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INTERNATIONAL JOURNAL OF ENVIRONMENTAL SCIENCES Volume 1, No 5, 2011

© Copyright 2010 All rights reserved Integrated Publishing Association

Research article ISSN 0976 – 4402

Received on December, 2010 Published on January 2011 847

Changes in the composition of poultry farm excreta (PFE) by the cumulative influence of the age of birds, feed and climatic conditions and a

simple mean for minimizing the environmental hazard Iyappan.P 2 , Karthikeyan.S 1 , Sekar.S 1

1 Department of Biotechnology, Bharathidasan University, Tiruchirappalli – 620 024, 2 Department of Biotechnology, Muthayammal College of Arts & Science, Namakkal

[email protected]

ABSTRACT

Constant growth of poultry industries impose environmental burden due to the release of numerous noxious volatiles from the Poultry Farm Excreta (PFE). Development of suitable waste management practices depend on the understanding of the factors involved particularly by the cumulative influence of the age of the birds, climatic conditions and nature of feed used. The study highlights the degree of changes in the composition of PFE due to the cumulative actions of these factors. Furthermore, the accumulation of PFE also promoted the loss of various nutrients. It is demonstrated that, insitu drying in onfarm shadow condition itself minimize the loss of nutrients and hence the intensity of environmental hazards.

Keywords: Poultry manure, excreta composition, growth phase, feed, onfarm drying.

1. Introduction

Poultry industry is one of the largest and fastest growing sectors in livestock operation. The term ‘poultry’ applies to domesticated birds that are normally slaughtered and prepared for market. Poultry includes chickens, turkeys, ducks, geese, swans, pigeons, pea fowls, guinea fowls, pheasants, quails and other game birds, together with ratites such as ostriches and emus.

In poultry production, chickens are the major birds produced. In 2009, chicken meat accounted for 92.88% where as other birds accounted for 7.12% only (FAOSTAT, 2010). In egg production, 6.4 billion laying hens were produced globally in 2009. China is the leading egg producer (39.28%) followed by USA (5.25) and Brazil (4.22) (FAOSTAT, 2010). Asia is the leading eggproducing continent contributing to 64.3%. In 2009, hens are the major egg producing birds (97.78%) whereas other birds account only for 2.22% (FAOSTAT, 2010). Global production of laying hens has also increased steadily in the recent years. Laying hens were estimated to produce an average of 68g of dry manure/bird/day (Bell, 2002). About 6.4 billions of laying hens were reported worldwide during 2009 (FAOSTAT, 2010). This can account for the accumulation of about 0.435 million dry metric tonnes of manure/day or 158.8 million dry metric tonnes of manure/year. The problems arise from poultry industries are due to inadequate land facilities and insufficient knowledge of waste management practices. Most of the poultry producers store the poultry wastes in the form of open stocks piles, compost piles and basin or lagoons. These waste products stored for long time within farmhouses produce offensive odorous gases including ammonia and certain volatile compounds by mainly the action of microbes is a major problem in poultry farm industries. More than 160 volatile compounds have been identified as contributing to the odor from the poultry and livestock confinement facilities. Common odorous compounds include volatile fatty acids (acetic, propionic, butyric, isobutyric, isovaleric acids), ammonia, amines

Changes in the composition of poultry farm excreta (PFE) by the cumulative influence of the age of birds, feed and climatic conditions and a simple mean for minimizing the environmental hazard

Iyappan.P, Karthikeyan.S , Sekar.S International Journal of Environmental Sciences Volume 1 No.5, 2011

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(methylamine, ethylamine), phenolics/heterocycles (phenol, pcresol, indole, skatole) and sulfur compounds (Hydrogen sulfide, Dimethyl sulfide, Methyl mercaptan, Ethyl mercaptan, Diethyl sulfide) (Schaefer, 1977; Spoelstra, 1980).

Poultry Farm Excreta (PFE) contains all the nutrients required for plant growth. In research literature, work on the applications of PFE is minimal when compared to patent literature. An extensive survey of both commercial and patent literature was made to indicate the possible applications of PFE as fertilizer, feed and fuel (Sekar et al., 2010). The major concern and the limiting factor in the development of effective waste management strategy is due to the wide range in the composition of poultry manure (Edwards and Daniel, 1992): moisture (36.977.0%), macronutrients such as carbon (22.432.8%), total nitrogen (1.827.2%), total phosphorus (1.353.4%), potassium (1.253.25%), calcium (3.625.96%), magnesium (0.18 0.66%), sodium (0.20.74%) and the micronutrients like manganese (259379 ppm) iron (80 560 ppm), copper (3868 ppm) and zinc (298388 ppm). Further, the leaching of nutrients particularly N and P from PFE is another environmental problem causing surface and ground water contamination. Hence, proper scientific knowledge on the composition of poultry farm excreta is required for the development of technological packages for the production of valuable resources from PFE such as fertilizer, feed stuff for ruminants or generation of fuel or electricity.

The objectives of the present work are two fold: 1) to understand the cumulative effect of various factors like age of the birds, feed given to birds and the climatic conditions on the physicochemical composition of PFE and 2) to evaluate the effect of quick drying in an on farm shadow condition on the nutrient loss of PFE in order to evolve management practices.

2. Materials and methods

2.1 Experimental design and Sampling

The study was performed for the entire growth period of white leghorn layer chickens. The growth period of layer chickens were divided into three stages viz., chicks (day old to 8 weeks), growers (9 weeks to 18 weeks) and layers (19 weeks to 72 weeks). The PFE samples were collected from a highrise layer farm located at Namakkal (11.23°N 78.17°E), Tamilnadu, India. In order to cover the entire growth phase of chickens, the sampling of fresh manure was done in the middle of 2 nd , 4 th , 7 th , 10.75 th 14.25 th , 25 th , 35 th , 46 th , 55 th and 66 th weeks of bird’s growth. Accumulated PFE samples were collected at the end of each growth phases of the bird (chicks – accumulated for 8 weeks; growers accumulated for 10 weeks; layers accumulated for 6 months). Fresh samples were collected from the free falling excreta and accumulated samples were collected from the excreta accumulated on the floor in the same highrise farm site. The collected samples were kept at 4°C and transported to lab within 3 hours. The samples were then stored at 20°C for biochemical analysis. The analysis was carried out in duplicates. The feed composition given to different growth phases of layers are provided (Table 1). Climatic conditions such as temperature, relative humidity, wind speed and rain fall existing in the sampling site during the study period (November 2006 to February 2008) were recorded from regional metrological center, Namakkal (Figure 1 to 4).

To study the effect of natural drying in the onfarm condition, poultry manure samples from 27 weeks old layers were collected. The excreta were collected on the surface of polythene sheets and kept for 10 days in an open shadow condition away from the rearing shed during the peak summer period (May 2007) in the farm site itself. It thus facilitated drying as there



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were no further addition of excreta from above. From this, samples were analysed at two days intervals.

2.2 Biochemical Characterization of PFE

Moisture content was determined after oven drying 10g of sample at 110°C for 6 h [15]. Organic carbon was analysed following the oxidation procedure (Walkley and Black, 1932). The total nitrogen and total Kjeldahl nitrogen were analyzed by macroKjeldahl method. Samples were digested using a auto 2000 block digestion system, distilled in 2100 Kjeltec TM

distillation unit (FOSS Tecator, Sweden) and analyzed for nitrogen as described in Foss Tecator Application Note, AN300 (Bremner and Mulvaney, 1982; Foss Tecator, 2003). Total phosphorus was analyzed using VanadoMolybdic acid method (Tandon, 1993). Sulfur was determined by turbidimetric method (Rayment and Higginson, 1992). Boron was analysed by Azomethane H method (Helrich, 1990). The nutrients like total potassium, calcium, magnesium, sodium, iron, manganese, zinc, and copper were analysed by Unicam 939 atomic absorption spectrophotometer (APHA, 1998) (Unicam ® Limited, UK).

3. Results and discussions

Climatic conditions such as temperature, relative humidity, wind speed and rain fall existing in the sampling site during the study period (November 2006 to February 2008) were recorded (Figure 1 to 4). It indicates major changes in the climatic conditions according to the four major seasons. The nature of feed fed to different growth phases of birds were provided (Table 1) indicating the change in the diet composition according to the rearing stages of the birds.

10

15

20

25

30

35

40

Nov. 1 2006

Dec. 1 2006

Jan. 1 2007

Feb. 1

Mar. 1

Apr. 1

May. 1

Jun. 1

Jul. 1

Aug. 1

Sep. 1

Oct. 1

Nov. 1

Dec. 1 2007

Jan. 1 2008

Feb. 1 2008

Month

Temperature (C)

Max. Temp.

Min. Temp.

Figure 1: Temperature profile of the study area during the study period

10 20 30 40 50 60 70 80 90 100

Nov. 1 2006

Dec. 1 2006

Jan. 1 2007

Feb. 1

Mar. 1

Apr. 1

May. 1

Jun. 1

Jul. 1

Aug. 1

Sep. 1

Oct. 1

Nov. 1

Dec. 1 2007

Jan. 1 2008

Feb. 1 2008

Month

Relative humidity (%)

at 7.30 AM

at 2.30 PM

Figure 2: Relative humidity profile of the study area during the study period



850

0 2 4 6 8 10 12 14 16 18

Nov. 1 2006

Dec. 1 2006

Jan. 1 2007

Feb. 1

Mar. 1

Apr. 1

May. 1

Jun. 1

Jul. 1

Aug. 1

Sep. 1

Oct. 1

Nov. 1

Dec. 1 2007

Jan. 1 2008

Feb. 1 2008

Month

Wind speed (km/h)

Figure 3: Wind speed profile of the study area during the study period

105

4 0 0 0

38 28.3 7 9

103

40

262

35.2

166.5

5 0 0 25 50 75 100 125 150 175 200 225 250 275

Nov. 2006

Dec.

Jan. 2007

Feb.

Mar.

Apr.

May

Jun.

Jul.

Aug. Se

p. Oct.

Nov.

Dec.

Jan. 2008

Feb. 2008

Month

Rainfall (mm)

Figure 4: Rainfall of the study area during the study period

Table 1: Composition of feed fed to layers prior to sampling during the study period (November 2006 to February 2008)

Feed Ingredients

2 nd Week

4 th Week

7 th Week

10.75 th Week

14.25 th Week

18 th Week

25 th Week

35 th Week

46 th Week

55 th Week

66 th Week

Maize (kg/tonne) 545 545 545 560 455 500 500 490 310 485 480

Soya DOC (kg/tonne) 360 340 340 255 280 230 230 85

Groundnut cake (kg/tonne)

85

Full Fat Soya (kg/tonne)

20 40 40

Jower (kg/tonne) 70 100

Fish Meal (kg/tonne) 20 20 70 40 40 40 30 70

Rice Bran Oil (kg/tonne)

20 20 20 10 20 10 10 5

De oiled Rice Bran (kg/tonne)

40 40 40 60 100 130 130 85 180 105 155



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Rapeseed (kg/tonne) 30 30 30 30

Shell Grit (kg/tonne) 10 10 10 10 16 25 25 72 70 65 60

Calcite (kg/tonne) 6 5 5 3 7 12 12 30 30 30 30

Dicalcium PO4 (kg/tonne)

14 12 12 6 9 8 8 12 9

NaCl (kg/tonne) 3.5 3.5 3.5 2.5 3.5 3.0 3.0 3.5 3.5 3.0 2.0

Sunflower DOC (kg/tonne)

120

Bajra (kg/tonne) 50 100 50

Soya (kg/tonne) 230 140 60

Sun flower Oil (kg/tonne)

5

Oil fish (Mathi) (kg/tonne)

20

Lysine (kg/tonne) 0.5 0.4 0.4

Meat and bone meal (kg/tonne)

20

Trace minerals (kg/tonne)

1 1 1 1

DL Methionine (kg/tonne)

1600 1600 1600 1600 1600 1000 1 1

Bacitracin (g/tonne) 150 150 150 150 150 150 150

Cocktile Enzyme (g/tonne)

400 400 400 400 400 400 400 400 500 400 400

Kemenzyme (g/tonne)

Phytase – 2500 IU (g/tonne)

120 120 120 120 120 120 150 120 120 120

Choline Chloride (g/tonne)

1000 1000 1000 1000 1000 1000 1500 500 500 500

Liver Powder (g/tonne)

500 500 500 500 500 500 500 400 500 400 400

Lay Vit (g/tonne) 550 550 550 550 550 600 600 500 500 500

Toxin Binder (g/tonne)

500 500 500 500 500 500 500 2000 1000 1000 1000

Iron (ppm/kg) 60 60 60 60 60 60 60



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Manganese (ppm/kg) 80 80 80 80 80 80 80

Zinc (ppm/kg) 80 80 80 80 80 80 80

Selenium (ppm/kg) 0.3 0.3 0.3 0.3 0.3 0.3 0.3

Copper (ppm/kg) 15 15 15 15 15 15 15

3.1 Cumulative effect of growth phases/age of birds, feed composition and environmental conditions on the composition of fresh excreta The analysis showed that the moisture in fresh PFE ranges from 60 to 78% (Figure 5). Moisture levels in poultry waste can vary greatly depending on many factors, including variations in diet, age of the bird and digestive health, as well as management practices (Patterson and Lorenz, 1997). The content of organic carbon in the excreta steadily increases in the phase of chicks and remain almost stable in the phases of growers and layers (Figure 5). This could be because of the addition of fat soya and increase in rice bran in the feed starting from the stage of growers (Table 1). Similarly, nitrogen content of excreta increases in the phase of growers. This could be because of the increase in the content of fish meal in the feed (Table 1). The composition of manure is directly influenced by layer feed composition, and so higher levels of nitrogen in feed for example is expected to result in the presence of more nitrogen in the manure (Leeson and Summers, 2008). The presence of high amount of major nutrients in manure showed only a small portion of dietary nutrients are retained in the body of the bird. Patterson and Lorenz (1997) also indicated that 42.2% of N, 73.84% of P, 85.61% of K, 79.86% of Ca and 81.22% of Mg were excreted in manure. Using lower protein or lower phosphorus diets will invariably result in less of nitrogen and phosphorus appearing in the manure. Reducing the level of crude protein in pullet diets, as a means of reducing nitrogen in the manure was proposed (Leeson and Summers, 2008). Moreover, Nitrogen excretion can be dramatically reduced by supplying a balance of amino acids that more exactly meets the bird’s needs with minimum of excess, and also by providing these amino acids in a readily digested form (Leeson and Summers, 2008). The content of calcium steadily increased from beginning to end (Figure 5) which could be attributed to the successive increase in the addition of calcite and shell grit in the feed in addition to the addition of zover (bajra) (Table 1). The level of elements like phosphorus, magnesium, sodium, sulfur, iron, manganese and zinc showed either marginal increase or marginal decrease (Figure 5). This could be because of the addition of dicalcium phosphate, sodium chloride, trace mineral mix, iron, manganese and zinc salts into the feed (Table 1). The content of potassium showed a gradual decrease. The level of copper and boron showed gradual increase in the early phases of growth followed by a decrease (Figure 5). The increased manure copper levels indicate the vast majority of the dietary copper is not retained. Moreover, 6ppm of dietary copper intake resulted in the presence of 36ppm in dry matter of the manure (Leeson and Summers, 2008). Reevaluation of the levels of trace minerals fed to layers is necessary, because manure concentration of zinc and copper may come under closer scrutiny due to soil accumulation.

Changes in PFE composition during specific growth phases of chickens are obvious. The increase in moisture, organic carbon, zinc, copper and boron was found during chick phase (08 weeks). The content of total phosphorus, magnesium and sodium were found to decrease and the content of total nitrogen, potassium, calcium, sulfur, iron and manganese showed marginal increase or decrease during chick phase (Figure 5). In grower phase (918 weeks), the content of moisture, carbon, magnesium, sodium, sulfur, manganese, zinc and boron



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remain constant between growth phases (i.e., between chicks and growers) but, total nitrogen, phosphorus, calcium and copper showed increasing trend. Within the grower phase, the content of carbon, nitrogen, phosphorus, calcium, sodium, iron, manganese and copper showed marginal increase or decrease. Magnesium and boron were found to decrease during the growth phase but sulfur and zinc were found to increase during the growth phase (Figure 5). The content of moisture, carbon, nitrogen, magnesium, sodium, sulfur, iron, manganese and zinc showed marginal increase or decrease among the growth phases but potassium, copper and boron showed decreasing trend while phosphorus and calcium showed increasing trend. In layer phase (1972 weeks), moisture, carbon, nitrogen, phosphorus, potassium, magnesium, sodium, iron, manganese, zinc, copper and boron showed marginal decrease while calcium and sulfur showed an increase (Figure 5). Literature also suggest that the manure composition widely influenced by the (1) age and breed of chickens, (2) density of confinement, (3) feed conversion rate, (4) feed ration, (5) type and amount of bedding material, (6) moisture content of bedding material, (7) type of floor, (8) climatic conditions during litter accumulation, and (9) organic matter and N losses (Perkins et al., 1964; Edwards and Daniel, 1992). The observed changes might also be due to the changes in the physiological stages of the birds apart from feed and environment.



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Figure 5: Characterization of fresh excreta of layers (dry weight basis) probed for the entire growth stages (66 weeks) from chicks to layers (November 2006 to February 2008)

3.2 Characteristics of accumulated excreta

The level of moisture ranges from 7 to 31% (Figure 6). Although moisture is low, it is comparable in both chick and grower excreta. The level of moisture is the highest in the layer excreta and it is 23 times higher when compared to chicks and grower excreta (Figure 6). In the case of macronutrients such as nitrogen, phosphorus, potassium, calcium and sulfur, there is either a gradual or steep increase in the level in excreta in relation to the age of the chicks (Figure 6). However, the level of carbon showed a slight decrease when the birds pass on to grower and layer stages. In the case of sodium, the level was increased from chicks to grower stage but it was lowest in the layer stage (0.5%).

The level of magnesium remains almost the same (around 0.6%) in excreta of all the growth phases (Figure 6). In the case of micronutrients, the content of iron remained more or less similar in the growth phases. The level of manganese and zinc in the chicks excreta and growers are comparable but their level increased to almost two fold in the case of layer excreta (Figure 6). The element copper increases from chicks to grower phase but decreases to minimum in the case of layer excreta.

The content of boron showed gradual decrease from chicks to layer excreta (Figure 6). So the changes in the composition of excreta could be attributed to the practice of accumulating PFE in the farm site. The accumulation or retaining of PFE facilitates subsequent falling of fresh excreta over the top of the already accumulated excreta in the floor of the poultry farms. It causes retaining of moisture in the excreta promoting intense microbial activity.



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Figure 6: Characterization of accumulated excreta of layers (dry weight basis) probed for the entire growth stages (66 weeks) from chicks to layers (November 2006 to February 2008)



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3.3 Changes upon accumulation of excreta

There was a drastic reduction in moisture due to evaporative loss upon accumulation. In all the growth phases, all of the nutrients by and large showed a distinct reduction trend or close to the maximum values of the range in the fresh sample (Figure 5 and 6). It is an indication that there is a loss of nutrients upon accumulation. Various manure nutrient loss especially nitrogen loss occurs during accumulation and storage of manure (Fulhage and Pfost, 2002). For example, about 10% to 80% of manure nitrogen were lost during various manure storage practices including deep pit, anaerobic lagoons, etc. (Harrison, 2004). Changes in nutrient content or loss of nutrients can occur due to dilution (e.g., rainwater entering a liquid storage system), settling (e.g., phosphorus precipitation and accumulation in lagoon sludge), or gaseous loss (e.g., nitrogen volatilization) (Hansen, 2006). Loss of sulfur and nitrogen is either prominent or close to the minimum value of range in the fresh sample indicating possible odor volatilizations which contribute to noxious odor and atmospheric pollution.

3.4 Effect of onfarm shadow drying on the chemical composition of collected PFE

Onfarm shadow drying showed drastic reduction in moisture content from 70% to 10% in a matter of two days (Figure 7). Further it reached a bare minimum of 2.3% upon drying for eight days. During this period, the nutritional profile of various micro and macroelements showed fluctuations in their level but within a narrow range (Figure 7). It is an indication that quick drying of PFE will be helpful to preserve the profile of nutrients and also to further avoid pollution and its consequences.



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Figure 7: Effect of drying of collected excreta in the onfarm shadow condition on the composition of 27 weeks old layer excreta (dry weight basis)

4. Conclusions

The nutrient profile of PFE indicates its diverse and heterogeneous nature. It varies with the growth phases of birds, nature of feed and climatic conditions prevailing thereupon. Loss of nutrients is evident due to the accumulation of PFE. Presence of high content of moisture is the major factor in causing nutrient loss by supporting microbial activity. Quick drying is one of the most promising ways to minimize the loss of nutrients and also to avoid pollution and its consequences. Most of the nutrients resulting from poultry feeding operations is in manure and not in animal protein (Robinson and Beauchamp, 1982). Hence, proper utilization of



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manure nutrients is pivotal. This study will pave way further to investigate the microbiological impact on the heterogeneity of PFE composition.

5. Acknowledgement

The authors acknowledge University Grants Commission (UGC), Government of India for project funding and M/s. Samithurai Poultry Farms (P) Limited, Namakkal, India for enabling this work.

6. References

1. APHA. Standard methods for the examination of water and waste water. 20 th edition. American Public health Association. 1998, Washington DC.

2. Bell, D.D. Waste management, In: Commercial chicken meat and egg production. 5th Edition, (eds. D.D. Bell and W.D. Weaver Jr.), Kluwer Academic Publishers, 2002, Norwell, Massachusetts.

3. Bremner, J.M., and Mulvaney, C.S. Nitrogentotal. In: Methods of soil analysis: Part 2. Chemical and microbiological properties (eds. A.L. Page, R.H. Miller and D.R. Keeney). American Society of Agronomy, 1982, Madison, Wisconsin

4. Edwards, D.R., and T.C. Daniel. Environmental impacts of onfarm poultry waste disposal A review. Bioresource Technology 1992, 41, pp 9–33.

5. FAOSTAT Food and Agriculture Organization, Lives stock primary, http://faostat.fao.org/site/569/DesktopDefault.aspx?PageID=569 Accessed September 26, 2010.

6. Foss Tecator, The determination of nitrogen according to kjeldahl using block digestion and steam distillation. FOSS Analytical AB, Application note AN300. 2003, Sweden.

7. Fulhage, C.D., and Pfost. D.L. Fertilizer nutrients in livestock and poultry manure. 2002. University of Missouri Cooperative Extension Bulletin EQ351.

8. Hansen, D.J. Chapter 9: Manure as a nutrient Source. In MidAtlantic Nutrient Management Handbook, 2006. G. Evanylo, Ed.

9. Harrison, J.D. Nutrient Concentrations in Manure Storage Facilities Process Improvement for Animal Feeding Operations. Agriculture Environmental Management Systems, 2004. UtahState University.

10. Helrich, K. Official methods of analysis of the Association of Official Analytical Chemists. 15 th edition. P. 29. Association of Official Analytical Chemists Inc., 1990, Arlinkton, Virgenia.

11. Leeson, S., and Summers, J.D. Commercial poultry nutrition, 3 rd edn. Nottingham University Press, 2008, Nottingham, England.



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12. Patterson, P.H., and Lorenz, E.S. Nutrients in manure from commercial white leghorn pullets. Journal of Applied Poultry Research, 1997, 6:pp 247252

13. Perkins, H.E, Parker, M.B., and Walker, M.L. Chicken manure its production, composition, and use as a fertilizer. Bull. NS 123, Georgia Agricultural Experiment Station, 1964. Athens, GA.

14. Rayment, and Higginson,. Australian Laboratory Handbook of Soil and Water Chemical Methods. Inkata Press, 1992, Melbourne, Oxford.

15. “Recommended methods of manure analysis” http://uwlab.soils.wisc.edu/pubs/A3769.pdf Accessed on November 28, 2010.

16. Robinson, J.B., and Beauchamp, E.G. The resource conservation ethic applied to manure management. In: The Manure Management Handbook, Ont. Soil and Crop Imp. Ass., Ont. Min. of Agriculture and Food, Ont. Agricultural College, 1982. Canada.

17. Schaefer, J. Sampling, characterisation and analysis of malodours. Agriculture and Environment, 1977, 3:pp 121127.

18. Sekar, S., Karthikeyan, S., and Iyappan, P. Trends in patenting and commercial utilization of poultry farm excreta (PFE). World’s Poultry Science Journal, 2010, 66(4): 533572.

19. Spoelstra, S.F. Origin of objectionable odorous components in piggery wastes and the possibility of applying indicator components for studying odor development. Agriculture and Environment, 1980, 5:pp 241260.

20. Tandon, H. L. S. Methods of analysis of soils, plants, waters and fertilizers. Fertilizer Development and Consultation Organization, 1993, New Delhi, India

21. Walkley, A., and Black, I. A. An Examination of the Degtareff method for determining soil organic matter, and a proposed modification of the chromic acid titration method. Soil Science, 1934, 37(1): pp 2938.

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