Effect of de®cit irrigation on wheat and
opportunities of growing wheat on residual
soil moisture in southeast Zimbabwe
F.T. Mugabe
a,*, E.Z. Nyakatawa
b aChiredzi Research Station, P.O. Box 97, Chiredzi, Zimbabwe bDepartment of Plant and Soil Science, Alabama A&M University,P.O. Box 1208, Normal, AL 35762, USA
Accepted 31 January 2000
Abstract
Soil moisture from six sites in Romwe was measured at the end of the wet season (April) and at the end of the dry season (October) and the available water calculated. Results from a variety by irrigation trial run at Chiredzi Research Station for two seasons was used to assess the possibility of growing wheat on residual soil moisture in Romwe. Six wheat genotypes (P1, P2, Pote, Deka, Nata and Ruya) were grown under three irrigation regimes at Chiredzi Research Station during the 1996 and 1997 winter seasons. The irrigation regimes used were supplying irrigation water according to the crop water requirements, supplying three quarters of the crop water requirements and half of the crop water requirements at each irrigation day. Applying three quarters and half of the crop water requirements resulted in a yield decrease of 12 and 20% in 1996 and 7 and 20% in 1997 season, respectively. P2 gave the highest yields on average for the two seasons and was the least affected by de®cit irrigation. However, Deka gave the least decrease in yield when the three-quarters and half water requirements were supplied. Four of the sites in Romwe, where residual soil moisture was measured, had more than half the water required to meet the crop water requirements of wheat.
#2000 Elsevier Science B.V. All rights reserved.
Keywords:Wheat; De®cit irrigation; Yield stability; Residual soil moisture
1. Introduction
In Zimbabwe wheat is grown in winter (May±September) under irrigation, when the temperatures are low and favorable for seed yield and quality. Only commercial farmers,
*Corresponding author. Tel.:263-037-369
E-mail address: mugabe@africaonline.co.zw (F.T. Mugabe).
with irrigation facilities, can grow winter wheat. However, some smallholder farmers, especially in the wetter areas, grow wheat on residual soil moisture in wetlands (Adam et al., 1997). Normally the water requirements of wheat grown on residual soil moisture is not met resulting in low yields being attained.
There is very limited water loss from the soil surface during the cooler months when wheat is grown. All the available residual soil moisture can be used by a crop since dry soil conditions promote root elongation and branching (Sharma and Ghildyal, 1977). Wheat roots can abstract water from up to 2 m (Hurd, 1968; Prihar et al., 1977; Arora and Prihar, 1983) and most of the residual soil water in the soil profile can be utilized.
Wheat yields of the order of 2800 kg/ha have been reported by Mishra et al. (1995) with 50 mm of precipitation grown on residual soil moisture in India. It has been suggested that genotypes with lower yield potentials under test crop conditions but which are much more stress resistant should be used where water deficits are anticipated. It is quite possible that some released genotypes and promising wheat lines in the current Zimbabwe breeding program can withstand drought.
Water has not been taken as a major limitation in the Zimbabwe breeding programme; hence the drought tolerance of the released genotypes is not known. Farmers growing wheat on limited soil moisture do not know which varieties to choose in order to realize optimum yields. This study was conducted to compare the response of four released genotypes and two promising genotypes to water deficits and to assess the possibility of growing wheat on residual soil moisture.
2. Study sites and methods
2.1. Irrigation by variety trial at Chiredzi Research Station
The study was conducted at Chiredzi Research Station, (218S and 318E), at an altitude of 429 m a.s.l. in the southeast Lowveld of Zimbabwe during the winters of 1996 and 1997. The region lies in natural region V of Zimbabwe that is semi-arid, with mean annual rainfall of 500 mm with a seasonal range of 250±1000 mm. The natural regions are a classification of the agricultural potential of the country, from natural region 1, which represents the high altitude wet areas to natural region V which receives low and erratic rainfall averaging 550 mm per annum (Vincent and Thomas, 1960). The soils at Chiredzi Research Station are dark-reddish brown clays derived from basic gneiss and are classified as the Triangle B2 series and typic rhodstuff in Zimbabwean and USDA classification systems, respectively.
Four released wheat genotypes, Deka, Pote, Nata and Ruya and two promising varieties S85331-3H-0H-3H-0H (P1) and S86073-5H-0H-3H-0-1H-0G (P2) were grown at three regimes of drought stress in the 1996 and 1997 seasons. These were full, three-quarters and half of normal irrigation requirements. The full treatment was irrigating the genotypes at 50% allowable moisture depletion and applying water according to the crop water requirements. All the irrigation treatments were based on the open pan irrigation scheduling using the wheat standard crop factor progression of 0.3±1.0 (Cackett, 1972).
The land on which the study was carried out was ploughed and a basal fertilizer of 700 kg/ha compound X (20:10:5) was disced into the soil before planting. Sowing was done in the first week of May, that is the recommended sowing time for wheat in the Lowveld. The seeds were sown in rows 25 cm apart in 3.5 m3.5 m plots at a seed rate of 100 kg/ha. The experiment was designed as a 63 factorial in a split plot design with four replications. The irrigation level was the main plot factor whereas genotype was the sub-plot factor.
The site was irrigated to field capacity soon after planting and 21.8 mm emergence irrigation was applied about 7 days later. Thereafter, the irrigation treatments were imposed. Weed control was hand hoeing. Regular scouting for insect and disease was done throughout the duration of the study. Measurements taken included days to 50% flowering, days to maturity, stand counts at harvest, plant heights at harvest, number of ears/m2, number of spiklets/ear, 1000 seed weight and grain yield. The data were subjected to the ANOVA procedure using MSTA-C statistical package.
The yield stability of the wheat genotypes was assessed, using Eq. (1), whereYs is the yield stability of a given genotype when deficit irrigation is applied,Yg1is the geno-type yield grown without water stress andYg2is the genotype yield grown under water stress.
Ys
100 Yg1ÿYg2
Yg1
(1)
2.2. Assessment of residual soil moisture at Romwe
Romwe, in Chivi, lies in natural region IV. The average annual rainfall at Chendebvu dam, a rainfall station located 12 km to the north of the catchment, over the last 40 years (1952±1992) is 581 mm (Butterworth et al., 1995) with a standard deviation of 263 mm. The soils are light grey colored, sandy soils formed from granitic gneiss. These are kaolinitic, fersialitic soils (III 5P) according to the Zimbabwean soil classification system (Thompson and Purves, 1978). In many locations, a thick clay layer underlines light textured horizons.
One access tube (external diameter 45 mm) was installed to 120 cm in the center of each of seven sites chosen. A neutron probe (Wallingford MK III) was used to measure soil moisture. Counts were taken over 16 s at 10 cm intervals at the end of the wet season (5 April 1994) and at the end of dry season (8 September 1994). Soil samples were sent to Center of Nuclear Studies (Cadarache, France) for calibration of the neutron probe (Mugabe, 1998). Particle size on each horizon for the seven sites was measured at the Institute of Soils and Chemistry, Harare. Horizon delineation was based on color and observable textural and structural differences. In this study, percent sand refers to the combination of coarse, medium and fine sand. The pressure chamber method was used to determine the gravimetric moisture content at ÿ1500 kPa following the technique described by Black (1965). Saturated paste for each of the samples was prepared and replicated twice in rings that were placed in ceramic plate. The samples were left to equilibrate until there was no significant amount of water increase in the burettes collecting the outflow. After equilibrium the samples were transferred into weighed
aluminum foil. The weight of the wet soil plus aluminum foil was determined and the sample was dried at 1058C hours before reweighing.
3. Results and discussion
3.1. Irrigation levels
Table 1 shows the amounts of irrigation water applied for each treatment in 1996 and 1997. For each treatment similar amounts of irrigation water were applied in both years. Irrigating three quarters and half of the crop water requirements at each irrigation day resulted in water deficits of 16 and 31% in the 1996 season and 20 and 38% in the 1997 season, respectively. More rainfall was received in 1996 (60 mm) than 1997 (11 mm). In 1996 the mean monthly temperatures for June, July and August were lower than that of 1997 (Table 2).
At all the irrigation levels, the 1996 crop took longer to mature, had more ears/m2, and was taller because of cooler conditions (Cackett and Wall, 1971; Mashiringwani and Schwepppenhauser, 1991) (Table 2) compared to 1997. On average, yields were heavier in 1996 than in 1997.
Table 3 shows that supplying less water than the crop water requirements has no effect on days to 50% flowering, number of ears/m2, plant height and 1000 seed weight.
Table 1
Irrigation dates and amounts for the wheat trial and rainfall received during the winters of 1996 and 1997 1996 1997 Amount of water applied (mm) Comment
Full Three quarters Half
16/5 10/5 50 50 50 Planting
23/5 22/5 22 22 22 Emergence
14/6 17/6 50 38 25 Treatment
11/7 10/7 50 38 25 Treatment
8/8 8/8 50 38 25 Treatment
27/8 18/8 50 38 25 Treatment
Total 272 224 172
Rain 1996 60 1997 11
Table 2
Monthly maximum, minimum and mean temperatures (8C) from May to August 1996 and 1997 at Chiredzi Research Station
Year May June July August
Table 3
Effect of de®cit irrigation on wheat yields and yield attributes of six genotypes during the 1996 and 1997 winter seasonsa
Days to 50%
1996 1997 1996 1997 1996 1997 1996 1997 1996 1997 1996 1997
Irrigation level
Full 75.0a ±b 114.3a 112.0a 375a 350a 95.9a 87.5a 37.1a 41.9a 5537a 5290a
Three quarter 74.8a ± 112.1ab 110.5a 367a 345a 95.0a 86.8a 35.0a 40.7a 4862b 4894a Half 73.9a ± 109.7b 108.5b 358a 318a 93.8a 84.9a 34.9a 39.7a 4431b 4223b
Genotype
P1 65.8d 105.2c 108.0c 363ab 344a 91.2b 79.3c 41.6a 43.8a 4622bc 3895b P2 74.1bc 109.0b 110.0ab 324b 325ab 89.7b 84.2b 35.3b 40.8a 5513a 5150a Pote 73.0c 106.3bc 109.5bc 386ab 362a 95.4ab 88.4a 34.5b 42.3a 4538c 4875a Deka 79.8a 116.8a 111.6a 353b 296b 101.2a 91.7a 36.5b 41.9a 5101abc 5060a Nata 79.5a 117.0a 111.3ab 416a 355a 92.6b 84.8b 30.7c 32.8b 5144ab 5020a Ruya 75.3b 117.9a 111.4a 358ab 344a 99.4a 89.8a 35.6b 42.0a 4743bc 4834a Mean 74.4 ± 112.0 110.3 367 338 94.9 86.4 35.7 40.8 4944 4806 LSD 1.531 ± 3.18 1.875 62.2 47.3 5.812 3.557 3.703 6.141 585.6 575.6 CV% 1.44 ± 1.99 1.19 11.9 9.8 4.30 2.89 7.29 10.58 8.32 8.41
Interaction NS ± NS NS NS NS NS NS NS NS S S
Irrigating half of the irrigation water requirements resulted in significantly fewer days to maturity compared to irrigation according to the crop's water requirements and supplying three-quarters of irrigation in 1997. Ehdaie (1995) also reported that water stressed wheat matures earlier than that which is not stressed. In 1996, applying less water gave significantly lower yields compared to irrigating according to the crop water requirements. There was a 21 and 20% decline in yield when three quarters and half irrigation water were applied, respectively. In 1997, there were no significant yield differences between full irrigation and three-quarter water application. Applying half the irrigation water resulted in significantly lower yields than applying all the crop water requirements and applying three quarters irrigation water.
3.2. Wheat genotypes
There were significant differences in days to 50% flowering. Days to maturity, number of ears/m2, plant height, 1000 seed weight and grain yield among the six genotypes (Table 3). The promising, genotype P2, gave the highest yields in both seasons though it was not significantly different from most of the genotypes (Deka and Nata in 1996 and all except P1 in 1997).
In 1996 P2 performed better than all the genotypes at all the three irrigation regimes (Table 4) while in 1997 it out performed all the genotypes at full irrigation only. Of the released genotypes, Deka produced the highest yields in 1997 season at half irrigation and was second at three quarters irrigation in 1996. On average Deka yielded highest at both three-quarters and half irrigation in the two seasons. Yield stability of genotypes can be calculated by comparing yields of crop grown under deficit irrigation and that obtained at
Table 4
Mean yields (kg/ha) of six genotypes of wheat for 1996 and 1997 at three levels of irrigation
Irrigation level Genotype 1996 1997
Full P1 4938 4483
P2 5983 6011
Pote 5498 5382
Deka 5432 5232
Nata 5834 5681
Ruya 5541 4951
Three quarters P1 5153 4026
P2 5631 5118
Pote 3754 5236
Deka 5342 4999
Nata 4989 5002
Ruya 4305 4987
Half P1 3776 3176
P2 4926 4322
Pote 4364 4008
Deka 4528 4950
Nata 4609 4377
Ruya 4385 4565
full irrigation (without water stress). Table 5 shows the percent deviation in yield of the six wheat genotypes under deficit irrigation during the 1996 and 1997 seasons. Deka gave the least deviation at both three-quarters and half irrigation regimes and this implies that Deka is not as much affected by deficit irrigation compared to the other genotypes. This might not mean that it would give the highest yields under deficit irrigation. For example P2 (though it has an average deviation of 23%) yielded highest at three-quarters and half irrigation in 1996. In the same year Deka gave second and third highest yields, respectively, but had a smaller average deviation of 11%.
3.3. Possibility of growing wheat on residual soil moisture
Table 6 shows that there is lot of variation in the texture of the sites chosen, though they are all sandy at the surface except F5. The soils used for this analysis are sandy at the surface and clayey below 70 cm. In wet years the soils will be water logged in the summer season and very moist at the end of the wet season. Available moisture to 110 cm was more than 77 mm at the end of the rainy season in April for all the sites. The difference in soil moisture between 5 April and 8 September is water loss through soil evaporation, weed uptake or some limited drainage.
This water could be used to grow a winter crop like wheat or beans and the expected yield levels will depend on the ability of the crop grown to withstand drought. The sites had different total soil moistures and water remaining at permanent wilting point.
This depended on the individual site's texture. Four of the seven sites (F4, F7, F8 and F9) selected had soil moisture above half of the wheat crop water requirements.
The water available to the wheat grown on residual soil moisture can be more than the figures given in Table 6, for water can be withdrawn from depths of up to 2 m when wheat is grown under water stress conditions (Sharma and Ghildyal, 1977).
The results from the irrigation trial carried out at Chiredzi Research Station (Table 3) shows that wheat yields of more than 4000 kg/ha can be expected when wheat is grown on deficit irrigation of at least 170 mm. Mishra et al. (1995) recorded wheat yields of the order of 2800 kg/ha when they grew wheat on residual soil moisture with 50 mm of rain only. Work carried out by Chaudhary and Bhatnagar (1980) shows that frequent irrigations must be provided throughout the growing season to ensure maximum yield of
Table 5
Percent yield deviation from two de®cit irrigations for six genotypes during the 1996 and 1997 seasons Irrigation level Three quarters Half
Genotype 1996 1997 Average 1996 1997 Average
P1 ÿ4a 10 3 24 29 26
P2 6 15 10 18 28 23
Pote 32 3 17 21 26 23
Deka 2 4 3 17 6 11
Nata 14 11 13 21 22 21
Ruya 22 ÿ1a 10 20 8 14
aIndicates an increase in yield resulting from de®cit irrigation.
wheat. This would mean that the expected yields from wheat grown on residual soil moisture would be lower than those obtained from wheat grown on deficit irrigation where frequent irrigation is supplied. Table 7 shows that winter rains are expected in most of the years at Romwe and this would increase the water available to the crop.
Table 6
Clay, silt and sand contents of the sites, and available soil water at the end of the rainy season (April) and at the end of the dry season (September)
Site Depth Clay (%) Silt (%) Sand (%) Total available soil water in 110 cm profile (mm) 5 September
1994
8 November 1994
F4 0±13 11 11 78 240 178
13±44 24 11 65
44±79 53 13 33
79 34 10 55
F5 0±26 20 10 70 77 36
26±46 14 18 68
46 15 14 71
F6 0±9 4 7 89 106 54
9±16 4 7 89
16±85 8 9 83
85 3 6 91
F7 0±13 6 9 85 194 109
13±81 7 8 85
81 25 5 70
F8 0±26 4 3 93 212 127
26±78 8 9 85
78 38 7 55
F9 0±15 9 10 81 220 98
15±100 12 10 78
100 29 8 63
F10 0±19 9 13 78 147 101
19±74 7 9 84
74 33 5 62
Table 7
Romwe winter rainfall (mm) in 1994, 1995, 1996 and 1997
Months 1994 1995 1996 1997
May 2 64 38 1.5
June 0 1.5 13 1
July 13 5 35 29
August 14 8 13 0
Total 29 78 99 31
Deka recorded the least reduction at the two water levels even though it did not record the highest yields when optimum water was added. This suggests possible benefits of growing Deka on residual soil moisture, because of the yield stability at water levels below wheat crop water requirements.
Acknowledgements
The authors would like to thank Dr. M.D.S. Nzima and P. Nyamudeza for their critical review on this paper, Dr. N. Mashirinwani for providing us with wheat genotypes, Messers D. Maringa, D. Muzenda and Mhlanga, for their roles in trial management and data collection.
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