Faculty of Agriculture, University of Peradeniya, Peradeniya, Sri Lanka

Impact of the Cropping Systems of a Minor Dry Season on the Growth, Yields

and Nitrogen Uptake of Maize (Zea mays L) Grown in the Humid Tropics during

the Major Rainy Season

U. R. Sangakkara, W. Richner, F. Steinebrunner and P. Stamp

Authors’ addresses: Dr U. R. Sangakkara, Faculty of Agriculture, University of Peradeniya, 20400, Sri Lanka; Dr W. Richner,Ir F. Steinbrunner and Dr P. Stamp, Institute of Plant Sciences, ETH, Zurich 8092, Switzerland

With 3 tables

Received August 21, 2002; accepted September 9, 2002

Abstract

A field study evaluated the residual effect of a croppingsystem in two minor (dry) seasons on the productivitypattern and nitrogen utilization of a maize crop grown inthe subsequent major (wet) seasons. The cropping systemsestablished in the minor seasons, where evaporationexceeds rainfall, were either monocultures of maize (Zeamays L), or maize intercropped with either common bean(Phaseolus vulgaris L) or sunhemp (Crotolaria juncea L). Inaddition, monocultures of two green manures, namelysunhemp (Crotolaria juncea) or Tithonia (Tithonia diversi-folia), were established. The residues of maize and beansand the green manures were incorporated at the end of theminor season; at the onset of rains in the major season,maize crops were established on the same plots. Germina-tion of maize was not affected by the previous croppingsystem. In contrast, crop growth and yields of maize andnitrogen utilization were affected by the previous croppingsystem. Optimum growth and highest yields were procuredin maize that was grown after a green manure crop.Similarly, although the yields were high, the planting of agreen manure crop reduced nitrogen utilization by maize inthe major season, thereby indicating its potential contri-bution to sustainability, due to its lower mining of soilnitrogen. On the basis of the results of this two-year study,the impact of cropping systems in minor seasons on theproductivity of maize, a very important highland cereal inthe tropics, grown under rain-fed conditions in a majorseason, is presented.

Key words: maize — cropping systems — majorseason — minor season — yields — nitrogenutilization

Introduction

Over the past few decades, Asian cropping systemshave been intensified to procure multiple benefitsfrom available resources in order to increase

production and provide food for the increasingpopulations. This has meant that crops have beengrown in suboptimal conditions, leading to thedegradation of land and loss of soil quality(Toomsan et al. 2000, Devendra and Thomas2002). The lack of adequate fertilizer and organicmatter inputs to these soils, and the non-adoptionof suitable methods to maintain sustainability,compound the process of soil degradation, whichcould reduce yields with time (Weischet andCaviedes 1993).

Monsoon regions of Asia have a dry (minor) andwet (major) season, and different crops are culti-vated on the basis of available soil moisture(Devendra and Thomas 2002). Farmers generallyresort to monocultures in the major wet season,and grow rice in the lowlands and cereals and otherfood crops in the highlands. In regions withadequate rainfall in the dry (minor season), farmersattempt to cultivate some cereals, legume or otherfood crops, although, in most instances, moistureand other stress factors mean that the yields arevery low. This method has an adverse impact onsoils and sustainability as the mining of nutrientsleads to further land degradation. In contrast,farmers leave fields fallow in some regions withinadequate rainfall, which aggravates problems dueto the increased incidence of weeds in the major wetseason (Akobunbu et al. 1999). Hence, the crop-ping pattern of the minor season could have asignificant impact on the productivity of thefollowing major wet season, as the judiciousselection of a suitable cropping system to possiblyprovide some crop yield or income to farmers orgreen manures could be used to develop soil

J. Agronomy & Crop Science 189, 361—366 (2003)� 2003 Blackwell Verlag, BerlinISSN 0931-2250

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fertility and quality. Intercropping of a cereal withlegumes in the dry season or the planting of species,such as Tithonia diversifolia (Hemsl.) A Gray, anonlegume but important green manure (Gachengoet al. 1998, Jama et al. 2000), or a legume mono-culture suited to dry conditions, could help main-tain soil fertility and supply nitrogen (N) toimprove yields of crops in the succeeding majorseason (Bockman 1997, Kouyate et al. 2000;Whitmore 2000).

Because it is used in human food and animal feedas seed or biomass, maize (Zea mays L) is the mostimportant highland cereal in the humid tropics ofAsia (Devendra and Thomas 2002). However,much of the research on maize and its productivityin cropping systems has concentrated either onAfrica or the semiarid regions of Asia (e.g. Subbianet al. 2000; Pal and Sheshu 2001), although asignificant land area is planted with this species inthe humid regions of Asia (FAO 1999). The highrequirement of N for this crop means that success-ful cultivation of maize in these regions is depend-ant upon soil fertility (Kayuki and Wortmann2001). As N is often the most limiting nutrient intropical cropping (Subbian et al. 2000), the crop-ping system of the minor season could have asignificant impact on soil nutrients and quality, andtherefore directly affects the growth and yields of amaize crop in the major wet season.

Research on the humid tropics of Asia does nothighlight the impact of cropping systems of minorseasons on the productivity of crops, especiallymaize, in the major wet seasons, as the emphasis ison the culture of rice (e.g. Toomsan et al. 2000;Vitayakorn et al. 2000). However, the developmentof sustainable highland cropping systems is a keyfactor in improving the productivity of tropicalagro-ecosystems. Therefore, field studies were con-ducted over two major (wet) seasons (October 1999and 2000) in order to identify the impact of selectedpreceding intercropping systems, including greenmanuring, on the productivity of a maize cropgrown during the wet season. The cropping systemsselected in the minor dry season were based ontheir ability to produce harvestable products orprovide adequate green manures to the maize cropof the major (wet) season.

Materials and Methods

The experiments were carried out at the experimental farmof the University of Peradeniya, Sri Lanka, during themajor (wet) season that corresponds to the North-east

monsoon of 1999 ⁄2000 and 2000 ⁄ 2001. The site is locatedat 8�N, 81�E, at 421 m above mean sea level, in the mid-country, intermediate zone of the country. The soil ofthe site is an Ultisol (Rhododhult) (Panabokke 1996), witha pH (1 : 2.5 H2O) of 6.4 ± 0.26, total N content of95 mg g)1 of soil, P 14.9 mg g)1 of soil and K 8.2 mg g)1

of soil, with a CEC of 25.61 ± 2.87 meq ⁄100 g of soil. Theorganic C content in the 0–40 cm layer of soil was 0.85 %.

The 15 years mean rainfall (1985–2000) over the wetseason (October–February) was 885 ± 31.6 mm whilethe mean evaporation over the same period was563 ± 18.5 mm.

At the onset of the minor (dry) season (May) in 1999,land was prepared and plots of 10 · 4 m demarcated forthe five treatments. Maize (Zea mays L) open pollinated cvRuwan was planted at the recommended spacing of60 · 30 cm. The plots selected for intercropping wereplanted with rows of maize, with beans (Phaseolus vulgarisL cv Wade), which is a food legume, or sun hemp(Crotolaria juncea L), planted in-between the rows of maizeat the same time. The spacing between the legume plantsvaried between 5 and 7 cm. In addition, monocultures ofCrotolaria and Tithonia, which were to be used as greenmanures, were also established in similar plots. Thespacings adopted were 30 · 15 cm for Tithonia and30 · 7.5 cm for sun hemp monocultures.

The fertilizer applied prior to planting of maize wasequivalent to 25 kg N, 45 kg P and 30 kg K (Departmentof Agriculture 1989). The maize crop was additionallysupplied with 45 kg N at 30 days after planting. Noadditional fertilizer was added to the beans or Crotolariaintercrops, and the P and K rates supplied to the maizewere added to the two green manure plots. The plots weremanually weeded twice.

After the fresh pods of beans had been harvested45–55 days after planting, the residue was incorporatedinto the soil manually. The intercropped Crotolaria bio-mass was incorporated at flowering (50–55 days after it wasplanted in the same way as the beans). After harvestingmaize seeds, the straw was removed as per normal practice.The biomass of the two green manure monocultures(Crotolaria and Tithonia) was incorporated at this time.Biomass and N contents of all incorporated material weremeasured using plant samples from 1 · 1 m quadrats.

In late October 1999, maize was again planted in all plotsin order to carry out the main experiment. The spacing andrates of fertilizer were the same as that used for the maizecrop of the previous minor (dry) season. The experiment,which had five treatments, was conducted within a Rand-omized Block design at two sites, with two replicates persite.

After harvesting the maize in February 2000, thetreatments of the previous minor season were establishedin the same plots and managed in the same way that theywere in 1999. After it had been harvested, maize mono-cultures were planted again in all plots in late October 2000(during the major (wet) season) and managed as before.

The measurements made in both wet seasons were asfollows:

362 Sangakkara et al.

• The germination of maize using 1 · 1 m quadrats at15 days after planting;

• The biomass of 5 maize plants per plot at silking stage;

• Seed yield (at 10 % moisture content) and crop biomassof maize at maturity;

• N contents of seeds and straw (including cob) (Micro-Kjeldhal method).

The data were subjected to an analysis of varianceappropriate to the experimental design. Significant differ-ences between treatments were separated by Fisher’s LSDtest.

Results and Discussion

The use of a legume intercrop or a green manuremonocrop in the preceding season had no impacton the germination and establishment of maize inthe major (wet) seasons, and all plots achieved80 % germination. The anticipated populations of40–45 plants per 10 m)2, was obtained, possiblydue to the availability of adequate soil moistureduring the wet season. The availability of adequatesoil moisture seems to mask the benefits of incor-porating organic matter at a time when seedlingsare dependent on stored reserves. In contrast,organic matter is generally useful in the long-termto improve soil properties in drier seasons andregions (Subbian et al. 2000), which could sustainthe germination of crops.

Dry matter accumulation and crop growth ratesbetween silking and harvest were affected by thepreceding cropping systems of the dry (minor)seasons (Table 1). In the first season, at the silkingand harvest, the biomass of maize plants grownafter a monoculture of maize or an intercrop ofCrotolaria were lower than in plants grown after anintercrop of beans. This resulted in lower cropgrowth rates in these plots, which could be partlyattributed to the lower quantity of biomass addedby the Crotolaria (81 gm)2) in comparison to the

beans (108 g m)2). Hence, the quantity of biomassadded seems to influence the rate of vegetativegrowth in the succeeding maize crop, although theCrotolaria biomass is higher in quality. In thesecond season, the differences in biomass at thesilking and harvest of the plants grown after amaize monocrop or an intercrop of beans orCrotolaria were reduced. In contrast, the benefitsof intercropping a legume were seen in the cropgrowth rates. As in season one, the use of beans inthe preceding season induced the developmentof higher growth rates. The crop growth rates ofmaize plants in plots that contained an intercrop ofCrotolaria increased significantly from season one.This suggests that the benefits of intercroppingCrotolaria develop over time. Therefore, in theshort term, intercropping with beans in the pre-ceding season may provide more conducive condi-tions for the better vegetative growth of the maizecrops that follow.

A comparison of the impact of all green manuresshowed that the highest biomass accumulation waswhen maize was planted in plots that containedmonocrops of Crotolaria and Tithonia in theprevious season. This resulted in higher cropgrowth rates in both wet seasons. This clearlyimplied that the use of an organic manure, with itshigh biomass (320 and 615 gm)2 of Crotolaria andTithonia, respectively), alone has a significantimpact on the growth of the succeeding maizecrop. More importantly, the effects of these greenmanures increased over time. In season one, thecrop growth rates of maize planted in plots thatcontained monocrops of Crotolaria and Tithoniawere 26 % and 28 % greater than those of plantsgrown after a maize monocrop. In the secondseason, the increments in crop growth rates due tothe monocrops of Crotolaria and Tithonia were28 % and 31 % respectively, which was an incre-

Table 1: Impact of the preceding cropping system on biomass of maize plants at silking and harvest, and cropgrowth rate CGR (g.day)1) between the two samplings

Cropping system

Season 1 Season 2

Silking g.m)2 Harvest CGR g.day)1 Silking g.m)2 Harvest CGR g.day)1

Maize monocrop 436 1065 12.58 458 1101 12.86Maize + phaseolus beans 477 1144 13.34 468 1137 13.38Maize + crotolaria 459 1090 12.62 461 1118 13.14Crotolaria monocrop 521 1315 15.88 534 1362 16.56Tithonia monocrop 548 1354 16.12 556 1401 16.90LSD (P ¼ 0.10) 18.41 94.58 0.41 28.30 66.07 0.34

Minor Dry Season’s Impact on Maize during the Rainy Season 363

ment over the previous season. The impact of thegreen manure crop in the preceding season wasagain evident in the crop growth rates rather than interms of total biomass production during bothseasons. This implies that the benefits of growing agreen manure in the minor (dry) seasons couldaccumulate over time, as Subbian et al. (2000)demonstrated is also the case in semiarid condi-tions, by promoting the rate of biomass accumula-tion of maize grown in the wet season. It also showsthe benefit of incorporating a greater quantum ofbiomass, which not only could provide nutrients butalso improve soil properties (Zebarth et al. 1999).However, the quality of organic matter cannot benegated, as Crotolaria has a high N content whileTithonia has high contents of both both N and Pthrough its biomass (Jama et al. 2000), which arefactors that could also influence plant growth.

Seed yields of maize (Table 2) highlight theimpact of the preceding cropping system underhumid tropical conditions. The seed yields fromplots containing maize moncultures were the low-est, especially in season two. While this could be aseasonal effect, the loss of sustainability by contin-ued cropping, which, in addition, could also resultin disease problems (Sauerborn et al. 2000), cannotbe ignored. The supply of a green manure as anintercrop or a monocrop in the preceding seasonincreased yields significantly The only exceptionwas in season one when the beans were inter-cropped in the preceding season, which did notinduce a significant yield increment over that ofmonoculture plots. This was an interesting phe-nomenon, as biomass production was increased toa greater extent by a bean intercrop than aCrotolaria intercrop in the preceding season(Table 1). It also implies that a Crotolaria inter-crop stimulates yield increments rather than bio-mass, and that causal factors need further study.

The impact of the intercropped or monocroppedgreen manures on maize yields was similar in bothseasons. However, there was an increment in themagnitude of the benefits in season two. Anintercrop of beans in the preceding seasonincreased yields of maize in the two wet seasons,by 3 % and 16 %. The use of a Crotolariaintercrop in the earlier season increased maizeyields over the monoculture plots by 10 % inseason one, while the increment was 24 % inseason two. This again implied that the benefits of aCrotolaria intercrop in the preceding season occursover time, and stimulates yields to a greater extentthan plant biomass (Table 1). However, the mostsignificant impact was again observed with themonocrops of the green manures in the precedingseason. In the first season, the use of Crotolaria andTithonia monocrops in the dry season increasedmaize yields by 15 % and 26 %, respectively, overthat of the monoculture plots. In the secondseason, the yield increments due to Crotolariaand Tithonia were 39 % and 51 %, respectively,over the maize monoculture plots. This effect, alsoreported in Africa by Sanchez (1999), again high-lights the value of a green manure cultivated in thedry season rather than leaving the land fallow, andillustrates the accumulation of beneficial effectsover time. This could be attributed to the effectsthat the green manures have on soil and also theprovision of nutrients, which enhances the growthof maize under tropical farming conditions (Jamaet al. 2000, Kayuki and Wortmann 2001).

In contrast to the seed yields, harvest indices inseason one were not affected by the croppingsystem of the preceding season (Table 2). Theywere similar, which suggested that the benefits ofusing a green manure in the preceding season occurin terms of increments in both seed and strawyields. A monocrop of maize in the preceding

Table 2: Seed yield (g.m)2) and harvest index (HI) of maize as affected by the precedingcropping system

Cropping system

Season 1 Season 2

Seed yield g.m)2 HI Seed yield g.m)2 HI

Maize monocrop 381 0.36 342 0.32Maize + phaseolus beans 392 0.34 398 0.35Maize + crotolaria 422 0.38 425 0.38Crotolaria monocrop 440 0.34 477 0.35Tithonia monocrop 481 0.35 516 0.37LSD (P ¼ 0.10) 18.1 0.046 29.7 0.014

364 Sangakkara et al.

season reduced the harvest indices in season two,which was similar to that observed in yields. Theharvest indices of the maize crop grown in plotsthat either contained a green manure intercrop or amonocrop were greater than that of the maizemonoculture plots. While the harvest indicesbetween the green manure treatments were similar,the results suggest the development of the beneficialeffects of incorporating the manures on seed yields,which would lead to economic returns. Again,longer term studies would highlight greater benefitsin terms of increased seed yields over that of straw.

A cropping pattern in a tropical farming systemneeds to consider the long-term sustainability,especially in relation to mining of nutrients. Nitro-gen is the most important and beneficial nutrient intropical cereal farming (Subbian et al. 2000), andlegumes and ⁄or green manures thus form animportant component of these systems (Sauerbornet al. 2000, Kayuki and Wortmann 2001). Contin-ued cropping of maize removes a large quantity ofN from the cropping system in both seasons(Table 3). Generally, all of the N applied is nottaken up by maize, especially in a wet season whereleaching could be high. The results thus suggest thatthe maize crop mines soil N under such conditions,depleting the system of an important nutrient. Thisleads to the loss of productivity of continued cerealmonocultures, as seen in many studies in the tropics(Reddy et al. 1994, Whitmore 2000). The non-incorporation of organic matter aggravates the lossof soil quality and sustainability.

The use of a green manure as an intercrop ormonocrop in the preceding season reduces the Nuptake efficiency by maize in the wet seasons,although there are no changes in the patterns of

reduction between treatments. This suggests thatthe patterns of uptake of N do not vary much withtime, although longer term experiments may proveotherwise. The reduction of N uptake is greaterwith the use of Crotolaria than with beans.However, both legumes add N to the systemthough its biomass and possibly by N fixation,which could also be taken up by the succeedingcrop (Dakora and Keya 1997, Boddy et al. 1997).Planting Tithonia, which has a high N and Pcontent (Jama et al. 2000), and which adds thehighest biomass at the end of the preceding season,reduces the N uptake efficiency by maize in the wetseasons to the greatest extent, whilst inducing theproduction of the highest yields (Table 2). Thisimplies that the maize crop removes a lowerquantity of N when planted after Tithonia, thuspossibly retaining the N status of the soil necessaryto maintain soil fertility and sustainability forsubsequent seasons. However, the results suggestthe ability of the added organic matter through alegume or Tithonia to retain N in the soil systemfor subsequent crop use as shown by Drinkwateret al. (1998) and Poudel et al. (2001).

The study illustrates that the growth and yield ofa maize crop in the major (wet) season in lowfertile soils, commonly found in most farmingsystems in the tropics, is affected by the precedingcrop grown in the minor (dry) season. Whilegermination of the maize in the major season is notaffected by the previous cropping season, cropgrowth is influenced significantly by the use of anintercrop of a legume in the cultivation of a maizeor a green manure crop in the preceding season.The green manure can be planted as an improvedfallow, as recommended by Sanchez (1999) forAfrican conditions, without keeping the landunused if stress factors, such as soil moisture, doesnot allow the cultivation of a cash crop. Thus, theeconomic yields that are the primary interest forthe farmer increase, and, furthermore, the studysuggests possible cumulative effects over a numberof seasons. More importantly, the use of a greenmanure as an intercrop or a monocrop in thepreceding season reduces the removal of soil N,thus influencing the fertility of the soil, which isone of the most important parameters of successfulagriculture in the tropics. The study also high-lights the importance of Tithonia as a greenmanure, as this crop can be grown under marginalconditions in seasons when crops cannot becultivated, in imparting beneficial effects to thesucceeding crop.

Table 3: Nitrogen uptake efficiency by maize on thebasis of N supplied by the organic matter of thepreceding season and fertilizers from the current season

Cropping system

N uptake efficiency*

Season 1 Season 2

Maize monocrop 0.85 0.89Maize + phaseolus beans 0.74 0.78Maize + crotolaria 0.69 0.71Crotolaria monocrop 0.65 0.61Tithonia monocrop 0.51 0.53Sx 0.09 0.12

*N uptake efficiency (%) ¼ Total N removed by theplant at harvest/Total N supplied by fertilizer of thepresent season and organic matter.

Minor Dry Season’s Impact on Maize during the Rainy Season 365

Acknowledgements

The authors express their gratitude to the Federal Instituteof Technology (ETH), Zurich and the University ofPeradeniya, for financial assistance. Technical assistancewas provided by Mr S Kandaploa, Ms S Bootawatte andMr C Samarasekera over a period of three years. Theauthors also acknowledge the help of the Federal Instituteof Technology (ETH) Zurich, who provided the facilitiesand funds for compiling this paper.

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