Soil water use, biomass accumulation and grain yield ...

Soil water use, biomass accumulation and grain yield of no-till winter wheat on the Canadian prairies D. R. Domitruk1, B. L. Duggan, and D. B. Fowler2

Can. J. Plant Sci. Downloaded from pubs.aic.ca by University of Saskatchewan on 06/04/15 For personal use only.

Crop Development Centre, University of Saskatchewan, Saskatoon, Saskatchewan, S7N 5A8, Canada. Received 20 January 2000, accepted 19 June 2000. Domitruk, D. R., Duggan, B. L. and Fowler, D. B. 2000. Soil water use, biomass accumulation and grain yield of no-till winter wheat on the Canadian prairies. Can. J. Plant Sci. 80: 729–738. Higher water use efficiency provides no-till-seeded winter wheat with an advantage over spring-sown crops in western Canada. However, like all crops, winter wheat (Triticum aestivum L) is subject to large yield losses due to drought. This study was undertaken to identify the effect of weather and crop soil water status on water use, aboveground biomass production and grain yield of no-till winter wheat grown on the Canadian prairies. Five winter wheat cultivars were grown over a 3-yr period at a total of 17 sites scattered across the different climatic zones of Saskatchewan. Both the establishment and expression of grain yield potential were limited by drought in these dryland environments. Early-season moisture was required to set up a high grain yield potential while low ET and high precipitation during grain filling were necessary to secure yield. Rapid growth under cool temperatures during April and early May consumed much of the available water in the top 50-cm of the soil profile and large ET deficits, as a consequence of a continuous decline in available water, characterized drought stress in most trials. While stored soil water at greenup was not sufficient to support a crop, there was growing season rainfall at all trial sites and improvements in water availability led to higher grain yields and an increased range in mean environmental grain yield. Rainfall had its greatest influence on grain yield during tillering, while atmospheric conditions and soil water content were more important from heading to anthesis. Because environmental differences in drought stress were related to the volume and distribution of growing season precipitation, some dryland environments were exposed to intermittent stress while stress was terminal in others. Therefore, to be successful, winter wheat cultivars and management systems for the Canadian prairies must be able to accommodate variable patterns of growing season water availability. Key words: Triticum aestivum L., evapotranspiration, precipitation, water use, biomass, grain yield Domitruk, D. R., Duggan, B. L. et Fowler, D. B. 2000. Utilisation de l’eau du sol, production de biomasse verte et rendement grainier chez le blé d’hiver en régime de semis direct dans les Prairies canadiennes. Can. J. Plant Sci. 80: 729–738. Sa meilleure efficacité d’utilisation de l’eau confère au blé d’hiver installé en semis direct un avantage sur les cultures de printemps dans l’ouest du Canada. Toutefois comme toutes les cultures, le blé d’hiver peut subir de graves manques à produire par suite de la sécheresse. Nos travaux avaient pour objet de cerner les effets du temps et de l’état hydrique du sol et des cultures sur l’utilisation de l’eau, sur la production de biomasse verte et sur le rendement grainier du blé d’hiver cultivé en régime de semis direct dans les Prairies canadiennes. Cinq cultivars étaient utilisés sur une période de trois ans,à un total de 17 emplacements répartis dans les différentes zones climatiques de la Saskatchewan. Dans des conditions de culture sèche, la sécheresse avait un effet limitant à la fois sur la levée des semis et sur l’expression de la productivité grainière. Un approvisionnement en eau suffisant était nécessaire en début de printemps pour assurer les bases d’une haute productivité en grain mais, pour que celle-ci se concrétise, il fallait en plus une combinaison de faible ET et de fortes précipitations durant le stade de formation du grain. La croissance rapide réalisée sous les températures fraîches d’avril et du début de mai épuisait une bonne partie de l’eau biodisponible dans les 50 cm supérieurs du sol et c’est les grands déficits hydriques dus à ET, résultant de la diminution continue des ressources en eau, qui dans la plupart des essais causaient les stress de sécheresse. Les réserves en eau du sol à la reprise de la croissance du printemps n’étaient pas suffisantes pour porter une culture, mais les pluies tombées durant la période de croissance à tous les emplacements amélioraient les disponibilités en eau, autorisant des rendements grainiers plus élevés et un accroissement de l’écart des rendements moyens en fonction de l’environnement. Relativement au rendement grainier, c’est surtout au stade du tallage que les pluies avaient le plus d’influence, alors que de l’épiaison à l’anthèse se sont les conditions atmosphériques et la teneur en eau du sol qui étaient plus importantes. Du fait que les différences environnementales affectant la gravité des stress hydriques étaient reliées aux quantités et à la répartition des précipitations durant la saison de croissance; le stress pouvait être intermittent à certains emplacements et définitif à d’autres. Pour ces raisons, le succès de la culture du blé d’hiver dans les Prairies canadiennes dépend de l’ aptitude des cultivars et des systèmes culturaux à tirer parti des profils variables de disponibilité en eau durant la saison de croissance. Mots clés: Triticum aestivum L., semis direct, blé, stress de sécheresse, évapotranspiration, précipitations, utilisation de l’eau, biomasse, rendement grainier

1Present

address: Soils and Crops Branch, Agriculture Resources Section, Manitoba Agriculture, Carman, Manitoba, Canada R0G 0J0. 2To whom correspondence should be addressed ([email protected]).

Abbreviations: EMY, environmental mean yield; ET, evapotranspiration 729


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Fowler 1990). In a similar study carried out under severe drought stress, 100% of the total biomass for the season was present at anthesis (Johnston and Fowler 1992). In areas like the Australian wheat belt, where the crop water balance tends to decline during the growing season, too much growth prior to anthesis increases the likelihood of late-season drought stress (Passioura 1983). Entz and Fowler (1989) suggested that this type of stress can occasionally prevent normal grain development in winter wheat in Saskatchewan. However, growing season weather patterns on the Canadian prairies are variable with a high rainfall period normally falling in the late spring or early summer, which coincides with the sensitive stem elongation to early grain-filling period of winter wheat (Entz and Fowler 1988). Because growing season precipitation is variable in both timing and amount, it is difficult to optimise plant breeding and crop management strategies for this region. With this problem in mind, the objective of this study was to determine the effect of weather and crop soil water status on water use, aboveground biomass production, and grain yield of no-till winter wheat grown on the Canadian prairies.

No-till seeded winter wheat (Triticum aestivum L.) is adapted to the Canadian prairies because of its relatively efficient use of water (Entz and Fowler 1991). However, like all crops grown in this semiarid region, winter wheat is often subject to large yield losses due to drought stress and broadly adapted cultivars that are produced using flexible management systems are required to minimize cropping risks. Therefore, an understanding of the relationships between water availability and crop productivity in western Canada is a necessary prerequisite when planning cropping strategies. Yield-weather models have shown that a large part of the variation in cereal grain yields in western Canada can be explained by differences in rainfall, soil moisture and the evaporative potential of the atmosphere (Robertson 1974; Williams et al. 1975). Lehane and Staple (1965) reported that growing season precipitation and soil water accounted for most of the variation in spring wheat yields in southern Saskatchewan. However, subsequent studies by Campbell et al. (1988) indicated that more variation in spring wheat yield was explained by growing season evapotranspiration (ET) than by precipitation. In winter wheat, Entz and Fowler (1989) also reported that growing season precipitation failed to account for a significant portion of the environmental variability in grain yield. Their analyses attributed a large portion of the variation observed in Norstar grain yield to crop water balance during the latter stages of floral development. Pre-anthesis crop growth influences the potential grain yield of cereal crops. Field experiments in Saskatchewan have shown that Norstar winter wheat accumulates 57–89% of total aboveground biomass by anthesis and demonstrates a rapid early-season response to moisture (Darroch and

MATERIALS AND METHODS Five hard red winter wheat genotypes (Norstar, Norwin, S86-736, S86-784 and S86-808) were selected for evaluation in this study on the basis of unique characteristics identified in trials conducted by the University of Saskatchewan Crop Development Centre. These cultivars were grown in 17 field trials that were located throughout Saskatchewan from 1989 to 1991 (Table 1). Details on the performance of the cultivars and the soil type of the trial locations have been reported in a related publication (Domitruk et al. 2001).

Table 1. Trial location and monthly precipitation, Class “A” pan evaporation, and average daily temperature for field trials conducted in Saskatchewan, 1989 to 1991 Evapouration (mm)z

Precipitation

Temperature (°C)

Location

Year

May

June

July

Aug

May

June

July

Aug

Total

May

June

July

Aug

Clair Clair Clair Elrose Elrose Saskatoon Saskatoon Saskatoon Mikado Mikado Mikado Outlooky P.Plain Prestony Shaunavon Shaunavon Shaunavon

1989 1990 1991 1989 1990 1989 1990 1991 1989 1990 1991 1991 1989 1991 1989 1990 1991

44 2 60 78 24 61 35 70 31 8 76 82 103 84 88 59 90

95 5 132 32 54 39 52 135 83 32 85 175 64 163 74 32 156

29 28 119 1 58 37 61 56 42 44 38 55 72 61 47 97 31

33 0 33 56 0 1 4 19 106 54 0 5 66 33 65 13 13

227 216 NA 198 186 195 168 189 231 152 NA 223 192 199 191 172 195

208 236 NA 219 243 213 219 196 204 202 NA 209 174 196 222 279 188

271 218 NA 281 254 261 207 230 255 164 NA 234 208 230 280 247 252

200 210 NA 233 188 NA 217 255 240 167 NA 261 161 255 225 268 272

908 882 NA 933 873 NA 813 872 931 689 NA 928 735 881 920 966 909

19.3 17.1 17.4 18.9 18.6 18.1 17.9 18.6 19.8 17.0 18.1 17.4 18.7 18.6 16.8 17.2 16.8

20.6 23.9 22.4 24.7 24.0 23.4 23.9 22.2 20.7 23.1 23.0 21.5 21.6 22.2 22.5 22.9 20.3

26.5 23.4 24.1 29.5 25.4 27.2 23.4 24.3 26.7 23.4 24.2 23.7 25.2 24.3 27.5 24.5 25.8

24.4 24.2 26.5 25.9 26.3 NA 25.3 27.8 24.6 24.3 26.6 28.4 24.4 27.8 24.1 26.1 28.5

59 30

83 52

52 27

29 31

196 22

214 25

239 32

225 36

867 79

18.0 0.9

22.5 1.3

25.2 1.7

26.0 1.6

Mean Standard deviation

zClass “A” pan evaporation obtained yIrrigation sites. NA, not available.

from the nearest Environment Canada weather station.

DOMITRUK ET AL. — RESPONSE OF NO-TILL WINTER WHEAT TO DROUGHT STRESS

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Table 2. Extractablez soil water at spring greenup measured gravimetrically (0–10 cm) and by neutron probe (10–110 cm) Depth (cm)


Site-year

Year

Site ID

0–10

10–30

30–50

50–70

70–90

Preston Saskatoon Outlook Mikado

1991 1991 1991 1991

1 2 3 4

0.0 9.0 7.7 16.4

47.7 39.9 39.7 42.0

Mikado Clair Mikadoy P.Plainy Clair Shaunavon Shaunavon

1989 1991 1990 1989 1989 1989 1991

5 6 7 8 9 10 11

2.4 10.7 15.7 2.1 2.4 1.4 14.3

24.0 34.3 35.4 26.0 21.9 10.1 33.6

Saskatoon Shaunavon Saskatoon Elrose Clair Elrosey

1989 1990 1990 1990 1990 1989

12 13 14 15 16 17

10.2 20.1 32.3 32.1 31.1 1.1

34.3 26.4 25.2 34.9 28.2 35.9

(mm of soil water) Irrigated/low stress sites 36.0 44.8 42.1 46.3 42.7 53.7 32.7 11.3 19.0 27.2 19.0 28.0 Intermittent drought stress sites 20.2 13.9 11.7 24.7 23.8 29.4 25.8 17.8 13.8 19.8 17.6 18.0 15.5 13.6 11.7 6.9 9.8 10.1 21.6 13.5 15.5 Terminal drought stress sites 23.5 24.3 15.5 18.6 13.9 16.8 12.2 13.4 9.1 9.5 4.9 3.7 22.5 26.9 27.3 14.4 5.8 4.8

12.3 11.1

31.7 9.0

22.2 9.8

Mean Standard Deviation

18.6 11.2

19.4 13.2

90–110

Total

43.8 46.3 27.1 35.0

214.4 237.9 137.5 167.6

8.8 36.6

81.0 159.5 108.5 83.7 72.7 48.8 121.0

7.6 10.5 22.5 5.4 19.6 8.3 10.2 24.9

113.2 115.4 110.5 95.3 160.9 62.0

21.9 14.2

122.3 52.0

z

Calculated by subtracting the lowest volumetric water content measured in the field from the volumetric water content measured at spring greenup (after Ritchie 1981). y Measured to 70- to 90-cm depth only.

All trials were rainfed except Outlook and Preston in 1991, which were irrigated with an additional 75 mm of water in the fall shortly after seeding and again in the spring and summer so as to achieve twice the normal monthly precipitation. The Porcupine Plain 1989 trial was seeded into fallow. The rest of the trials were direct-seeded with a small plot hoe-press drill in late August or early September into standing canola (Brassica campestris L.), durum wheat (Triticum durum L.), barley (Hordeum vulgare L.) or mustard (Sinapis alba L.) stubble. The seeding rate was 75 kg ha–1. Phosphate fertilizer (11-51-0) was applied with the seed at a rate of 30 kg ha–1 P2O5. Nitrogen fertilizer (34-00) was top-dressed at a rate of 100 kg ha–1 N as soon as equipment was able to travel on the field in the spring. Broadleaf and grassy weeds were controlled using recommended post-emergent herbicides. The experimental design of all trials was a randomized complete block with three replicates. Plot size was 2.4 m × 6 m and row spacing was 20 cm. One half of each plot of each genotype in each replicate was designated for sampling during the growing season while the other half was left undisturbed until harvest. After removing approximately 30 cm from each end at maturity, the undisturbed half of each plot was harvested with a self-propelled small plot combine for grain yield determination. The outside two rows of each plot were not harvested. Crop development stages were divided into four periods; DP1 (tillering, Zadoks stages 24–30), DP2 (stem elongation, Zadoks stages 31–45), DP3 (heading to anthesis, Zadoks stages 45–65), and DP4 (anthesis to maturity, Zadoks stages

65–95). Aboveground biomass that accumulated during each development period was determined by removing plants from a 25 × 100-cm area of each plot that was designated for biomass sampling. Samples were dried at 60°C for 48 h in forced-air chambers and weighed to determine dry matter yield. Measurements of crop water balance were taken for each treatment at approximately 10- to 14-d intervals from early May to harvest so that each development period could be further subdivided. In general, the measurement frequency depended on the rate of phenological development. Soil water was measured using the neutron scattering method (Model 3300; Troxler Laboratories; Triangle Park, NC). One neutron access tube was located at the Centre of each sample area. Volumetric water content was measured at 20-cm increments from 10 to 110 cm. Water in the top 10 cm of the soil was determined gravimetrically then converted to volumetric values. The lowest volume of water present during the growing season was considered the lower limit of plant-available water (Ritchie 1981). Evapotranspiration was calculated over each sampling interval by adding rainfall (and irrigation) to the difference in soil water content between sampling dates. Minimum and maximum air temperatures and rainfall were monitored daily at each site. Class ‘A’ pan evaporation was obtained for each site from the nearest Environment Canada weather station. Some sites were equipped with a tipping rain bucket (Model RG2501, Sierra Misco Inc., Berkeley, CA) and a data logger (Model CR-21, Campbell Scientific Inc., Logan, UT).


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RESULTS AND DISCUSSION Monthly precipitation, pan evaporation, and average daily temperature for all sites is given in Table 1. Natural variation in weather, plus the addition of water through irrigation at some sites, caused a wide variation in crop water balance and, thus, in the timing and intensity of drought stress. Sites were rated as irrigated/low-stress, intermittent stress, or terminal stress (Table 2). Limited soil water resulted in an unfavorable crop water balance in the trials that were classified as terminal or intermittent stress. Crop-available water stored in the soil profile at spring greenup ranged from 48.8 mm at Shaunavon in 1989 to 237.9 mm under irrigation at Saskatoon in 1991 (Table 2). Based on previous

estimates of growing season ET, which ranged from 200 to 365 mm for winter wheat in Saskatchewan (Entz and Fowler 1989; Johnston and Fowler 1992), water available in the soil at spring greenup in most cases was insufficient to produce a commercial crop. The amount of available water at greenup decreased with depth in soil profiles measured in 1989 and 1990 but was evenly distributed throughout the profiles in 1991. Irrigated/low-stress environments had greater soil water reserves than the dryland sites. The highest concentration of water was at the 10- to 30-cm depth in the dry sites (Fig. 1). Below this, approximately 12 mm of water was available in each 20 cm of soil depth. Most of the water was extracted

Fig. 1. Average pattern of soil water extraction from greenup to maturity in terminal stress, intermittent stress and irrigated environments



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30 25 20 15 10 5 0

Fig. 2. Crop water regimes throughout the growing period for Elrose 1989, Clair 1989 and Preston 1991 representing terminal stress, intermittent stress and irrigation treatments, respectively.

from 10–30 cm before anthesis in the lowest rainfall environments and the crop exploited extra water when postheading precipitation occurred. Variation was observed among environments in the quantity of water extracted from below 50 cm. Only environments that received near normal precipitation (1950–1980 average) extracted water below 50 cm, even though similar amounts of water were available at those depths in all dryland environments. Environmental differences in soil water extraction reflected differences in rainfall distribution and the crop water balance. For example, continuous exploitation of soil water by ET in the absence of frequent precipitation resulted in an increasingly unfavorable crop water balance at Clair and Elrose in 1989 (Fig. 2). As a result, while general-

Fig. 3. Aboveground biomass accumulation from 1 May until physiological maturity in terminal and intermittent stress environments in 1989 and 1990, and irrigated/high rainfall (low stress) and dryland environments in 1991 (see Table 2 for trial locations).

ly restricted in these terminal and intermittent stress environments, ET was even more curtailed after heading. This limitation was not observed at the irrigated Preston site in 1991. Drought stress was more likely and most severe in environments where low soil water reserves at greenup, particularly at the 10- to 30-cm depth, were coincident with low rainfall and high evaporative demand during floral organ formation (DP1) in June. A similar crop water balance pattern was observed in each terminal or intermittent stress environment regardless of the frequency of re-wetting. This pattern is consistent with the “water user” concept for crops in semiarid environments.


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Fig. 4. Relation between aboveground biomass at maturity and growing season evapotranspiration for five genotypes in 15 trials grown between 1989 and 1991 in Saskatchewan.

Three patterns of aboveground biomass accumulation were observed. Under terminal stress, aboveground biomass accumulation was terminated between heading and anthesis (Fig. 3). Where rainfall and drought stress were intermittent throughout crop development, aboveground biomass accumulated at a relatively constant rate until maturity. Irrigation and high rainfall environments allowed the crop to increase in biomass through to maturity. Aboveground biomass accumulation was directly related to water consumption (Fig. 4) while water use efficiency (unit of aboveground biomass produced per unit of water consumed) became more variable as ET and aboveground biomass increased. Environmental variation in loss of water to evaporation or loss of dry matter to respiration resulted in variation in the water use efficiency of aboveground biomass production. Nevertheless, the pattern of crop growth was generally reflected in the pattern of water consumption. Variation in the crop water balance was related to variability in the water supply and a crop water deficit that was typical of the semiarid nature of the prairie region was experienced over the entire growing season in all rainfed environments. Grain yield is more sensitive to stress during some developmental stages than others (Baier and Robertson 1967; Day and Intalap 1970; Fischer 1973; Passioura 1977; Warrington et al. 1977; Musick and Dusek 1980; Entz and Fowler 1988). As a result, empirical approaches, which use mean grain yield as an integrated index of season-long environmental stress, require that the underlying cause of grain yield variation be established. In the present study, differences in the crop water balance during crop development contributed to environmental variation in grain yield. Simple linear correlation analysis established the associa-

tions between the mean yield of all genotypes in each environment (environmental mean yield) and individual weather variables measured during each development period. Environmental mean yield (EMY) was positively correlated with precipitation, soil water in all regions of the profile, and the total water supply (Table 3). Environmental mean yield was negatively correlated with water in the top 10 cm of the soil profile at greenup. A positive correlation was observed between EMY and mean air temperature during DP1–DP2 and between EMY and growing degree-days during DP1 and DP1 through DP2. Negative correlations were observed between EMY and maximum air temperature in June and also between EMY and maximum air temperatures for the period including DP2 to DP4. Environmental mean yield was negatively correlated with Class “A” pan evaporation during floral development (EDAY3, JUNEE). In these studies, EMY was positively associated with total available water during all development periods and negatively associated with aerial demand for water at anthesis. Linear regression on seasonal weather indices indicated that variation in EMY reflected environmental differences in crop water supply (Fig. 5). A problem associated with regression in the current context is that levels of the independent variables are often associated with time. Also, due to the uncontrolled nature of field studies, precipitation and soil moisture are confounded because soil moisture content increased with irrigation and rainfall. The effect of multiple environmental factors on grain yield was addressed by simultaneously assessing several components of crop water balance through stepwise multiple linear regression. In terms of water supply, water stored 30–50 cm below the soil surface at greenup (AWATC1) together with total growing season precipitation (GSP) best


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Table 3. Simple linear correlations between weather variables and EMY EDAY3z –0.72**

E1-3z 0.57*

JUNEEz –0.64*

P1 0.52*

P1-2 0.57*

P2-3 0.66**

AWATA1 –0.55*

AWATC1 0.76**

AWATP1 0.55*

Atmospheric conditions AT1-2 AMT2-4 0.54* –0.58* Precipitation P3-4 P1-3 0.75** 0.72** Stored water AWATD1 AWATE1 0.66** 0.70**

JUNEMT –0.61* PJUNE 0.79**

GDD1 0.57*

GDD1-2 0.54*

GSP 0.75**

SWS 0.81**

AWATF1y 0.69**


z, y

n = 12, 14, respectively, otherwise n = 15; *,** P < 0.05, P < 0.01, respectively. EDAY3, mean daily Class “A” pan evaporation DP3. E1-3, total Class “A” pan evaporation DP1-DP3. JUNEE, total Class “A” pan evaporation during June. AT1-2, mean daily temperature DP1-DP2. AMT2-4, mean maximum daily temperature DP2-DP4. JUNEMT, mean maximum daily temperature during June. GDD1, total growing degree days (5°C base) DP1. GDD1-2, total growing degree days (5°C base) DP1 through DP2. P1, precipitation DP1. P1-2, precipitation DP1 through DP2. P2-3, precipitation DP2 through DP3. P3-4, precipitation DP3 through DP4. P1-3, precipitation DP1 through DP3. PJUNE, precipitation during June. GSP, growing season precipitation. SWS, seasonal water supply from stored soil water and GSP. AWATP1, total profile available water at greenup. AWATA1, available water in the top 10 cm at greenup. AWATC1, available water in the 30-50 cm region of the profile at greenup. AWATD1, available water in the 50-70 cm region of the profile at greenup. AWATE1, available water in the 70-90 cm region of the profile at greenup. AWATF1, available water in the 90-110 cm region of the profile at greenup.

explained the variation in EMY (Y = 780 + 54.6(AWATC1) + 12.4(GSP); r2 = 0.76**). In terms of water demand, evaporative demand from heading to anthesis (EDAY3) played a larger role in determining EMY (Y = 6485–525.7(EDAY3); r2 = 0.51**) than evaporative demand during other periods. Considering all water supply and demand variables simultaneously, variation in total water supply (SWS) and in mean daily evaporation from heading to anthesis (EDAY3) best explained variation in EMY (Y = 3371.9 + 5.7(SWS) – 315.3(EDAY3); r2 = 0.80**). Abiotic stress affects the expression of yield potential during the entire crop development cycle. Therefore, EMY was regressed on weather data from the four development periods to determine the season-long influence of air temperature, pan evaporation, soil water content and rainfall on grain yield. However, the sensitivity of grain yield to certain crop water variables may change as the crop develops. Thus, the environmental sums of squares from the analysis of variance was partitioned into portions attributable to individual weather variables during each development period, each time ignoring the other weather variables. Growing season precipitation and average maximum daily temperature accounted for 88 and 48% of the environmental sums of squares for grain yield, respectively. Precipitation during tillering and from heading to anthesis accounted for 36.3 and 25.7% of environmental variation in grain yield, respectively. Season-long variation in average maximum daily

temperature was less influential, although variation in temperature from anthesis to maturity, during grain filling, contributed 34% of the grain yield environmental sums of squares. Variation in average daily Class “A” pan evaporation over the four development periods accounted for 91.8% of environmental variability in grain yield. Daily evaporation during anthesis was most important in this respect accounting for 44.9% of environmental variation in grain yield. Springtime variation in available soil water at incremental depths accounted for 86.5% of the environmental variation in grain yield. Variation in the amount of water available in the top 50 cm had the greatest influence on grain yield. Grain yield was positively related to total growing season ET (Fig. 5). However, total water use varied with precipitation distribution and evapotranspiration in this water-limited environment was a function of water supply. For example, when the soil was frequently wetted, as with irrigation, losses to evaporation were higher. A negative association (P < 0.10) was observed between EMY and the proportion of growing season evapotranspiration that occurred before heading (r = –0.50) and before anthesis (r = –0.49). Crops that did not receive rainfall after heading consumed approximately 75% of their total water supply before anthesis. In more favourable environments, the crops consumed 36–72% during the same period. Rainfall after heading increased post-anthesis evapotranspiration in some



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Fig. 6. Evapotranspiration during pre-anthesis, Zadoks 45-90 and post-anthesis periods according to yield rank (1 = highest) of the environment.

Fig. 5. Linear relation between EMY and (a) precipitation, (b) total water supply from precipitation and soil water and, (c) growing season ET for 15 environments (Clair 1989 and 1990, Elrose 1989 and 1990, Saskatoon 1989, 1990 and 1991, Mikado 1989, 1990 and 1991, Outlook 1991, Preston 1989 and 1991 and Shaunavon 1989 and 1990) in Saskatchewan.

dryland environments. Otherwise, ET was especially restricted during grain development due to insufficient water supply. Environmental mean yield was positively correlated with both pre- and post-anthesis ET (Fig. 6).

Tillering and elongation significantly depleted the supply of soil water, especially at the 10- to 30-cm depth. As a result, most of the profiles associated with dryland trials had less than 75 mm of available water during the initial stages of floral development. Low soil water supplies and limited or non-existent rainfall created a large ET deficit from this stage through to grain filling and maturity. Stress at this time limited both the establishment and expression of yield potential. Low-stress crops consumed 175–225 mm of water during these periods compared with less than 100 mm for dryland crops. As a result, ET deficit from heading to maturity was highly correlated with EMY (r = 0.87***). Aboveground dry matter at anthesis was not an accurate predictor of EMY (r2 = 0.32, NS). In contrast, there was a strong positive relationship between EMY and aboveground dry matter at maturity (Fig. 7). However, this relationship was biased by the presence of four highly productive



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Fig. 7. Relation between EMY and aboveground biomass at maturity (AGDM) for 15 environments in Saskatchewan from 1989 to 1991 (see Fig. 5 for trial locations).

environments that produced grain yields between 5.0 and 5.5 t ha–1. Environments that produced 6–8 t ha–1 aboveground dry matter produced a wide range of grain yield (1.7–3.2 t ha–1). In summary, large ET deficits characterized drought stress and variation in the intensity and duration of drought was largely responsible for differences in yield among trials. As reported in earlier studies (Entz and Fowler 1989; Johnston and Fowler 1992), stored soil water was not sufficient to support the winter wheat crop throughout the growing season. Rapid growth under cool temperatures during April and early May consumed much of the available water stored in the top 50 cm of the soil profile with the result that differences in drought stress were related to the volume and distribution of growing season precipitation. Variable growing season rainfall distribution meant that some trials were exposed to intermittent stress while stress was terminal in others. In terms of grain yield, precipitation had its largest influence during tillering while atmospheric conditions and soil water content were more important from heading to anthesis. These observations show that the distribution of growing season rainfall is the primary determinant of no-till winter wheat growth, water use and yield in the western Canadian prairie environment. For this reason, dryland winter wheat cultivar development and management strategies in this region should proceed within a framework designed for crops that have the ability to respond to variable patterns of growing season water availability. Baier, W. and Robertson, G. W. 1967. Estimating yield components of wheat from calculated soil moisture. Can. J. Plant Sci. 47: 617–630.

Campbell, C. A., Zentner, R. P. and Johnson, P. J. 1988. Effect of crop rotation and fertilization on the quantitative relationship between spring wheat yield and moisture use in southwestern Saskatchewan. Can. J. Soil Sci. 68: 1–16. Darroch, B. A. and Fowler, D. B. 1990. Dry matter production and nitrogen accumulation in no-till winter wheat. Can. J. Plant Sci. 70: 461–472. Day, A. D. and Intalap, S. 1970. Some effects of soil moisture stress on the growth of wheat (Triticum aestivum L.em Thell.). Agron. J. 62: 27–29. Domitruk, D. R., Duggan, B. L. and Fowler, D. B. 2001. Genotype-environment interaction of no-till winter wheat in western Canada. Can. J. Plant Sci. 81: (in press). Entz, M. H. and Fowler, D. B. 1988. Stress periods affecting productivity of no-till winter wheat in Western Canada. Agron. J. 80: 987–992. Entz, M. H. and Fowler, D. B. 1989. Influence of crop water environment and dry matter accumulation on grain yield of no-till winter wheat. Can. J. Plant Sci. 69: 367–375. Entz, M. H. and Fowler, D. B. 1991. Agronomic performance of winter versus spring wheat. Agron. J. 83: 527–532. Fischer, R. A. 1973. The effect of water stress at various stages of development on yield processes in wheat. Pages 233–241 in R. D. Slaytner, ed. Plant response to climatic factors. Proc. 1970 Uppsala Symp., Uppsala, Sweden, UNESCO, Paris. Johnston, A. M. and Fowler, D. B. 1992. Response of no-till winter wheat to nitrogen fertilization and drought stress. Can. J. Plant Sci. 72: 1075–1089. Lehane, J. J. and Staple, W. J. 1965. Influence of soil texture, depth of soil moisture storage, and rainfall distribution on wheat yields in southwestern Saskatchewan. Can J. Soil Sci. 45: 207–219. Musick, J. T. and Dusek, D. A. 1980. Planting date and water deficit effects on development and yield of irrigated winter wheat. Agron. J. 72: 45–52.

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Passioura, J. B. 1977. Grain yield, harvest index and water use of wheat. J. Aust. Inst. Agric. Sci. 43: 117–120. Passioura, J. B. 1983. Roots and drought resistance. Agric. Water Manage. 7: 265–268. Ritchie, J. T. 1981. Soil water availability. Plant Soil 58: 327–338. Robertson, G. W. 1974. Wheat yields for 50 years at Swift Current, Saskatchewan in relation to weather. Can. J. Plant Sci. 54: 625–650.

Warrington, I. J., Dunstone, R. L. and Green, L. M. 1977. Temperature effects at three development stages on the yield of the wheat ear. Aust. J. Agric. Res. 28: 11–27. Williams, G. D. V., Joynt, M. I. and McCormick, P. A. 1975. Regression analyses of Canadian prairie crop-district cereal yields, 1961–1972, in relation to weather, soil and trend. Can. J. Soil Sci. 55: 43–53.