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Agriculture, Ecosystems and Environment 84 (2001) 227–243 Implications of livestock feeding management on soil fertility in the smallholder farming systems of sub-Saharan Africa R.J. Delve a,b,* , G. Cadisch a , J.C. Tanner b , W. Thorpe b , P.J. Thorne c , K.E. Giller a,1 a Wye College, University of London, Wye, Ashford, Kent, TN25 5AH, UK b International Livestock Research Institute (ILRI), P.O. Box 30709, Nairobi, Kenya c Natural Resources Institute (NRI), Chatham, Kent, ME4 4TB, UK Received 16 November 1999; received in revised form 21 June 2000; accepted 27 July 2000 Abstract The role of livestock in nitrogen cycling in mixed crop–livestock farming systems of sub-Saharan Africa was explored. Cattle were fed a range of diets to investigate the effects on partitioning of nitrogen between urine and faeces and on the chemical composition of the manures produced. The trade-offs in efficiency between using the feed resources as a direct soil amendment for crop production compared with feeding to livestock and use of the manure as a fertiliser are discussed. Increased dry matter (DM) and nitrogen intake of a poor quality basal diet (barley straw) was achieved by supplementation with 15 and 30% of DM offered as Calliandra calothyrsus, Macrotyloma axillare or poultry manure. Urinary-N excretion for the basal diet (0.5 mg kg -1 liveweight (W) per day) was similar to C. calothyrsus at 15 and 30% supplementation (1.3 and 0.8 mg kg -1 W per day, respectively) and M. axillare at 15 and 30% supplementation (0.4 and 0.6 mg kg -1 W per day, respectively). In contrast, feeding poultry manure, a supplement containing highly degradable N, resulted in larger excretions of excess rumen ammonia as N in the urine, 17.5 and 23.2 mg kg -1 W per day for 15 and 30% supplementation, respectively. Diets containing the largest rate of C. calothyrsus supplementation had the lowest digestibility of N in the acid detergent fibre (ADF) and neutral detergent fibre (NDF) fractions. This was reflected in faeces from cattle fed diets supplemented with C. calothyrsus, which had substantially greater amounts of N bound to fibre (ADF and NDF) fractions than faeces from the other diets. When incubated in leaching tubes prunings of C. calothyrsus showed net N mineralisation from week 2, whereas barley straw, M. axillare and poultry manure immobilised N for >28, 24 and >28weeks, respectively. Faeces derived from supplementation with C. calothyrsus and M. axillare resulted in shorter nitrogen immobilisation in leaching tubes (16 weeks) than supplementation with poultry manure (24 weeks) when compared with faeces derived from animals fed straw only (28 weeks). Similarly, reduced N uptake from 10-week-old maize plants was observed in pots to which faeces had been added compared with the control treatment. A second crop of maize had increased N uptake. Feeding poor quality crop residues like barley straw to animals produces manures with a decreased capacity to immobilise mineral N in the soil. This was shown * Corresponding author. Present address: TSBF/CIAT, P.O. Box 6247, Kampala, Uganda. Tel.: +256-41-566415; fax: +256-41-567635. E-mail address: [email protected] (R.J. Delve). 1 Present address: University of Zimbabwe, Box MP167, Harare, Zimbabwe. 0167-8809/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII:S0167-8809(00)00244-9

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Page 1: Implications of livestock feeding management on soil fertility in the smallholder farming systems of sub-Saharan Africa

Agriculture, Ecosystems and Environment 84 (2001) 227–243

Implications of livestock feeding management on soil fertility inthe smallholder farming systems of sub-Saharan Africa

R.J. Delvea,b,∗, G. Cadischa, J.C. Tannerb,W. Thorpeb, P.J. Thornec, K.E. Gillera,1

a Wye College, University of London, Wye, Ashford, Kent, TN25 5AH, UKb International Livestock Research Institute (ILRI), P.O. Box 30709, Nairobi, Kenya

c Natural Resources Institute (NRI), Chatham, Kent, ME4 4TB, UK

Received 16 November 1999; received in revised form 21 June 2000; accepted 27 July 2000

Abstract

The role of livestock in nitrogen cycling in mixed crop–livestock farming systems of sub-Saharan Africa was explored.Cattle were fed a range of diets to investigate the effects on partitioning of nitrogen between urine and faeces and on thechemical composition of the manures produced. The trade-offs in efficiency between using the feed resources as a directsoil amendment for crop production compared with feeding to livestock and use of the manure as a fertiliser are discussed.Increased dry matter (DM) and nitrogen intake of a poor quality basal diet (barley straw) was achieved by supplementationwith 15 and 30% of DM offered asCalliandra calothyrsus, Macrotyloma axillareor poultry manure. Urinary-N excretionfor the basal diet (0.5 mg kg−1 liveweight (W) per day) was similar toC. calothyrsusat 15 and 30% supplementation (1.3and 0.8 mg kg−1 W per day, respectively) andM. axillare at 15 and 30% supplementation (0.4 and 0.6 mg kg−1 W per day,respectively). In contrast, feeding poultry manure, a supplement containing highly degradable N, resulted in larger excretionsof excess rumen ammonia as N in the urine, 17.5 and 23.2 mg kg−1 W per day for 15 and 30% supplementation, respectively.Diets containing the largest rate ofC. calothyrsussupplementation had the lowest digestibility of N in the acid detergentfibre (ADF) and neutral detergent fibre (NDF) fractions. This was reflected in faeces from cattle fed diets supplemented withC. calothyrsus, which had substantially greater amounts of N bound to fibre (ADF and NDF) fractions than faeces from theother diets. When incubated in leaching tubes prunings ofC. calothyrsusshowed net N mineralisation from week 2, whereasbarley straw,M. axillare and poultry manure immobilised N for >28, 24 and >28 weeks, respectively. Faeces derived fromsupplementation withC. calothyrsusandM. axillare resulted in shorter nitrogen immobilisation in leaching tubes (16 weeks)than supplementation with poultry manure (24 weeks) when compared with faeces derived from animals fed straw only(28 weeks). Similarly, reduced N uptake from 10-week-old maize plants was observed in pots to which faeces had been addedcompared with the control treatment. A second crop of maize had increased N uptake. Feeding poor quality crop residueslike barley straw to animals produces manures with a decreased capacity to immobilise mineral N in the soil. This was shown

∗Corresponding author. Present address: TSBF/CIAT, P.O. Box 6247, Kampala, Uganda. Tel.:+256-41-566415; fax:+256-41-567635.E-mail address:[email protected] (R.J. Delve).

1Present address: University of Zimbabwe, Box MP167, Harare, Zimbabwe.

0167-8809/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved.PII: S0167-8809(00)00244-9

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228 R.J. Delve et al. / Agriculture, Ecosystems and Environment 84 (2001) 227–243

with faeces derived from feeding ruminants a diet of only barley straw, which had a faster N mineralisation rate than freshbarley straw, a shorter and smaller N immobilisation stage in leaching tubes and gave greater N uptake in maize grown inpots. These results indicate that N losses in urine from livestock may be less important than previously thought. Most of theN is excreted in the faeces, which must be conserved and managed to maximise nutrient cycling. © 2001 Elsevier ScienceB.V. All rights reserved.

Keywords:Nutrient cycling; Soil fertility; Diet quality; Manure quality; N mineralisation; Crop–livestock interactions; Sub-Saharan Africa;Kenya

1. Introduction

With intensification of mixed crop–livestock sys-tems in sub-Saharan Africa, the quantity and qualityof feed resources has decreased through the lossof communal grazing areas and increased pressureon arable land for food production. Livestock grazelargely poor quality grasses or are fed low quality cropresidues, for example, maize (Zea maysL.) stoverand barley (Hordeum vulgoseL.) straw. A decline infeed quality is generally associated with increases inthe indigestible fractions in the feeds, for example,lignin and cellulose–lignin complexes, as well as areduction in their N content (Leng, 1990). Farmersin Kenya often cannot afford purchased supplementsand instead improve diet quality and provide addi-tional dietary nitrogen through the use of tree andforage legumes as supplements to poor quality basaldiets (e.g. Abu et al., 1992; Abule et al., 1995; Cop-pock and Reed, 1992; Getachew et al., 1994). Twosuch legumes areMacrotyloma axillareandCallian-dra calothyrsus, these legumes vary in their dietaryquality with M. axillare being high in N and low inpolyphenols (tannins) whilstC. calothyrsusis high inboth N and polyphenols. In this study, barley straw(an example of a low quality basal diet) was supple-mented withM. axillare, C. calothyrsusand poultrymanure (a source of non-protein N).

The varying quality (e.g. nitrogen, lignin and tan-nin contents) of these diets may influence feed intake(Barry and Duncan, 1984), feed digestibility (Hanleyet al., 1992; Norton and Ahn, 1997), partitioning ofnutrients to ruminant tissues and partitioning of ex-creted nitrogen between urine (Fassler and Lascano,1995; Tanner, 1988) and faeces (Powell et al., 1994;Somda et al., 1995). Tannins can reduce palatability,intake and dry matter (DM) and nitrogen digestibil-ity through precipitation of salivary or feed proteins

(Perez-Maldonado et al., 1995) and the binding to anddeactivation of enzymes (McLeod, 1974). These draw-backs can be offset by the benefit of using legumeswithin the mixed farming system, as it has been shownthat feeding browse species to ruminants can shift ex-cretion from urinary-N to faecal-N (e.g. Fassler andLascano, 1995; Reed, 1986; Tanner, 1988). Where lessN is excreted in the form of urea in urine, less willbe lost through volatilisation and leaching and the re-sulting larger amounts of more recalcitrant N in thefaeces may be collected, stored and recycled throughthe farming system.

A choice must be made between the allocation of or-ganic resources, whether they should be used for live-stock feed or as organic fertilisers in crop production.Animal manures are of major importance in nutrientcycling but generally of poor quality to supply plantnutrients (Giller et al., 1997) and organic resourcesvary widely in their ability to provide nutrients directlyfor crop growth (Cadisch and Giller, 1997). Althoughmanures provide other nutrients, the major effects areon N cycling and this is the focus of this study. Thisstudy was conducted to explore the role of livestockin managing nutrient cycling in crop–livestock farm-ing systems. A number of hypothesis were explored,whether: (1) nutrient supply capacity of high qualitymaterials is reduced by passage through livestock; (2)nutrient supply capacity of low quality materials isimproved by passage through livestock and (3) ma-nure quality could be manipulated by supplementingthe diet fed to livestock. The study had three stages:(1) an animal feeding experiment in which differentquality diets were fed to cattle; (2) a comparison tonutrient release characteristics of the organic materialsapplied directly to soil or from the manures resultingfrom each of these diets; (3) crop growth experimentsexamining the utility of the organic materials and theanimal manures over two crop cycles.

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R.J. Delve et al. / Agriculture, Ecosystems and Environment 84 (2001) 227–243 229

2. Materials and methods

2.1. Animals, feeding and management

The study was conducted at Muguga ResearchStation in the Central Highlands of Kenya in 1997–1998. Seven Friesian-Ayrshire steers (bodyweight246 ± 26 kg) were fed a daily basal diet of barleystraw (25 g DM per kg liveweight (W)) alone or with15 or 30% of the DM offered either asC. calothyrsus,M. axillare or poultry manure.C. calothyrsus(leavesand stems (<5 mm diameter)) were collected from8 month re-growth,M. axillare (whole plants) were cutat 12-month-old. Poultry manure for feeding was pur-chased from a local farmer and prepared by screeningwood shaving based poultry manure through a 5 mmmesh. Each of the seven steers received five of theseven diets. The animals were weighed weekly andthe DM offer of basal diet and supplements was recal-culated according to this new weight. The steers weregiven a 14-day adaptation period at the start of the ex-periment and each time the diet was changed, duringwhich time they were kept in large individual pens(8 m by 4 m). The adaptation period was followed bya 6-day period in metabolism units. The metabolismunits were situated on the first floor of a purposebuilt housing unit, with faecal and urine collection onthe ground floor. Fresh supplements (C. calothyrsusandM. axillare) were cut each morning, chopped toa length of approximately 10 cm and offered within1 h of cutting. The previous days refusals were col-lected from the feed containers and the floor beforefeeding. Supplements were offered first at 08:00 h,the unchopped barley straw basal diet was split, halfbeing offered after complete consumption of the sup-plement in the morning and the remainder at 16:00 h.Water and mineral lick were available ad libitum.

2.2. Sampling procedure and analysis

Feed offers, feed refusals and faeces were weighed.Duplicate sub-samples (500 g) were dried for 48 h at60◦C in a forced draft oven, ground to pass a 1 mmsieve and bulked over the 6 day collection period.Urine was collected daily directly into acid (250 ml;2 M H2SO4) and a sub-sample (50 ml) frozen; sam-ples were bulked over 6 days prior to analysis. Rumenliquor (25 ml) was collected by suction using a plas-

tic tube inserted into the rumen cannulae. All feed,refusals and faecal samples were analysed for: nitro-gen (Kjeldahl digestion followed by steam distilla-tion; Tecator); acid detergent fibre (ADF; measurescellulose); neutral detergent fibre (NDF; measureshemicellulose and cellulose) and acid detergent lignin(ADL; measures lignin) (Ankom Technology Corp.,Fairport, USA; Van Soest et al., 1987); NDF-N andADF-N (Ankom incubation followed by nitrogen de-termination); total ash (4 h at 500◦C); total extractablepolyphenols (0.2 g/50 ml, Folin-Ciocalteau (Con-stantinides and Fownes, 1994)) and protein bindingcapacity (Dawra et al., 1988). Urine (5 ml) was anal-ysed for urinary-N by Kjeldahl digestion followedby steam distillation. Plant and faecal samples wereextracted in cold water and analysed for NO3-N andNH4-N using a colourimetric assay (Allen, 1989).Total soluble organic N was analysed by Kjeldahl di-gestion and total soluble C following an adapted wetdigestion using potassium dichromate (Dalal, 1979).

2.3. Incubation procedure

Soil samples were collected from 0 to 20 cm layerof experimental plots at KARI-Muguga Research Sta-tion, Kenya, air-dried and ground to pass through a2 mm sieve. The soil was a Nitisol, with pH (water)6.3, total C (Walkley-Black) 31 g kg−1, total N (Kjel-dahl) 3.1 g kg−1, sand 270 g kg−1, silt 430 g kg−1, clay250 g kg−1, Ca 13.2 cmol kg−1, Mg 1.4 cmol kg−1

and K 1.4 cmol kg−1 (extracted in NH4OAc). Nmineralisation was measured using leaching tube in-cubations following a modified method of Stanfordand Smith (1972). Air-dried soil (50 g) was mixedwith acid-washed sand (1:2 w/w) and plant and faecalsamples added to give equal additions of nitrogen(150mg N per tube). No plant or faecal sample wasadded to the control tubes. A layer of sand was addedon top of the soil–sand mixture to avoid soil distur-bance on addition of the leaching solution. Distilledwater was added to each tube to bring the soil–sandmixture close to water holding capacity (WHC), after1 h suction was applied to bring the mixture to ap-proximately 80% WHC. The top was covered looselywith aluminium foil to permit gaseous exchange andto reduce evaporative losses. The 12 treatments withfive replicates were randomised in racks and placedin a growth room at 27◦C. Leaching using 150 ml

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230 R.J. Delve et al. / Agriculture, Ecosystems and Environment 84 (2001) 227–243

of a solution containing 1 mM CaCl2, 0.9 mM KCl,0.1 mM KH2PO4 and 1 mM MgSO4 (Cassman andMunns, 1980) adapted for acid soils (Cadisch et al.,1994) was used to remove all mineral N after 1, 2, 3,4, 6, 8, 12, 16, 20, 24 and 28 weeks and the weightof leachate recorded. Post-leaching the tube was leftovernight to drain and suction applied to each tube toreturn the water content to 80% WHC. NO3-N andNH4-N were determined as above by autoanalyserand net N mineralisation calculated by subtracting theN mineralisation in the soil only control tube fromthe N mineralisation in the organic material amendedtubes.

2.4. Glasshouse pot experiment

Equal amounts of N as plant, faecal samples or ureafertiliser were mixed with 4 kg of soil (as describedabove) and added to a pot (20 cm diameter) at a rateof 9.6 mg N per kg soil (equivalent to 25 kg N perha). The rate of N addition was calculated as an av-erage rate added by farmers from field surveys con-ducted in the Central Highlands of Kenya (Kagwanja,1996; Kihanda, 1996). No plant litter, faeces or fer-tiliser were added to the control pots. Distilled waterwas added to each pot to bring the soil-organic mat-ter mixture to approximately 80% WHC. Five repli-cates were randomised and placed in a glasshouse.Four maize seeds were planted per pot and 1 week af-ter germination thinned to two. At weekly intervals,the amount of water required daily to maintain 80%of WHC was calculated and this amount added for theremainder of the week. After 10 weeks the shoots wereharvested and the N content of the shoots determined.After the first harvest, the pots were again planted withmaize seeds and the same protocol repeated for a fur-ther 10 weeks to investigate the residual effect of thetreatments.

2.5. Statistical analysis

The feeding experiment design was a partially bal-anced incomplete latin square design with five peri-ods. Each of the seven steers received five of the sevendiets. The N mineralisation experiments were ran-domised complete block designs. Data were analysedby the SAS general linear models procedure (SASInstitute Inc., 1988). The rate constant for N minerali-

sation/immobilisation (c) was calculated by fitting thepercent of N remaining to a single exponential decaymodel, Nt = N0 e−kt , where Nt , N remaining at timet; N0, N remaining at time zero (100%);k, N releaserate constant (percent per week) andt, time. Linearregression equations were calculated between the Nrelease rate constants (k) calculated for immobilisa-tion and mineralisation and the chemical componentsof the incubated samples to identify the main factorsaffecting N release from the plant materials and faeces.

3. Results

3.1. Chemical composition of plant and faecalsamples

Barley straw contained significantly (P < 0.05)smaller concentrations of N, ADF-N and NDF-N butsignificantly larger amounts of hemicellulose, NDFand cellulose compared with the other feeds (Table 1).C. calothyrsusprunings contained significantly (P <

0.05) greater concentrations of N, ADF-N, NDF-N,lignin, soluble N (nitrate+ammonia), soluble organicN, as well as the largest amounts of N bound to fi-bre (ADF-N and NDF-N) (Table 1).C. calothyrsusalso had significantly more extractable polyphenolsand protein binding capacity than the other plantmaterials.M. axillare plants contained significantly(P < 0.05) more soluble C compared with the othersamples. The analysis of the faeces showed lowerconcentrations of the soluble components comparedwith the plant materials (Table 1). Faeces derivedfrom diets supplemented withC. calothyrsushadsignificantly (P < 0.05) greater concentrations of N,ADF, bound fibre nitrogen (ADF-N and NDF-N) andlignin (ADL) compared with other faeces. Significantincreases in faecal NDF-N and the proportion of ex-creted N bound to NDF were found with the higherrate ofC. calothyrsussupplementation (Table 1).

3.2. Intake, excreta production and partitioning ofexcreted nitrogen

Organic matter (OM) intake and N intake (Table 2)were increased by both supplementation and supple-mentation rate for all diets compared with the basalstraw diet. Animals fed with all treatments except

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R.J. Delve et al. / Agriculture, Ecosystems and Environment 84 (2001) 227–243 233

Fig. 1. Rumen ammonia concentrations (mg NH3-N per l) with time for each diet (15% supplementation rate) after the morning feed(08:00 h).

15% M. axillare and 15% poultry manure producedmore faeces (P < 0.05) than those fed on straw alone(Table 2). Urinary-N excretion was small for the basaldiet and with legume supplementation of the basaldiet (Table 2). Only poultry manure resulted in sig-nificantly (P < 0.05) larger urinary-N excretion thanwith the straw only diet. In general, a small percent-age of ingested N was excreted in the urine (Table 2).In the first 4 h supplementation resulted in increasedrumen ammonium concentrations (P < 0.05) com-pared with feeding barley straw alone, the effect wasstrongest with poultry manure (Fig. 1). Faecal-N ex-cretion was significantly larger (P < 0.05) for allsupplemented diets compared with the straw only diet.For all supplements the 30% rate of supplementationresulted in significantly more (P < 0.05) faeces thanthe 15% rate (Table 2). The type of supplementation(P < 0.05) but not rate (P > 0.05) influenced OMdigestibility (Table 3).C. calothyrsusat both ratesof supplementation showed a significant (P < 0.05)reduction in ADF and ADL digestibility comparedwith the straw only diet. An apparent enrichment ofnitrogen in ADF in the manure was found with mostdiets. M. axillare was the only supplement to showgreater (P < 0.05) digestibility of NDF-N comparedwith the other supplements (Table 3).

3.3. Leaching tube incubation experiment

Of the plant materials,C. calothyrsuspruningsshowed net N mineralisation from week 2 with 28%of added N mineralised by week 28. In contrast,M.axillare and poultry manure did not show net mineral-isation until 24 and more than 28 weeks, respectively(Fig. 2A). Barley straw showed strong immobilisationof N throughout the course of the experiment. All fae-cal samples exhibited net N release after 1 week, withonly the faeces produced from diets containing 30%M. axillare showing net N mineralisation at 3 weeks(1.6%, Fig. 2B). From 4 weeks all faecal samplesimmobilised N, the time to net mineralisation variedfrom 17 to more than 28 weeks.

The nitrogen release data was divided into netimmobilisation and net mineralisation phases for cal-culation of the release rate constants. The derivedrate constants showed significant differences betweenplant materials (Table 4), barley straw exhibited astrong negative rate constant reflecting the high lev-els of observed immobilisation (Fig. 2). For faecalsamples the derived rate constants for immobilisa-tion showed significant differences between differentquality faeces (P < 0.05, Table 4). Faeces derivedfrom supplementation with 15%M. axillare and

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Table 3Influence of diet quality on digestibility for steers fed supplemented dietsa

Digestibility (%)

Organicmatter

Acid detergentfibre

Nitrogenin ADF

Acid detergentlignin

Neutral detergentfibre

Nitrogenin NDF

Barley straw only 50 54 −25 18 59 1015% C. calothyrsus 49 48 −20 8 56 2330% C. calothyrsus 52 47 −43 2 56 1815% M. axillare 56 56 −6 19 61 3730% M. axillare 53 54 23 25 59 5115% Poultry manure 47 51 −17 14 57 1930% Poultry manure 46 50 −14 11 56 21S.E.D. (df= 24) 2b 2 20 4 2 8

ANOVAFeed ∗∗∗ ∗∗∗ ∗ ∗∗∗ ∗∗ ∗∗∗Rate ns ns ns ns ns nsFeed∗ rate ns ns ns ns ns ns

a OM, organic matter; W, liveweight; S.E.D., standard error of the difference; df, degrees of freedom;∗, P < 0.05; ∗∗, P < 0.01; ∗∗∗,P < 0.001; ns,P > 0.05.

b df = 17.

poultry manure had significantly lower rates of im-mobilisation (−0.007 and−0.019, respectively) thanthe corresponding 30% supplementation rate (−0.028and−0.038, respectively). There were no significantdifferences (P > 0.05) between faeces derived frombarley straw, 15 and 30%C. calothyrsusand the 15%rate of supplementation ofM. axillare and poultrymanure. The derived rate constants for mineralisationbetween 8 and 28 weeks showed no significant differ-ences between the different quality faeces (Table 4).

Table 4Calculated rate constants (percent per week) for immobilisation and mineralisation for plant materials and faeces incorporated into the soila

Plant material Immobilisationphase(1–6 weeks)

Mineralisationphase(6–28 weeks)

Faeces Immobilisationphase(2–8 weeks)

Mineralisationphase(8–28 weeks)

C. calothyrsus 0.011 0.013 Barley straw only −0.017 0.006M. axillare −0.037 0.001 15% Supplement ofC. calothyrsus −0.014 0.006Poultry manure −0.028 −0.007 30% Supplement ofC. calothyrsus −0.012 0.003Barley straw −0.079 −0.026 15% Supplement ofM. axillare −0.007 0.002

30% Supplement ofM. axillare −0.028 0.00815% Supplement of poultry manure −0.019 −0.00130% Supplement of poultry manure −0.038 −0.005

S.E.D. (df= 12) 0.010 0.006 0.007 0.006

ANOVAFeed ∗∗∗ ∗∗∗ ∗∗ ns

a S.E.D., standard error of the difference; df, degrees of freedom;∗, P < 0.05; ∗∗, P < 0.01; ∗∗∗, P < 0.001; ns,P > 0.05.

During the initial stages (1–6 weeks) regressionanalysis indicated significant positive linear rela-tionships between plant material rate constants andN, total soluble N (TSN), ADF-N and nitrate-N(Table 5). After 6 weeks N, ADF-N, TSN and nitrate-Nwere again significantly related to the rate constantas were NDF-N and PBC (Table 5). For the immobil-isation phase of the different quality faecal samples(2–8 weeks) a significant negative linear equation wasonly found with the C:N ratio (y = 0.06− 0.0034x)

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Fig. 2. Net N mineralisation (percent of recovery) from plant materials (A) and faeces (B) incubated in leaching tubes.

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Table 5Linear regression equations for selected parameters (x) of the plant materials and the N release rate constant (y) during the leaching tubeincubationa

Immobilisation stage (1–6 weeks) Mineralisation stage (6–28 weeks)

Parameter (x) Regression equation Parameter (x) Regression equation

Total N 0.024x − 0.09 (R2 = 0.98)∗ Total N 0.011x + 2.7 (R2 = 0.99)∗∗TSN 0.00002x − 0.09 (R2 = 0.90)0.10 ADF-N 0.006x − 0.03 (R2 = 0.96)∗ADF-N 0.013x − 0.08 (R2 = 0.88)0.10 TSN 0.000007x − 0.03 (R2 = 0.94)∗NO3 0.00006x −0.06 (R2 = 0.82)0.10 NDF-N 200.4x + 7.3 (R2 = 0.92)∗

PBC 3559x + 163.27 (R2 = 0.88)0.10

a TSN, total soluble N; ADF-N, nitrogen in ADF; NO3, nitrate-N; NDF, neutral detergent fibre; NDF-N, nitrogen in NDF; PBC, proteinbinding capacity;∗, P < 0.05; ∗∗, P < 0.01; ∗∗∗, P < 0.001.

accounting for 71% of the variation (data not shown).No significant relationship was found between the re-gression analysis and the faecal mineralisation rateconstant for any measured parameter between 8 and28 weeks.

3.4. Glasshouse pot experiment

At the first harvest after 10 weeks, N fertiliser andprunings ofC. calothyrsusand M. axillare were theonly treatments to have significantly greater shoot Ncompared with the soil control. Maize plants grownon soils amended with barley straw and all faecaltreatments produced maize plants had less (P < 0.05)shoot N compared with the soil control (Fig. 3). Nyield of maize shoots was decreased in all treatmentsat the second harvest except where barley straw wasincorporated into the soil. No differences (P > 0.05)in N content were observed when comparing soil withfaeces, soil with plant materials or plant materialswith faeces. Cumulative data showed that the N-richplant materials gave maize shoots with higher N con-tent (P < 0.05) when compared with the differentfaeces treatments (Fig. 3). The percentage N recov-ery between the leaching tube experiment and theglasshouse pot experiment showed a close linear rela-tionship after 10 weeks (R2 = 0.79, Fig. 4A) and forthe cumulative N recovery after 20 weeks (R2 = 0.65,Fig. 4B) but an outlying point exerted an overridinginfluence on the regressions. There was no significantrelationship between the N recoveries for the second10 week harvest in the glasshouse pot experimentand the leaching tube experiment (data not shown,R2 = 0.03).

4. Discussion

4.1. Characterisation of feed and faecal samples

The choice of basal diet and the range of qualitybetween the three diet supplements, provided vary-ing concentrations of N, tannins and fibre fractioncontents (Table 1), justifying their inclusion to inves-tigate the effect of diet quality on nutrient partitioningbetween urine and faeces and faeces quality. Thechemical analysis of barley straw was consistent withother reported analyses of barley straw (Givens et al.,1989; Norton and Ahn, 1997). For example, valuesof ADF, NDF and lignin found here were 485, 711and 59 mg kg−1, respectively (Table 1), comparedwith 510, 784 and 64 reported by Norton and Ahn(1997). A similar value for the PBC ofC. calothyrsus(248mg BSA per mg sample) was reported by Han-dayanto et al. (1995). The content of faecal N thatwas soluble in neutral detergent solution was largestfor faeces produced from the poultry manure supple-mented diets (Table 1), indicating higher excretion ofmicrobial and endogenous N in the faeces. The ob-served reduction in digestibility of lignin and ADF inC. calothyrsussupplemented diets compared with thestraw only diet (Table 3) may be not only due to thepoorer digestibility of this legume per se, but may bepartly an artifact of the methodology used to measurethese fibre fractions. Gains in ADF and lignin havebeen reported by Norton and Ahn (1997) to resultfrom the formation of tannin–carbohydrate complexesas well as tannin–protein complexes. Furthermore,lignin and tannin complexes with macromolecules areinsoluble and can appear in the lignin fraction during

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Fig. 3. Nitrogen uptake in maize plants, first harvest (A); second harvest (B); total (first and second) N harvest (C) and percent of Nrecovery of added N (D) in the glasshouse pot experiment.

analysis (Van Soest et al., 1987). The greater recalci-trance of these fractions leads to greater amounts ofN in recalcitrant forms in the manure which will haveimportant implications for N release from the faeceswhen added to the soil. Other authors have foundlarger negative effects on lignin digestibility; Nortonand Ahn (1997) found 55–250% more lignin in thefaeces than was consumed for a diet of barley strawsupplemented withC. calothyrsusand Osbourn et al.(1971) found increases in lignin content of between25 and 140% when feeding sainfoin (Onobrychisviciifolia).

4.2. Intake, excreta production and partitioning ofexcreted nitrogen

Increased OM intake and nitrogen intake (Table 2)of supplemented diets was also observed by otherauthors (e.g. Abu et al., 1992; Abule et al., 1995;Kitalyi and Owen, 1993). Reduced feed intake dueto condensed tannins which has been reported dueto feedingof Lotus pedunculatus(Barry and Duncan,1984) orC. calothyrsus(Norton and Ahn, 1997) wasnot found during this experiment when feeding theC.calothyrsusat either supplementation rate (Table 2).

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Fig. 4. Relationships between percent of N recovery in the leachingtube and pot glasshouse experiments at 10 weeks (A); 20 weeks(B) and total N recovery (C).

A small amount of N was excreted in the urine (lessthan 1% of total excreted N) and there were no signifi-cant differences (P > 0.05) between the barley straw,C. calothyrsusandM. axillare diets. No influence ofdietary polyphenols on the amounts of N excretion inthe urine was detected, presumably due to the smallamounts of urinary-N excretion. In contrast, poultrymanure showed larger amounts of N excretion in theurine compared with the other supplements (Table 2).The urine and manure excreted by animals fed highlydigestible diets are more susceptible to N losses thanexcreta from diets containing greater amounts ofroughage (Powell and Williams, 1993). Although, noresponse of feeding the tannin-richC. calothyrsuswas observed during this experiment, other authorshave reported reductions in urinary-N excretion whenfeeding browse species. The extent of this shift inexcretion varied with the browse species fed. For ex-ample, Tanner (1988) found a reduction of between28 and 52% in urinary-N excretion depending on the

Acacia species fed. More than twice the amount ofN was excreted as urine by animals fed cowpea hay,than from animals fedAcacia tortilis pods, or nosupplement (Coppock and Reed, 1992). Substitutionof Cratylia argenteain a basal diet mixture (60%Brachiaria dictyoneura: 40% C. argentea) with 8 or16% of the high tannin containingFlemingia macro-phylla, reduced urinary-N excretion by 8 and 35%,respectively (Fassler and Lascano, 1995).

Concentrations of rumen ammonium increased fol-lowing dietary supplementation with forage legumes(Getachew et al., 1994), and this increase was relatedto the degradability of the legume nitrogen. Satterand Slyter (1974) suggested that 50–80 mg NH3-Nper l rumen fluid is optimal for maximising micro-bial growth yield and this has been widely accepted.However, recent studies have indicated that the op-timum concentration may be more than double theseconcentrations for poorly digestible, N deficient for-ages in cattle (Leng, 1990). The rumen concentrationsreported here are within this range only for poultrymanure supplement in the first 5 h after feeding thesupplement, all other supplements resulted in ammo-nium concentrations below this minimum threshold(Fig. 1). Therefore, urinary-N excretion would be ex-pected to be small given the below-optimum rumenammonium concentrations, as all blood ammonia willbe recycled and little excess ammonia will be availablefor excretion (Fig. 1 and Table 2). The larger excre-tion of urinary-N found when supplementing the poorquality diet with poultry manure is likely to be dueto its higher degradability and hence rapid fermen-tation in the rumen. It was expected that ammoniumconcentrations would be smaller in the rumen, wherea diet of high tannin content (C. calothyrsus) wasfed, sinceC. calothyrsustannins are known to bindto protein in the rumen (Lowry et al., 1996; McLeod,1974) but this was not observed. Rumen ammoniumconcentrations were small and not significantly dif-ferent betweenC. calothyrsusandM. axillare (Fig. 1)explaining the similar outputs of urinary-N excre-tion between the legume supplemented diets. Otherauthors have reported larger rumen ammonium con-centrations. For example, Silva and Orskov (1988)reported values of 27 mg NH3-N per l rumen fluid insheep fed on untreated barley. In contrast, Abule et al.(1995) reported much larger concentrations of 48 mgNH3-N per l rumen fluid for unsupplemented t’eff

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straw and from 154 to 218 mg NH3-N per l rumenfluid when calves were fed t’eff straw supplementedwith the forage legumes cowpea and lablab. In simi-lar, Bos taurascattle to those used in this experiment,Hunter and Siebert (1985) reported rumen ammoniumconcentrations of 36 mg NH3-N per l rumen fluid ona diet of unsupplemented Pangola grass (Digitariadecumbens) hay. The most likely explanation for thesmall urinary-N excretion found in this study is thatthe steers used in this experiment (March 1997) hadjust survived the failure of the 1996 short rains. Thepasture on the farm at Muguga had been of poorquality throughout this drought and the animals hadnot received any dietary supplementation. It is there-fore likely that the animals were under-nourished atthe beginning of the experiment, which may explainthe high N retention and low urinary-N loss for alldiets. This condition of seasonal under-nourishmentis common throughout sub-Saharan Africa as boththe quantity and quality of feed supplied to cattle arefar below optimum in most cases (Renard, 1997). Ittherefore follows that supplementation with legumesto increase OM and N intake with livestock survivingat or below maintenance will not affect urinary-N ex-cretion until the condition of the animals improves. Ifa corresponding increase in the faecal-N componentof the excretion occurs at the same time then the Ncycling benefit to the farming system is increased.

While supplementation withC. calothyrsusdidnot influence the partitioning of excreted N betweenurine and faeces, feeding of this legume resulted in anincreased N content of the faeces of 186% for 15%supplementation rate and 250% for 30% supplemen-tation rate compared with the barley straw only diet(Table 3). Similarly, Perez-Maldonado and Norton(1996) found that feedingC. calothyrsus(75 g kg−1

total extractable condensed tannins) to sheep andgoats increased faecal-N excretion when comparedwith a control diet of Pangola grass (D. decumbens)but had no effect on urinary-N excretion.

4.3. Effect of diet quality on manure quality andsubsequent N release

C. calothyrsusand poultry manure, both had lowTSC:TSN ratios (9 and 5, respectively) and thesematerials would therefore be expected to result in netN mineralisation when added to the soil, but only

C. calothyrsusaddition actually resulted in net N min-eralisation (Fig. 2).C. calothyrsusshowed net miner-alisation of 28% of added N mineralised by week 28,and 18% of the added N was mineralised by week 14,comparable results with those of Handayanto et al.(1995) for the same species. The higher C:N observedin barley straw andM. axillare resulted in net N im-mobilisation. A total C:N ratio of 20 is often used as aguide to whether immediate net N immobilisation ormineralisation is expected (e.g. Senesi, 1989). Thereis no comparable accepted ratio for soluble compo-nents, but the C:N ratio of bacteria is around 6–8, andas 45–50% of carbon is lost during decomposition,then a C:N greater than 12–16 should result in N im-mobilisation. This is the result found withM. axillareand barley straw, which had C:N ratios of 17 and 24,respectively, in the water soluble fraction. The immo-bilisation of N observed with poultry manure can onlyresult from a limited availability of N to satisfy micro-bial demand. It follows therefore, that although thereis a low measured C:N ratio in the soluble fraction,there must be a significant pool of C which is readilyavailable for microbial degradation, but not includedin the cold water extractable soluble C pool. The dif-ference in mineralisation patterns observed betweenC. calothyrsusand poultry manure may result fromtheir differences in ADF and hence in the availabilityof the hemicellulose and cellulose fractions to degra-dation by the microbes. Quemada and Cabrera (1995)incubated stems of wheat, oat and rye straw and re-ported net N mineralisation after 9 weeks for oat andrye and after 14 weeks for wheat. Smith and Sharpley(1990) found less drastic effects on soil N availabilitywhen high C:N crop residues were left on the soilsurface than when they are incorporated, which maypartially explain the stronger immobilisation observedhere.

Addition of faecal samples to soil resulted in a smallamount of net N mineralisation over the first 2 weeks,which was greater than the amounts of mineral Npresented in the faeces. No significant differenceswere found between the soluble components of thefaecal samples (Table 1) except for the concentrationsof total soluble organic N but there was no correla-tion between the concentrations of TSN and initial Nmineralisation observed. The C:N ratio of the solublefraction in the faecal samples mostly ranged from 10to 17 and all additions resulted in similar small rates

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of N release, which would be expected given theassumption above of a critical C:N ration of 12–16.The only faecal sample to have a higher C:N ratio ofthe soluble fraction (20) was faeces which resultedfrom feeding barley straw only (Table 1) and additionsof these faeces to the soil resulted in strong immobil-isation of N after only 2 weeks (Fig. 2). Again, thisresult was most likely due to the presence of readilyavailable C that was not measured in the cold waterextractable C pool. All faecal samples resulted in netN immobilisation between 3 and 8 weeks. Mineralisa-tion figures reported by Kristensen (1996) for faecesfrom barley straw supplemented diets at 4 weeks (−15to −41%) and 12 weeks (−2 to−28%) compared wellwith figures reported here of−4 to −23% and−4to −28% for 4 and 12 weeks, respectively (Fig. 2B).Compared with the fresh barley straw incubated in theleaching tubes the faeces derived from barley strawonly diet had a faster mineralisation rate (Table 4)and showed less net N immobilisation (Fig. 2).

Several researchers have suggested that the initialN concentration of the plant materials regulates Nmineralisation (e.g. Frankenberger and Abdelmagid,1985; Tian et al., 1992). Other reports of strong rela-tionships between the rate of N mineralisation to otherquality factors, for example, the C:N ratio (Franken-berger and Abdelmagid, 1985), lignin+ polyphenolratio (Tian et al., 1992) and (lignin+ polyphenol): Nratio (Handayanto et al., 1995) were not supported bythe current study, where these factors showed little re-lationship with the N release constants. Other factorsanalysed here provided better correlation between theN mineralisation rate constants and initial plant residuequality, for example, TSN, ADF-N and nitrate-N(Table 4). Kristensen (1996) reported that the rate ofN release from manures produced from a range of dif-ferent quality diets was negatively related to the NDF,ADF and lignin C:N of the manures. Similar resultswere found in this study with a negative relationshipbetween C:N and N mineralisation, best explainingthe observed results. Others have reported close neg-ative relationships between C:N and N mineralisationin faeces (Floate, 1970; Serna and Pomares, 1991),whereas, Castellanos and Pratt (1981) found no sig-nificant correlation between the C:N and N minerali-sation for a range of fresh and stored animal faeces. Inthe data reported here, none of the measured chemicaland derived parameters explained a significant amount

of the variation in N mineralisation between faecaltreatments.

4.4. Implications of organic resource use strategyfor N cycling

Addition of C. calothyrsusprunings gave the high-est N uptake by maize apart from fertiliser (Fig. 3).There was a close linear relationship between theN mineralised during the leaching tube experimentand the total N recovery in shoots and roots in theglasshouse pot experiment (Fig. 4) at the first harvestat 10 weeks (R2 = 0.79) but not at the second harvestafter 20 weeks (R2 = 0.03). The lack of correlation atthe second harvest was caused by the positive N re-covery in most of the treatments in the pot experiment(Fig. 3) compared with the N immobilisation foundfor all treatments exceptC. calothyrsus, in the leach-ing tube experiment (Fig. 2A and B). Similarly toresults reported here (Table 4), there are several otherreports of higher N recovery from incorporation ofplant material than from faeces derived from the sameplant materials (Catchpoole and Blair, 1990; Barrow,1961; Floate, 1970) but this only holds for high qualityplant materials. If the low quality barley straw addeddirectly to the soil is compared with the application offaeces derived from the barley straw diet the oppositewas true. The faeces resulting from feeding only bar-ley straw gave a lower cumulative net immobilisation(Fig. 2), higher mineralisation rates (Table 4) and ahigher N uptake by maize in pots (Fig. 3) comparedwith fresh barley straw incorporated into the soil.

5. Conclusions

Urinary-N excretion from animals which are fed alow quality basal diet with supplements in smallholderfarming systems in the tropics may be smaller than ex-pected particularly during and after drought periods,which suggests that less urinary-N than that commonlyassumed is exposed to losses through ammonia volatil-isation and leaching. Supplementation of the low qual-ity diet increased faecal production and N content ofthe faeces. If these faeces are conserved on-farm andutilised as the primary fertiliser source, increased fae-cal excretion with a larger faecal-N content will in-crease N additions to the soil. The increased amount of

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N bound to the NDF fraction may reduce the releaseof the faecal-N when the faeces are added to the soil.Bound N in association with polyphenols has beenshown to be more slowly released, thereby restrictingthe supply of soil mineral N for immediate use by thecrop (Handayanto et al., 1997). However, this slowerN release could produce better synchrony of N supplyto crop demand if the N is released at the correct timebut if N release is too slow, or if less is released thanis required by the crop, then N deficiency of the cropmay result. In this study all materials of either plantor faecal origin showed low rates of N release overthe 28 weeks of the incubation.C. calothyrsuswhichreleased the most N only resulted in 28% recovery ofN, faecal samples from supplementation with 15%C.calothyrsusand 30%M. axillare also released similaramounts of N but after a long immobilisation phase ofover 17 weeks. It therefore follows that only freshC.calothyrsuswould provide N to a growing crop in thefirst season, whereas, applications of other materialscould decrease crop production through immobilisa-tion of soil N. The option of feeding the plant mate-rials to ruminant livestock and then adding the faecesto the soil did increase the rate of N mineralisationas shown by the shorter and smaller N immobilisationstage in leaching tubes, and produced larger N uptakein maize grown in pots for faeces from barley strawand poultry manure supplemented diets. The resultsindicate that the lower the quality of the feed material,the more beneficial the application of faeces, is overdirect application of the plant material to the soil.

Increased knowledge of the interactions that occurwhen plant materials and faeces of different qualityare applied to soil can lead to various strategies forthe management and optimal utilisation of these ma-terials, including the method and time of applicationand the application of mixtures of different quality(Mafongoya et al., 1998). Thus, it is concluded thatlow quality plant materials should be first fed to live-stock and the faeces collected and used as a fertiliser.High quality plant materials can provide useful directsources of N for plants, though farmers may chooseto feed these to animals to improve meat and milkproduction. Application of faeces of a range of quali-ties resulted in short term N immobilisation that couldhave serious implications for N supply and hencecrop production in the first season after application.However, manures also provide other nutrients (e.g.

cations and P) and may alter soil structure and WHCin sandy soils which will provide alternative benefits.

Acknowledgements

The support given by the Centre Director and thestaff of KARI National Agricultural Research Centre,Muguga, and the financial support of the Departmentfor International Development (Project R6283) aregratefully acknowledged. The information and viewsexpressed are solely those of the authors.

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