Wheat Management in Warm Environments: Effect of ...

Reprinted from Agronomy Journal Vol. 91, No. 6

Wheat Management in Warm Environments: Effect of Organic and Inorganic Fertilizers, Irrigation Frequency, and Mulching Mohamad Badaruddin. Matthew P. Reynolds,* and Osman A. A. Ageeb

ABSTRACT Wheat (Triticum aestivum L.) yields are reduced by heat stress in many countries worldwide. The main objective of this study was to determine whether modifications to currently recommended crop management practices could improve wheat yield at hot-environment sites in three countries. Field trials were conducted between 1991 and 1993 in Sudan and Bangladesh by their respective national agn'cultural research systems (NARS) and in Mexico by the International Maize and Wheat Improvement Center (CIMMYT). Crop establishment and other plant characteristics were measured to help understand the basis of improved performance and to identify potential diagnostic traits. Management factors included (i) irrigation, (ii) inorganic fertilizer (NPK), (iii) organic fertilizer in the form of farmyard manure (NM),(iv) straw mulch, (v) genotype, and (vi) sowing date. Control treatments represented recommended practices and gave yields of 3.6 t ha-', averaged across all environments. Considering main effects, N M (10 t ha-') gave the highest yield response (14%) and approximately equivalent levels of NPK gave the lowest (5.5%), suggesting that organic fertilizer provided growth factors in addition to nutrient content. Mulch and extra irrigation increased yield in Sudan and Mexico, which are hot environments with low relative humidity, but not in hot, humid Bangladesh. In Mexico, extra inputs were more beneficial under hotter, spring-sown conditions than for winter sowings, with responses of 17 and 8% to extra NPK, 17 and 14% to N M , and 11 and 6% to mulch, respectively. Comparison of heat-tolerant (Glennson 81) and heat-sensitive (Pavon 76) genotypes showed that the heat-tolerant genotype was generally more responsive to additional inputs. Improved performance in response to inputs was generally associated with better stand establishment, and with significant increases in plant height, grain per unit area, and aboveground biomass; in Mexico, there was also an association with higher canopy -temperature depression and light interception. The possibilities of realizing the benefits of applied organic matter and mulch treatments through residue retention and reduced tillage are briefly discussed.

w

HEAT is traditionally grown as a cool-season crop, but with the increased availability of more widely adapted semidwarf germplasm, wheat production has expanded into warmer regions of countries where production had been restricted to higher altitudes or cooler latitudes. Wheat, which is one of the most broadly adapted cereal crops, is cultivated on about 7 million ha in the subtropics under continual heat stress, defined as having a mean daily temperature greater than 17S°C in the coolest month of the cycle (Fischer and Byerlee, 1991). While genetic sensitivity to high temperature stress is acknowledged as an important constraint to yield in the subtropics and tropics, relatively little work

M. Badaruddin, Bangladesh Agric. Res. Inst., P.D. Nashipur, P.O. Box 6057, Gulshan, Bangladesh; M.P. Reynolds, Int. Maize and Wheat Improvement Ctr. (CIMMYT), Mexico (mailing address: Apartado 370, P.O. Box 60326, Houston, TX 77205); O.A.A. Ageeb, Sudan Agric. Res. Corp., P.O. Box 126, Wad Medani, Sudan. Received 24 Nov. 1997. *Corresponding author ([email protected]). Published in Agron. J. 91:975-983 (1999).

has been conducted on ameliorating the effects of heat stress through management practices. Optimal crop growth requires a nonlimiting supply of resources (water, nutrients, and radiation) and, as temperatures rise, the demand for growth resources increases due to higher rates of metabolism, development, and evapotranspiration (Rawson, 1988). If growth resources are limited under heat stress, then the size of plant organs such as leaves, tillers, and spikes is reduced (Fischer, 1984). The apparent sensitivity of metabolic processes to heat stress in the field environment (Reynolds et al., 1998), coupled with the reduced length of life cycle at high temperature (Midmore et al., 1984), explains why grain yield is strongly associated with total plant biomass in hot environments (Reynolds et al., 1994a). These interactions make crop management factors critical to sustaining wheat yields in warm environments. A few studies have shown benefits of specific management factors under stress. For example, the application of farmyard manure (FYM) has been reported to improve soil physical and chemical conditions and to help conserve soil moisture (Sattar and Gaur, 1989; Gill and Meelu, 1982; Tran-Thuc-Son et al., 1995). One-time application of FYM (10-15 t ha-') increased wheat yields for up to three successive crop cycles, when applied in conjunction with inorganic N fertilizers, and for up to four years with the addition of P fertilizers under hot and h m i d conditions in Bangladesh (Mian et al., 1985). Under high-temperature conditions, volatilization of N fertilizers such as NH3 is more likely, and further decreases wheat yield compared with the application of equivalent N in organic forms such as FYM (Tran-ThucSon et al., 1995). Straw mulch is another agronomic input with the potential to ameliorate stress by reducing evaporation of moisture from the soil and increasing infiltration rate (Lal, 1975). It has also been reported to lower soil temperature (Benoit and Kirkhoun, 1963), while a negative consequence can be to impede seedling emergence (Chopra and Chaudhary, 1980). Surface soil temperatures can exceed air temperature by 10 to 15°C if the soil surface is bare and radiation intensity is high, and straw mulch in such environments may increase seedling emergence and survival (Fischer, 1984). Since wheat growth under warm conditions is highly sensitive to management, judicious combinations of management factors could have substantial benefits on performance by improving crop establishment as well as the availability of water and nutrients during subsequent growth stages. Abbreviations: FYM, farmyard manure; GDD, growing degree days; GS, growth stage; NARS, national agricultural research system; PAR, photosynthetically active radiation; SED, standard error of difference between means.

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T h e present study was conducted t o provide information from warm environments o n the response of wheat t o such management factors (i.e. mulching and application of FYM) and, in addition, t o elevated levels of inorganic fertilizer a n d increased irrigation frequency. T h e research was conducted collaboratively by t h e NARS wheat programs of Sudan and Bangladesh and by t h e International Maize a n d W h e a t Improvement Center (CIMMYT) based in Mexico. Objectives were as follows: (i) determine whether modifications t o recommended crop management practices can significantly improve grain yield; (ii) measure crop establishment and other morphological and physiological traits with a view t o understanding t h e basis of improved performance, as well as identifying potentially useful diagnostic plant traits; and (iii) identify management practices that might b e implemented t o maximize wheat yields in hot growing environments. T h e six management factors studied were irrigation, inorganic fertilizer (NPK), organic fertilizer in the form of farmyard manure (FYM), straw mulch, genotype, a n d sowing date.

MATERIALS AND METHODS Growing Environments The experiments were conducted at Dinajpur, Bangladesh, during the winter seasons of 19961991 to 1992-1993, and at Wad Medani, Sudan, in the winters of 1991-1992 to 1992-1993. In Mexico, the experiments were conducted in both winter and spring seasons (19961991 through 1992-1993) at the CIMMYT experiment station in Tlaltizapin, Morelos, central Mexico. The Dinajpur, Bangladesh, site is 40 m above sea level (26" N, 92" E). Wheat was sown on 10, 12, and 12 December in 1990,1991, and 1992, respectively, and had 100 d mean growing cycle from emergence to physiological maturity. The Wad Medani, Sudan, site is 411 m above sea level (14" N, 33" E). Wheat was sown on 11 and 9 November in 1991 and 1992, respectively, and had a mean growing cycle of 90 d. The Tlaltizapin, Mexico, site is 940 m above sea level (18" N, 90" W). For the winter cycle, wheat crops were sown on 5 Dec. 1990, 5 Dec. 1991, and 19 Nov. 1992. For the later spring sowings, crops were sown on 27 Feb. 1991, 4 Mar. 1992, and 20 Feb. 1993. The winter- and spring-sown crops had mean cycle lengths of 100 and 80 d, respectively. Soil at the Dinajpur, Bangladesh, site was a noncalcareous piedmont alluvium (Tista flood plain) up to 3 m deep. Soil analyses for the 0- to 30-cm depth showed 23 to 28% clay, 48 to 53% silt, 24% sand, 5.6 to 6.0 pH, 4.4 to 6.2 g kg-' organic C, 0.41-0.63 g kg-' total Kjeldahl N, 1.6 to 7.45 mg kg-' available Olsen P, and 0.12 to 0.15 cmol, kg-' exchangeable K. Soil at the Sudan site was a fine, montmorillonitic, isohyperthermic soil with a pH from 8.0 to 8.3 and a rooting depth up to about 40 cm. This soil had total Kjeldahl N of 0.943 g kg-', available Olsen P of 3.5 mg kg-', and exchangeable K of 0.285 cmol, kg-'. At the Tlaltizapin site in Mexico the soil was a calcareous Vertisol (isothermic Udic Pellustert) with a rooting depth of about 30 to 40 cm. Soil properties in the 0- to 30cm depth were pH of 7.5 to 7.6, organic matter of 1.75 to 2.55%. total Kjeldahl N of 0.107 to 0.138 g kg-', available Olsen P of 2.4 to 6.9 mg kg-', and exchangeable K of 0.11 to 0.13 cmol, kg-'. Daily growing degree days (GDD) were calculated using the following formula and conditions: GDD = [T,,, + T,,,]l

2 - 1O0C,where T,,, is the maximum daily temperature and Td, is the minimum daily temperature. For T,, > 40°C, T,, was set to 40°C; for Tmi,< 1O0C,the value was set to 10°C.

Experiment Treatments Management treatments were set up to compare recommended agronomic practices with the use of additional inputs. Fertility treatments comprised (i) control (normal NPK rates recommended for each specific site), (ii) control + 10 t ha-' farmyard manure (FYM), (iii) control + 50% extra N, NP, or NPK. The control fertility treatment had 1 2 M W 2 0 kg ha-' N-P-K-S in Bangladesh, 8 H 3 - 0 kg ha-' N-P-K in Sudan, and 200-50-0 kg ha-' N-P-K in Mexico. The extra NPK treatment consisted of 50% extra N and 50% extra N and P in Bangladesh and Sudan, respectively. In Mexico it consisted of 50% extra N and P plus K and trace elements at the following rates: 35 kg ha-' K plus 1 kg ha-' of a trace element formula (Fertiquel BASE 11.5% S, 7.5% MgO, 5.0% Fe, 3.0% Zn, 2.0% Mn, 0.5% Cu, 0.2% Bo, and 0.002% Mo). Urea, triple superphosphate, and muriate of potash were applied as the source of N, P, and K, respectively. All chemical fertilizers and FYM were applied during land preparation. In the 1992-1993 cycle in Mexico, elemental S was applied at 1 t ha-', as a soil fungicide and to potentially offset the high soil pH. The NPK contents in FYM samples were 5 to 15 g kg-' N, 4 to 8 g kg-' P, and 5 to 19 g kg-' K in Bangladesh; 104.5-9.3 g kg-' N-P-K in Sudan; and 12.85-2&16 N-P-K in Mexico. With 100% decomposition, FYM could add 100,60, and 120 kg of N, P, and K, respectively, in Bangladesh soil. Similarly, FYM can add 100,5, and 93 kg of N, P, and K in Sudan and 130,20, and 160 kg in Mexico soil. However, the amount of NPK release to soil depends entirely on the rate of decomposition of FYM for a particular environment. The mulch treatment was the same at all locations, and consisted of an application of chopped straw at 2.5 t ha-', spread evenly over the surface of plots immediately after sowing. Irrigation treatments were as follows. In Bangladesh: (i) control (flood irrigation every 25 d after sowing), (ii) control + 1 extra irrigation at boot stage, and (iii) control + 1 extra irrigation during the grain-filling stage. In the 1990-1991 cycle, only the first and third treatments were used. In Sudan: (i) control (flood irrigation every 14 d), (ii) flood irrigation every 10 d, and (iii) flood irrigation every 7 d. In Mexico (Feb. 1991): (i) control (approximately 600 mm by sprinkler, applied at 10-d intervals); (ii) control + extra 150 mm duringgrain filling. In Mexico for 1991-1992 and 1992-1993 for both sowing dates: (i) control ( ~ 6 0 0mm), (ii) Stress 1 (approximately 450 mm), 0 There was no irrigation treatment and (iii) Stress 2 ( ~ 2 5 mm). for the crop sown in December in 1990-1991 in Mexico.

Genotype Two cultivars, heat-tolerant Glennson and heat-sensitive Pavon, were used in Mexico to observe the interaction of management factors with genotype. (Heat tolerance had been established based on international testing at warm locations; Reynolds et al., 1994a.) The best-adapted cultivar from Bangladesh ('Kanchan'), and Sudan ('Debeira') were used for all trials at their respective locations.

Yield, Yield Components, and Morphological Traits Yield was estimated after physiological maturity by harvesting all but the outer rows of each plot and excluding at least 0.5 m from either end of the rows. The harvest area was 3.6

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BADARUDDIN ET AL.: WHEAT MANAGEMENT IN WARM ENVIRONMENTS

m2. Total dry weight and grain weight from 100 spike-bearing culms was measured for calculating harvest index, spike density, and grains per spike. Kernel weight was estimated on a sample of 200 grains for calculating grains per square meter. The Zadoks growth scale was used to take phenological data (Zadoks et al., 1974). The number of days to growth stage (GS) 65 (anthesis) and those to GS 92 (maturity) were measured when the crop had approximately 50% spikes flowered and 50% spikes turned yellow, respectively. Ground cover was estimated visually at both GS 14 and GS 65. Initial plant counts were taken between GS 10 and GS 12, when germination appeared to have ended.

Canopy Temperature Depression, Light Interception, and Chlorophyll Content Canopy temperature depression was mcasured only in Mexico during the spring-sown growing cycle in 1993. Canopy temperature was measured using a hand-held infrared thermometer (Model AG-42, Telatemp Corp., Fullerton, CA) with a field of view of 2.5". The effect of soil temperature was avoided by holding the infrared thermometer at an appropriate angle and distance from the plot and where it could view 100% of the crop canopy. Two measurements of canopy temperature depression were taken from either end of the plot by holding the infrared thermometer 1 m from the edge of the plot and approximately 0.50 m above the plants, with an angle of 30" from the horizontal. Chlorophyll content and interception of photosynthetically active radiation (PAR) was measured at GS 65 (anthesis) during the spring-sown growing cycle at Tlaltizapln, 19901991. The chlorophyll measurements were taken with a selfcalibrating chlorophyll meter (Model SPAD 502, Minolta, Japan) on 15 flag leaves per plot. Percent interception of PAR was calculated using measurement of incident radiation above the canopy, and light penetration to the base of the canopy, close to solar noon.

Experiment Design and Statistical Analysis The treatments were laid out in the field in a split-plot factorial arrangement with three replications. The plot size was 10 m2 in Bangladesh and 8 m2 in Sudan and Mexico. Statistical analyses were made with the MSTAT statistical program (Nissen, 1984). The analyses of variance for all characters studied were made separately for each environment (three environments for Bangladesh, two for Sudan, and six for Mexico). Pooled analyses were carried out within location across years andlor seasons and across environments for FYM, inorganic nutrition (N, NP, NPK), and straw mulch treatments. In Mexico, planting date was considered a separate experiment location, and analyses of variance were calculated accordingly. Correlation was calculated for morphological and physiological traits separately. The treatment means were compared using standard error of difference (SED) between means for equally replicated treatment. The SED was computed as where r is the number of replications for the SED = (2~%)'~, treatments and s2 is the error mean square in the analysis of variance (Gomez and Gornez, 1984, p. 187-207).

RESULTS Weather Bangladesh has a hot, humid climate; Sudan and Mexico are hot and dry. Sudan had the highest average temperature for the growing cycle (26.7OC) and Bangla-

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desh had the lowest (18.5"C). Although temperatures during January are usually lower in Bangladesh than the other three environments, the rise in temperature from January to March is steeper, approximately 1°C per week (Fig. 1). In Sudan this trend is reversed, if anything, although temperatures d o not change drastically during the cycle. In Mexico, mean growing-season temperature was approximately 3°C greater in the spring-sown cycle than the winter-sown cycle. Among the three years in Mexico, 1991-1992 was the coolest and 1992-1993 was the hottest. Comparing the spring sowing cycles, however, the 1993 cycle was cooler than that of 1991. The year 1992-1993 was the coolest in Bangladesh. In Sudan, the temperature during 19921993 was higher than that of 1991-1992 growing cycle. The growing degree days (GDD) were calculated for a reference year in each environment. The GDD from sowing to anthesis and anthesis to maturity were 569.4 and 575.0 in Bangladesh (1991-1992), 773.3 and 528.4 in Sudan (1991-1992), 773.4 and 345.2 in winter cycle Mexico (1992-1993), and 812.1 and 301.6 in spring cycle (1993), respectively. In comparison with water applied from irrigation (>500 rnm),precipitation is very low in all environments during the wheat cycle. The growing-season total precipitation is about 53 mm in Bangladesh (30-year average). In Mexico, the long-term mean for total precipitation in the winter-sown cycle is approximately 15 mm, and approximately 70 mm for the spring-sown cycle, which falls mostly during grain ripening. Sudan has negligible precipitation in the winter growing season.

Grain Yield Effect of Environment

Grain yield across all environments and years averaged 3.7 t ha-'. Grain yields were similar in Bangladesh for all three years, averaging 2.9 t ha-'. In Sudan, yields averaged 3.65 t ha-', but were 34% lower in 1992-1993 than in 1991-1992; this was due to the later sowing date (later by 20 d), since temperatures during the season was similar for both years. In Mexico, higher temperatures in the spring-sown cycle reduced yields relative to the winter cycle by about 30% in both 1990-1991 and 19911992, and by 16% in 1992-1993, which was cooler than other spring cycles. Performance across environments was not significantly associated with accumulated G D D +Mexico winter +Bangladesh

10

+Sudan

/ Nov

Dec

Jan

spring

+Mexico

,

Feb

-

I

,

Mar

Ap

May

Growing cycle(month)

Fig. 1. Monthly mean temperatures during wheat growing cycles in Sudan, Bangladesh, and Mexico, 1991-1993.

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AGRONOMY JOURNAL, VOL. 91, NOVEMBER-DECEMBER 1999

during grain filling, preheading, or over the cycle as a whole. Main Effects of Management Factors Across all environments, the application of FYM resulted in the highest yield response, averaging 4.1 t ha-', compared with 3.5 t ha-' for controls. Application of extra inorganic nutrition (50% over recommended rates) gave average yields of 3.8 t ha-' (Table 1). The average yield with the straw mulch treatment was 3.9 t ha-'. By country, the respective responses to FYM and NPK were 24 and 16% in Bangladesh, 12 and 3% in Sudan, and 14 and 8% in the Mexico winter cycle and 17 and 17% in the Mexico spring cycle. The effect of using straw mulch was 8% in Bangladesh, 9% in Sudan, and 6 and 11% in Mexico for winter and spring sowings, respectively (Table 1). Additional irrigation over recommended frequencies, resulted in increased yields in Sudan and Mexico (Table 1). In Sudan, increasing irrigation frequency from 14 d to 10 d increased yields lo%, while frequencies of 7 d resulted in an increase of 29%. In Mexico, application of 600 mm of water gave control yields of 3.1 t ha-', while an extra 150 mm during grain filling gave a 13% yield increase (3.5 t hap1). Although an extra irrigation did not have any main effect in Bangladesh, extra irrigation with a deep tillage treatment increased yield by 11% (data not shown).

Management Factor Effects on Plant Traits Farmyard Manure Application of FY M resulted in a significant increase (average 21 %) in final aboveground biomass in all environments (Tables 2 and 3). It also resulted in a small but significant increase in harvest index (average 2.5%) in Bangladesh and Sudan (Table 2). FYM was weakly Table 1. Effect of year, management factor and irrigation within each environment on grain yield of wheat during 1990-1991 t o 19921993. Grain yield Bangladesh

Sudan

Mexico (winter)

Mexico (mrind

Mean

t ha-' Year 1990-1991 3.0 4.5 2.9 35 1991-1992 2.7 4.4 4.2 3.0 3.6 1992-1993 3.0 23 5.5 4.6 4.0 SED 0.04 0.06 0.19 0.19 Management factor 25 3.4 Control 4.8 35 3.5 Extra N or NPK 2.9 3.5 5.2 4.1 3.8 WM 3.1 .3.8 5.5 4.1 41 Mulch 2.7 3.7 5.1 3.9 3.9 SED for comparing management factor mean = 0.16. SED for comparing management factor X environment mean = 033. Irrigation level 2.90 3.1 3.1 Normal Extra It 2.94 3.4 Extra 2$ 2.% 4.0 3.5 SED NS 0.1 0.06 -

associated with improved stand establishment in all environments, manifested as improved plant dry weight at GS 14 in Sudan (Table 2) and Mexico (Table 3), and more plants per unit area in Bangladesh (Table 2). In Mexico, FYM was associated with significant increases in spike number, and number of grains per unit area (Table 3). Extra NPK Extra inorganic nutrients had a significant effect on biomass in those environments where yield was also boosted: i.e., in Mexico (Table 3) and Bangladesh, but not in Sudan (Table 3). In Mexico the effect of extra NPK was similar to but weaker than that of FYM for most traits measured, with the notable exception of early stand establishment, which was not affected by NPK (Table 3). Flag leaf chlorophyll content was consistently higher in this treatment. Straw Mulch Mulching significantly increased harvest index and was associated with a slight increase in 1000-grain weight in Sudan (Table 2). In Mexico it was weakly associated with improved crop establishment, increased spike number, grains per unit area, and aboveground biomass at maturity (Table 3). Extra Irrigation Extra irrigation had no significant effect on plant traits or yield in Bangladesh (Table 2). In Sudan, increased grain yield in response to extra irrigation was associated with higher harvest index and a tendency for higher 1000-grain weight relative to the control (Table 2). With extra irrigation in Mexico (spring 1991), yield increases of 13% were associated with similar increases in grains per unit area and biomass at maturity, as well as a 2-d delay in maturity (not shown).

Interaction of Management Factors Interaction of FYM with Sowing Date and Irrigation Farmyard manure had significant interactions with other factors, namely, sowing date and irrigation (Fig. 2 and 3). Averaged over two cycles, FYM increased grain yield by 13% in the winter sowing in Mexico, while for the warmer (heat-stressed) spring-sown cycle FYM increased yields by 23% (Fig. 2), indicating an enhanced benefit of FYM under progressively hotter conditions. FYM also showed a significant interaction with moisture stress. FYM increased yield by 13% without moisture stress, and by 24% with moisture stress in the wintersown cycle in Mexico (Fig. 2). In the hotter spring-sown cycle, however, moisture stress reduced yields considerably and the benefit of FYM was smaller (10%) than without moisture stress (Fig. 2). In Sudan, FYM interacted with mulch, increasing grain yield by 23% when applied with mulch (not shown), compared with a 12% increase without mulch (Table 1).

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t One additional irrigation at gain filling in Bangladesh; irrigation at ., 10-d intervals in Sudan. S Two additional irrigations at booting and grain filling in Bangladesh; irrigation at 7-d intervals in Sudan; extra 150 mm urigation during grain filling in Mexico.

Interaction of Genotypes with FYM and Mulch Heat-tolerant Glennson and heat-sensitive Pavon were included in 1991-1992 in Mexico. At the winter

BADARUDDlN ET AL.: WHEAT MANAGEMENT IN WARM ENVIRONMENTS

Table 2. Effect of management factors o n different traits of wheat in Bangladesh and Sudan, 1991-1992.t Plants m-I

Plant DW (GS 14)

Ground cover (GS 14)

Days to anthesis (GS 65)

no.

mg

Oh

d

Control FYM Extra N Deep plow Extra irrigation SED Corr. with yield (r)

68.4 67.9 68.1 68.1 68.1

Control FYM Extra N Mulch Extra irrigation SED Corr. with vield ( r )

59.4 60.1 59.8 59.8 59.8 0.12 -

0.20

0.26~s

Plant ht.

Kernels spike-I

1000gain wt.

Biomass at harvest

Harvest index

cm Bangladesh 76 78 82 79 79 1.7 057** Sudan 77 81 77 77 79

no.

g

t ha-'

70

2.4 0.36**

*,**

Significant at the 0.05 and 0.01 probability levels, respectively. t DW, dry weight; GS, growth stage on the Zadoks scale (Zadoks el al., 1974); SED, standard error of difference.

sowing date, the heat-tolerant line showed yield increases of 13 and 7%, respectively, in response to FYM and mulch, while no significant increases in yield were associated with these treatments for the heat-sensitive genotype Pavon (Fig. 4). For the spring sowing, Glennson showed 23 and 12% increases in yield in response to FYM and mulch, respectively, and Pavon showed 15 and 13%. Overall, these data suggest that response to management factors in warm environments may depend on choice of cultivar.

was also seen to be more beneficial under increased moisture stress but the interaction was not significant in the second year (data not shown). Similarly, in Sudan the straw mulch was more beneficial with greater intervals between irrigation (i.e., 17, 6, and 1% for 14-, lo-, and 7-d intervals, respectively; data not shown), but the effect was seen only in the second of the two years.

Interaction of Extra NPK with Sowing Date There was significant interaction between fertility factors and sowing date in Mexico, with the hotter environment consistently eliciting a greater response in all fertility treatments. Response to extra NPK compared with the control level of fertility was 8% at the normal sowing date and 17% under hotter late-sown conditions (Fig. 3). When considering responses over the zero N treatment as a baseline, the interaction was even more marked for all treatments (Fig. 3).

Interaction of Straw Mulch with Sowing Date and Irrigation The mulch treatment showed an interaction with sowing date at TlaltizapBn, in much the same way as FYM. Yield increases in response to mulch were consistently greater under the hotter spring-sown cycle than for winter sowings (Fig. 3 and 4). In one cycle, straw mulch

Table 3. Response of wheat yield and other traits to management treatments for two sowing dates (winter 1992 and spring 1993) in TlaltizapBn, Mexico.+

Management treatment

Hawest Spikes Yield Biomass index m-'

Grains mike-I no.

Grains m-2

Heieht

Days to Days to anthesis maturity (GS 65) (GS 90)

Chlorophyll (GS 65)

Plant DW Plants (GS 17) m-'

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Winter-sown Control (200-50-0) Control + FYM (10 t ha-') Control + Mulch (2.5 t ha-') Extra NPK (300-75-25) No applied N (0-504) Mean* Spring-sown Control (200-50-0) Control + FYM(10 t ha-') Control + Mulch (2.5 t haU1) Extra NPK (300-75-25) No applied N (0-50-0) Mean* SED for significant fador Date mean Treatment mean Lnteraction f GS, growth stage on the Zadoks scale (Zadoks el al., 1974); DW, dry weight; FYM,farmyard manure; SED, standard error of difference. $ Excluding zero N.

no.

210 272 278 210 200 242 215 195 200 217 179 198 5.0 7.6 10.7

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...

.

.

SED = .

Control (600rnrn)

............

Moisture stress

Control (600mm)

(250mm)

Moisture Slrcss

(25Ornm)

Winter cycle

Spring cycle

Fig. 2. Response of wheat yield to presence (+FYM) or absence (-FYM) of farmyard manure (10 t ha-') at two irrigation levels (600 and 250 mm) and two sowing cycles (winter and spring) at Tlaltizaphn, Mexico, averaged for 1991-1992 and 1992-1993.

DISCUSSION Effect of FYM and NPK Of the management treatments used in this study, FYM had clear and consistent effects on augmenting grain yields across heat-stressed environments (Table 1). Chemical analyses of FYM indicated that it generally added more N, P, and K than did the inorganic nutrition extra NPK treatments (see Materials and Methods, above). Although it is debatable whether all of the nutrients in the FYM became available to the croD duringu the cycle, it is possible that increased response to the FYM compared with the NPK treatment is explained by the higher nutrient levels present in the FYM for Bangladesh. However, in Mexico, the extra N (130 kg ha-l) and P (20 kg ha-') in the FYM was not sufficiently different from that in the NPK treatment (N:100 kg ha-' and P:25 kg hap1) to explain the difference in yield response for the winter-sown cycle (Table 1). Similarly, in Sudan the response to FYM was greater than would be expected from a simple linear response to N, in comparison with the response from the extra NPK treatment. Soil analyses for K suggest that responses to FYM were not associated with K limitation, and the use of micronutrients in the NPK treatments in Mexico SUEgested that these were not explaining the enhanced rzsponse of performance to organic fertilizer. Nonetheless, in Mexico, FYM increased N, P, Ca, Mg, and Zn I3Winter

Control

FYM

Extra NPK

Effect of Mulch Mulch enhanced yield in all the environments, and especially where temperatures were high early in the cycle: i.e., Sudan and Mexico-spring cycle (Table 1). This is expected, since the principal effect of mulch Winter tolerant Spring tolerant

$Spring

Mulch

content in flag leaves by 7, 21, 5, 22, and 9% over controls in the 1992-1993 winter cycle (data not shown). It is interesting that while NPK had similar (though weaker) effects as FYM on yield and most other traits, NPK had no effect on early stand establishment (Table 3). In addition, the physiological parameters canopy temperature depression and light interception showed higher response to organic than inorganic fertilizer (Table 4). Overall, the data support the notion that organic fertilizer can provide growth factors in addition to those purely related to nutrient content. Many researchers (Sattar and Gaur, 1989; Gill and Meelu, 1982; TranThuc-Son e t al., 1995) have reported that FYM increased wheat grain yield through improvement of soil water holding capacity, physical and chemical conditions, reduction of volatilization of nitrogenous fertilizers to NH3gas, and greater availability of plant nutrients. Although widespread use of animal manure is not practical for most agronomic crops, the extremely high responses to this factor pose the important question of whether other means of applying organic matter, such as from residue retention or green manure (Sayre and Moreno Ramos, 1997; Reynolds et al., 1994b), might provide similar benefits. Taking into consideration the positive responses to both organic fertilizer and mulch treatments in this study, it seems that more sustainable yield increases in hot areas may be possible by moving towards reduced tillage systems (assuming that soilborne pathogens are not exacerbated). The combination of residue retention. and reduced soil movement would potentially increase soil organic matter levels, thereby increasing nutrient and water holding capacity, as well as decreasing the rate of evaporation from the soil by increasing ground cover. Such integrated approaches to improving crop management are likely to become more important in the future, especially as water availability becomes more limited.

Zero N

Fig. 3. Response of wheat yield to five management treatments under optimally irrigated conditions for winter and spring sowing dates at Tlaltizaphn, Mexico, 1992-1993: Control (200-50-0 N-P-K), FYM (control + 10 t ha-' farmyard manure), mulch (control + 2.5 t ha-' straw mulch), extra NPK (300-75-3s N-P-K + trace elements), and zero N (C50-0 N-P-K).

CI Winter sensitive

Spring sensitive

Control FYM Mulch Fig. 4. Response of two wheat lines (heat-tolerant Glennson and heatsensitive Pavon) to three management treatments under optimally irrigated conditions for winter and spring sowing dates at Tlaltizap h , Mexico, 1991-1992: Conlrol (204k50-0 N-P-K), FYM (control + 10 t ha-' farmyard manure), and mulch (control + 2.5 t ha-' straw mulch).

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would be to reduce soil temperatures before full ground cover, at least during the day. Thermistors placed at 3 cm under the soil (approximately the depth of the apical meristem during vegetative growth) during early establishment in Mexico, showed temperatures of up 3°C lower under mulch during the day, though average daily temperatures were similar across treatments due to higher night temperatures under mulch (Reynolds, unpublished data). This would explain why mulching may reduce some of the effects of heat stress, without affecting the rate of development of the crop (Table 3). In Mexico, mulching was associated with improved crop establishment, as well as yield and final biomass (Table 3). In Sudan, mulching was associated with higher kernel weight and harvest index, the same traits as those associated with extra irrigation (Table 2), suggesting that mulch may have played a role in conserving soil moisture. This supports the idea that, as well as improving crop establishment, mulching may provide added benefits under heat stress (when water deficits are likely to occur). From a practical point of view, post-sowing mulching is not a feasible practice in agronomic crops; however, there may be economic benefits to mulching in warm climates if technology for direct drilling into previous crop residues is developed.

Effect of Irrigation While resources did not permit good quantification of irrigation water supplied in this study, response to extra irrigation was clear in Mexico and especially Sudan (Table I), the sites with the highest evaporative demand. Extra irrigation had no benefit in Bangladesh where high relative humidity reduces vapor pressure deficit. Benefits from increased irrigation frequency will be a viable option only where infrastructure permits relatively uninterrupted access to water in irrigated wheat zones. In the meantime, the clear benefit of extra moisture supports the need to develop water-conserving technologies such as reduced tillage or mulching through residue retention.

Interaction of Management Factors The most significant interactions were those between sowing date and additional inputs (in Mexico), which generally indicated a more favorable response to extra inputs under warmer conditions (Fig. 2 4 ) . This supports the hypothesis of Rawson (1988) that good crop

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management is even more critical under warmer conditions. The main exception was under moisture stress, where FYM was less beneficial for the hotter spring sowing than for the cooler winter one (Fig. 2). Nonetheless, this was somewhat consistent with data from Pakistan where FYM reduced wheat yields under conditions of severe moisture stress (Hatam et al., 1994). Another important interaction was that between genotype and management. In general, the heat-tolerant genotype Glennson 81 was more responsive t o management (Fig. 4) than the heat-sensitive Pavon 76, illustrating the importance of evaluating genotype X management interactions when establishing agronomic practices. Another interesting interaction was that observed between straw mulch and moisture availability. Although data was not very consistent among years, it indicated (in Sudan and Mexico) that mulch was more beneficial where soil moisture was limiting. This idea was supported by the fact that mulch treatment in Sudan improved both kernel weight and harvest index, traits that were similarly affected by the extra irrigation treatment (Table 2).

Developmental Basis of Yield Improvement The data presented indicate that management factors may permit superior performance through improving crop establishment initially, through increased tillering, and through higher spike density and consequently more grains per unit area (Tables 3 and 4). The data in general do not indicate a strong effect of management on kernel size, nor on duration of the overall crop cycle. However, there were exceptions. For example, treatments affecting moisture deficit (i.e., irrigation and mulching) in Sudan seemed to have a small effect on kernel size, and FYM delayed maturity by 3 or 4 d in Mexico. Management factors also increased height in most situations by up to 7 cm. Under heat stress, plant growth requires an increased supply of nutrients and water to satisfy its accelerated rate of development (Rawson, 1988). In these experiments, FYM had the most consistently beneficial effect on yield. Application of FYM was associated with increased number of seedlings per unit area, early plant vigor, number of spikes per unit area, and final biomass (Table 3), as well as greater light interception and higher canopy temperature depression between growth stages 30 (tillering) and 65 (anthesis) (Table 4). These same traits are also associated with improved heat tolerance when responses of

Table 4. Effect o f management factors o n light interception and canopy temperature depression at different growth stages under normal irrigation at s ~ r i n gsowing. Tlaltiza~an.Mexico. 1993. Light interception Management factor Farmyard manure Straw mulch Extra NPK Control SED Corr. with yieId (r)t

Max tiller (GS 30) 57 55 5550 0.1 0.73**

Boot stage

6 s 40) 74

66 69 55 0.1 0.82**

Canopy temperature depression

Heading (GS 50)

Anthesis (GS 65)

Max filler (GS 30)

Boot stage (GS 40)

Heading (GS 50)

Anthesis (GS 65)

81 71 72 60 0.1 0.84**

76 74 71 67 0.1 0.45**

10.4 9.4 9.4 7.1 0.04 OAO**

6.7 6.7 6.8 5.4 0.34 OAF*

9.3 9.1 9.1 7.7 0.26 0.33**

9.7 8.7 9.1 7.8 0.28 0.47**

t Correlation based on 7 treatments (26 dl): zero N, extra N, and Bagas 10 t ha-' in addition to the four listed above.

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genotypes are compared (Reynolds et al., 1994a). While both FYM and NPK treatments improved biomass and yield components in a similar way, FYM treatment was associated with improved stand establishment, which inorganic nutrition did not affect (Table 3).

Physiological Basis of Yield Improvement Some of the physiological parameters associated with response to management factors in this study, especially simply measured traits, might lend themselves as diagnostic tools in extension work. The transpiring surfaces of irrigated wheat plots may be several degrees below the ambient temperature (Amani et al., 1996). Canopy temperature depression was increased by 29, 21, and 23% relative to the control by N M , mulch, and extra NPK, respectively, averaged across GS 30 and GS 65 in Mexico. The same management factors affected interception of PAR between GS 30 and GS 65, with average values being 24, 15, and 15% higher for FYM, mulch, and extra NPK, respectively; these physiological parameters were significantly correlated with plot yields (Table 4). Chlorophyll content of flag leaves measured at GS 65 were slightly higher for the FYM and NPK treatments in Mexico in the 1992-1993 winter-sown cycle. These same traits have been shown to be associated with heat tolerance in contrasting genotypes cultivated at hot wheat growing locations world wide (Reynolds et al., 1994a, 1998). Fischer (1984) reported that temperatures ranging from 20 to 35OC had little effect on photosynthesis. Current environments were largely within the range of 2035°C (Fig. 1); nonetheless, yields were responsive to what was considered supraoptimal management. While maximum rate of leaf photosynthesis may not be inhibited by high temperature under controlled conditions, there are many factors in a field environment that may reduce daily assimilation. These include photoinhibition and reduced stomata1 conductance when radiation levels are extremely high. Management factors may ameliorate these problems through increasing rooting capacity and leaf nutritional status. Many researchers (Fischer, 1985; Sofield et al., 1977; Chowdhury and Wardlaw, 1978; Tashiro and Wardlaw, 1990) have found that heat stress reduces number of grains per unit area, 1000grain weight, and some early growth parameters. The present study showed that improved management can reverse the detrimental effect of heat on many of these growth parameters, especially on early crop establishment, grain number, and total aboveground biomass.

Economic Basis of Yield Improvement We did not attempt to analyze the economic basis of these management factors, only to establish their biological value. Nonetheless, data indicate that recommended levels of fertilizer, whether organic or otherwise, were not generally sufficient to meet the crop's requirement. Average yield responses to NPK and FYM at a given site were as much as 17 and 2496, respectively (Table I), suggesting that even economic yields might be improved through better crop nutrition in hot re-

gions. The economic basis of increasing irrigation frequency is more complex, for two reasons. Firstly, irrigation schemes, such as the one in the Gezira of central Sudan, lack the flexibility to permit farmers to irrigate at will. Water is usually available only at set times in a given area as water is passed systematically through the whole irrigation scheme. Secondly, water availability is declining in many regions of the world so the expectation of increasing economic returns through increased irrigation may be upset if water prices rise dramatically. As already noted, attaining the benefits of mulching, and perhaps increased soil organic matter, may be possible through a combination of residue retention and reduced tillage practices. Nonetheless, significant investment will be required on the part of the national agricultural research systems and their governments, or western-sponsored agricultural agencies, if such practices are to become a reality in developing countries.

CONCLUSION Objective 1: Can modificationsto recommended crop management practices significantly improve grain yield? Results indicate that wheat yields in warm environments can be raised significantly by modifying agronomic practices. Yields were increased, in relation to currently recommended practices, by application of animal manure and straw mulch, as well as from increased levels of inorganic nutrients and irrigation frequency. Objective 2: Measure crop establishment and other morphological and physiological traits with a view to understanding the basis of improved performance, as well as identifying potentially useful diagnostic plant traits. Improved management was most commonly associated with better stand establishment, higher grain number and increased crop biomass. Physiological traits such as light interception, canopy temperature depression, and flag leaf chlorophyll content were improved by additional management factors. These traits could be potentially useful in the development and evaluation of improved management strategies. Objective 3: Identify management practices that might be implemented to maximize wheat yields in hot growing environments. Overall, the application of animal manure had the largest and most consistent effect on yield. Some of the benefits associated with extra organic matter may be provided by practicing residue retention from previous crops and reduced tillage. Such integrated approaches to crop and soil management in abiotically stressed environments are becoming increasingly relevant in light of diminishing water supplies in many agroecosystems. ACKNOWLEDGMENTS

We are grateful to Dr. M.A. Razzaque, Director, Wheat Research Center of Bangladesh, for providing facilities to conduct this research in Bangladesh. We also sincerely acknowledge Dr. Craig A. Meisner, Dr. D.W. Meyer, and Dr. K.D. Sayre for their assistance and suggestions relating to the manuscript.

BADARUDDIN E T AL.: W H E A T MANAGEMENT IN WARM ENVIRONIMENTS

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