Agroforestry Forum 共2005兲 63: 299–306 © Springer 2005
299
Weed seedbank response to planted fallow and tillage in southwest Nigeria F. Ekeleme1, D. Chikoye2,* and I. O. Akobundu3 1Michael
Okpara University of Agriculture, Umudike, PMB 7267, Umuahia, Nigeria; 2International Institute of Tropical Agriculture, Ibadan, Nigeria; 3Liberty Place, Apt # 1, Windsor Mill, MD 21244–2060, USA; *Author for correspondence (email:
[email protected]; tel.: 234-2-241-2626; fax: 234–2–241–2221) Received 26 May 2003; accepted in revised form 7 April 2004
Key words: Bush fallow, Minimum tillage, Mounding, Planted fallow, Redundancy analysis
Abstract Planted fallows are an alternative to the unsustainable bush fallow for improved soil and weed management in the tropics. However, the interactive effects of planted fallows and tillage on the weed seedbank are not well documented in the tropical environment. The effect of fallow type and tillage on the weed seedbank in the soil was assessed in 1995 and 1996 at Ibadan, southwest Nigeria. The planted fallow species consisted of a herbaceous legume 共Pueraria phaseoloides兲 and three woody legumes 共Acacia auriculiformis, Leucaena leucocephala, and Senna siamea兲. Natural bush fallow and continuous cassava/maize plots were controls. Tillage treatments were minimum tillage and mounding. Continuous maize/cassava plots had the largest weed seedbank in both years. After six years of continuous fallow, the weed seedbank was 86% lower in A. auriculiformis, 79% in P. phaseoloides, 68% in S. siamea, 53% in L. leucocephala, and 35% in natural bush fallow plots than in continuously cultivated plots. Compared to minimum tillage, mounding reduced the seedbank by 47% in 1995 and 66% in 1996. Redundancy analysis showed that tillage contributed significantly to the variance in species composition. Euphorbia hyssopifolia, E. heterophylla, and Cynodon dactylon showed no preference in terms of tillage. Perennial and annual grasses 共Digitaria horizontalis, Eleusine indica, Paspalum orbiculare, Cynodon dactylon兲 with Cyathula prostrata and Desmodium scorpiurus, an annual and perennial broadleaf, respectively, were most abundant in the seedbank of continuously cultivated plots. There were more annual broadleaf weeds in the seedbank of planted fallow plots than in the control plots. Species diversity of the seedbank was greatest in plots under minimum tillage. Mounding as a seedbed preparation method, especially within the improved fallow system, could reduce the high weed pressure experienced by smallholder farmers in southwest Nigeria.
Introduction In the forest/savanna transition zone of southwest Nigeria, high weed pressure is a major constraint to crop production in smallholder farms. Farmers spend more time and labour in removing weeds from their crop than in any other farm operation. About 70% of family labour is expended on weeding 共Chikoye et al. 2001兲. Traditionally, smallholder farmers reduced weed infestation and improved soil productivity through long fallow periods of up to 10 years after a short period of cropping. Such long fallows act as a
weed-break by depleting the weed seed population in the soil and suppressing weeds that survive from the previous cropping cycle 共De Rouw 1995兲. Intensification of cropping systems in response to marketing opportunities and population pressure has reduced fallow periods drastically 共0–3 years兲, with consequences such as a loss of organic matter and an increase in weed pressure. Intensification of crop production also causes changes in the structure and composition of the agricultural weed flora 共Eggers 1984兲.
300 Short-term planted fallows that employ herbaceous cover crops or agroforestry practices such as alley cropping have been suggested as alternatives to the traditional bush fallowing for intensified and sustainable crop production in the tropics 共Kang et al. 1997兲. Planted fallows have been reported to facilitate rapid amelioration of soil fertility of degraded arable land as well as reduce weed populations 共Akobundu et al. 1999; Kang et al. 1997兲. Planted fallows reduce weed infestation by shading weeds surviving after crop harvest and by reducing the weed seed population in the soil 共Akobundu et al. 1999; Ekeleme et al. 2000; Chikoye et al. 2001兲. Weed seedbank depletion by planted fallows may be ascribed to loss of weed seed viability, predation and non-recruitment of new seeds 共Adedeji 1984兲. Tillage is among the many factors that are likely to affect the ability of planted fallows to suppress weeds. Tillage practices may be broadly classified into conservation and conventional tillage systems 共Antapa and Angen 1990兲. Conservation tillage is based on the principle of causing the least disturbance and leaving some plant residue on the soil surface. In smallholder farms in Africa, conservation tillage practices include no-till where vegetation is slashed and burned without any tillage, and minimum tillage where shallow cultivation is carried out without turning the soil upside down. Conventional tillage systems include ridging or mounding where plant residues are incorporated into the soil using hand-hoes, animal, or tractor-mounted implements. The effect of tillage on weed density and composition is well-documented in the temperate regions of the world. It has been reported to cause shifts in weed communities, species composition, and weed densities. For example, Shrestha et al. 共2002兲 found that weed density was greater in conventional tillage than in no-tillage systems. The authors attributed the higher weed density to ploughing which could have brought weed seeds from lower soil profiles to a depth that was favorable for germination and emergence. Tillage also alters the size and composition of the seedbank 共Swanton et al. 2000兲. In no-till systems, most weed seedlings emerge from the seedbank near the soil surface 共 ⬍ 10 cm soil depth兲 while in conventional tillage systems, a high proportion of the seedbank is located in deeper soil layers 共10–30 cm兲. Information on the effect of tillage on the seedbank in the tropics is sparse. Because the seedbank is the main source of weed infestation in arable fields, there is a need to understand how tillage affects it, especially in smallholder agri-
culture. There is also a need to understand how tillage combined with planted fallows affects the size and composition of the seedbank. We hypothesize that weed abundance and diversity will be higher in minimum tillage than in mounded treatments. The objective of the study was to assess the effect of planted fallow and type of tillage on the weed seedbank of an Alfisol under rehabilitation.
Materials and methods Study site This study was conducted during the 1995 and 1996 cropping seasons at the International Institute of Tropical Agriculture 共IITA兲 in Ibadan, Nigeria 共7°30’ N, 3°54’ E兲. The seedbank was monitored in a multidisciplinary trial set up in 1989 to evaluate the effects of various fallow species on soil productivity. The soil type was an Alfisol. IITA is within the forest/savanna transition zone and has a bimodal rainfall pattern. The rainy season starts in March and ends in November. Mean annual rainfall was 1436 mm in 1995 and 1322 mm in 1996. Experimental design and treatments Prior to this study, the experimental site had been mechanically cleared and cultivated continuously for over 10 years, which led to soil compaction and soil erosion. Kang et al. 共1997兲 and Salako et al. 共2001兲 have described the experimental site in detail. In 1989, treatments consisting of 15 planted fallow species, natural bush fallow, and continuous maize/cassava plots were set up in a randomized complete block design. Natural bush fallow and continuously cropped plots were controls. The overall plot size for each species was 12 m ⫻ 24 m. In 1995, each plot of the surviving planted fallow species 共Acacia auriculiformis, Leucaena leucocephala, Senna siamea, Pueraria phaseoloides兲 and the control treatments was divided along its length into three 8 m ⫻ 12 m subplots. One subplot of each treatment 共planted fallow and control兲 was cleared and cultivated in 1995 and 1996. The third subplot was left fallow. There were 12 subplots in each treatment. Each subplot was further subdivided into two sub-subplots of 6 m ⫻ 8 m plots, which were either mounded or received the minimum tillage treatment. There were 24 sub-subplots in each treatment, which were replicated three
301 times. Data reported in this study were collected from sub-subplots cleared and cultivated in 1995 and 1996. Minimum soil scarification using a hoe was carried out all sub-subplots receiving the minimum tillage treatment. Mounds 共25 cm in height and with a base of 85 cm兲 were made in the other sub-subplot. Cleared vegetation was burned before cropping. Maize 共cv. DMR–SR-W T28843兲 and cassava 共cv. TMS 30572兲 were planted in each plot in May 1995 and 1996. Maize was seeded at a population of 40,000 plants ha–1 and cassava at 10,000 plants ha–1. Each plot was weeded with hoes at 3 and 8 weeks after planting 共WAP兲. The second weeding was carried out before maize harvest 共August in both years兲. Seedbank sampling Seedbank sampling was performed in August of 1995 and 1996 after maize harvest. The reason for sampling in August was to evaluate the two tillage systems after all weed control measures had been imposed. Twenty-four soil cores were randomly collected from each sub-subplot with a precision auger 共7.4 cm in diameter兲 at a depth of 20 cm. In sub-subplots where mounds were made, 12 soil cores were taken from the furrow between mounds and 12 soil cores from the mounds, and bulked. Soil from each sub-subplot was bulked to form one sample. Soil samples were air dried and passed through a 2 mm sieve to remove large stones and root fragments. Large weed seeds that could not pass through the sieve were sorted out and returned to the sieved soil sample. The sieved soil samples were placed in three trays of 13 cm diameter and 3 cm depth in the glasshouse for germination 共maximum temperature ⫽ 35 ºC; minimum temperature ⫽ 30 ºC兲. Germinating weed seedlings were identified, counted, and then removed. Seedlings that could not be identified were transplanted into wooden boxes filled with sterile soil and allowed to grow to maturity before identification. Samples were watered as necessary. Soil samples were dried for two weeks and then stirred to stimulate germination of weed seeds below the 3 cm soil layers. The experiment was terminated at the end of 3 months when seedling germination ceased. Chikoye and Ekeleme 共2001兲 reported that 3 months was enough time for the germination of viable non-dormant weeds.
Statistical analyses Analysis of variance 共ANOVA兲 was used to test the effect of fallow type and tillage system on the total seedbank using PROC MIXED model of SAS 共Littel et al. 1996兲. Means were separated using the standard error of the mean. Data were square root 共兹x⫹1兲 transformed to improve the homogeneity of variance before ANOVA. The effect of tillage on species composition was assessed in 1996 by Redundancy Analyses 共RDA兲 using CANOCO 3.1 共Ter Braak 1990兲. RDA is the linear method of direct ordination. In the analysis, replicates were used as covariables and nominal treatments 共tillage and fallow type兲 as environmental variables. The ordination was scaled by Euclidean distance because the emphasis was on the difference in species composition among tillage type. In the joint plot 共species ⫻ tillage ⫻ fallow type兲, weed species associated with a particular tillage or fallow type lie close to the centroid of the tillage or fallow type 共Ter Braak and Prentice 1988兲. A statistical test of significance of the analysis was carried out by Monte Carlo simulation 共Manly 1990兲. The importance of each axis on the ordination diagram was determined by the magnitude of the eigenvalues 共兲. The ordination diagram was drawn using the software CanoDraw 共Smilauer 1992兲. Species diversity 共H兲 of the seedbank was estimated for each fallow type and tillage system using Shannon and Wiener index: H ⫽ – ⌺ 共ni /N兲 log2 共ni /N兲 where ni is the number of seeds of species i and N is the total number of seeds. Maize and cassava yield data were reported in Kang et al. 共1997兲.
Results and discussion Weed seedbank size Fallow type, tillage system, and year significantly influenced the weed seedbank 共P ⱕ 0.01兲. There was also a significant interaction between fallow type and tillage, and between tillage and year 共P ⱕ 0.01兲. There was no interaction between fallow type and year 共P ⬎ 0.05兲. Averaged over the tillage systems, A. auriculiformis had the lowest seedbank followed by P. phaseoloides and S. siamea 共Table 1, Table 2兲. L. leucocephala had a larger seedbank than the other
302 Table 1. : Effect of fallow type and tillage system on weed seedbank and species diversity in southwestern Nigeria in 1995 and 1996. 1995 Fallow type
1996
Minimum tillage
Mounding
Weed seedbank 共no. m 兲 4366 1120 15495 3198 2955 5616 6922 6017 18051 8307 25559 14856 12225 6519 1584 Species diversity index 0.83 0.53 1.21 0.79 0.47 0.42 0.98 0.94 1.23 1.12 0.90 0.96 0.94 0.79 0.09
Mean
Minimum tillage
Mounding
Mean
2743 9347 4286 6470 13179 20208
4005 16137 7656 8657 39517 34335 18384 5745
2238 3092 4270 5286 5919 17079 6314
3121 9614 5963 6972 22718 25707
–2
A. auriculiformis L. leucocephala P. phaseoloides S. siamea Bush fallow Continuous cropping Mean S.E.D 共 ⫾ 兲 A. auriculiformis L. leucocephala P. phaseoloides S. siamea Bush fallow Continuous cropping Mean S.E.D 共 ⫾ 兲
Table 2. Comparison of average 共year兲 seedbank density in different fallow type in southwestern Nigeria. 共Values in the matrix represent probabilities associated with paired comparisons兲.
Aa Ll Pp Ss Nf Cc
Aa
Ll
Pp
Ss
Nf
Cc
–
0.0080 –
0.3003 0.1013 –
0.1136 0.2219 0.6187 –
0.0030 0.6721 0.0446 0.1054 –
0.0008 0.3147 0.0135 0.0341 0.5459 –
1 Aa ⫽ Acacia auriculiformis; Ll ⫽ Leucaena leucocephala, Pp ⫽ Pueraria phaseoloides, Ss ⫽ Senna siamea, Nf ⫽ Natural bush fallow, Cc ⫽ Continuous cropping.
fallow types but this was smaller than in continuously cropped and natural fallow plots. In both years, continuously cultivated plots had the highest seedbank and this was significantly different from fallowed plots, except for plots cultivated after L. leucocephala and natural bush fallow 共Table 2兲. After six years of fallow, the weed seedbank was reduced by 86% in A. auriculiformis, 79% in P. phaseoloides, 68% in S. siamea, 54% in L. leucocephala, and 35% in natural bush fallow plots, compared with continuously cultivated plots which had been under maize/cassava intercrop since 1989. The larger seedbank size in continuously cultivated plots could be due to the recruitment of new weed seeds produced by the dominant weed community and these seeds accumulated
3282 0.68 1.00 0.45 0.96 1.18 0.93 0.87
1.34 1.85 1.32 1.58 1.16 0.78 1.34 0.17
4826 1.30 1.61 1.00 0.86 1.01 1.18 1.16
1.32 1.73 1.16 1.22 1.09 0.98 1.25
in the soil over time. High seedbank densities in continuously cultivated fields have been reported in southwest Nigeria 共Akobundu et al. 1999兲. Akobundu et al. 共1999兲 reported greater weed densities in continuously cultivated fields than in fields that went through a fallow period before the cultivation phase. Ekeleme et al. 共2000兲 noted that high weed density leads to high weed seed return in continuously cultivated fields. Conversely, the small weed seedbank observed in fallowed plots supports previous studies that demonstrated that fallow vegetation and fallow– crop rotation prevented the continuous re-seeding of weeds and reduced viable weeds in the soil 共De Rouw 1995兲. The planted fallow systems were superior to the natural bush fallow in reducing the weed seedbank. This trend may be attributed to a better plant canopy cover in planted fallow plots during the fallow period than in the natural bush fallow plot. For example, Ekeleme et al. 共2000兲 reported good groundcover 共 ⬎ 70%兲 by P. phaseoloides 18 months after planting in a similar agroecology. Good canopy cover may have reduced the amount of light reaching the weed seed, prevented soil evapotranspiration and thus kept the seed environment moist 共Teasdale and Mohler 1993兲. These conditions may have increased hydration of the seed environment and created favorable conditions for rotting of weed seeds, which reduced the soil seedbank. Allelopathic effects of leachates from both the canopy and litter on the weed
303 communities during the fallow period may also have contributed in reducing the seedbank in the planted fallow plots. Various research reports have demonstrated the allelopathic potentials of some of the species used in this study 共Bora et al. 1999兲. Under experimental concentrations, extract from leaf mulches of S. siamea and L. leucocephala have been shown to inhibit the germination of certain plant species by allelopathy 共Kamara et al. 1999兲. There was a significant interaction between tillage and sampling year 共P ⫽ 0.0088兲. Compared with minimum tillage, mounding reduced the weed seedbank by 47% in 1995 共P ⫽ 0.0057兲 and 66% in 1996 共P ⫽ 0.0003兲. These results are consistent with other studies that reported that any tillage systems that turn the soil 共mounding in our study兲 reduces the weed seedbank better than minimum tillage 共Baldoni et al. 1999兲. The low weed seedbank in mounded plots may be attributed to the emergence of more weed seeds exposed on mound surfaces compared with plots under minimum tillage. In mounding, weed seed brought to the soil surface are stimulated to germinate and consequently reduce the seedbank. Mounding which involves heaping of soils may have also buried some weed seeds beyond the soil depth at which we sampled. Mulugeta and Stoltenberg 共1997兲 have reported differences in the vertical distribution of weed seeds in the soil as a result of different tillage systems. In the minimum tillage system, most weed seeds are on the soil surface. This results in an increased seedling density in the early stages of conservation farming. But over time, the majority of the seeds germinate and the soil seedbank gets exhausted. In this study, minimum tillage had 33% larger weed seedbank after two years while it was 3.2% smaller in mounded plots. Averaged over tillage system, the weed seedbank significantly increased in the second year of cultivation in all the treatments 共P ⫽ 0.0001兲. This trend may be due to weed seed return to the seedbank by species that probably escaped control during the first year of cropping irrespective of type of tillage. Species composition The effect of tillage and fallow on species composition is shown in Figure 1. The first and second axes with eigenvalues 共兲 0.16 and 0.14 account for 53% of the variance in species composition. The Monte Carlo permutation test showed an overall significant difference in species composition 共P ⫽ 0.01兲.
Centroids of tillage systems are shown in the upper right-hand side 共minimum tillage兲 and lower lefthand side 共mounding兲 of the ordination diagram. The change in abundance of a particular species as a result of tillage can be deduced by projecting its point on a line connecting the two tillage systems. On this line, the point of change is the origin, which corresponds to the mean value in the original data. For example, Cynodon dactylon, Euphorbia hyssopifolia, and E. heterophylla, which are clustered around the origin of the axes, showed no preference in terms of tillage. There were more species in plots under minimum tillage than in plots with mounds. The abundance of most species decreased with mounding. Plots that had mounds were dominated by Triumfetta cordifolia, Ipomoea involucrata, Cyathula prostrata, Oldenlandia corymbosa, Paspalum orbiculare, Desmodium scorpiurus and Eleusine indica. Plots under minimum tillage were dominated by Chromolaena odorata, Laportea aestuans, Solanum nigrum, Talinum triangulare, Chloris pilosa, Tridax procumbens, Indigofera hirsuta, Portulaca oleracea, Brachiaria lata, Pouzolzia guineensis, Euphorbia hirta, Spermacoce ocymoides, Spigelia anthelmia, Panicum maximum, Amaranthus viridis and Leptochloa caerulescens. Our findings agree with the work done by Barberi et al. 共1998兲, which showed more weeds in reduced tillage than in systems based upon conventional ploughing. Perennial and annual grasses 共Digitaria horizontalis, E. indica, P. orbiculare, C. dactylon兲 and two broadleaf weeds, i.e., C. prostrata, an annual and D. scorpiurus, a perennial, were most abundant in the seedbank of continuously cultivated plot. This weed community was different from that in planted and natural bush fallow, which was dominated by broadleaf weeds. Acacia auriculiformis and P. phaseoloides plots has similar weed composition which was composed of Physalis angulata, Acalypha ciliata, S.ocymoides, and E. hirta. Plots with L. leucocephala were dominated by O. oleracea, T. procumbens, and S. anthelmia. Senna siamea plots were dominated by Ageratum conzyoides, Leptochloa caerulescens, Commelina benghalensis, and Lindernia diffusa. Akobundu et al. 共1999兲 reported more grass weed population in continuously cropped systems than in systems where fallowing was practiced. The natural bush fallow plot was closely associated with C. odorata, T. triangulare and S. nigrum. Chromolaena odorata was more abundant in the natural bush fallow plot than the other two species. This result correlates well with field observation of the above
304
Figure 1. Ordination diagram of a Redundancy Analysis 共RDA兲 showing species composition within a tillage systems and fallow type in 1996. MT ⫽ minimum tillage and MD ⫽ mounding centroids. Shaded triangles represent plots where 1 ⫽ Pueraria phaseoloides 共Roxb.兲 Benth., 2 ⫽ Acacia auriculiformis A. Cunn. ex Benth., 3 ⫽ Leucaena leucocephala 共Lam.兲 de Wit, 4 ⫽ Natural bush fallow, 5 ⫽ Senna siamea Lam., 6 ⫽ Continuous cropping. Species are: Aca cil ⫽ Acalypha ciliata Fosk., Age con ⫽ Ageratum conyzoides L., Ama vir ⫽ Amaranthus viridis L., Bra lat ⫽ Brachiaria lata 共Schumach.兲 C.E. Hubb., Chl pil ⫽ Chloris pilosa Schumach., Chr odo ⫽ Chromolaena odorata 共L.兲 King & Robins., Cya pro ⫽ Cyathula prostrata 共L.兲 Bl., Cyn dac = Cynodon dactylon 共L.兲 Pers., Dig hor ⫽ Digitaria horizontalis Willd., Des sco ⫽ Desmodium scorpiurus 共Sw.兲 Desv., Ele ind = Eleusine indica 共L.兲 Gaertn, Eup hir ⫽ Euphorbia hirta L., Eup het ⫽ Euphorbia heterophylla L., Eup hys ⫽ Euphorbia hyssopifolia L., Hib spp ⫽ Hibiscus sp., Ipo inv = Ipomoea involucrata Beauv., Lap aes ⫽ Laportea aestuans 共L.兲 Chew, Lep cae ⫽ Leptochloa caerulescens Steud., Lin dif ⫽ Lindernia diffusa 共L.兲 Wettst., Old cor = Oldenlandia corymbosa L., Phy ang ⫽ Physalis angulata L., Pou gui ⫽ Pouzolzia guineensis Benth., Pan max = Panicum maximum Jacq., Por ole ⫽ Portulaca oleracea L., Pas orb ⫽ Paspalum orbiculare Forst., Sid ver ⫽ Sida veronisifolia L., Sol nig ⫽ Solanum nigrum L., Spe ocy ⫽ Spermacoce ocymoides Burm., Spi ant ⫽ Spigelia anthelmia L., Tal tri ⫽ Talinum triangulare 共Jacq.兲 Willd., Tre gui ⫽ Trena guineensis L., Tri cor ⫽ Triumfetta cordifolia A.Rich., Tri pro ⫽ Tridax procumbens L.
ground vegetation before land preparation in 1995 when C. odorata contributed more than 40% the total vegetation biomass 共Kang et al. 1997兲. In general there were more annual broadleaf weeds in the seed-
bank of planted fallow plots than in the control plots. This trend supports the work of Udensi et al. 共1999兲 who reported a shift in species composition of a weed community dominated by perennials to that domi-
305 nated by annual broadleaf weed population after management with cover crops.
is number IITA/03/JA/123 from the International Institute of Tropical Agriculture.
Species diversity References Species diversity decreased with mounding in both years 共Table 1兲. Species diversity was generally lower in 1995 than 1996, suggesting the emergence of new species after the 1995-cropping season 共Table 1兲. For example, in the minimum tillage plots the average number of species was 17 ⫾ 1.73 in 1995 and 21 ⫾ 1.64 in 1996. Whereas in the plots with mounds, the average number of species was 10 ⫾ 1.82 in 1995 and 11 ⫾ 1.69 in 1996. In plots under minimum tillage, species diversity was lowest in P. phaseoloides and highest in natural bush fallow in 1995 共Table 1兲. This pattern was reversed in 1996 共Table 1兲. Barberi et al. 共1998兲 reported that the diversity of weed communities in the seedbank was slightly greater in systems based on conservation tillage than in those based on ploughing. The high diversity index in plots under minimum tillage suggest that the system allows the build up of a more diverse group of weed species in the seedbank and this may lead to the emergence of potential problem weeds in future crops.
Conclusion This study showed that after six years of continuous fallow the weed seedbank was smaller in all planted fallows than in continuously cultivated treatments. The size of the weed seedbank varied with type of planted fallows with A. auriculiformis and P. phaseoloides having the largest impact and natural bush fallow the least impact. There were more weed species in plots under minimum tillage than in plots with mounds. There were more annual broadleaf weeds in the seedbank of planted fallow plots than in the control plots. Species diversity of the seedbank was greatest in plots under minimum tillage. Mounding as a seedbed preparation method, especially within the improved fallow system, could reduce the high weed pressure experienced by smallholder farmers in southwest Nigeria.
Acknowledgements We thank Udensi E. Udensi, John Ogazie, and Kayode Sayaolu for technical assistance. This manuscript
Adedeji F.O. 1984. Population dynamics of Aspilia africana in lowland bush fallows following shifting cultivation agriculture in southern Nigeria. Acta Oecologica 5: 315–320. Akobundu I.O., Ekeleme F. and Chikoye D. 1999. Influence of fallow management systems and frequency of cropping on weed growth and crop yield. Weed Research 39: 241–256. Antapa P.L. and Angen T.V. 1990. Tillage practices and residue management in Tanzania. In : Pushparajah E. Latham M. and Elliot C.R. 共eds兲, Organic matter management and tillage in humid and subhumid Africa. Bangkok, Thailand: International Board for Soil Research and Management, 1990. IBSRAM Proceedings no. 10. Baldoni G., Catizone P. and Viggiani P. 1999. Long-term effects of primary tillage and herbicide input on weed seedbank. Italian Journal of Agronomy 3: 7–18. Barberi P., Cozz.ini A., Macchia M. and Bonari E.C. 1998. Size and composition of the weed seedbank under different management systems for continuous maize cropping. Weed Research 38: 319–334. Bora I.P., Singh J., Borthakur R. and Bora E. 1999. Allelopathic effect of leaf extract of Acacia auriculiformis on seed germination of some agricultural crops. Annals of Forestry 7: 143–146. Chikoye D., Manyong M.V., Carsky R.J., Ekeleme F., Gbehounou G. and Ahanchede A. 2001. Response of speargeass 共Imperata cylindrica兲 to cover crops integrated with handweeding and chemical control in maize and cassava. Crop Protection 19: 481–487. Chikoye D. and Ekeleme F. 2001. Weed flora and soil seedbank in fields dominated by Imperata cylindrica in the moist savannah of West Africa. Weed Research 41: 475–490. De Rouw A. 1995. The fallow period as a weed-break in shifting cultivation 共tropical rain forest兲. Agriculture, Ecosystems, and Environment 54: 31–43. Eggers T. 1984. Changes in the field vegetation. Schweizerische Landwirtschaftliche Forschung 23: 47–60. Ekeleme F., Akobundu I.O., Isichei A.O., and Chikoye D. 2000. Influence of fallow type and landuse intensity on weed seed rain in a forest/savanna transition zone. Weed Science 48: 604–612. Kamara A.Y., Akobundu I.O., Sanginga N. and Jutzi S.C. 1999. Effects of mulch from 14 multipurpose tree species 共MTPs兲 on early growth and nodulation of cowpea 共Vigna unguiculata L.兲. Journal of Agronomy and Crop-Science 182: 127–133. Kang B.T., Salako F.K., Akobundu I.O., Pleysier J.I. and Chianu J.N. 1997. Amelioration of a degraded Oxic Paleustalf by leguminous and natural fallows. Soil Use and Management 13: 130– 136. Littel R.C., Milliken G.A., Stroup W.W. and Wolfinger R.C. 1996. SAS system for mixed models. Statistical Analysis Systems Inc. Cary, North Carolina, USA. 633p. Manly B.F.J. 1990. Randomization and Monte Carlo methods in biology. Chapman and Hall, London. UK. Mulugeta D. and Stoltenberg D.E. 1997. Weed and seedbank management with integrated methods as influenced by tillage. Weed Science 45: 706–715.
306 Salako F.K., Hauser S., Babalola O. and Tian G. 2001. Improvement of the physical fertility of a degraded Alfisol with planted and natural fallows under human tropical conditions Soil Use and Management 16: 1–8. Shrestha A., Knezevic S.Z., Roy R.C., Ball-Coelho B.R. and Swanton C.J. 2002. Effect of tillage, cover crop, and crop rotation on the composition of weed flora in a sandy soil. Weed Research 42: 76–87. Smilauer P. 1992. CanoDraw. Ithaca, Microcomputer Power, New York, USA. Swanton C.J., Shrestha A., Knezevic S.Z., Roy R.C. and Ball-Coelho B.R. 2000. Influence of tillage type on vertical weed seedbank distribution in a sandy soil. Canadian Journal of Plant Science 80: 455–457.
Teasdale J.R. and Mohler C.L. 1993. Light transmittance, soil temperature, and soil moisture under residues of hairy vetch and rye. Agronomy Journal 85: 673–680. Ter Braak C.J.F. 1990. CANOCO – a FORTRAN program for canonical community ordination by 共partial兲 共detrended兲 共canonical兲 correspondence analysis, principal components analysis, and redundancy analysis version 3. 10, Microcomputer Power, Ithaca, New York, USA. Ter Braak C.J.F. and Prentice I.C. 1988. A theory of gradient analysis. Advances in Ecological Research 18: 271–317. Udensi E.U., Akobundu I.O., Ayeni A.O. and Chikoye D. 1999. Management of cogongrass 共Imperata cylindrica兲 with velvetbean 共Mucuna pruriens var. utilis兲 and herbicides. Weed Technology 13: 201–208.
