impact of water stress on maize grown off-season in a

Drought Stress

Department of Rural Engineering, University of Sao Paulo, Piracicaba, SP, Brazil

Impact of Water Stress on Maize Grown Off-Season in a Subtropical Environment

C. M. T. Soler, G. Hoogenboom, P. C. Sentelhas, and A. P. Duarte

Authors� addresses: Dr. C. M. T. Soler (corresponding author; e-mail: [email protected]), Department of Rural Engineering,University of Sao Paulo, Av. Padua Dias 11, CP.9. CEP 13.418-900, Piracicaba, SP, Brazil. Current Address: 1109 Experiment

Street, Department of Biological and Agricultural Engineering, The University of Georgia, Griffin, GA 30223, USA; Prof.G. Hoogenboom, 1109 Experiment Street, Department of Biological and Agricultural Engineering, The University of Georgia,Griffin, GA 30223, USA; Assoc. Prof. P. C. Sentelhas, Department of Exact Science, University of Sao Paulo, Av. Padua Dias

11, PO Box 9. 13.418-900, Piracicaba, SP, Brazil; Dr. A. P. Duarte, Apta Medio Paranapanema, Rodovia Assis-Marilia km 397,Caixa Postal 263 – Assis, SP, Brazil

With 6 figures and 4 tables

Received July 19, 2006; accepted February 20, 2007

Abstract

During the last decade, the production of off-season maizehas increased in several regions of Brazil. Growing maizeduring this season, with sowing from January throughApril, imposes several climatic risks that can impact cropyield. This is mainly caused by the high variability ofprecipitation and the probability of frost during thereproduction phases. High production risks are also par-tially due to the use of cultivars that are not adapted to thelocal environmental conditions. The goal of this study wasto evaluate crop growth and development and associatedyield, yield components and water use efficiency (WUE) formaize hybrids with different maturity ratings grown off-season in a subtropical environment under both rainfedand irrigated conditions. Three experiments were conduc-ted in 2001 and 2002 in Piracicaba, state of Sao Paulo,Brazil with four hybrids of different maturity duration,AG9010 (very short season), DAS CO32 and Exceler (shortseason) and DKB 333B (normal season). Leaf area index(LAI), plant height and dry matter were measured approxi-mately every 18 days. Under rainfed conditions, the soilwater content in the deeper layers was reduced, suggestingthat the extension of the roots into these layers was aresponse to soil water limitations. On average, WUE variedfrom 1.45 kg m)3 under rainfed conditions to 1.69 kg m)3

under irrigated conditions during 2001. The average yieldvaried from 4209 kg ha)1 for the hybrids grown underrainfed conditions to 5594 kg ha)1 under irrigated condi-tions during 2001. Yield reductions under rainfed condi-tions were affected by the genotype. For the hybrid DKB333B with a normal maturity, yield was reduced by 25.6 %while the short maturity hybrid Exceler was the leastimpacted by soil water limitations with a yield reduction ofonly 8.4 %. To decrease the risk of yield loss, theapplication of supplemental irrigation should be considered

by local farmers, provided that this practice is not restrictedby either economic considerations or the availability ofsufficient water resources.

Key words: dry matter accumulation — growthanalysis — leaf area — water deficit — water useefficiency — yield components

Introduction

In recent years, maize (Zea mays L.) has becomeone of the main alternative crops for the fall-wintergrowing season in the subtropical central-westernand south-eastern regions of Brazil. According toLanders (1994), maize grown off-season presentsimportant advantages for the farmers, including amore rational use of agricultural equipment andhuman resources, more protection of the soil due tothe existence of crops for a longer period during theyear, better prices for maize during the off-seasonharvest time and better weed control for thefollowing summer crop. However, water deficits,sub-optimum temperatures and short days arecommon during the off-season, resulting in apotential reduction in maize yield (Farinelli et al.2003). In the subtropical and tropical regions ofBrazil, off-season maize is mainly grown underrainfed conditions, causing a high variability inyield (Caramori et al. 1999). High production risksand low yields could also be caused by the use ofnon-adapted cultivars to the environmental condi-tions of this growing season, as the available genetic

J. Agronomy & Crop Science 193, 247—261 (2007) doi:10.1111/j.1439-037X.2007.00265.x� 2007 The AuthorsJournal compilation � 2007 Blackwell Verlag, BerlinISSN 0931-2250

U.S. Copyright Clearance Centre Code Statement: 0931–2250/2007/9304–0247 $15.00/0 www.blackwell-synergy.com

material is usually obtained from selections con-ducted during the normal growing season fromSeptember to December (Quiessi et al. 1999).Farinelli et al. (2003) evaluated 15 commercialcultivars in south-eastern Brazil in both normaland off-season periods. They found that the localenvironmental conditions reduced the yield duringthe off-season and that different cultivars weresuitable for each type of growing season. Similarly,Goncalves et al. (1999) reported that a set ofcultivars differed with respect to the adaptabilityand stability of grain yield for maize grown off-season in southern Brazil. Kamara et al. (2003)found that some genotypes selected for high yieldunder normal growing conditions in Nigeria alsoshowed a high level of dry matter accumulation andpartitioning of assimilates to the grain under mildwater-deficit conditions. According to Kamaraet al. (2003), the improved genotypes that per-formed better under water-deficit conditions shouldbe recommended for release in areas prone tointermittent drought. However, Zaidi et al. (2004)found that selection for mid-season drought toler-ance in tropical maize resulted in morpho-physio-logical changes that proved advantageous underboth drought and low-N stress, without significantyield penalties under optimal input conditions.One important aspect for new production prac-

tices is to define a proper maturity rating for thecultivars grown in a specific region or environment.Early maturing cultivars have become increasinglypopular in regions where limited water availabilityrequires irrigation or where an early harvest isdesired (Howell et al. 1998). For example, in someregions, cool or wet weather could hamper anddelay sowing, reducing the total number of poten-tial growing days and forcing some growers toconsider earlier maturing hybrids. Begna et al.(2001) found that early-maturing genotypes mayachieve earlier canopy closure and better use of thelight available during the growing season, which iscritical for short growing season environments.Crop establishment, determined by shoot and rootgrowth during early growth, was considered a vitalcomponent in procuring desired plant populationsand high yields in rainfed tropical smallholderfarming systems (Sangakkara et al. 2004). In sometemperate climatic regions, long-term research hasshown that adapted full-season maize hybrids oftenproduce the highest yield (Iragavarapu 1999).Studies conducted by Thompson (1986) revealedthat, for maize grown during the normal season,years with low temperatures were, in general,

associated with the highest yields. However, Wilsonet al. (1995) reported that potential yields for maizecould be negatively affected if the mean temperatureduring the growing season is <18 �C.Another approach to improve and stabilize

production is the application of supplementalirrigation. However, as water resources are becom-ing scarcer in many subtropical regions due to thecompetition with residential and industrial sectors,studies are needed to determine the effect ofirrigation on yield and the improvement in thewater use efficiency (WUE) of the major agronomiccrops. Studies on WUE are mainly associated withthe normal growing season (spring and summermonths). For example, Howell et al. (1995) repor-ted that the WUE for fully irrigated maize rangedfrom 1.27 to 1.35 kg m)3 and Sadras and Calvino(2001) reported a value of 1.17 kg m)3. Forirrigated maize grown under cool winter conditionsin India, Mishra et al. (2001) reported values ofWUE that ranged from 1.17 to 1.75 kg m)3. Cropgrowth in commercial fields usually requires max-imizing grain yield on limited available waterresources, which results in maximizing the ratioof yield to evapotranspiration. Many authors havestudied the impact of water deficits on growth anddevelopment of maize grown during the normalseason, i.e. during the spring and summer months.Most studies suggest that water shortage during thevegetative phase reduces leaf area growth (NeSmithand Ricthie 1992a), internode elongation (Novoaand Loomis 1981), leaf and stem weight (Eck 1984)and radiation use efficiency (Earl and Davis 2003).According to Kiniry and Ritchie (1985), the mostcritical period for yield determination for plantsunder drought stress is between the two weeks priorto and after silking. Drought stress prior to silkingcan cause a failure of ear development, whiledrought stress after pollination can cause a reduc-tion in the number of kernels (Harder et al. 1982).Grant et al. (1989) found that water stress prior tosilking did not decrease kernel number per ear, butearly grain development drought resulted in asignificant reduction in kernel number. Andradeet al. (1999) demonstrated that a limited partition-ing of dry matter to reproductive tissues during thecritical period around silking resulted in a lownumber of established kernels.The number of kernels per plant depends on the

number of ears per plant and the number of kernelsper ear that reach physiological maturity. Pandeyet al. (2001) found that grain yield was primarilycorrelated with the kernel number per unit land

248 Soler et al.

area and then with kernel weight when deficitirrigation was imposed during the vegetative andreproductive phases. Little information existsabout maize grown off-season under rainfed con-ditions in subtropical environments, such as thecentral-western and south-eastern regions of Brazil,but the advantages of identifying and using the bestmaize hybrids adapted for these conditions arecrucial to obtain high and stable yields.The goal of this study was to evaluate the growth

and development of four maize hybrids with differ-ent maturity ratings grown off-season under rainfedand irrigated conditions in a subtropical environ-ment and to determine which hybrids were bestadapted for this environment. Specific objectivesincluded the determination of plant growth anddevelopment as a function of thermal time, the ana-lysis of the impact of drought stress on plant devel-opment and yield, and a comparison of WUE ofmaize grown under rainfed and irrigated conditions.

Materials and Methods

Field experiments

Three field experiments were conducted at the �EscolaSuperior de Agricultura Luiz de Queiroz� of the Universityof Sao Paulo, in Piracicaba ()22�43¢ latitude, )47�25¢longitude, 580 m above the sea level), state of Sao Paulo,Brazil, during 2001 and 2002. The climate of the regionaccording to the Koppen’s classification is Cwa: subtropicalwith a rainy summer and dry winter. The soil of theexperimental site was classified as a Typic Eutrudox, a darkred soil with a clay texture.

One experiment was conducted in 2001 under irrigatedconditions and two experiments were conducted in 2002, oneunder rainfed and one under irrigated conditions. Allexperiments had a randomized complete block design withthree replicates for 2001 and four replicates for 2002. Themaize hybrids used in this study were: AG9010, a very shortseason hybrid, DASCO32 and Exceler, short season hybridsand DKB 333B, a normal season hybrid. These hybridsnormally produce a high yield with high quality grains (semi-flint) that are acceptable for the Brazilian market. Thesowing dates were 15 March in 2001 and 13 March in 2002.Each plot was 20 m long and 2.4 m wide, with four rows perplot. The seeds were planted in rows and thinned to fourplants per meter of row at the V2 stage. The row spacing was0.80 m and the density was 50 000 plants ha)1. All experi-ments were fertilized at sowing with 380 kg ha)1 of formula8-28-16 for the irrigated experiments and 250 kg ha)1 for therainfed experiment. A side dress of 108 kg ha)1 urea wasapplied at the V5–V6 stage and the irrigated experimentswere also fertilized with a side dress of 108 kg ha)1 urea atthe V10 stage. Pest and weeds were controlled following thelocal technical recommendations. In general, the manage-ment practices used in the experiments were similar to the

practices commonly used by the farmers in this subtropicalregion. In the irrigated experiments, water was applied via acentre pivot to avoid drought stress. The amount of waterapplied was based on crop evapotranspiration, estimatedfrom a class A pan evaporation (Allen et al. 1998).

Plant measurements

Leaf area and plant height of three plants for each plot weremeasured approximately every 18 days. The length andmaximumwidth of every leaf was measured by hand and theleaf area was estimated as the product of length and widthmultiplied by 0.75, similar to the procedures described byMcKee (1964). Plant height was measured from groundlevel to the top leaves without stretching them. Above-ground dry matter was measured approximately every18 days, starting at 21 days after sowing until final harvest,by sampling 1 m of plants from the central rows of eachplot. Final harvest was conducted manually on two rows of8 m each of the two central rows of the plot. The totalnumber of ears and plants were counted and the number ofears per plant was then determined. The harvested ears wereshelled and the percentage grain moisture was determined ina laboratory. Yield was corrected with 0 % of moisture.Twelve ears from each plot were selected randomly todetermine the number of grains per ear. The harvest index(HI) was estimated by dividing grain dry matter by totalaboveground dry matter. The stems were chopped beforedrying. Additional information with respect to the plantmeasurements details can be found in Soler et al. (2005).

Equations for determining thermal time, water

balance, WUE and yield reductions

The analysis of maize growth and development wasconducted as a function of thermal time (�Cd) as expressedin eqn 1.

TT8 ¼Xn

j¼1Tmax þ Tmin

2� Tb; ð1Þ

where TT8 ¼ thermal time (�Cd), Tmax ¼ maximum airtemperature and Tmin ¼ minimum air temperature. Tb isthe base temperature below which no development occurs;it was assumed to be 8 �C (Kiniry 1991); n is the number ofdays for which the thermal time is determined.

The Penman–Monteith equation was used to estimatereference evapotranspiration, as parameterized by Allenet al. (1998) in eqn 2.

ETo ¼0:408DðRn � GÞ þ c 900

Tþ273 u2ðes � eaÞDþ cð1þ 0:34u2Þ

; ð2Þ

where ETo ¼ reference evapotranspiration (mm day)1),Rn ¼ net radiation at the crop surface (MJ m)2 day)1),G ¼ soil heat flux density (MJ m)2 day)1), T ¼ air tem-perature (�C), u2 ¼ wind speed at a height of 2 m (m s)1),es ¼ saturated vapour pressure (kPa), ea ¼ actual vapourpressure (kPa), es)ea ¼ vapour pressure deficit (kPa), D ¼slope of vapour pressure curve (kPa �C)1) and c ¼ psychr-ometric constant (kPa �C)1).

Impact of Water Stress on Maize Grown Off-Season 249

Daily maximum, minimum, and average air temperature,rainfall, incoming solar radiation and wind speed data wereobtained from an automatic weather station, located in thevicinity of the experimental area (�1 km).

The crop coefficients (Kc) for maize were used to estimatecrop evapotranspiration as shown in eqn 3. Values of Kc

were set equal to 1 for the first 55 days, to 1.2 from days 57to 96 after sowing, and to 0.6 from day 97 to physiologicalmaturity, based on Allen et al. (1998) for maize grown in asubtropical region.

ETc ¼ KcETo ð3Þ

For the rainfed experiment conducted in 2002, the effectof soil water stress on ETc was estimated using the waterstress coefficient (Ks), as presented in eqn 4.

ETa ¼ KsETc; ð4Þ

where ETa ¼ actual evapotranspiration (mm day)1). Toestimate Ks, the daily water balance for the root zone wascalculated (eqn 5).

TAWr;i ¼ TAWr;i�1 þ Pi �ROi � ETa;i �DPi; ð5Þ

where TAWr,i ¼ Total Available Soil Water (TAW) in theroot zone at the end of day i (mm), TAWr,i)1 ¼ TotalAvailable Soil Water (TAW) in the root zone at the end ofthe previous day, i ) 1 (mm), Pi ¼ precipitation on day i(mm), ROi ¼ runoff from the soil surface on day i (mm),ETa,i ¼ crop evapotranspiration on day i (mm), DPi ¼water loss out of the root zone by deep percolation on day i(mm). ROi and DPi were not differentiated and simplycalculated as the water lost after the profile was at fieldcapacity.

As the soil water content decreases, water becomes morestrongly bound to the soil matrix and is more difficult toextract. When the soil water content drops below athreshold value, soil water can no longer be transportedquickly enough towards the roots to respond to thetranspirational demand and the crop begins to experiencedrought stress. The fraction of TAW that a crop can extractfrom the root zone without suffering drought stress is thereadily available soil water (RAW, mm; Allen et al. 1998).

RAW ¼ pTAW; ð6Þ

where p ¼ the average fraction of TAW that can bedepleted from the root zone before moisture stress (reduc-tion in ET) occurs. The factor p was set at 0.55 asrecommended by Allen et al. (1998) for maize.

Ks is a dimensionless transpiration reduction factordependent on available soil water that varies between 0 and1. For the fraction of water that is readily available, the Ks

factor is considered to be equal to 1. Thereafter, water islimiting and ETa <ETc, and Ks decreases in proportion tothe amount of water that remains in the root zone (Allenet al. 1998).

It was assumed that there were no water limitations inthe irrigated experiments based on the irrigation schedulingmethod that was used. Therefore, accumulated ETa for thecrop was equal to accumulated ETc during the growingseason. In the experiments conducted in 2002, the soil water

content was determined three times by gravimetric method,once prior to sowing and twice during the vegetativeperiod. Soil moisture was also monitored every two dayswith three groups of tensiometers installed at four differentdepths, including 0.2, 0.4, 0.55 and 0.7 m.

Water use efficiency (kg m)3) was determined as theratio of grain yield (Y, kg ha)1) to ETa (mm) as expressedin eqn 7.

WUE ¼�

YETa

�� 0:1 ð7Þ

Water use efficiency was estimated for all treatments. Inaddition, WUEi)r, which reflects the yield per unit of waterused by the crop above that of rain, was estimated usingeqn 8:

WUEi�r ¼�ðYi � YrÞ

ðETai � ETarÞ

�� 0:1; ð8Þ

where Yi ¼ the yield (kg ha)1) under irrigated conditions,Yr ¼ the yield (kg ha)1) under rainfed conditions, ETai ¼crop evapotranspiration under irrigated conditions (¼ETc)and ETar ¼ actual evapotranspiration under rainfed con-ditions.

To quantify the impact of water deficit on yield, the yieldreduction (Yred) for each hybrid was estimated using eqn 9:

Yred ¼�1�

�Yr

Yi

�� 100 ð9Þ

Statistical analysis

A statistical analysis was conducted to determine the effectof hybrid and irrigation on yield and yield components.This included an analysis of variance to analyze eachexperiment individually and general linear models (GLM)to analyze the three experiments as a group (SAS Institute2001). For each treatment, average and standard error ofthe mean were estimated. Simple and multiple linearregression analyses were performed to determine therelationship between yield and yield components and yieldand ETa.

Results

Weather and soil conditions

During the 2002 growing season, higher tempera-tures were recorded than during the 2001 growingseason. During the first 70 days of the growingseason, i.e. from sowing to the start of flowering,the average maximum and minimum air tempera-tures for all hybrids were 28.5 �C and 16.1 �C,respectively, in 2001, while in 2002 these temper-atures were 29.7 �C and 17.7 �C (Fig. 1a).Total cumulative precipitation and irrigation and

the number of days with precipitation and irriga-tion events for the 2001 and 2002 growing seasonsare illustrated in Fig. 1b,c. Seasonal rainfall was

250 Soler et al.

212 mm in 2001 and 363 mm in 2002, while totalirrigation was 334 mm in 2001 and 261 mm in2002. The 2002 season mainly had dry spells inApril during vegetative growth and in June when

grain filling occurred. Rain during the beginningand middle of May 2002 brought relief, just in timefor the most critical period around flowering.For the rainfed experiment conducted in 2002,

the tensiometers installed at a soil depth of 0.2 mwere the first ones to show a decrease in the soilmatrix potential (Fig. 2a). This was an indicationof the rapid consumption of water in the top soillayers that occurred around 26 days after sowing.Due to the extremely low soil water content duringsome periods, the tensiometers were unable tomeasure the matrix potential. The matrix potentialat a depth of 0.4 m started to show a decrease atapproximately 30 days after sowing, while for thedepth of 0.55 m the decrease in matrix potentialwas observed around 36 days. Finally, for thedepth of 0.7 m, a decrease in matrix potential wasobserved at 41 days after sowing. This was alsoconfirmed using the gravimetric method to monitorthe soil water content. The low values of watercontent measured on 18 and 39 days after sowingin the rainfed experiment indicated that cropgrowth was limited by soil water availability duringthe vegetative phase and that the soil water contentdecreased until a depth of 0.6 m (Fig. 2c). For theirrigated experiment conducted in 2002, only thetop soil layer at a depth of 0.2 m showed somevariation in tensiometer readings, while for thedeeper layers the values remained constant nearfield capacity, indicating that the maize plants werenot exposed to drought stress and water deficits(Fig. 2b).

Plant height, leaf area development and dry matter

accumulation

In general, the difference in plant height betweenthe hybrids for the rainfed and irrigated experi-ments became evident around 600 �Cd (V12–V13),which occurred after the rainfed crop had experi-enced several periods of drought stress (Fig. 3).These stages coincided with the rapid stem elonga-tion phase (Birch et al. 2002); water stress duringthis period has been reported to reduce internode’slength and, therefore, plant height (Bennouna et al.2004). The hybrid Exceler was the tallest underboth irrigated and rainfed conditions, with a heightof 2.30 and 2.27 m, respectively. The heights for thehybrids DKB 333B (2.13 m) and DAS CO32(2.17 m) were very similar for irrigated conditionsand the reduction in height under rainfed condi-tions was <5 % (Fig. 3). For hybrid AG9010, theheight was 2.00 m under irrigated conditions and

(a)

Day of year74 105 135 166 197 228

Tem

pera

ture

(°C

)

5

10

15

20

25

30

35

Tmax 2002 Tmax 2001 Tmin 2002 Tmin 2001Tavg 2002 Tavg 2001

Silking

Day of year

80 100 120 140 160 180 200 220 240

80 100 120 140 160 180 200 220 240

Irrig

atio

n an

d pr

ecip

itatio

n (m

m)

0

30

60

90

Acc

umul

ated

wat

er (

mm

)

0

200

400

600

800

Date3/1/2001 4/1/2001 5/1/2001 6/1/2001 7/1/2001 8/1/2001 9/1/2001

PrecipitationIrrigationAccumulated precipitation and irrigationAccumulated precipitationSowing

PhysiologicalMaturity

(b)

Silking

Irrig

atio

n an

d pr

ecip

itatio

n (m

m)

0

30

60

90

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Day of year

Acc

umul

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wat

er (

mm

)

0

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800

Date

3/1/2002 4/1/2002 4/1/2002 6/1/2002 7/1/2002 8/1/2002 9/1/2002

PrecipitationIrrigationAccumulated precipitation

SowingPhysiological

Maturity

(c)

Fig. 1: Maximum (Tmax), minimum (Tmin) and average(Tavg) air temperature for the 2001 and 2002 growingseasons (a); total irrigation, precipitation, and totalaccumulated precipitation and irrigation during the2001 (b) and 2002 (c) growing seasons for Piracicaba,state of Sao Paulo, Brazil


1.75 m under rainfed conditions; the most signifi-cant reduction in height was found for this hybrid,as it was reduced by 13 % under rainfed whencompared with irrigated conditions. The very short

season maturity rating of AG9010 could havedetermined that critical periods during the veget-ative phase were probably more exposed to waterdeficits, resulting in a considerable reduction inplant height.For all experiments, leaf area development of all

hybrids was very slow during the first part of thevegetative stage, followed by an intensive increaseafter 400 �Cd (V6–V7) until flowering, when themaximum leaf area index (LAI) was measured atapproximately 900 �Cd after sowing (Fig. 4). Thefinal LAI measurement, conducted around1250 �Cd for the hybrids grown under rainfedconditions in 2002, showed a decrease in LAI thatwas more evident than for the irrigated experi-ments. The hybrids DKB 333B and DAS CO32had the highest LAI for irrigated conditions in2001, with a maximum LAI of 3.75 and3.4 m2 m)2, respectively, and the lowest values forrainfed conditions in 2002, i.e. 3.07 and2.87 m2 m)2, respectively. The hybrids AG9010and Exceler had very similar values for LAI in allexperiments. The very short season hybrid AG9010had the lowest maximum LAI, which only variedfrom 2.6 to 2.8 m2 m)2 for both the rainfed andirrigated experiments (Fig. 4).Aboveground biomass accumulation followed a

sigmoid curve and most of the biomass accumula-tion occurred from 400 �Cd (V6–V7) to approxi-mately 1250 �Cd (grain filling). There was adecrease in the aboveground biomass accumulationrate from approximately 1250 �Cd to final harvestfor all four hybrids (Fig. 5). Mainly prior to thestart of flowering, the biomass accumulation for allhybrids grown in the 2001 experiment was higherthan for the 2002 experiments. For the 2002experiments the normal maturity hybrid, DKB333B and the very short season maturity, AG9010,had the largest differences in total dry matterbetween irrigated and rainfed conditions at harvest.

Yield and yield components

On average for all three experiments, Excelerhad the highest yield (expressed as 0 % moisture),i.e. 5343 kg ha)1 (Table 1). The interaction bet-ween hybrid and experiment was not significant(P > 0.05), meaning that the yield performance forthe four hybrids was similar in each one of the threeexperiments. The highest average yield was foundin 2001 for the irrigated experiment, i.e. 5594 kgha)1. There was a considerable difference betweenthe average of the irrigated and rainfed experiments

Day of year90 100 110 120 130 140 150

90 100 110 120 130 140 150

Mat

rix p

oten

tial (

kPa)

–100

(a)

(b)

(c)

–80

–60

–40

–20

0

Date

4/3/2002 4/18/2002 5/3/2002 5/18/2002 6/2/2002

0.2 m0.4 m0.55 m0.7 m

Silking

Soil depth:

Day of year

Mat

rix p

oten

tial (

kPa)

–100

–80

–60

–40

–20

0

Date

4/3/2002 4/18/2002 5/3/2002 5/18/2002 6/2/2002

0.2 m0.4 m0.55 m0.7 m

Silking

Soil depth:

Sowing date: March, 13th 2002

Soil water content (cm3 cm–3)

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40

Dep

th (

cm)

0

20

40

60

80

100

Irrigated (March 13th)

Rainfed (March 13th)

Irrigated (April 2nd)

Rainfed (April 2nd)

Irrigated (April 23th)

Rainfed (April 23th)

Fig. 2: Temporal and spatial variation of the matrixpotential for the 2002 rainfed (a) and irrigated experi-ment (b); soil water content measured by gravimetricmethod during the vegetative phase of the maize cropfor the irrigated and rainfed experiments in 2002 (c)

252 Soler et al.

of 2002, i.e. 910 kg ha)1. The high yield obtainedwith the hybrid Exceler for the 2002 rainfedexperiment was remarkable. It produced4859 kg ha)1, considerably more than the hybridsAG9010, DAS CO32 and DKB 333B (Table 1).During 2002, some rains that occurred during thebeginning and middle of May brought relief from

drought stress, which was just in time for the mostcritical period around flowering.Soil water limitations during the vegetative and

reproductive phases of the rainfed experimentconducted in 2002 caused an average yield reduc-tion of 17.8 % when compared with the irrigatedexperiment conducted in 2002. It is important to

AG9010

0 200 400 600 800 1000 1200 1400

Pla

nt h

eigh

t (cm

)

0

50

100

150

200

250(a) (b)

(c) (d)

Pla

nt h

eigh

t (cm

)

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250

Pla

nt h

eigh

t (cm

)

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RainfedIrrigated

RainfedIrrigated

DKB 333B

RainfedIrrigated

RainfedIrrigated

Exceler

0 200 400 600 800 1000 1200 1400

Pla

nt h

eigh

t (cm

)

0

50

100

150

200

250DAS CO32

Thermal time – (TT8 ) (°Cd)Thermal time – (TT8 ) (°Cd)

Thermal time – (TT8 ) (°Cd)

0 200 400 600 800 1000 1200 1400

Thermal time – (TT8 ) (°Cd)

0 200 400 600 800 1000 1200 1400

Fig. 3: Average plant heig-ht as a function of thermaltime for the four hybrids ofthe irrigated and rainfedexperiments conducted in2002: AG9010 (a), DKB333B (b), Exceler (c) andDAS CO32 (d)

AG9010

0

1

2

3

4

5DKB 333B

0

1

2

3

4

5

Exceler

LAI (

m2 m

–2)

LAI (

m2 m

–2)

LAI (

m2 m

–2)

LAI (

m2 m

–2)

0

1

2

3

4

5

Irrigated 2001Irrigated 2002

Rainfed 2002


Rainfed 2002


Rainfed 2002


Rainfed 2002

DAS CO32

Thermal time – (TT8) (°Cd)

0 200 400 600 800 1000 1200 1400


0 200 400 600 800 1000 1200 1400


0 200 400 600 800 1000 1200 1400


0 200 400 600 800 1000 1200 14000

1

2

3

4

5

(a) (b)

(c) (d)Fig. 4: Leaf area index(LAI) as a function of ther-mal time for the fourhybrids, AG9010 (a), DKB333B (b), Exceler (c) andDAS CO32 (d). The silkingstage occurred around900 �Cd for the very shortseason hybrid (AG9010)and 1040 �Cd for the nor-mal season maturity hybrid(DKB 333B)


note that different amounts of fertilizer wereapplied to the irrigated and rainfed experiments,which could be a potential cause for the variationin yield. However, it was not considered to beapplicable because the main limiting factor in the

rainfed experiment was the availability of soil water(Fig. 2a,b). When comparing the 2002 irrigatedand rainfed experiments, the normal maturityhybrid DKB 333B was the most affected by thereduced availability of soil water, as the yield

AG9010 DKB 333B

Exceler

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4000

6000

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Irrigated 2001Irrigated 2002Rainfed 2002




DAS CO32

Thermal time – (TT8) (°Cd)0 500 1000 1500 2000 2500




(a) (b)

(c) (d)

Fig. 5: Aboveground bio-mass as a function of ther-mal time for the fourhybrids, AG9010 (a), DKB333B (b), Exceler (c) andDAS CO32 (d)

Table 1: Average and standard error of the mean (S.E.M) for yield, yield components and harvest index

Yield (kg ha)1; average) Harvest index (average)

Irrigated Rainfed

Average

Irrigated Rainfed

AverageHybrids 2001 2002 2002 2001 2002 2002

AG9010 5028 4986 4044 4686 0.41 0.42 0.44 0.42DKB 333B 5801 5139 3822 4921 0.42 0.37 0.37 0.39DAS CO32 5684 5047 4109 4947 0.46 0.37 0.34 0.39Exceler 5863 5306 4859 5343 0.48 0.40 0.41 0.43Average 5594 5119 4209 0.44 0.39 0.39S.E.M. 182 111 206 130 0.01 0.01 0.01 0.01

Kernels per ear (kernel number per ear) Kernel weight (mg kernel)1)AG9010 414 427 401 414 241 231 232 234DKB 333B 413 417 394 407 252 251 225 242DAS CO32 449 432 416 431 248 228 228 234Exceler 453 469 417 446 255 238 238 243Average 432 436 407 249 237 231S.E.M. 8 7 7 5 4 3 4 2

Ears per plant (n) Kernels m)2 (kernel number m)2)AG9010 1.06 1.08 0.90 1.0 2191 2318 1801 2103DKB 333B 1.14 0.98 1.02 1.0 2366 2078 2028 2157DAS CO32 1.06 1.12 0.96 1.0 2362 2308 2011 2227Exceler 1.16 0.98 1.04 1.0 2622 2286 2152 2353Average 1.10 1.03 0.98 2386 2247 1998S.E.M. 0.03 0.02 0.02 0.02 84 57 59 44

254 Soler et al.

reduction was 25.6 %. The hybrid Exceler was theleast impacted by the reduced availability of soilwater with a yield reduction of only 8.4 %. Becausethe hybrids that were evaluated had differentgrowing season durations and time to maturity, itis possible that water stress affected the differentphenological periods of each hybrid.The HI defines the relationship between grain

yield and total aboveground biomass and is anindicator of the proportion of the dry matter that isallocated to the reproductive plant components.The hybrids AG9010 and Exceler had the highestaverage HI (0.42 and 0.43), indicating that a largerproportion of dry matter was allocated to grainsfor these two hybrids when compared with theother two hybrids (Table 1). The short seasonhybrids Exceler and DAS CO32 had the highestHI for the irrigated experiment conducted in 2001,i.e. 0.48 and 0.46, respectively. The highest averageHI was obtained in the 2001 irrigated experiment,i.e. 0.44, probably due to favourable weather andsoil conditions. The average HI was not differentbetween the irrigated and rainfed experimentsconducted in 2002 (Table 1).The analysis of the average number of kernels

per ear for each hybrid for the three experimentsshowed that the short season hybrids Exceler andDAS CO32 had, on average, a larger number ofkernels per ear, i.e. 446 and 431, respectively(Table 1). No significant interaction was foundbetween hybrid and experiment. The averagenumber of kernels per ear for the irrigated andrainfed experiments conducted in 2002 was 436 and407, respectively. For the 2002 irrigated experi-ment, the number of kernels per ear for the hybridExceler was 469, a remarkably high value incomparison with the other hybrids, which hadbetween 417 and 432 kernels per ear.The average kernel weight among hybrids for the

three experiments was similar, varying between 234and 242 mg per kernel (Table 1). The averagekernel weight was higher for the irrigated experi-ments than for the rainfed experiment. The inter-action between hybrid and experiment was notsignificant. The analysis showed remarkable highvalues for the hybrids DKB 333B (251 mg) andExceler (238 mg) for the 2002 irrigated experiment.The kernel weight varied between 228 and 238 mgfor the 2002 rainfed experiment and between 228and 251 mg for the 2002 irrigated experiment.The average ears per plant among the hybrids

for the three experiments showed no variation(1.0; Table 1). The highest average number of ears

was found for the 2001 irrigated experiment, i.e. 1.1ears per plant for all four hybrids. No significantinteraction between hybrid and experiment wasfound. There were no important differences bet-ween the average number of ears per plant for theirrigated and rainfed experiments conducted in2002.The short season hybrid Exceler had a higher

average number of kernels per unit land area, i.e.2353 kernels m)2. The average number of kernelswas 2157 and 2227 m)2 for DKB 333B and DASCO32, respectively. No significant interactionbetween hybrid and experiment was found. Thehighest number of kernels per unit land area wasfound in the 2001 irrigated experiment, i.e. 2386kernels m)2. On average, the number of ker-nels m)2 for the irrigated experiment conducted in2002 was higher (2247 kernels m)2) than that forthe rainfed experiment (1998 kernels m)2). For the2002 rainfed experiment, the hybrid Exceler hadthe highest number of kernels m)2, i.e. 2152, whilethe hybrid AG9010 had the lowest number ofkernels m)2, i.e. 1801.

Relationship between yield and yield components

A multiple regression analysis for the four hybridsfor all three experiments showed that the kernelnumber per unit land area was the yield componentthat was most correlated with yield, followed byindividual kernel weight. The inclusion of these twoyield components in the linear regression analysisdetermined a higher F and lower P-values thanusing the three yield components, e.g. kernels perear, ears per plant and individual kernel weight(Table 2). However, the regression analysis bet-ween the three main yield components (kernels perear, ears per plant and individual kernel weight)and yield revealed that individual kernel weightwas the yield component with the highest correla-tion coefficient (0.69).When analysing each individual hybrid, it was

found that for the very short season hybrid,AG9010, and the short season hybrid, Exceler, thekernel number m)2 was mainly related to yield, witha correlation coefficient of 0.84 and 0.76, respect-ively (Table 3). For the hybrids DKB 333B andDAS CO32, the individual kernel weight was themain component that determined yield, with acorrelation coefficient of 0.86 and 0.80, respectively.The number of ears per plant and the number ofkernels per ear had a low variation and were only ofsecondary importance for determining the grain


yield of the four hybrids. The analysis of the threeindividual experiments showed that for rainfedconditions kernel weight was the main componentthat was correlated with yield (r ¼ 0.80). This seemsto indicate that water stress during the grain fillingwas the most important yield-limiting factor.

Relationship between yield and evapotranspiration

Total crop evapotranspiration during the growingseasons of 2001 and 2002 was very similar, with anaverage total of 330 and 329 mm, respectively(Table 4). However, for the rainfed conditionslower values for actual evapotranspiration (ETa)were obtained, which ranged from 285 mm for thevery short season hybrid AG9010 to 298 mm for

the normal maturity hybrid DKB 333B. In thisstudy, the coefficient of determination betweenyield and actual evapotranspiration (ETa) was 0.67(Fig. 6). The slope of the regression line showedthat for each mm of actual evapotranspiration theexpected yield increase was 26.8 kg ha)1.The hybrid Exceler had the highest WUE

(kg m)3) with an average of 1.69 kg m)3 for thethree experiments, followed by the hybrids DASCO32 – 1.56 kg m)3, and AG9010 – 1.52 kg m)3,and the lowest value was found for the hybridDKB 333B – 1.49 kg m)3. The WUE was thehighest for the irrigated experiment conducted in2001, which means that in 2001 more grain masswas produced per mm of water when comparedwith 2002 (Table 4). As evapotranspiration was

Table 2: Linear regression analysis for yield and yield components

Coefficients Standard error t-Stat P-value

Linear regression: 2 yield componentsIntercept )6705.4 1594.1 )4.2 0.0023Kernels number m)2 1.8 0.4 4.7 0.0011Kernel weight 31.8 8.0 4.0 0.0033R2 0.91Standard error 228.9F 43.9Significance F 2.27E-05

Linear regression: 3 yield componentsIntercept )11 359.5 1687.5 )6.7 0.0001Kernel weight 32.0 7.6 4.2 0.0031Kernel number per ear 12.4 3.2 3.8 0.0049Ears per plant 3306.3 1052.4 3.1 0.0138R2 0.92Standard error 217.5F 33.1Significance F 7.36E-05

Table 3: Correlation coefficients between yield and yield components for the four maize hybrids grown underrainfed and irrigated conditions

HybridKernels

(number m)2)

Kernelweight

(mg kernel)1)

Kernelsper ear

(number ear)1)Ears

per plant (n)

AG9010 0.84 0.30 0.50 0.75DKB 333B 0.67 0.86 0.75 0.46DAS C032 0.22 0.80 0.25 0.14Exceler 0.76 0.51 0.41 0.59Experiment

Irrigated 2001 0.53 0.52 0.35 0.51Irrigated 2002 0.25 0.24 0.19 0.04Rainfed 2002 0.37 0.80 0.40 0.18

Overall 0.55 0.69 0.46 0.45

256 Soler et al.

very similar between the two years, the differencesin WUE between the irrigated experiments weredue to the higher yields obtained in 2001. In 2002,a lower WUE was found for the rainfed,i.e. 1.45 kg m)3, than for the irrigated conditions,i.e. 1.56 kg m)3.The average yield per unit of water used by the

crop above that of rain (WUEi)r) in 2002 was2.36 kg m)3. The short season hybrid Exceler hadthe lowest value, i.e. 1.18 kg m)3, suggesting thatthe response to irrigation was not as evident as forthe other hybrids. For the other three hybridsAG9010, DKB 333B and DAS CO32, the WUEi)r

was >2.4 kg m)3. In contrast to Exceler, thesethree hybrids were more sensitive to water deficitsand, therefore, irrigation should be considered toavoid important yield losses.

Discussion

Differences in average temperature were foundbetween the two growing seasons in this study thatimpacted maize development. The higher temper-atures of the 2002 growing season caused a morerapid development, resulting in a reduction in thenumber of days from sowing to flowering whencompared with 2001 (Soler et al. 2005). Differencesin temperature associated with differences in otherweather variables can explain, in part, the differ-ences in yield between the two irrigated experi-ments that were conducted in different years.The impact of reduced soil water availability was

least pronounced on the hybrid Exceler, which hadonly an 8.4 % yield reduction. NeSmith andRicthie (1992a) reported yield losses that rangedfrom 15 % to 25 % as a result of drought stressduring the pre-anthesis stage. For severe waterdeficits during the grain-filling period, NeSmithand Ritchie (1992b) found that yield reductionsranged from 21 % to 40 %, with kernel weightbeing the most affected component.The hybrids AG9010 and Exceler had the highest

HI average values (0.42 and 0.43; Table 1). Theclimatic conditions during the fall and winterseasons in this subtropical region could potentiallylimit maize yield, causing a low HI. For maize, theaverage HI was reported as 0.52 for one temperateregion (Kiniry et al. 2004), but in another study itranged from 0.46 to 0.58 (Kiniry et al. 1997).However, for tropical maize grown in Mexico andMalawi, Hay and Gilbert (2001) found that themagnitude of the HI was not highly heritable andvaried inconsistently with season, management andenvironment. This was mainly due to the vulner-ability of the grain-setting processes after floweringto soil water deficits (Hay and Gilbert 2001).There were differences in the number of kernels

per ear between hybrids for the average of the three

Table 4: Actual evapotranspiration for irrigated and rainfed conditions (ETa); water use efficiency (WUE) andwater use efficiency considering irrigation only (WUEi)r) for the four maize hybrids grown under rainfed andirrigation conditions

Hybrid

ETa (mm) WUE (kg m)3)

WUEi)r(kg m)3)

Irrigated –2001

Irrigated –2002

Rainfed –2002

Irrigated –2001

Irrigated –2002

Rainfed –2002

AG9010 320 317 285 1.57 1.57 1.42 2.93DKB 333B 341 344 298 1.70 1.49 1.28 2.84DAS CO32 329 328 290 1.73 1.54 1.42 2.47Exceler 329 328 290 1.78 1.62 1.67 1.18Average 330 329 291 1.69 1.56 1.45 2.36

y = –3537.7+26.8x r 2= 0.67r = 0.82SE = 408 kg ha–1

ETa (mm)

260 280 300 320 340 360

Yie

ld (

kg h

a–1)

3000

4000

5000

6000

7000

Irrigated – 2001Irrigated – 2002Rainfed – 2002

Fig. 6: Correlation between yield and actual evapo-transpiration (ETa)


experiments. For rainfed conditions, the hybridExceler showed more number of kernels per ear(469) than the other hybrids (Table 1). Previousstudies conducted by Bergamaschi et al. (2004)found that for maize exposed to drought during theflowering and grain-filling phases there were, onaverage, 300 kernels per ear and for maize grownwith sufficient soil moisture during these periodsthere were 600 kernels per ear during the normalseason of southern Brazil. For maize grown off-season in south-eastern Brazil, it was found thathybrids planted at high populations showed lowervalues of kernels per ear than at low plantpopulations (Duraes et al. 1995). Vilhegas et al.(2001) evaluated 12 hybrids and found that thegrain production was strongly influenced by theprecipitation for the different sowing dates thatwere studied.The kernel weight of the different hybrids, on

average, was similar for the three experiments.However, higher average values for kernel weightwere found for the irrigated than the rainfedexperiments (Table 1). Borras and Otegui (2001)reported that differences in kernel weight resultedfrom changes in kernel growth rate, indicating thatcompetition for assimilates among kernels tookplace during the whole grain-filling period. Theseresults agree with an earlier study conducted byNeSmith and Ricthie (1992a), who found that thedecrease in yield for plants grown under waterdeficits was due to a reduction in the number ofwell developed kernels and that this potentiallycould reduce the average kernel weight. Theyreported a kernel weight of 350 mg for plants witha low number of kernels per plant and 150 mg forplants with a high number of kernels per plant.On average for these three experiments, the

hybrid Exceler had the highest number of kernelsper area (Table 1). Andrade et al. (1996) reportedthat maize yield was highly correlated with thenumber of kernels per unit land area, a combina-tion of the number of kernels per ear and thenumber of ears per m2. Eck (1986) found that waterdeficits during vegetative growth reduced kernelnumber, but had little impact on weight per kernel.Kernel numbers were not affected by water deficitsduring grain filling unless severe deficits wereimposed early at the start of grain filling. Carcovaet al. (2000) found that for the Pampa conditionsof Argentine the kernel number was highly depend-ent on ETa around silking. When low values ofETa, e.g. 2 mm day)1, were measured aroundsilking the number of kernels was around

1800 m)2 and when high values of ETa (e.g. 5.5mm day)1) were measured, a high value of kernelswas obtained, e.g. 5000 m)2.Low values of ETa were found for the hybrids

grown in the rainfed experiment, which rangedfrom 285 mm for the very short season hybrid to298 mm for the normal maturity hybrid DKB 333B(Table 4). Other studies have shown that ETa ishighly variable. For instance, for maize grown off-season in India, ETa was 223 mm for rainfed and409 mm for irrigated conditions (Mishra et al.2001). However, higher values for ETa are com-monly reported in the literature for maize grownduring the summer in subtropical and temperateregions, e.g. 354 (Tyagi et al. 2003) and 673 mm(Howell et al. 1998). This shows that there is a cleardependency of ETa on the local weather and soilwater conditions.The hybrid Exceler had the highest value of

WUE in the three experiments. The WUE waslower for the hybrids grown under rainfed condi-tion than the irrigated conditions of 2002. Theseresults agreed with previous studies conducted byPanda et al. (2004), who reported a decreasingvalue for WUE of maize when irrigations weredelayed in comparison with more frequent irriga-tions. Sadras and Calvino (2001) also found thatthe WUE decreased with increasing water deficit inmaize. According to Zhang et al. (1998), theincrease in WUE under supplemental irrigation incomparison with rainfed conditions is associatedwith the increased leaf area and its effect on theratio of soil evaporation to crop transpiration.Cooper et al. (1987) reported that an increase inWUE can result from changes in the transpirationand/or evaporation/transpiration relationship andHI under a given climatic condition.It is important to note that WUE is a result of the

relationship between yield and actual evapotran-spiration, and as such, some errors could haveoccurred that are mainly related to the assumptionsthat were made to estimate the actual evapotran-spiration. However, this information is valuable asa first approach and it could be a base for furtherstudies of maize grown off-season in other sub-tropical regions. For maize grown off-season, it isimportant to improve and stabilize production andfor this, the application of supplemental irrigationshould be considered. However, other managementstrategies should be evaluated, such as the use ofcultivars that are adapted to the environmentalconditions of this growing season, e.g. an earlymaturity hybrid, early sowing date, optimum plant

258 Soler et al.

population and irrigation strategies aiming at highWUE.

Conclusions

The results from this study showed that maizegrown off-season under rainfed conditions in asubtropical environment has, in some cases, a lowerLAI, plant height and biomass accumulation thanunder irrigated conditions. Due to a limited avail-ability of soil water, the rainfed crop showed adecrease in the soil water content of the deeper soillayers, suggesting that an increase in rooting depthand supplemental root water uptake from thedeeper soil layers were the plant’s response towater deficit.Maize grown under rainfed condition was less

efficient in terms of water use than under irrigatedconditions. The four hybrids in this study respon-ded differently to soil water limitations, showingthat genotypic differences exist in response todrought stress. The normal maturity hybrid DKB333B was affected the most by soil water deficit,while the short maturity hybrid Exceler was affec-ted the least. The hybrid Exceler also had a highyield, number of kernels per unit land area, HI andWUE.The application of supplemental irrigation

should be considered for maize grown off-season,as it can mitigate the yield reduction due to boththe temporal and spatial variability in precipita-tion, provided that this practice is not prevented byeconomic limitations. However, other managementstrategies should also be considered to improve andstabilize the production of maize grown off-season,especially some hybrids that seem to be adapted tothis type of environmental conditions.The information obtained from this study is not

only useful for farmers, but also for modellers, whofrequently need to evaluate their models in con-trasting environments in order to be able to applythe models for practical purposes. Further researchshould include the evaluation of additional culti-vars as well as the economic feasibility of irrigationfor maize grown off-season.

Acknowledgements

This work was supported in part by a fellowship for thefirst author from the �Fundacao de Amparo a Pesquisa doEstado de Sao Paulo� (FAPESP), Sao Paulo State, Brazil(Process 00/09050-0) and by State and Federal fundsallocated to Georgia Agricultural Experiment Stations

Hatch projects GEO01654. The authors wish to thank thestaff from the �Departamento de Producao Vegetal�,ESALQ/USP, for the assistance with the field experiments.

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