Available online at www.sciencedirect.com

Bioresource Technology 99 (2008) 5626–5633

Fertilizing value of broiler litter: Effects of drying and pelletizing

M.E. Lopez-Mosquera a,*, F. Cabaleiro a, M.J. Sainz a, A. Lopez-Fabal a, E. Carral b

a Departamento de Produccion Vegetal, Universidad de Santiago de Compostela, E-27002 Lugo, Spainb Departamento de Biologıa Celular y Ecologıa, Universidad de Santiago de Compostela, E-27002 Lugo, Spain

Received 9 January 2007; received in revised form 17 October 2007; accepted 18 October 2007Available online 3 December 2007

Abstract

The effects of drying and pelletizing on the properties of broiler chicken litter, obtained from a farm in northwest Spain, were inves-tigated. The drying and pelletizing process reduced among-batch variability in dry matter content, electrical conductivity, urea N, and K,S, Na, Fe, Cu and Cd contents, but increased among-batch variability in total N, ammonium N, nitrate N, total P and pH. N formcontents in the pelletized product could be estimated with reasonable accuracy on the basis of dry matter content. Cr, Cu and Cd con-tents were all significantly lower in the dried pelletized product than in fresh litter, whereas Pb content was significantly higher. The driedpelletized product is of course clearly preferable to the fresh product as regards storage and handling, however, our results suggest a needto optimize the production process with the aim of reducing possible contamination during manufacture, and of minimizing variability inN form contents, P content and pH.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Broiler litter; Fertilizing value; Rice hull; Heavy metals

1. Introduction

Broiler chicken (Gallus gallus domesticus) productionhas undergone steady growth both in developed and espe-cially in developing countries during the past 40 years(Sims et al., 2005). In Europe, despite some countries expe-rienced a decrease in 2003 (Benelux, due to avian influenzaoutbreaks) and 2004 (Portugal, as a result of aflatoxin con-tamination), the production of broiler meat is expected tofurther increase in response to increased domestic con-sumption (Polet, 2005). France, Spain and United King-dom are the main broiler chicken producing countries inEurope. The total European census of broiler chickens in2002 was about 5900 million (Turner et al., 2005), whichresults in an estimated production of 6.5–8.3 million Mgif considering that 1000 birds produce 1.1–1.4 Mg of litter(Collins, 1996).

0960-8524/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.biortech.2007.10.034

* Corresponding author. Tel.: +34 982 252231; fax: +34 982 241835.E-mail address: ellomo@usc.es (M.E. Lopez-Mosquera).

Chicken litter largely comprises chicken faeces, the bed-ding material (e.g. sawdust, woodshavings, rice hulls orpeanut hulls), feathers, and remains of feed. It containsorganic matter and mineral nutrients (N, P, K), and hasbeen long-used as fertilizer and soil amendment for numer-ous economically important crops, including maize (Mosset al., 2001), soybean (Adeli et al., 2005), pasture (Kingeryet al., 1993) and horticultural species (Rubeitz et al., 1998).Litter from broiler houses is periodically removed andstored in covered facilities or outdoors until transportationand field application, what is usually done in the autumn–winter period prior to ploughing although broiler litter canbe also applied successfully in spring (Nicholson et al.,1999).

Yet fertilizer requirements are seasonal, chicken litter isproduced at a roughly constant rate throughout the year.Thus litter often accumulates and either must be storedor results in incorrect timing or excessive land application.Storage is often so problematic that the litter is dumped aswaste. Inadequate storage or field application of fresh broi-ler litter may lead to contamination problems including

Table 1Physical and chemical properties of the rice hull used as bedding in thefarm studied

General properties Micronutrient and heavymetal content (mg kg�1)

Dry matter (%) 92.80 Co 2.80pH 6.64 Cu 2.60C:N ratio 79.83 Fe 108.00Loss on ignition (% d.w.) 14.60 Mn 429.00Total C (% d.w.) 37.02 Zn 22.00Total N (% d.w.) 0.46 Cd <0.02P (% d.w.) 0.002 Ni <0.10K (% d.w.) 0.29 Pb <0.10Ca (% d.w.) 0.16 Cr 10.80Mg (% d.w.) 0.11Na (% d.w.) 0.26S (% d.w.) 0.07

M.E. Lopez-Mosquera et al. / Bioresource Technology 99 (2008) 5626–5633 5627

nitrogen emission to the atmosphere, chemical and micro-bial contamination of water bodies, and odour nuisance(Sims and Wolf, 1994). In the UK, for example, an esti-mated 19% of agricultural N emissions (Pain et al., 1996)and 25% of cases of agricultural odour nuisance (MAFF,1992) are due to chicken litter.

A promising approach for reducing volume, reducingoffensive odours and facilitating storage of chicken litteris drying and pelletizing. The reduced moisture content ofthe dried pelletized organic wastes means that they are eas-ier to store, transport, and apply to soil (John et al., 1996).In the case of chicken litter, these techniques may help toobtain a readily stored product that does not leach anddoes not have bad odour, thus increasing its environmentalacceptability and financial value. In addition, drying andpelletizing may reduce the significant pathogen load andantibiotic residue content of fresh litter (Sims and Wolf,1994).

Several previous studies have investigated the benefits ofdrying chicken litter as regards handling and N contamina-tion risk (Wood and Hall, 1991; Mondini et al., 1996).However, up to our knowledge, there has been no studyof the effects of drying and pelletizing on fertilizer value.The aim of the present study was to investigate the effectsof drying and pelletizing on the basic physicochemicalproperties, nutrient contents, heavy metal contents, patho-gen contents, and antibiotic residue contents of broilerchicken litter.

2. Methods

2.1. Farm characteristics

Litter was obtained from a broiler chicken farm in Por-tomarın (Galicia, NW Spain), consisting of three 2000 m2

concrete-floored houses with a stocking density of 20birds/m2. At the start of each 60-day production cycle,the floor is covered with 5 kg/m2 of rice hull. Rice hull (aby-product of rice-growing in Mediterranean areas ofSpain) is widely used in chicken farms in northern Spain,replacing the traditional pine sawdust or shavings. Ricehull is a highly efficient moisture absorbent (Sweeten,1988), and constitutes an important source of carbon inchicken litter used as fertilizer. Table 1 lists the principalcharacteristics of the rice hull used. It is a material withnear-neutral pH, high C:N ratio, and relatively high Mn,Fe, Zn and Cr contents.

At the end of each production cycle, after removal of thebirds, the litter is removed with a loading shovel and piledunder cover, and the floors are washed and disinfected.Fresh rice hull is then introduced for the next cycle.

2.2. Sampling procedure: fresh litter

Fresh litter was sampled at the end of four productioncycles, from all three sheds, immediately after removal withthe loading shovel. Specifically, 10 samples each of 200 g

were obtained from random points in each pile, at depthsof 50–100 cm. These 10 samples were then pooled. Thetotal number of final 2-kg composite samples of fresh litterwas thus 12 (4 cycles · 3 sheds).

2.3. Sampling procedure: dried pelletized litter

The broiler litter obtained at the end of each productioncycle was processed to obtain the pelletized product. Thelitter was dried and pelletized in a plant located on thefarm, being the only industry producing dried pelletizedbroiler litter in Spain. The fresh litter is first passed througha drying tunnel at 250 �C, then triturated in a hammer mill,then homogenized, then pelleted in a granulator press toobtain pellets of 5 mm diameter and 12–14 mm length.For the present study, eight 200-g samples of dried pellet-ized product were obtained hourly over 8 h during the pro-cessing of the litter from each house. These eight sampleswere then pooled. The total number of final 1.6-kg compos-ite samples of pelletized litter was thus again 12 (4cycles · 3 sheds).

2.4. Chemical analyses

Subsamples of fresh litter and pelletized litter were oven-dried at 105 �C for 24 h, for moisture determination. Theremaining fresh litter was air-dried, then sieved through a2-mm-mesh sieve to remove feather fragments. Theremaining pelletized litter was ground to a particle diame-ter of about 2 mm.

Physicochemical analyses of fresh and dried pelletizedlitter were as follows. pH and electrical conductivity weredetermined in a 1:5 solid:water extracts. Total C, N, andS were determined in sieved samples by dry combustionwith a LECO-2000 autoanalyser. Ammonium (NH4-N),nitrate (NO3-N), urea N and organic N were determinedby AOAC standard methods (AOAC, 1970, 1980).

Total P, K, Ca, Mg, Na, B, Fe, Mn and Mo contentswere determined in extracts obtained by digestion of 2-mm-sieved samples with concentrated sulphuric acid and

5628 M.E. Lopez-Mosquera et al. / Bioresource Technology 99 (2008) 5626–5633

33% hydrogen peroxide at 360 �C (Thomas et al., 1967). Pwas determined by the molybdenum blue method (Chap-man and Pratt, 1961); B by the azomethine-H method(Wolf, 1974); and K, Na, Ca, Mg, Fe, Mn and Mo byatomic emission/absorption spectrophotometry.

Cu, Co, Zn, Cd, Ni, Pb and Cr were determined byatomic absorption spectrophotometry in extracts obtainedby digestion of samples with 70% nitric acid for 30 min ina Milestone Ethos 900 microwave oven (Tessier et al.,1979). The digestion procedure was quality-controlled byparallel analysis of the BCR� certified reference materialBCR 144 R.

Microbiological analyses were performed as per Pascualand Calderon (2000) to test for faecal streptococci, totalenterobacteria and Salmonella. Tests for antibiotic residueswere as per Currie et al. (1998).

2.5. Statistical analyses

The data were subjected to analysis of variance (normaldata) or Kruskal–Wallis analysis and Mann–Whitney Utests (non-normal data). Linear regression analysis wasused to investigate the possible utility of dry matter content

Table 2Physical, chemical and biological properties of the fresh broiler litter, showingprevious studies

Parameter Mean ± SD (n = 12) Previ

Moisture content (%) 26.0 ± 4.4 19.5Ash (% d.w.) 18.0 ± 0.1 8.9Electrical conductivity (dS m�1) 9.9 ± 2.4 6.3pH(H2O) 8.5 ± 0.2 6.3Organic C (% d.w.) 38.2 ± 1.0 29.3Total N (% d.w.) 6.5 ± 0.7 2.6C:N ratio 6.0 ± 0.6 6.4Ammonium N (% d.w.) 0.5 ± 0.1 0.3Nitrate N (% d.w.) 0.3 ± 0.04 0.0Organic N (% d.w.) 5.4 ± 0.6 0.3Urea N (% d.w.) 0.2 ± 0.04 0.7P (% d.w.) 1.7 ± 0.4 0.6K (% d.w.) 2.8 ± 0.5 0.7Ca (% d.w.) 2.0 ± 0.5 0.8Mg (% d.w.) 0.7 ± 0.1 0.2S (% d.w.) 0.5 ± 0.04 0.2Na (% d.w.) 1.6 ± 0.6 0.7Fe (mg kg�1) 737.6 ± 217.1 529.0Mn (mg kg�1) 349.6 ± 39.4 125.0B (mg kg�1) 20.1 ± 3.2 23.0Cu (mg kg�1) 71.3 ± 11.9 22.7Zn (mg kg�1) 261.2 ± 18.9 54.0Cd (mg kg�1) 1.5 ± 0.2 2.4Ni (mg kg�1) <0.1 7.6Pb (mg kg�1) <0.1 14.6Cr (mg kg�1) 27.3 ± 5.3 8.5

Salmonella spp. 0 AbseFaecal streptococci 72 E6 CFUTotal enterobacteria 10 E3 CFU

a 1, Beegle (1997); 2, Brown et al. (1993); 3, Brown et al. (1994); 4, CummisWood (1996); 8, Fulhage and Pfost (1994); 9, Gordillo and Cabrera (1997a); 1et al. (1992); 13, Kunkle et al. (1981); 14, Malone (1992); 15, Marshall et al. (199Chambers (1993); 19, Stephenson et al. (1990); 20, Wood et al. (1999); 21, Zu

as a predictor of nutrient contents. All analyses were per-formed with the statistics package SPSS.

3. Results and discussion

3.1. Characteristics of the fresh and pelletized litter

In what follows, we consider mean properties of thefresh litter and the pelletized litter; note though that someproperties showed variation over time (i.e. among the fourproduction cycles), as will be considered in Section 3.2.

Table 2 summarizes the principal characteristics of thefresh litter. Total N content is high (6.5%), and NPK ratiois 3.8:1.0:1.6. N levels in chicken litter are generally thoughtto reflect the characteristics of the feed (Wood and Hall,1991); in the farm studied here, protein content in the feedranged from 22.6% at the start of the production cycle to19.9% at the end. N content was largely organic N(83%), as obtained by Carballas (1996) for fresh litter from20 farms in northwest Spain. In view of the low C:N ratio,this organic N can be considered as readily mineralized(Alexander, 1967). Most of the inorganic N was ammo-nium N, again in line with the results of Carballas (1996).

ranges of mean values for physical and chemical parameters reported in

ously reported range Sourcesa

–30.6 6, 9, 10, 11, 18, 20–54.4 5, 13, 9, 19–12.6 9, 10, 20–8.4 6, 9, 10, 11,16, 20–38.8 6, 9, 10, 11, 15, 16, 20–5.3 1, 2, 3, 6, 7, 8, 9, 10, 11, 12, 15, 16, 20, 21–11.8 6, 7, 9, 10, 11, 15, 16, 20–1.0 8, 9, 10, 11,15,16, 20, 2105–0.1 9, 10, 11, 15, 20–3.3 8, 9, 10–1.1 9, 10, 16–3.9 2, 3, 5, 8, 10, 11, 12, 16, 18, 20, 21, 17, 19–5.2 2, 3, 5, 8, 11, 12, 16, 18, 20, 21, 19–6.1 2, 3, 5, 11, 18, 20, 19–0.9 2, 3, 4, 5, 11, 16, 18, 20, 19–0.8 16, 18, 20, 19

5, 11–2982.0 1, 3, 5,12, 18, 19–667.0 3, 5, 11, 18, 20, 19–125.0 4, 5, 19–1003.0 3, 5, 11, 18, 20, 19, 14–680.0 2, 3, 5, 11, 12, 18, 20, 19, 14

5–181.5 5, 11–55.0 5, 11

5

nt in 25 gg�1

g�1

et al. (1993); 5, Edwards et al. (1995); 6, Ekinci et al. (2000); 7, Flynn and0, Gordillo and Cabrera (1997b); 11, Henry and White (1993); 12, Koon8); 16, Nicholson et al. (1996); 17, Payne and Donald (1991); 18, Smith andblena et al. (1991).

M.E. Lopez-Mosquera et al. / Bioresource Technology 99 (2008) 5626–5633 5629

Although diverse physiological, environmental andmanagement factors can influence litter composition, ourresults are similar to those obtained in other studies(Table 2). However, total N, organic N, nitrate N andCr contents were relatively high, while B, Pb and espe-cially Cu and Ni levels were relatively low. The Cr contentis attributable to the high Cr content of the rice hull bed-ding (Table 1). The low B, Pb, Cu and Ni contents areprobably attributable to the composition of the feed, par-ticularly as regards Cu salts, antibiotics and coccidiostatics(Sims and Wolf, 1994).

Table 3 shows the principal characteristics of the pellet-ized litter, and also minimum and/or maximum permittedvalues for each parameter under current Spanish legislationfor fertilizers (BOE, 1998). The pelletized product has highN content (5.2% dry weight, 80% as organic N), as well ashigh P and K contents. NPK ratio is 3.2:1.0:1.6. As in thefresh litter, most of the inorganic N was ammonium N.C:N ratio was low, in line with the high N content, andagain the organic N can thus be considered readily miner-alizable. All of these characteristics indicate that the pellet-

Table 3Physical, chemical and biological properties of the dried pelletized broiler litfertilizer, (b) ‘‘organic fertilizer with specified secondary element/s’’, and (c) ‘‘

Parameter Mean ± SD(n = 12)

(a) Criteria: organicfertilizer (OF)

(b) AddiOF + se

Moisture content (%) 10.1 ± 4.1 < 35.0Ash (% d.w.) 18.5 ± 0.5Electrical conductivity

(dS m�1)11.1 ± 1.1

pH (H2O) 7.9 ± 0.4Organic C (% d.w.) 36.8 ± 1.9 >17.4Total N (% d.w.) 5.2 ± 1.0C:N ratio 7.3 ± 1.6 3–15Ammonium N (%

d.w.)0.5 ± 0.1

Nitrate N (% d.w.) 0.4 ± 0.05Organic N (% d.w.) 4.2 ± 1.1 >2.0Urea N (% d.w.) 0.2 ± 0.04P (% d.w.) 1.6 ± 0.3 >0.44K (% d.w.) 2.6 ± 0.3 >0.83Ca (% d.w.) 1.9 ± 0.4 >2.1

Mg (% d.w.) 0.6 ± 0.1 >1.2

S (% d.w.) 0.6 ± 0.01 >2.0

Na (% d.w.) 1.2 ± 0.3 >2.2

Fe (mg kg�1) 792.2 ± 135.9Mn (mg kg�1) 187.0 ± 19.4B (mg kg�1) 13.9 ± 2.3Cu (mg kg�1) 63.6 ± 4.3 <450Mo (mg kg�1) 12.3 ± 3.5Zn (mg kg�1) 259.8 ± 36.0 <1100Cd (mg kg�1) 0.02 <3Ni (mg kg�1) < 0.1 <120Pb (mg kg�1) 26.6 ± 2.7 <150Cr (mg kg�1) 7.4 ± 2.4 <270Salmonella spp. 0 absent in 25 gFaecal streptococci 0 <1.0 · 103 MPN g�1

Total enterobacteria 0 <1.0 · 102 CFU g�1

Antibiotics negative

Criteria NOT met are indicated in bold.

ized product has high fertilizer value, and indeed theproduct meets all current Spanish legislative requirementsfor marketing as an organic fertilizer (Table 3). At the sametime, and as noted, the low moisture content of the pellet-ized form greatly facilitates storage, transport and mecha-nized application in the field.

Heavy metal contents in the pelletized litter were in allcases much lower than the legally permitted maximum val-ues for organic fertilizers (Table 3). Neither Salmonella,faecal streptococci nor enterobacteria were detected, indi-cating that drying at 250 �C eliminates these pathogens;likewise, no antibiotic residues were detected. In addition,and although no objective tests were performed, the pellet-ized product clearly showed less offensive odour than thefresh product. These results thus indicate that the driedproduct is fully acceptable for fertilizer use as regards con-tamination risk. Additionally, the product meets Spanishlegislative requirements for marketing as ‘‘containing theoligoelements Mo and Zn’’ (for horticulture) and ‘‘contain-ing the oligoelements Fe, Mn, Cu, Mo and Zn’’ (for pas-ture and extensive crops) (Table 3).

ter, showing Spanish legislation criteria for consideration as (a) organicorganic fertilizer with specified oligoelement/s’’ (BOE, 1998)

tional criteria:condary element/s

(c) Additional criteria: OF + oligo/s (p:pasture, h: horticulture)

>5000(p), >200(h)>1000(p), >100(h)>100

>100(p), >20(h)>10>100(p), >20(h)

5630 M.E. Lopez-Mosquera et al. / Bioresource Technology 99 (2008) 5626–5633

3.2. Variability over time

A frequently noted characteristic of fresh animal man-ures is marked variability in composition. The factors giv-ing rise to this variability include stock-related factors (age,variety, stocking density), as well as factors relating to feed-ing, litter and losses of nutrients, especially N (for review,see Edwards and Daniel, 1992). Here, with a view to theuse of pelletized chicken litter as a fertilizer, we have inves-tigated variability among the four consecutive productioncycles in the properties of the fresh and pelletized litters.

Table 4 summarizes the results of this analysis. Parame-ters that did not show significant among-cycle variability ineither fresh or pelletized litter were ash content, total C,organic N, Ca, Mg, Mn, B, Ni, Zn and Cr. One parameter,C:N ratio, showed significant variability in both products.Parameters that showed significant variability in fresh butnot pelletized litter were dry matter, electrical conductivity,urea N, K, S, Na, Fe, Cu and Cd. Parameters that showedsignificant variability in pelletized but not fresh litter werepH, total N, ammonium N, nitrate N and total P.

By comparison with previous studies (Stephenson et al.,1990; Edwards and Daniel, 1992; Nicholson et al., 1996),the number of parameters which did not show significantvariability in the fresh product is large; however, these pre-

Table 4Temporal variability (i.e. variability among the four production cycles) in the

Category Parameter

(a) No significant temporal variability in either product AshTotal COrganic NCaMgMnBNiZnCrPb

(b) Significant variability in both products C:N ratio

(c) Significant variability in the fresh but not the pelletizedproduct

Dry matter cElectrical conUrea NKSNaFeCuCd

(d) Significant variability in the pelletized but not the freshproduct

pHTotal NAmmoniumNitrate NP

Parameters are grouped into four categories: (a) without significant temporal va(c) with significant variability in the fresh but not the pelletized product, and*Significant variability (p < 0.01); n.s. = not significant.

vious studies considered litter obtained from differentcycles and from different farms, whereas in the presentstudy we considered only litter from a single farm.

As noted, the pelletization process reduced variability innine parameters, however, it increased variability in fiveparameters (pH, total N, ammonium N, nitrate N and totalP). This is perhaps attributable to variability in the temper-ature reached during the drying process, suggesting that itmay be important to control this factor carefully. Dryingtemperature is probably especially important for nitrogen,since NH3 volatilization increases exponentially with tem-perature (Wood and Hall, 1991).

3.3. Effects of drying and pelletizing on composition

Table 5 shows the analysis of differences in mean com-position between the fresh and pelletized products. Dryingand pelletizing reduced moisture content by over 60%,from an average of 26.1% in the fresh product to an aver-age of 10.1% in the pelletized product. The reduced mois-ture content and associated volume reduction andpelletization greatly facilitate storage, transport and appli-cation. In contrast, the storage and application of freshmanures typically gives rise to diverse environmental prob-

fresh and dried pelletized litters

F or v2 value, freshproduct

F or v2 value, pelletizedproduct

F = 0.1 n.s. v2 = 8.3 n.s.v2 = 7.3 n.s. v2 = 8.9 n.s.F = 4.0 n.s. F = 6.1 n.s.F = 5.7 n.s. F = 0.1 n.s.F = 4.8 n.s. F = 5.2 n.s.F = 9.3 n.s. F = 3.3 n.s.F = 1.1 n.s. F = 3.0 n.s.F = 6.1 n.s. F = 2.1 n.s.F = 4.9 n.s. F = 2.8 n.s.F = 3.3. n.s. F = 10.3 n.s.– F = 1.4 n.s.

F = 7.4* F = 11.5*

ontent F = 208.9* F = 9.5 n.s.ductivity F = 123.5* F = 3.9 n.s.

F = 90.4* F = 8.8 n.s.F = 6.7* F = 3.3 n.s.F = 79.8* F = 6.1 n.s.F = 27.8* F = 0.8 n.s.F = 6.9* F = 0.4 n.s.F = 8.9* F = 2.0 n.s.F = 9.0* F = 2.0 n.s.

v2 = 9.8 n.s. F = 902.8*

F = 5.1 n.s. F = 8.3*

N v2 = 6.4 n.s. F = 19.9*

v2 = 8.1 n.s. F = 27.1*

F = 4.8 n.s. F = 7.2*

riability in either product, (b) with significant variability in both products,(d) with significant variability in the pelletized but not the fresh product.

Table 5Differences in physical and chemical properties between the fresh product and the dried pelletized product, showing (a) F or U values for the between-product comparison and (b) the difference between means (e.g. the mean ash content of the pelletized product was 0.5 units higher than that of the freshproduct)

Category Parameter (a) F or U value (b) Pelletized vs. fresh(difference between means)

No temporal variability Ash (% d.w.) F = 1.8 n.s. +0.5Organic carbon (% d.w.) U = 27.0* �1.4Total N (% d.w.) F = 13.9* �1.3Ca (% d.w.) F = 0.4 n.s. �0.1Mg (% d.w.) F = 2.0 n.s. �0.1Mn (mg kg�1) U = 6.0* �162.6B (mg kg�1) F = 29.2* �6.2Zn (mg kg�1) F = 0.2 n.s. �1.4Pb (mg kg�1) — +26.6

Temporal variability Dry matter (% d.w.) Fmin = 777.2* (3 cyc) +15.1pH Fmin = 1008.2* (2 cyc) �0.6Electrical conductivity (dS m�1) Fmax = 12.7 n.s. +1.2C:N ratio Fmax = 15.7 n.s. +1.3Organic N (% d.w.) Fmax = 13.8 n.s. �1.2Ammonium N (%d.w.) Fmax = 5.1 n.s. 0.0Nitrate N (% d.w.) Fmax = 10.3 n.s. +0.1Urea N (% d.w.) Fmax = 6.3 n.s. 0.0P (% d.w.) Fmax = 1.8 n.s. �0.1K (% d.w.) Fmax = 16.6 n.s. �0.2S (% d.w.) Fmin = 70.6* (4 cyc) +0.1Na (% d.w.) Fmax = 8.2 n.s. �0.4Cu (mg kg�1) Fmin = 25.3

*(2 cyc) �7.7

Cr (mg kg�1) Fmin = 32.0*

(2 cyc) �19.9Cd (mg kg�1) Fmin = 174.0* (3 cyc) �1.5

For parameters not showing temporal variability (see Table 4), the two products were compared by considering the samples obtained from the fourdifferent cycles (cyc) simply as replicates (n = 12); for parameters showing temporal variability, comparisons were performed cycle by cycle (n = 3 in eachcase), and an overall significant difference between the fresh and pelletized product was defined to be present when significant differences were detected intwo or more of the four cycles; for these latter parameters, the value shown is the maximum F value for nonsignificant comparisons, or the minimum F

value for significant comparisons, with the number of individual significant values in brackets.*Significant difference (p < 0.01); n.s. = not significant.

M.E. Lopez-Mosquera et al. / Bioresource Technology 99 (2008) 5626–5633 5631

lems, including N emission to the atmosphere and leachingto groundwater (Sims and Wolf, 1994).

Drying and pelletization likewise led to a significantreduction in total C and especially total N content,although C:N ratio was not significantly affected. On aver-age, the pelletized product showed 4% less total C and 20%less total N than the fresh product. These reductions prob-ably reflect the high temperature (250 �C) to which thefresh product was subjected. Wood and Hall (1991) alsodetected a significant reduction in total C after heatingfresh chicken litter to 40–60 �C, much lower temperaturesthan that applied in the present study. In the case of totalN, and as noted, it is well known that losses increase withincreasing drying temperature, largely as a result of NH3

volatilization (Gale et al., 1991; Wood and Hall, 1991).However, we did not detect differences between the freshand pelletized products in ammonium N, nitrate N or ureaN.

pH remained basic but was significantly lower in the pel-letized product (mean 7.9) than in the fresh product (mean8.5). It is known that the oxidation of the organic matter isan acidifying process (McBride, 1994). The decrease in pHwas possibly due to the release of H+ that are associatedwith organic anions during drying and pelletization. In

any case, the alkalinity of the pelletized product makes itespecially suitable for neutral or acid soils.

Wood and Hall (1991) found that the P, K, Cu, Fe andZn contents of chicken litter were not affected by dryingtemperatures of up to 60 �C. Henry and White (1993)reported an increase in the contents of various elementsafter composting fresh litter, which they attributed simplyto weight reduction. In the present study, by contrast, heat-ing at 250 �C led to significant reductions in B, Mn, Cu andCr contents with respect to the fresh product, together withan increase in Pb content (Table 5). This latter may beattributable to contamination by the apparatus used inthe drying and pelletizing process.

Despite great variability in the composition of poultryfeeds, Nicholson et al. (1996) have suggested that the drymatter content of the litter is a good predictor of total N,P, K, Mg and S contents expressed with respect to freshweight. The results of linear regression of these contentson dry matter content, in both the fresh and pelletized lit-ters, are presented in Table 6. The dry matter content of thepelletized product was a good predictor of the variousnitrogen contents, but not of other parameters. This sug-gests that simple determination of dry matter contentmay be effective for estimating the total N content of pellet-

Table 6Results of linear regressions with dry matter content (DM) as candidate predictor variable and chemical properties as dependent variables

Litter Regression equation (n = 12) r2 F value p value

Fresh Total N = �5.37 · 10�3 DM + 6.89 0.00 0.01 0.940Pelletized Total N = �0.22 · DM + 24.37 0.64 18.01 0.002

Fresh N-org = �2.8 · 10�2 DM + 7.49 0.04 0.37 0.560Pelletized N-org = �0.18 · DM + 20.49 0.60 14.94 0.003

Fresh N-ammon = 1.20 · 10�2 DM � 0.40 0.23 2.91 0.119Pelletized N-ammon = �1.40 · 10�2 DM + 1.71 0.62 16.58 0.002

Fresh N-urea = 5.97 · 10�3 DM � 0.23 0.46 8.40 0.016Pelletized N-urea = �8.5 · 10�3 DM + 0.97 0.68 21.41 0.001

Fresh N-nitr = 1.35 · 10�3 DM + 0.02 0.22 2.73 0.129Pelletized N-nitr = �9.33 · 10�3 DM + 1.19 0.69 22.19 0.001

Fresh Total P = �3.71 · 10�2 DM + 4.40 0.18 2.20 0.171Pelletized Total P = �1.40 · 10�2 DM + 2.86 0.04 0.41 0.539

Fresh K = 5.75 · 10�2 DM � 1.47 0.24 3.11 0.108Pelletized K = �8.33 · 10�2 DM + 10.01 0.29 4.16 0.069

Fresh Mg = 1.35 · 10�2 DM � 0.32 0.35 5.34 0.043Pelletized Mg = �8.5 · 10�3 DM + 1.38 0.11 1.18 0.303

Fresh S = �5.10 · 10�3 DM + 0.85 0.26 3.48 0.092Pelletized S = 1.53 · 10�3 DM + 0.44 0.20 2.50 0.145

All analyses were performed with n = 12 (3 sheds · 4 cycles).

5632 M.E. Lopez-Mosquera et al. / Bioresource Technology 99 (2008) 5626–5633

ized chicken litter. This may be of particular value as abasis for rapid quality-control tests during the drying/pel-letizing process.

In conclusion, dried pelletized chicken litter has morestable nutrient characteristics than the fresh product. Ncontent can be estimated from dry matter content. It doesnot contain faecal bacteria and does not have significantlynoxious odour. From a practical point of view, the pellet-izing process eases storing, transport and field applicationof the broiler litter, and can also facilitate the incorpora-tion of other components into the final product, includingfor example mineral elements, herbicides or nitrificationinhibitors.

Acknowledgements

The authors thank Juan Carlos Serrano, head of Avi-porto S.L., for facilitating sampling of fresh and pelletizedlitter. We also thank Cristina Vazquez and Susana Dopicofor technical assistance in sample analysis. This work wasfinanced by the Directorate General for Research of theSpanish Ministry of Science and Technology (projectAGL2000-04-81).

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