Response of six vegetable crops
to irrigation schedules
M. Imtiyaz
*, N.P. Mgadla, B. Chepete,
S.K. Manase
Department of Agricultural Research, Regional Agricultural Research Of®ce, P.O. Box 151, Maun, Botswana
Accepted 22 October 1999
Abstract
Field studies were carried out for 2 years (1995 and 1996) on sandy soil in the north western region of Botswana to examine the effect of irrigation schedules (18 mm of water in each irrigation at 11, 22, 33, 44 and 55 mm of cumulative pan evaporation, CPE) on marketable yield, irrigation production ef®ciency and economic return of cabbage, spinach, rape, carrot, tomato and onion under sprinkler irrigation. The higher mean marketable yield of cabbage (71.65 t/ha), spinach (33.53 t/ha), rape (73.22 t/ha), carrot (56.76 and 38.39 t/ha), tomato (46.81 t/ha) and onion (56.05 t/ha) were recorded for irrigation scheduled at CPE of 22, 11, 22, 22, 22 and 11 mm, respectively. The irrigation at CPE of 22 mm resulted in higher irrigation production ef®ciency of cabbage
(11.32 kg/m3), spinach (3.35 kg/m3), carrot (9.83 and 6.66 kg/m3), tomato (5.90 kg/m3) and onion
(6.26 kg/m3), but rape (12.03 kg/m3) gave higher irrigation production ef®ciency at CPE of 33±
55 mm. Irrigation scheduled at 22 mm CPE resulted in a higher net return of 47 131, 104 398, 76 691 and 93 192 P/ha for cabbage, rape, carrot and tomato, respectively, but spinach and onion
gave a higher net return of 27 086 and 83 934 P/ha at 11 mm CPE(1 US$4.55 P). Irrigation
scheduled at 22 mm CPE gave a higherB/Cratio of 2.92, 1.94, 5.40, 4.98, 4.91 and 4.82 for
cabbage, spinach, rape, carrot, tomato and onion, respectively. Seasonal water applied and
marketable yield of vegetable crops exhibited quadratic relationships (R20.85±0.99). The ®tted
regression models attained the maximum yield, net return andB/Cratio at CPE of 16±18 mm. The
results revealed that rape is the most remunerative crop followed by tomato, onion, carrot, cabbage
and spinach.#2000 Elsevier Science B.V. All rights reserved.
Keywords:Sprinkler irrigation; Irrigation scheduling; Marketable yield; Net return; Vegetable crops
*Corresponding author. Tel.:267-660327; fax:267-663761.
1. Introduction
Botswana is a semiarid country with limited water resources. Water is the major limiting factor for crop production in most of the agricultural regions of Botswana. Water for irrigation, domestic and industrial needs is increasing considerably to meet the demands for a growing population. Presently, 80% of vegetables are imported from neighbouring countries. However, the government of Botswana through its financial assistance programme is trying to encourage farmers to grow vegetable and fruit crops in order to reduce imports. Therefore, it is necessary to develop efficient, reliable and economically viable irrigation management strategies for effective use of the existing limited water resources. Improper irrigation management practices do not only waste scarce and expensive water resources but also decrease marketable yield and economic return.
The irrigation scheduling which determines the amount and frequency of irrigation is governed by many complex factors, but climate plays a major role. Therefore, it is important to develop irrigation scheduling techniques under prevailing climatic condition. Numerous studies were carried out in the past on the development and evaluation of irrigation scheduling techniques under a wide range of irrigation system and manage-ment, soil, climate and crop conditions (Hagan and Laborde, 1964; Jensen et al., 1970; Imtiyaz and Shiromani, 1990; Wanjura et al., 1990; Imtiyaz et al., 1992; Steele et al., 1997). The meteorological-based irrigation scheduling approach, such as pan evaporation replenishment, cumulative pan evaporation (CPE), and ratio between irrigation water and cumulative pan evaporation etc., was used by many researchers due to its simplicity, data availability and higher degree of adaptability at the farmers level (Prihar et al., 1974; Singh, 1987; Pawar et al., 1991; Singh and Mohan, 1994; Imtiyaz et al., 1995, 1999; Singh et al., 1997). In Botswana, evaporation from USWB Class A open pan is being systematically recorded and readily available for irrigation scheduling.
Cabbage, spinach, rape, carrot, tomato and onion are the most important vegetable crops in Botswana. Due to lack of proper irrigation scheduling techniques, the average yield of these vegetable crops is low because of excess or deficit soil moisture regimes. The quick-coupling sprinkler irrigation is the most common method of irrigation for vegetable crops in Botswana, but information on economic viability of this system is lacking. Therefore, the objectives of the present study were to examine the effect of irrigation schedules on marketable yield, irrigation production efficiency and economic return of vegetable crops.
2. Materials and methods
The field experiments were conducted during the winter/summer crop growing season of 1995 and 1996 at the Etsha 6 experimental station of the Department of Agricultural Research, Botswana (198370S, 22
8170E, 964 m above MSL). The mean monthly maximum air temperature, minimum air temperature, relative humidity, wind velocity and pan evaporation during the crop growing season (June to November) ranged from 22.7 to 36.88C, 4.8±228C, 35.3±75.2%, 1.0±1.9 m/s and 3.4±11.3 mm/day,
respectively. The rainfall during the month of October and November in 1995 was 8.7 and 42.8 mm and in 1996 was 1.6 and 40.0 mm, respectively. According to rainfall and potential evapotranspiration, the climate in this part of the country has been classified as semiarid with mild winters and hot summers. The soil in the experimental field was sandy with low organic matter content. The soil moisture content at field capacity (ÿ0.03 MPa) and wilting point (ÿ1.5 MPa) was 0.1058 and 0.0345 m3
/m3, respectively. The average bulk density of soil was 1.41 g/cm3. The plant available soil moisture was 72 mm/m.
The experiment was laid out in a randomised block design with three replicates. The experimental plots were sub-divided for six vegetable crops. The size of plots for cabbage, spinach, rape, carrot, tomato and onion were 45, 45, 45, 25, 45 and 25 m2, respectively. A buffer zone spacing of 2.0 m was provided between the crops. Prior to planting, the experimental field for vegetable crops received 94.3 kg/ha phosphorus (P2O5) and 62.9 kg/ha potash (K2O). The tomato and carrot received an additional amount of 62.5 kg/ha K2O. Before planting, the experimental field for cabbage, spinach, and rape received 50 kg/ha nitrogen, whereas carrot, tomato and onion received 63, 75 and 42 kg/ha nitrogen, respectively. Spinach (Var. Fordhook Giant) and carrot (Var. Brazillia) were sown on the 30th June in both 1995 and 1996 with plant and row spacings of 0.15 m0.3 m and 0.05 m0.4 m, respectively. Cabbage (Var. Grandslam), rape (Var. Giant Essex), tomato (Var. Sixpack) and onion (Var. Texas Grano 502 PRR) seedlings were transplanted from 11th to 14th July in 1995 and 1996 with plant and row spacings of 0.5 m0.5 m, 0.3 m0.4 m, 0.3 m0.75 m and 0.1 m0.3 m, respec-tively. The experimental plots for cabbage, rape and spinach received 50 kg/ha of nitrogen after 3, 5 and 8 weeks of transplanting. The experimental plots for tomato and onion received 37.5 and 42 kg/ha of nitrogen after 4 and 8 weeks of transplanting, but carrot received 21 kg/ha of nitrogen after 8 weeks. Spinach and carrot emerged during the 7th to 9th of July and thinning was done after 3 weeks to maintain desired plant population. During the first 3 weeks, the crops were irrigated daily at 75% of pan evaporation losses in order to establish newly planted seedlings.
The experiment consisted of five treatments formed by irrigation schedules at CPE (cumulative pan evaporation) of 11, 22, 33, 44 and 55 mm during the entire crop growth period with the depth of 18 mm at each irrigation. The above-mentioned CPE values were selected in order to create excess and deficit soil moisture regimes as well as match the frequency of irrigation, i.e. days. The cumulative pan evaporation was calculated as a sum of daily 7-years recorded evaporation from USWB Class A open pan. The pan is located at the climate station adjacent to the irrigation experimental field with moderate grass cover. The irrigation in the respective treatments were applied when CPE reached approximately 11, 22, 33, 44 and 55 mm. The irrigation during the crop growing season was applied by quick-coupling sprinkler irrigation system.
Cabbage, spinach, rape, tomato, carrot and onion were harvested from 4th to 15th October, 13th September to 2nd November, 31st August to 2nd November, 1st October to 16th November, 2nd to 4th October and 14th to 16th November in both 1995 and 1996. For economic analysis, both fixed and operating cost were included. The total production cost, gross revenue and net return under different irrigation schedules were estimated on the following assumptions:
Salvage value of the components0
Useful life of engine, pump, pump house and irrigation system12 years Interest rate14%
Repair and maintenance5% No. of crops/year1
The fixed cost including engine, pump, pump house and quick-coupling sprinkler irrigation system was calculated. The annual fixed cost was calculated by the following approach (James and Lee, 1971):
CRF i 1i
n
1inÿ1
Where CRF is the cost recovery factor;iis the interest rate (fraction) andnis the useful life of the component (years).
Annual fixed cost/haCRFfixed cost/ha
The operating cost, including labour, land preparation, seeds, fertilizers, chemicals, diesel and oil, and repair and maintenance, was calculated. The total cost of production under different irrigation schedules was estimated by adding fixed and operating cost. The gross revenue for different irrigation schedules was estimated taking into account the marketable yield and market price of cabbage, spinach, rape, carrot, tomato and onion. Subsequently, the net return under different irrigation schedules was calculated considering total cost of production and gross revenue. The benefit cost ratio (B/C) under different irrigation schedules was calculated as follows:
B
C
Gross revenue P=ha Total cost of production P=ha
3. Results and discussion
3.1. Yield and irrigation production ef®ciency
In both years, maximum heads/m2, head weight and marketable yield of cabbage were recorded when irrigation was applied at cumulative pan evaporation (CPE) of 22 mm (Table 1). Further increase in irrigation level resulting from CPE of 11 mm did not increase the marketable yield significantly. Irrigation at CPE of 33, 44 and 55 mm reduced the marketable yield by reducing heads/m2and head weight. The results revealed that the head weight (28±66%) was more affected by irrigation levels compared to heads/ m2 (12±42%). Irrigation at CPE of 22 mm resulted in higher irrigation production efficiency. A further increase or decrease in irrigation levels resulting from CPE of 11, 33, 44 and 55 mm reduced the irrigation production efficiency significantly.
In both years, irrigation at CPE of 11 mm produced a significantly higher marketable yield of spinach (Table 2). Marketable yield decreased significantly with decrease in irrigation levels resulting from CPE of 22, 33, 44 and 55 mm. However, irrigation at CPE of 22 mm gave a higher irrigation production efficiency because yield reduction (20.8%)
was less than the seasonal water application (45.3%). The difference in irrigation production efficiency between CPE of 22 and 33 mm was statistically similar. Irrigation production efficiency decreased severely (41±64%) with irrigation at CPE of 44 and 55 mm due to severe reduction in marketable yield.
In 1995, maximum marketable rape yield was obtained with irrigation at CPE of 11± 33 mm, but in 1996 irrigation at CPE of 33 mm resulted in a significantly lower yield compared to CPE of 11 and 22 mm, probably due to variation in climatic conditions (Table 3). In both years, marketable yield decreased significantly with decrease in irrigation levels resulting from CPE of 44 and 55 mm. This is due to the fact that irrigation at CPE of 44 and 55 mm reduced the plant growth as well as increased the non-marketable yield. In both 1995 and 1996 seasons, irrigation at CPE of 33 to 55 mm gave a significantly higher irrigation production efficiency because yield reduction was less compared to seasonal water application. Irrigation at CPE of 11 mm resulted in minimum irrigation production efficiency because it increased the seasonal water application considerably (74.4%) without any significant improvement in marketable yield.
In both years, irrigation at CPE of 11 and 22 mm produced higher total and root yield of carrot (Table 4). Marketable yield decreased significantly with decrease in irrigation Table 1
Effect of irrigation schedules on yield, yield components and irrigation production ef®ciency of cabbage in 1995 and 1996
11 1115 1140 73.34 74.26 3.75 3.79 1.96 1.97 6.58 6.51 22 629 636 70.91 72.39 3.71 3.74 1.91 1.94 11.26 11.38 33 467 474 45.51 45.23 3.27 3.28 1.39 1.38 9.75 9.54 44 377 384 26.02 24.69 2.65 2.55 0.98 0.97 6.90 6.43 55 323 330 14.71 12.97 2.21 2.09 0.67 0.63 4.55 3.93 LSD(P0.05) ± ± 3.04 7.05 0.21 0.22 0.06 0.14 0.39 1.22
Table 2
Effect of irrigation schedules on yield and irrigation production ef®ciency of spinach in 1995 and 1996
Treatment (irrigation
11 1421 1482 33.25 33.81 2.34 2.28
22 773 816 27.42 25.72 3.55 3.15
33 557 600 19.79 16.79 3.55 2.80
44 449 474 9.22 9.02 2.05 1.90
55 395 402 4.98 4.68 1.26 1.16
LSD(P0.05) ± ± 2.09 2.77 0.30 0.43
levels resulting from CPE of 33, 44 and 55 mm. This is due to the fact that irrigation at CPE of 33, 44 and 55 mm reduced the plant growth as well as increased the non-marketable yield. Maximum irrigation production efficiency was obtained at CPE of 22 mm and it decreased significantly with a decrease in irrigation levels. Irrigation at CPE of 11 mm reduced irrigation production efficiency considerably (44%) because it increased the seasonal water application by 43% without significant improvement in marketable yield.
In both 1995 and 1996 seasons, irrigation at CPE of 11 and 22 mm produced higher fruit weight and marketable yield of tomato (Table 5). Marketable yield decreased significantly with a decrease in irrigation levels resulting from CPE of 33, 44 and 55 mm due to reduction in fruit weight and marketable number of fruits. In both years, higher irrigation production efficiency was obtained with irrigation at CPE of 22 and 33 mm, thereafter it decreased significantly, because yield reduction was higher than seasonal water application. Irrigation at CPE of 11 mm in which seasonal water application was maximum, reduced the irrigation production efficiency considerably (51%) because it increased the seasonal water application without significant improvement in yield. Table 3
Effect of irrigation schedules on yield and irrigation production ef®ciency of rape in 1995 and 1996
Treatment (irrigation
11 1421 1482 72.93 73.24 5.13 4.94
22 773 816 72.12 74.31 9.33 9.11
33 557 600 68.11 65.61 12.23 10.94
44 449 474 57.90 55.43 12.90 11.69
55 395 402 50.99 46.27 12.91 11.51
LSD(P0.05) ± ± 5.19 6.45 1.03 1.07
Table 4
Effect of irrigation schedules on yield and irrigation production ef®ciency of carrot in 1995 and 1996
Treatment (irrigation
95 96 Total Root Total Root
95 96 95 96 95 96 95 96
11 986 1032 57.01 56.21 38.23 37.12 5.77 5.45 3.86 3.60 22 557 600 56.31 57.21 38.09 38.68 10.11 9.54 6.86 6.45 33 413 438 36.73 37.72 24.49 24.99 8.89 8.61 5.93 5.71 44 341 366 15.45 15.54 10.22 10.31 4.53 4.24 2.99 2.82
55 305 312 5.77 6.62 3.86 4.44 1.89 2.12 1.27 1.42
LSD(P0.05) ± ± 3.96 3.90 2.32 2.71 0.82 0.78 0.47 0.49
In 1995 maximum bulb yield of onion was recorded with irrigation at CPE of 11 mm, but in 1996 yield difference between CPE of 11 and 22 mm was statistically similar (Table 6). Marketable bulb yield decreased significantly with a decrease in irrigation levels resulting from CPE of 33 to 55 mm due to a reduction in bulb weight. In 1995, irrigation at CPE of 33 mm resulted in higher irrigation production efficiency, but in 1996 the difference between 22 and 33 mm of CPE was non-significant. The results clearly indicated that irrigation above 33 mm of CPE reduced the irrigation production efficiency significantly due to higher yield reduction compared to seasonal water application. Furthermore, irrigation at CPE of 11 mm, in which seasonal irrigation was maximum, resulted in minimum irrigation production efficiency because increase in yield was considerably less than the seasonal water application.
Despite of some variation, the overall results showed that a fixed amount of 18 mm of irrigation (IW) application at cumulative pan evaporation (CPE) of 22 mm resulted in higher marketable yield and irrigation production efficiency of vegetable crops (Tables 1±6). Imtiyaz et al. (1999) reported similar results for broccoli, cabbage, carrot and rape under drip irrigation. Singh and Mohan (1994) reported a reduction in sugarcane yield when irrigation was applied beyond a IW/CPE ratio of 1.0. Singh et al. (1997) Table 5
Effect of irrigation schedules on yield, fruit weight and irrigation production ef®ciency of tomato in 1995 and 1996
11 1421 1482 44.58 42.90 82.15 80.67 3.14 2.89
22 773 816 47.69 45.92 82.11 83.02 6.17 5.63
33 557 600 36.47 38.25 64.95 67.17 6.55 6.38
44 449 474 18.16 18.20 54.74 51.49 4.04 3.84
55 395 402 12.55 10.93 47.03 45.49 3.18 2.72
LSD (P0.05) ± ± 2.94 4.00 3.58 5.24 0.52 0.55
Table 6
Effect of irrigation schedules on bulbs yield and irrigation production ef®ciency of onion in 1995 and 1996
Treatment (irrigation
11 1421 1482 57.16 54.94 4.02 3.71
22 773 816 49.96 49.31 6.46 6.05
33 557 600 40.80 37.97 7.33 6.33
44 449 474 25.15 24.60 5.60 5.19
55 395 402 20.59 18.31 5.21 4.55
LSD(P0.05) ± ± 3.49 6.01 0.58 0.86
observed that the reduction in plant growth and yield of palmarosa at higher irrigation levels resulted from IW/CPE ratios of 1.1 to 1.5.
3.2. Water supply and marketable yield
The relationship between seasonal water applied and marketable yield of cabbage, spinach, rape, carrot, tomato and onion are shown in Fig. 1. In both years, seasonal water applied and marketable yield of vegetable crops exhibited quadratic relationships (R20.85±0.99). Cabbage, spinach, rape, carrot, tomato and onion attained maximum marketable yields of 86.33, 35.81, 80.70, 70.55 (total) and 47.40 (root), 57.18 and 61.82 t/ha at the seasonal water application of 890, 1248, 1103, 785, 1083 and 1190 mm, respectively, and thereafter it tended to decline (Fig. 1). Further analysis of regression models revealed that the above-mentioned maximum yield for different crops correspond to CPE of 16±18 mm. The quadratic water applied yield relationship results were probably due to poor aeration and nutrient-leaching caused by excessive soil moisture. Imtiyaz et al. (1994, 1995, 1996, 1999) reported the quadratic relationships between seasonal water applied and marketable yield of cabbage, carrot, onion, tomato, green pepper, broccoli, rape and okra under sprinkler and drip irrigation. Many researchers reported quadratic crop water production functions for vegetable and field crops under a wide variety of irrigation systems and regimes, soil and climatic conditions (Musick et al., 1976; Singh, 1987; Stone et al., 1996; Farah et al., 1997; Howell et al., 1997; Tiwari and Reddy, 1997).
3.3. Economic return
The total production cost, net return and benefit cost ratio (B/C) for different crops under different irrigation schedules are presented in Table 7. The total cost of production increased with increase in irrigation levels. The total cost of production varied amongst the crops mainly due to difference in crop growing period and labour requirements for harvesting and packaging etc. Labour costs in performing major farm activities contributed 40±64% to total cost of production depending upon the crops and irrigation levels. The fixed cost, repair and maintenance, and pumping cost contributed 8±27.5%, 3.1±7.7% and 6.2±12.9%, respectively, to total cost of production. Cabbage, rape, carrot and tomato gave the higher net return at CPE of 22, whereas spinach and onion resulted in higher net return at CPE of 11 mm. Irrigation at CPE of 55 mm resulted in considerable economic loss from cabbage, carrot and spinach because total cost of production exceeded the gross revenue. The benefit cost ratio (B/C), which indicates gross revenue per unit investment, was also influenced by irrigation schedules. Irrigation at CPE of 22 mm gave higherB/Cratio for cabbage, spinach, rape, carrot, tomato and onion (Table 7). However, according to the fitted regression models (yield versus water applied/irrigation schedules), all vegetable crops attained the maximum gross revenue, net return andB/Cratio at CPE of 16±18 mm. The overall results clearly show that rape is a more remunerative crop followed by tomato, onion, carrot, cabbage and spinach. Imtiyaz et al. (1999) reported similar results for cabbage, broccoli, rape and carrot under drip irrigation.
Fig. 1. Relationship between water applied and marketable yield of cabbage (a), spinach (b), rape (c), carrot (d), tomato (e) and onion (f).
Table 7
Economic analysis of sprinkler-irrigated vegetable crops under different irrigation schedules (average data of 2 years)a
Treatment (irrigation at CPE, mm)
Total production cost (P/ha) Net return (P/ha) B/Cratio
Cabbage Spinach Rape Carrot Tomato Onion Cabbage Spinach Rape Carrot Tomato Onion Cabbage Spinach Rape Carrot Tom Onion
11 31678 31592 31292 26268 31371 28166 42122 27086 96607 67920 77979 83934 2.33 1.86 4.09 3.59 3.49 3.98 22 24519 24028 23728 19272 23808 20602 47131 22470 104398 76691 93192 78668 2.92 1.94 5.40 4.98 4.91 4.82 33 22140 21514 21214 16914 21293 18088 23230 10494 95791 44936 53427 60682 2.05 1.49 5.52 3.66 3.51 4.36 44 20929 20235 19935 15749 20015 16810 5426 ÿ4275 79229 9914 25435 32940 1.26 0.79 4.97 1.63 2.27 2.96 55 20201 19486 19186 15044 19265 16060 ÿ6361 ÿ11033 65917ÿ4669 10085 22840 0.69 0.43 4.44 0.69 1.52 2.42
aThe prices of cabbage, spinach, rape, carrot, tomato and onion are taken at 1.0, 1.75,1.75, 2.50, 2.50 and 2.0 P/kg, respectively, 1 US$4.55 P (Botswana Pula).
M.
Imtiyaz
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45
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4. Conclusion
Evaporation from USWB Class A open pan is the most common and simple approach for scheduling irrigation for field, vegetable and fruit crops. Field extension personnel can prepare an irrigation scheduling calender taking into account the long-term daily pan evaporation data that can be easily adopted by the farmers. The experimental results clearly indicated that a fixed depth of 18 mm of irrigation application at cumulative pan evaporation of 16±18 mm (June to November) is optimum for sandy soil in order to achieve maximum marketable yield, net return and benefit cost ratio from sprinkler-irrigated cabbage, spinach, rape, carrot, tomato and onion under semiarid climate of north western region of Botswana.
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