6. RESPONSE OF COWPEA (Vigna unguiculata (L.) Walp) TO VARYING WATER
6.2 Materials and Methods
The methodology adopted in this study is described in the following sub-sections. This included the description of the experimental designs and procedures as well as data collection and analysis.
6.2.1 Study area and experimental design
The study was carried out in tunnels at the Controlled Environment Facility (CEF) of UKZN, Pietermaritzbug Campus (29.580 S, 30.420 E) and UKZN’s Ukulinga Research farm (29.670 S, 30.410 E.) The experiment at CEF was carried out in a glasshouse with raised beds measuring 11 m long, 0.75 m wide and 0.75 m high. The soil texture was loam (42.3% sand, 33.3% silt, 24.4%) with bulk density of 1.36 g cm-3. At Ukulinga, the experiment was carried out in a 12 m by 5 m tunnel where the soil texture was clay (24.3% sand, 23.6% silt and 52.1% clay) with
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bulk density of 1.23 g cm-3. The tunnel at Ukulinga had open ends to replicate as much as possible the field conditions with free movement of air.
The experiment was laid out in a split-plot design arranged in randomized complete block design. The main block was the irrigation type (SDI and MTI) while the sub-plots were the three water regimes replicated three times. The water regimes imposed consisted of full irrigation to meet the crop water requirement (100% ETc), and DI of 70% ETc and 40% ETc.
The drip emitters and Moistube tapes were installed at a depth of 15 cm. The experimental layouts are shown in Figures 6.1 and 6.2 for CEF and Ukulinga, respectively.
Figure 6.1 Experimental layout for CEF experiment
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Figure 6.2 Experimental layout for Ukulinga experiment
Cowpea (brown mix variety) was planted on 14th February 2018 at CEF and 25th May 2018 at Ukulinga. The spacing was 50 cm between rows and 30 cm within rows giving a density of 66667 plants ha-1. Soil fertility test conducted at Cedara Agricultural College indicated that the soil did not have nutrient deficiency at Ukulinga but soil at CEF required phosphorus at a rate of 60 kg ha-1 Single Superphosphate (10.5% P). The DI treatments were introduced 21 days after planting (DAP) at CEF and 30 DAP at Ukulinga when the crops were fully established.
Other agronomic management practices such as weed, pest and disease control were done accordingly based on recommended best practices.
6.2.2 Determination of crop water requirements
The procedure for determination of crop water requirements was as follows;
a) The crop water requirements (ETc) for each crop growth stage were determined using potential evapotranspiration and crop coefficients as describe in Equation 5.4 (Chapter 5).
b) The net irrigation requirement (Inet) was the same ETc since rainfall was zero c) The volume of irrigation water was computed using Equation 6.1
𝑉 = 10 × ∑𝑛𝑖=1d (6.1)
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Where V = volume of water (m3 ha-1), d= irrigation depth (mm), and n = number of water applications derived from the irrigation interval.
d) The net irrigation depth was computed using Equation 6.2
dnet = ρ * (FC – PWP) * D (6.2)
Where dnet = The net irrigation depth (mm), D = The depth of the root zone of the crop (mm), and ρ = allowable depletion level which was taken as 50%, FC = field capacity, PWP = permanent wilting point. The root zone was limited by the soil layer thickness which was 0.60 m and thus this was taken as the effective rooting depth.
e) The gross irrigation depth can then be calculated using Equation 6.3
dgross = dnet/Ea (6.3)
Where dgross = The gross irrigation depth (mm) and Ea = The field application efficiency which was taken as 90% for both MTI and SDI.
f) The irrigation interval (T) computed using Equation 6.4
T = dnet/ETc (6.4)
Where ETc is the crop water requirement per decade
The different water regimes were applied by varying the irrigation interval in such a way that the total amount of irrigation was 100%, 70% and 40% of ETc. MTI was supposed to be continuous i.e. water applied throughout the cropping cycle but due to infrastructural challenges, the flow regulation was not sufficiently low enough to allow for continuous water application. Therefore, the water application was applied intermittently ranging from as low as 3 days continuously per decade to 8 days per decade for 40% ETc and 100% ETc, respectively.
6.2.3 Data collection and analysis
Weather data was obtained inside the tunnel using HOBO data logger sensors (Onset Computer Corporation, USA). The variables measured were temperature, relative humidity, solar radiation. Wind speed was measured using Kestrel 3000 anemometer (Nielsen-Kellerman, Inc.
USA). Soil water content was measured weekly using Water Mark sensors (Irrometer Inc.
USA) and MPS-2 sensors (Decagon, Inc. USA) installed at 10 cm, 20 cm and 40 cm depths.
The soil water potential values obtained were calibrated using gravimetric measurements (Figure A.1). At Ukulinga, the water content was measured using PR2/6 profile probe (Delta- T Ltd, UK). Leaf area index (LAI) was measured weekly using LAI 2200 canopy analyser (LI- COR Inc. USA). Time to 50% flowering was determined by counting the number of flowered
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plants. Determination of yield components was done by sampling 10 plants per plot excluding border plants. All the pods were harvested from each plant and counted then shelled for yield analysis. Above ground biomass was determined by cutting each of the 10 plants per plot then weighing after air drying.
The harvest index (HI) was computed using Equation 6.5 (Cisse, 2001).
𝐻𝑎𝑟𝑣𝑒𝑠𝑡 𝑖𝑛𝑑𝑒𝑥 =𝐺𝑟𝑎𝑖𝑛 𝑦𝑖𝑒𝑙𝑑 (𝑘𝑔 ℎ𝑎−1
𝐵𝑖𝑜𝑚𝑎𝑠𝑠 (𝑘𝑔 ℎ𝑎−1) × 100% (6.5)
The reported data was analysed using ANOVA with the help of GenStat® version 18 (VSN International, Hemel Hempstead, UK). Separation of means of significant variables were done using Least Significant Differences (LSD) at 5% significance level. Correlation analysis was carried out on growth and yield components to determine the relationship between variables.