 2).

Physical and Biochemical Properties

Composting resulted in increases in nitrogen concentrations and decreases in volatile

solids, total C and C:N ratio (Table 3). However, substantial amounts of organic matter

(VS>72%) remained in the final composts (Table 3). Carbon loss during composting was

substantial. From 45 to 74% of the total C was lost from the sawdust amended and from 54 to

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79% from the straw amended windrows (Table 3). The moisture content decreased or increased

depending on rainfall and amount of water added during turning. Ammonia concentrations

decreased more than 60% in all sawdust windrows, but rose in two of four straw amended

windrow (DM/ST3 and DM/ST4). Nitrate was detectable (>0.5 ppm) in only 3 of the stabilized

composts (Table 3). The pH of the composts changed by less than 0.5 unit in 7 of 8 windrows

during composting. The pHs of all of the stabilized composts ranged from 7.7 to 8.6 (Table 3).

All of the stabilized composts exhibited physical properties and nutrient concentrations that

would make them excellent soil amendments and fertility sources (Tables 1 and 3). Additional

information on plant growth in DM/ST1 and DM/SD1 amended potting mixes is presented in

Wang et al., (2003).

A mass balance analysis on nitrogen indicated that the sawdust-amended composts lost 8-

26% whereas the straw amended windrows lost 15 to 43% of the initial nitrogen (Table 3). The

quantities of phosphorus lost were from 12 to 21% for sawdust amended and 1 to 38% for straw

amended windrows. Potassium losses were from 17 to 25% for sawdust amended and 14 to 39%

for straw amended windrows (Table 3). These losses were most likely due to runoff and leachate

losses.

The rates of CO2 evolution in compost windrows were initially high (~3 mg CO2/g

VS/h). The rate of CO2 evolution decreased rapidly (DM/SD1) to <1 mg CO2/g VS/h within 23

days in the sawdust amended windrow. In the straw amended windrow, a lag of more than 60

days was observed before compost stability reached a similar level of stability (Fig. 4). This

finding is consistent with the temperature and windrow oxygen concentration data showing a

marked lag period in the composting of straw amended dairy manure. The reason for this lag are

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unclear but may have to do with free air space and temperature differences, structural limitations,

an initial resistance to wetting and decomposition by straw, and management practices.

Discussion

The results of this study demonstrate the changes which occur during full scale composting

of dairy manure and provide useful information for the design and sizing of full scale dairy

manure composting facilities. This includes the amount of manure generated, the initial moisture

content of the manure, the quantities of amendment required, the extent of bulk density, volume

and free air space changes, nutrient, wet and dry matter losses, and the factors which influence

nitrogen loss during composting. This information is critical to the sizing and design of

composting systems for dairies that is not widely available in the literature.

The results clearly show that overall carbon and nutrient losses during composting were

similar for straw and sawdust amendments, the two amendments most commonly used by

farmers (Tables 2 and 3). However there were clear differences during composting using the two

different amendments. One difference was that the two amendments and the initial composts

made using them had very different bulk densities (Fig. 3). As a result the initial sawdust

amended windrows contained on average 935 ±184 (kg manure/m) while the straw amended

windrows contained 377 ±109 (kg manure/m). This difference resulted in windrows 2.5 times

longer on average when straw versus sawdust amendment was used. This difference translated

into a significantly increased compost pad size requirement for straw and, ultimately, to an

increase in composting capital and operating costs. However, actual pad size requirements also

depend on decomposition rates, compost retention times and the ability to build windrows of

larger cross-sectional areas. The high free air space of the straw windrows, low initial process

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temperature, and high observed oxygen concentrations indicate that larger windrows could be

built with straw than sawdust. Unfortunately, the dimensions of turning equipment often limit

windrow size.

Substantial differences in windrow temperatures were observed with straw versus sawdust

amended windrows (Fig. 1). All of the sawdust but none of the straw amended windrows reached

temperatures required to meet guidelines for pathogen destruction during composting (USEPA,

1989; USDA, 2002). Both the EPA and USDA Organic Program rules state that, “producers

using a windrow system must maintain the composting materials at a temperature between 131 F

and 170 F for 15 days, during which time, the materials must be turned a minimum of five

times”. The differences in temperature profiles may be related to differences in free air space and

the initial resistance of straw to biodegradation. The straw-amended windrows all had higher

initial free air space values (76-95%) than the sawdust-amended windrows (62-66%) (Fig. 3).

The higher free air space in the straw-amended compost may have allowed for greater convective

air flow through the windrow leading to greater heat loss and a lower rate of temperature

increase (Fig. 1). This increased airflow may also have contributed to greater variations in

temperatures recorded in the straw-amended windrows (Fig. 1). The straw amended windrows

DM/ST2, DM/ST3 and DM/ST4 also had low initial moisture contents (53-62%) which may

have been suboptimal. As the bulk density of straw-amended windrows DM/ST1 and DM/ST3

began to increase and the straw lost its structure, higher temperatures (> 50º C) were maintained

and a decrease in oxygen concentrations was observed (Figs. 2 and 3). However, two of the

straw amended windrows did not reach temperatures above 50º C during the entire composting

process (DM/ST2 and DM/ST4). These straw amended also exhibited the smallest changes in

volume, bulk density and free air space (Table 2, Fig. 3). A contributing factor may have been

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that these two windrows exhibited high initial C:N ratios (39 and 50, respectively) that decreased

the rate of organic matter decomposition as well (Fig. 5).

Differences in the biodegradability of the carbon in the two amendments also may have

affected windrow temperatures. Cellulose in sawdust breaks down slowly and evenly due to

limited accessibility (Stone et al., 2001). This may have contributed to the constant even

temperatures and rate of decomposition observed in the sawdust amended windrows. Straw was

initially resistant to degradation possibly due to its larger particle size and structure and it’s waxy

hydrophobic coating (Ward et al., 2000). However, once the straw lost its physical integrity, the

free air space of the windrow decreased, and the underlying cellulose in the straw degraded more

rapidly. Stability data (Fig. 4) for straw show a lag followed by a rapid decrease in stability on

day 50. A slower, steady decrease in stability in the sawdust amended windrow supports these

ideas (Fig. 4). Despite these differences, all of the composts were similar in appearance and color

with little indication of the amendment type used.

A large reduction (65-93%) was observed (Fig. 3, Table 4) in the volumes of the straw-

amended windrows (DM/ST1 to DM/ST4) while sawdust amended windrows showed somewhat

less volume losses of 33 to 79% (Table 4, Fig. 3). Factors contributing to this effect were

probably physical chopping of the straw by the windrow turner initially and extensive

decomposition of the straw during the process which further reduced windrow free air space and

increased bulk density. The small particle size of the sawdust and initially higher bulk densities

of the windrows resulted in less volume reduction.

A mass balance on the manure removed from the dairy barn through the composting

process showed that composting reduced the mass of material that must be transported by the

farmer even when the addition of amendments is considered (Fig. 6). Dairy cows (one animal

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unit [AU] = 1000 lbs) usually generate approximately 1720 kg of dry manure per year

(Veehuizen et al., 1991) that translates into 10,118 kg wet manure/AU/yr at a moisture content of

83% (Table 1). When this manure was composted, the amount of stable compost generated from

one dairy animal unit ranged from 2100 to 4700 wet kg/yr (1321 to 2810 kg dry/yr; Fig. 6). On a

volume basis this corresponds to from 7 to 24 m3/AU/yr for sawdust and from 12 to 35 m3/AU/yr

for straw amended composts. In six of the eight windrows the weight of stabilized compost

produced was less than 55% of that of the manure removed from the barn, even when

considering the amendments and precipitation inputs (Fig. 6). The decrease in weight during the

composting process was caused by loss of moisture and volatile solids. In the best cases for straw

and sawdust amendments ((DM/SD1 and DM/ST1) an 83% mass reduction was realized (Table

4). In one case, moisture mismanagement led to an increase in water weight in the compost

(DM/ST4) after composting. This could have been avoided by the use of covers (Lufkin et al.,

1995) which would have resulted in much lower final moisture contents. However, this finding

illustrates the difficulty of maintaining optimal moisture contents during outdoor composting.

Assuming that moisture management was more optimal and that all of the windrows had final

moisture contents of 40%, then a weight decrease of more than 60% (compared to the manure

removed from the barn) would have been realized in all eight windrows (Fig. 6). This translates

into a 50-80% reduction in the weight of material that must be transported and applied during

utilization to provide the same amount of nitrogen as the original manure. This finding is

especially important when considering the distances that farmers must transport manure for land

application and for comparisons with liquid manure handling systems where water is added to

improve flow properties resulting in major increases in the weight of manure as well as

transportation costs (NRAES, 2001). The reduced weight and stability of compost, as compared

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to liquid manure, also means that the compost can be stored and transported more easily to

distant nursery and residential value-added markets. The costs and availability of amendments

and labor costs associated with composting may offset some of these benefits, however.

Nitrogen loss is an important consideration during composting from both a nutrient

conservation standpoint and since atmospheric ammonia and nitrous oxides have been linked to a

variety of adverse environmental and health effects. Future clean air rules may limit ammonia

and greenhouse gas emissions from farms (Bicudo et al. 2002). Manures contain high levels of

ammonia (Table 1) and nitrogen loss is usually attributed to ammonia volatilization and leaching

(Dewes, 1999; Barrington et al., 2002; Gibbs et al., 2002; Eghball et al., 1997) and to nitrous

oxide and nitrogen volatilization (He et al, 2002: Veeken et al., 2002). Nitrogen losses occur

during many phases of manure handling including during accumulation and storage in the barn,

during removal, mixing, processing and finally during and after land application. All of these

emissions contribute to adverse effects (Kirchmann and Lundvall, 1998). It is difficult to

compare overall nitrogen losses from different manure handling systems to minimize ammonia

losses (Gibbs et al., 2002) since in some systems the majority of the losses occur during land

application (liquid manure and anaerobic digests) while for others it occurs during processing

and storage (solid storage and composting). Some aspects of the composting process such as

high temperatures, convective aeration (Michel et al., 1996), high porosity (Veeken et al., 2002;

Lopez-Real and Baptista, 1996), and pH values (>8.0) as compared to liquid manures, would be

expected to facilitate ammonia volatilization from composts (Dewes, 1999). For example,

nitrogen losses ranging from 35 to 75% have been reported during composting of hog manure

(Veeken et al., 2002; Michel et al., 2001; Barrington et al. 2002) and from 9 to 68% during the

composting of cattle manure (Gibbs et al., 2002; Eghball et al., 1997). However, Dewes (1999)

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showed that lower overall emission of ammonia occurs over long periods when manure is

composted than when it is stored as a liquid due to biological immobilization of nitrogen. In

addition, denitrification can result in substantial quantities of nitrogen loss via nitrous oxides

and/or nitrogen gas from oxygen limited areas of a compost pile (Veeken et al., 2002 and He et

al 2002). Results of this study indicate that as little as 7% and as much as 43% of the total initial

nitrogen was lost during dairy manure composting in eight different windrows using two

different amendments (Table 4). There appeared to be no effect of amendment type used on

percent nitrogen loss (Table 4). However, the initial C:N ratio of the composts, which varied

from 25:1 to 51:1, correlated significantly and linearly (R2=0.78) with the loss of total Nitrogen

(Fig. 5). For example, compost with a starting C:N ratio of 25 (DM/ST1) lost 32% of its initial

nitrogen, while two windrows with starting C:N ratios of 50 (DM/SD2 and DM/SD4) lost only

8% and 7%, respectively (Fig. 5). The C:N ratio has also been shown to be an important factor

for minimizing nitrogen loss during the composting of poultry manure (Hansen et al., 1993;

Ekinci et al., 1997; Ekinci et al., 2002) yard trimmings (Michel and Reddy, 1998) and cattle

manure (Eghball et al., 1997). However, in studies where large percentages of nitrogen were lost,

initial C:N ratios were relatively low (Tiquia et al., 2002; Gibbs et al, 2001; Michel et al., 2001).

This indicates that there may be a potential to manipulate windrow C:N ratios to substantially

reduce nitrogen volatilization during manure composting. However this could potentially affect

decomposition rate and the length of time necessary for production of stable composts. For

example, the initial C:N ratio was also significantly linearly correlated (R2=0.87) with total

carbon loss (Fig. 5) indicating that organic matter decomposition may have been slowed at

higher C:N ratios. Still, to minimize nitrogen loss during dairy manure composting, it may be

advisable to prepare composts with initial C:N ratios of 40:1 to 50:1.

17

Conclusions

o Dairy manure composting with sawdust and straw led to extensive reductions in

manure volume and weight even after considering the weight of the added

amendment. Many farmers haul manures up to 10 km (6 miles) from their farm to

avoid over-applying nutrients and reduce water pollution. By composting, farmers

can reduce the volume and weights to be hauled by 50 to 80%, based on equivalent

nitrogen values as compared to unamended raw dairy manure

o The initial sawdust amended windrows contained on average 935 ±184 (kg

manure/m) while the straw amended windrows contained 377 ±109 (kg manure/m).

This difference resulted in windrows 2.5 times longer on average when straw versus

sawdust amendment was used. Straw amendment resulted in greater volume

decreases than sawdust due to greater changes in bulk density and free air space and

higher oxygen concentrations in the windrow which meant that larger windrows

could potentially have been used.

o Straw amended dairy manure composts had low initial bulk densities with high free

air space values that led to lower temperatures and near ambient interstitial oxygen

concentrations during composting. None of the straw amended composts reached

pathogen guidelines of >55º C for 15 days despite exhibiting extensive volume and

volatile solids losses as well as physical changes. This problem may be solved by

increasing windrow size.

18

o All sawdust-amended composts reached temperatures >55 C in less than 10 days and

maintained these temperatures for more than 60 days thereby meeting pathogen and

weed seed destruction guidelines for windrow composting.

o Moisture management is critical to attaining manure weight reductions during

composting. Rainfall and moisture adjustments can result in composts with moisture

contents greater than the starting material (Table 3). Once the compost is stable, self

heating is not available to fuel evaporation of this excess moisture. Therefore, covers,

larger curing piles or barn storage should be used as composts become stable to

assure that excess moisture does not accumulate in stabilized composts and that final

composts have moisture contents of 40% or lower.

o The amounts of phosphorus and potassium lost during composting were from 14 to

39% and from 1 to 38%, respectively. These losses were most likely due to runoff

and leachate losses.

o From 7% to 38% of the initial total nitrogen was lost during composting. Somewhat

more loss of carbon (45-79%) was observed. There was a significant negative

correlation between C:N ratio and nitrogen loss (R2=0.78) and carbon loss (R2=0.87)

during composting. To minimize nitrogen loss during dairy manure composting with

sawdust or straw amendments, a C:N ratio of 40:1 to 50:1 is recommended.

Acknowledgments

19

Salaries and research support was provided by funding from the Ohio Water

Development Authority appropriated to the Ohio Agricultural Research and Development

Center, The Ohio State University. The authors would like to thank Ping Wang and Charles

Changa for measuring carbon dioxide evolution, James Muracao for sampling assistance and

Harry Hoitink for editorial comments.

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23

TABLE 1.

Mean chemical properties of compost feed stocks.

Moist VS Total N

Total C C:N pH SS NH3N NO3N P K

% wet

% dry

% dry

% dry Ratio mMhos

cm mg kg

mg kg

mg kg

mg kg

DAIRY MANURE

Mean (n=34) 83 82.2 2.9 44 15 7.6 25 7329 <0.5 4520 27461

Std Deviation 3 7.1 0.6 3 3 0.4 5 4119 <0.5 459 6091

STRAW (Wheat)

Mean (n=10) 10 90.9 0.9 46 54 7.3 - <0.5 <0.5 1100 15678

Std Deviation 3 5.2 0.2 2 12 0.3 - <0.5 <0.5 225 1594

SAWDUST (Hardwood)

Mean (n=22) 9 99.5 0.2 47 254 4.5 - <0.5 <0.5 37 785

Std Deviation 3 0.6 0.1 2 103 0.9 - <0.5 <0.5 13 124

24

TABLE 2.

Total mass of manure, straw and sawdust used for composting and windrow volume and mass

balance. Values are on a wet weight basis.

Windrow Mass (kg)

Windrow ID Date formed Length (m) Manure Sawdust Straw Total Initial

TotalFinal

DM/SD1 5/2/01 19.1 23,027 7,214 0 30,241 5,273

DM/SD2 8/16/02 11.7 9,733 3,246 0 12,978 5,639

DM/SD3 8/23/02 10.8 9,674 3,237 0 12,910 6,546

DM/SD4 9/3/02 12.0 9,689 3,171 0 12,860 7,634

DM/ST1 5/9/01 30.8 16,382 0 7,314 23,696 4,000

DM/ST2 8/16/02 17.8 5,544 0 2,264 7,808 5,675

DM/ST3 8/23/02 15.1 5,643 0 2,486 8,130 4,252

DM/ST4 9/3/02 17.8 5,156 0 2,013 7,169 7,034DM/SD- dairy manure plus sawdust amendment, DM/ST- dairy manure plus straw amendment

25

TABLE 3.

Mean chemical properties of initial and final dairy manure (DM) composts made with sawdust (SD) or straw (ST) amendments.

Values reported are on a dry weight basis except moisture content which is a wet weight basis.

INITIAL

Volatile Total Total C:N Windrow

Moisture Solids Carbon N Ratio pH NH3-N NO3-N P K

ID (% wet) (%) (%) (%) (g/g) (mg/kg) (mg/kg) (mg/kg) (mg/kg)DM/SD1 65 ±1 91.3 ±1.0 46.4 ±1.2 1.4±0.1 32.9 ±0.9 8.8±0.1 1976 ±121 <0.5 1948 ±195 8753 ±756 DM/SD2 66 ±1 94.6 ±0.7 45.4 ±0.7 0.9 ±0.1 49.5 ±1.3 8.2 ±0.2 288 ±54 <0.5 1328 ±83 10303 ±157 DM/SD3 65 ±1 93.5 ±2.4 45.5 ±0.7 1.0 ±0.1 45.5 ±1.1 7.8 ±0.1 357 ±129 <0.5 1382 ±76 10775 ±283 DM/SD4 67 ±1 94.4 ±0.3 44.7 ±0.5 0.9 ±0.1 50.8 ±1.2 8.2 ±0.2 568 ±218 <0.5 1578 ±334 13219 ±4386 DM/ST1 67 ±2 82.9 ±0.6 44.1 ±0.6 1.8 ±0.1 25.1 ±1.2 8.2±0.1 238 ±60 <1.0 3512 ±57 20134 ±310 DM/ST2 62 ±3 94.3±0.5 44.7 ±0.4 1.1 ±0.1 39.0 ±1.1 8.2 ±0.1 71 ±23 <0.5 1716 ±157 20096 ±510 DM/ST3 53 ±4 94.2 ±0.2 43.7 ±4.4 1.2 ±0.1 35.2 ±3.8 6.4 ±0.4 161 ±22 <0.5 1940 ±621 16144 ±1565 DM/ST4 55 ±4 92.7 ±0.2

44.2 ±1.1

1.2 ±0.1 37.0 ±2.5 8.7 ±0.1 97 ±42

<0.5 1638 ±164

16184±4504

FINAL

Volatile Total Total C:N Windrow

Moisture Solids Carbon N Ratio pH NH3-N NO3-N P K

ID (% wet) (%) (%) (%) (g/g) (mg/kg) (mg/kg) (mg/kg) (mg/kg)DM/SD1 43 ±1 76.6 ±1.6 42.5 ±0.5 3.7 ±0.1 11.6 ±0.3 8.6 ±0.1 89 ± 8 90 ±14 5420 ±116 23045 ±410 DM/SD2 56 ±1 91.1 ±0.3 45.0 ±0.7 1.5 ±0.1 29.6 ±1.1 7.8 ±0.1 30 ±1 <0.5 2105 ±35 15351 ±320 DM/SD3 69 ±3 91.3 ±0.3 46.4 ±3.1 1.7 ±0.1 28.0 ±1.0 7.7 ±0.1 14 ± 2 <0.5 2553 ±303 17707 ±1780 DM/SD4 70 ±1 90.7 ±0.6 44.7 ±0.7 1.5 ±0.1 29.4 ±0.3 7.8 ±0.1 54 ±15 6 ±10 2301 ±128 19241 ±193 DM/ST1 49 ±1 71.6 ±6.6 35.1 ±0.5 3.9 ±0.5 8.5 ±1.1 8.3±0.1 116 ±90 128 ±99 8431 ±226 47671 ±1515 DM/ST2 77.±1 85.5±0.2 44.3 ±1.5 2.1 ±0.1 21.1 ±1.1 7.9 ±0.1 34 ±9 <0.5 3689 ±378 32118 ±458 DM/ST3 79 ±1 78.3 ±0.2 41.7 ±0.6 3.3 ±0.1 12.9 ±0.6 8.1 ±0.1 557 ±100 <0.5 6568 ±262 44072 ±265 DM/ST4 81 ±1 84.8 ±0.2 42.2 ±0.7 2.1 ±0.1 19.8 ±0.2 7.9 ±0.1 275 ±104 <0.5 3732 ± 154 34031 ±1220

TABLE 4.

Overall weight, volume, volatile solids, carbon and nutrient losses during dairy manure (DM)

composting with sawdust (SD) or straw (ST) amendments.

Windrow

ID

Volume

Wet

Weight

Dry

Weight

Water

Volatile

Solids

Total

Nitrogen

Total

Carbon

Total

P

Total

K DM/SD1 79% 83% 72% 89% 76% 26% 74% 21% 25%DM/SD2 40% 57% 44% 63% 46% 8% 45% 12% 17%DM/SD3 36% 49% 54% 47% 55% 24% 54% 16% 25%DM/SD4 33% 41% 46% 38% 48% 7% 46% 21% 21%DM/ST1 93% 83% 74% 88% 81% 43% 79% 38% 39%DM/ST2 65% 27% 54% 11% 58% 15% 54% 1% 26%DM/ST3 86% 48% 76% 22% 80% 38% 77% 19% 35%DM/ST4 71% 2% 59% -45% 63% 27% 61% 7% 14%

Values reported are losses as a percent of the initial quantity and were calculated using total weight, moisture and nutrient

concentrations. All values are on a dry weight basis except wet weight and water.

0

10

20

30

40

50

60

70

80

0 20 40 60 80 100 120 140 160

Day

DM/SD1DM/SD2DM/SD3DM/SD4

Sawdust

Temp. (C)

0

10

20

30

40

50

60

70

80

0 20 40 60 80 100 120 140 16

Day0

DM/ST1DM/ST2DM/ST3DM/ST4

Straw

Temp. (C)

FIGURE 1. Mean temperatures during windrow composting of dairy manure with sawdust and straw as amendments on four different starting dates. Data points represent an average of 18 measurements per windrow. Turning days are indicated in the materials and methods section.

28

0

5

10

15

20

25

0 20 40 60 80 100 120 140 160Day

Oxy

gen

(%)

DM/ST1DM/SD2DM/SD3DM/SD4

Sawdust

0

5

10

15

20

25

0 20 40 60 80 100 120 140 160Day

Oxy

gen

(%)

DM/ST1DM/ST2DM/ST3DM/ST4

Straw

FIGURE 2. Mean interstitial oxygen concentration during windrow composting of dairy manure with sawdust and straw as amendments made on four different starting dates. Data points represent an average of nine measurements per windrow. Turning days are as described in Materials and Methods.

29

0

100

200

300

400

500

0 20 40 60 80 100 120 140 160Days

Bul

k D

ensi

ty (k

g w

et/m

3 )

DM/ST1DM/ST2DM/ST3DM/ST4DM/SD1DM/SD2DM/SD3DM/SD4

0%

20%

40%

60%

80%

100%

0 20 40 60 80 100 120 140 160Days

Volu

me

(% o

f ini

tial)

50%

60%

70%

80%

90%

100%

0 20 40 60 80 100 120 140 160Days

Free

Air

Spac

e (%

)

FIGURE 3 Bulk density, volume and free air space of dairy manure compost windrows prepared with sawdust or straw amendments on four different starting dates. Data points represent an averages of three measurements per windrow.

30

0

1

2

3

4

5

6

0 20 40 60 80 100 120 140 16Compost Age (Days)

CO

2 evo

lutio

n ra

te(m

g C

O2-

C/g

VS/

d )

0

Dairy + StrawDairy + Sawdust

FIGURE 4. Compost stability levels for two dairy manure composts measured as mean CO2 evolution rate at 25 °C over a 3-day period. Values are means plus and minus one standard deviation for six replicate samples.

31

y = -0.013x + 0.7453R2 = 0.7666

0%

10%

20%

30%

40%

50%

20 30 40 50 60Initial C:N Ratio

DM/SD

DM/ST

Total N Loss(% of initial N)

y = -0.0147x + 1.1902R2 = 0.8596

40%

50%

60%

70%

80%

90%

100%

20 30 40 50 60Initial C:N Ratio

DM/SD

DM/ST

Total C

Loss(% of initial)

FIGURE 5. Relationship between initial C:N ratio of straw vesus sawdust amended dairy manure composts and nitrogen and volatile solids (carbon) losses during composting. Values are averages for 3 to 6 replicate samples. Error bars represent one standard deviation.

32

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

16,000

Manureweight

InitialCompost

FinalCompost

FinalCompost

(assuming40%

moisture)

Tota

l M

ass

(kg/

AU

/yr)

0246810121416

Tota

l Mas

s (to

n/A

U/y

r)DM/SD1

DM/SD2

DM/SD3

DM/SD4

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

16,000

Manureweight

InitialCompost

FinalCompost

FinalCompost

(assuming40%

moisture)

Tota

l M

ass

(kg/

AU

/yr)

0246810121416

Tota

l Mas

s (to

n/A

U/y

r)

DM/ST1

DM/ST2

DM/ST3

DM/ST4

FIGURE 6. Quantities (wet weight) of manure, initial compost and final dairy manure compost

generated per animal unit (AU/yr) using sawdust or straw amendments in windrows made on different days. Values were calculated based on initial and final compost weights and assuming a manure generation rate of 1720 kg dry manure/AU/yr.

33