Grain sorghum response to supplementary irrigations under post-rainy season conditions

Agricultural water management ELSEVIER

Agricultural

Water Management

33 (1997) 3 l-41

Grain sorghum response to supplementary irrigations under post-rainy season conditions S.M. Farah *, A.A. Salih, A.M. Taha, Z.I. Ali, I.A. Ali Agriculiurul

Research Corporation, Accepted

P.O. Box 126, Wud-Me&k,

29 August

S&n

1996

Abstract Irrigated sorghum (Sorghum bicolor L.) in the Sudan is grown during the rainy season with supplementary irrigation @I). A field study was conducted for 2 years (1993 and 1994) at the Gezira Research Station, Sudan, to evaluate effects of SI under the post-rainy season conditions on grain and forage yields, yield components, water use and its economics using three promising varieties. Addition of one SI in the first year (FY) and two SIs in the second year (SY) resulted in significant grain yield increases as compared with no SI and one SI. Whereas two SIs in the FY produced a non-significant increase, three SIs in the SY tended to decrease grain yields. Grain yield variations were positively correlated with the number of seedsmm2 and lOOO-grain weight. For forage, however, varieties differed significantly only in the FY. Addition of SI linearly increased forage yields and the differences were significant in the SY. Irrigation production efficiency (IRE) for grain yield generally decreased with addition of SI. No appreciable differences in grain yield were found between varieties or their interactions with SI in both seasons. Economic evaluation revealed the profitability of adding two rather than three SIs, whereby the investment in the second SI gave a 192-fold benefit. Keywords:

Sorghum;

Supplementary

irrigation;

Irrigation

production

effkiency;

Water economics

1. Introduction Grain sorghum (Sorghum bicolor L.) is the most important cereal in the Sudan. The area which is annually cultivated is about 2.1 million ha, of which 80% is completely rain-fed, whereas the remainder is given supplementary irrigation throughout the grow-

’ Corresponding Japan.

author, at: Arid Land Research

Center, Tottori University,

0378-3774/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SO378-3774(96)01283-S

1390 Hamasaka,

Tottori 680,

32

SM. Fnrah et ul./Apkultrmxl

Water Munugement

33 (1997) 31-41

ing season. Irrigation water is supplied by gravity from river flows or with lift irrigation from the rivers and ground water using pumps with diesel engines. The yields of the rain-fed subsector in the Sudan have progressively declined with time, owing to changes in both quantity and distribution of the rainfall (El-Karouri, 1986; Olsson and Rapp, 1991). This has necessitated the allocation of a greater area to sorghum in the irrigated subsector. However, the limited quantity of water available and the cost of its pumping make it mandatory that irrigation water be used efficiently. If satisfactory yields can be obtained with less water, irrigation production efficiency (IPE) will be increased. Levine and Bailey (1987) defined IPE as the ratio of crop production to water supplied, and stated that it can be measured at any level in the irrigation system. The goal in IPE would be to maximize physical output per unit of water. Eck and Musick (1979a) stated that sorghum, being both drought tolerant and highly responsive to added water, can be adapted to both dryland and irrigated conditions. The effects of moisture stress on grain sorghum in decreasing growth and yield are not only due to the magnitude of the stress, but also due to the stage of growth at which it is imposed (Salter and Goode, 1967). Lewis et al. (1974) tested the susceptibility of sorghum to moisture stress once at each of three growth stages: late vegetative to boot stage, the boot to bloom stage and the milk to soft dough stage. They found that grain yields were reduced by 17%, 34% and lo%, respectively. Eck and Musick (1979a) reported that stresses equivalent to a leaf water potential (LWP) of - 22.9 bars, beginning at boot stage for a period of 35days, reduced yields 43%, and LWP of - 21.7 bars for 27days, beginning at early boot or heading stage, reduced yields 27%, whereas a LWP of - 22.7 for the same period, beginning at early grain filling, reduced yields only 12%. The reductions in yield owing to stress initiated at early boot stage resulted from both reduced seed number and reduced seed weight, but when stress was imposed at heading or later, only seed weight was reduced. Testing effects of moisture stress on forage yield of sorghum at three growth stages-establishment O-30DAS (days after sowing), vegetative lag 37-55 DAS and flowering stage 56-75 DAS-it was found that the stage of vegetative lag was the most critical in its demand for assured water supply, whereas the other two stages could tolerate mild stress (Hukkari and Shukla, 1983). Irrigated grain sorghum in the Sudan is cultivated during the rainy season from June to October or November with supplementary irrigation @I). Nevertheless, although physiological maturity is reached around mid-October, farmers continue adding SI to the crop, anticipating that extra irrigations would result in higher yield. Such practice has very serious implications upon availability and cost of the water, which is left over for planting wheat in November and for cotton, which attains its maximum water demands during October-November. Garrity et al. (1982a) stated that in arid and semi-arid regions water is becoming scarce and expensive, and emphasized the point that where water is a limiting resource the objective of irrigation may shift from obtaining maximum yield to obtaining maximum economic production per unit of supplied water. Effects of irrigation during the post-rainy season must be known, but this information is not available for sorghum in the Sudan. The objective of this study was to evaluate the effects of SI under the post-rainy season conditions on yields of grain and forage, yield components, IPE and its economics using three sorghum varieties.

SM.

2. Materials

Farah

et al./Agricultural

Water Munagrment

33 (1997) 31-41

33

and methods

2.1. Experimental

site

Experiments were carried out at the Gezira Research Station Farm (GRSF), WadMedani, Sudan, in a cotton-wheat rotation during 1993 (FY) and 1994 (SY) seasons. Soil type at the site is a deep, heavy, fine montmorillonitic soil with .58-66% clay, 300-400 ppm nitrogen, 2-4 ppm available phosphorus, 0.5% organic matter, water infiltration rate of 1 mm h- ’ and pH 8.5. The climate is semi-arid, with mean rainfall of 250-350 mm year- ’, occurring mainly between May and September. Most experimental procedures and designs were identical in the two seasons unless stated otherwise. Experiments were laid out in a split-plot design in four replications. Main plots consisted of irrigation treatments. Sub-plots consisted of two varieties, ‘Wad Ahmed’ (WA) and ‘Ingaz’ (IN) in FY, and a third variety, ‘Tabat’ (TA), was added in SY. Sub-plots were 8 m rows of 8-9 m length with 0.8 m between rows. Each sub-plot was surrounded by an earth boundary 25 cm high, and sub-plots were separated by paths of 2m width to eliminate effects of lateral movement of water between the different irrigation treatments. 2.2. Meteorological

data

Rainfall, and maximum and minimum temperatures were collected from the meteorological station at GRSF about 300m away from the experimental plots. 2.3. Crop management Planting in both seasons was carried out in the third week of July by placing 4-5 seeds per hill and thinning the stand at 3-4 leaf stage to three seedlings per hill. A basal fertilizer of 86 kg N ha-l, as urea, and 43 kgP,O, ha-‘, as triple superphosphate, was applied uniformly at sowing and the initial irrigation, including rainfall, was 1326 m3 ha- ’ for all treatments. Supplementary irrigation (SI) was scheduled so as not to allow soil moisture depletion in the root zone (O-75cm) to exceed about 50% of the available water. Measurements of the soil moisture were made by means of a neutron probe through access tubes installed at the centre of the sub-plots to a depth of 1.2 m. In the FY, the first and second SI took place at the end of September and in mid-October, respectively. The volume of added water was about 850m3 ha-‘, which is the volume of water farmers usually apply per irrigation during the post-rainy season. In the SY, SI started about mid-September and ended by mid-October. The treatments which received two, one and no SI in the FY, or three, two and one SI in the SY are designated A, B, and C, respectively. 2.4. Harvest and yield measurements In both seasons grain and above-ground dry matter were determined in the six central rows of each plot, leaving 1.5-2.0m from either side as margin. The heads of plants

34

SM. Faruh

et al./Agriculturul

Water Munupment

33 (1997) 31-41

were cut at the tip of the peduncles and the remainder was harvested at ground level and then air dried. Grain yield was determined after threshing the heads. Forage yield was obtained by the weight of the rest of the plants. Total dry matter (TDM) was assessed from the head and the forage yields. The lOOO-seed weight was determined as the mean of three samples taken from each sub-plot. Number of seeds per square metre was calculated from grain yield, number of plants per unit area and the weight of 1000 seeds. Irrigation production efficiency (IPE) was calculated as the ratios of grain and TDM yields to water supplied (irrigation + rainfall). 2.5. Economic

evaluation

Economic evaluation was made using the partial budget and marginal analysis, at farm-gate prices. Net benefits were derived as the differences between gross benefits and variable costs of the volume of supplied water, which was related to the current water charges of the Ministry of Irrigation. Marginal rate of return (MRR) was calculated as a percentage, and reflects the relationship between variable costs and net benefits between two treatments. It equals the marginal net benefits divided by the marginal cost. The costs of the other inputs, which were the same for the different treatments, were not included in the analysis, as the objective was to test profitability of SI additions and not of sorghum production.

3. Results and discussion 3.1. Weather data The weather data (rainfall and temperature) for the two growing seasons are presented in Fig. 1. The FY was hotter during the growing season July-October and its rainfall lower, during July-August, than for the SY. There were 179.2mm and 199.4mm of precipitation from planting to about mid-September in the FY and SY seasons, respectively. The first SI in the SY commenced by mid-September whereas the crops of the FY did not require addition of water because of the higher precipitation in September and the smaller number of plants per unit area (Fig. 1, Table 1). The presence of fewer plants reduces the canopy area intercepting net radiation and therefore allows a greater fraction of this energy to be directed toward the soil. The net radiation received by the soil surface does not promote evaporation from the soil although its conversion to sensible heat might promote some canopy transpiration (Blum, 1972). Eck and Musick (1979a) also emphasized the importance of reducing plant population for sorghum when water stress is anticipated, as close plant spacing accelerates stress severity. 3.2. Yields Grain and forage yields, seed weights and numbers of seed and plants per square metre are presented in Table 1. Grain yields of the FY were generally lower than those

S.M. Farah et al./Agricultural

Water Management

33 (1997) 31-41

1993

May

June

July

Aug. Sept.

---O---

Fig.

I. Meteorological

35

I

Oct.

1993 min

data for 1993 (FY) and 1994 (SY) growing

seasons: (A) rainfall; (B) temperature.

of the SY, particularly in A and B treatments. The reductions may be attributed to the relatively higher temperatures of the FY (above 37”C), which exceeded those of the SY by more than 2°C as well as the lower rainfall of July (only 9mm), which might have subjected the crops of the FY to a greater moisture stress during the early growth period. Weather factors such as temperature and rain are important in determining yield performance of sorghum. Mohamed and Francis (1984), from studying the relative contribution of several weather variables during various growth stages of sorghum genotypes, found that variations in temperature and rainfall accounted for more than half of the environment and genotype-environment interaction sums of squares for yield, seed number and seed weight components of yield. Farah (1981) attributed the differences in grain yields of Vicia fuba, during two seasons, to variations in environmental conditions. The yields of the drier and warmer season as compared with those of the wetter and cooler season were nearly half under the wet irrigation regime (54%) and less than half under the medium and dry treatments (42%). Impediment of growth as a result of moisture stress on morphological, physiological and biochemical processes, as well as uptake and movement of nutrients to various parts of the plants, has been well documented (Marschner, 1986; Kramer and Boyer, 1995). In sorghum, Eck and Musick (1979a,b) found that plant water stress reduced dry matter and nutrient accumulation, particularly for nitrogen and phosphorus. The crops of the FT were also low in the number of plants per unit area, on average by 29.2%, as compared with those of the SY. Yield increases as a result of greater plant population densities were also reported for Vicia fuba (Ishag, 1973).

36

SM. Faruh

Table I Yield and yield components and 1994 (SY) seasons Treatment Irrigation

A

B

Variety

WA IN TA Mean WA IN

Mean

Mean WA IN TA Mean WA IN TA

Water Mumgement

33 (1997) 31-41

of grain sorghum as affected by variety and supplementary

Grain yield (kg ha- ’)

Forage yield (kgha-‘)X 1000

Seedsm’ (X 1000)

FY

SK

FY

SY

FY

4697 3898 4375 4317 a 4661 437s 4653 4563 a 2813 2795 2344 2650 b 4051 ’ 3689 ’ 3791 ’

10.36 9.02 _

15.88 Il.51 I I.90 13.10 a

13.6 14.6

3379 3295 3337 a 3066 3188

TA

C

et ul./Apkulturul

3092 a 2942 2915 2929 b 3129 ’

3110’

9.69 a 10.23 8.99 9.61 a IO.18 8.94 9.56 a 10.26 ’ 8.98 ’

12.05 11.15 10.12 Il.11 b 7.29 6.74 6.95 6.99 ’ 11.74’ 9.80 ’ 9.66 ’

_ 14.1 a 12.8 13.2 13.0 ab 12.2 12.9 _ 12.5 b 12.8 ’ 13.6 ’

irrigation

in 1993 (FY)

Wt. per 1000 seeds (g)

No. of plants (m -‘)

SY

FY

SY

FY

SY

21.1

24.9 23.8

19.2 19.5 = 21.7 19.4

24.4 a 23.9 23.7

22.1 21.7 22.8 22.2 a 21.6 22.6 22.7 22.3 a 17.0 16.1 18.4 17.2 b 20.2 ’ 20.1 ’ 21.3 ’

9.0 7.8 x.4 a 9.0 7.7 8.3 a 8.8 6.6 7.7 B 8.9 ’ 7.41

13.4

18.0

12.3 IO.1 IO.1 10.8 b 12.8 ’ 10.7’

-

11.0z

20.5 20.6 a 16.6 17.4 13.0 15.7 b 19.8 ’

18.3’ ’

17.6

23.8 a 24.0 22.7 23.4 b 24.3 ’ 23.1 ?

I I.1 12.1 12.2 il 12.8 10.9 10.8

I I.5 ab

WA, Variety ‘Wad Ahmed’; IN, variety ‘Ingaz’; TA, variety ‘Tabat’. A, Application of two irrigations in FY and three irrigations in SY; B, application of one irrigation in FY and two irrigations in SY; C, no irrigation in FY and one irrigation in SY. Means in the same column with different letters (irrigation) or numbers (variety) are significantly different at the 5% level according to Tukey-Kramer HSD.

The differences in grain yields of the three varieties were not statistically significant in both seasons. However, variety WA appeared to have a greater yield than the other two varieties under all irrigation treatments, mainly as a result of its superiority in number of plants and seeds per square metre (Table 1). On the other hand, effect of SI was significant in both seasons: one SI in the FY or two in the SY significantly increased the grain yield as compared with no SI or one SI in the two seasons, respectively. However, whereas addition of the second SI in the FY resulted in a non-significant increase in grain yield, the third SI had a tendency to decrease the yield in the SY. This decrease in grain yields with higher supplies of water were also noted by Garrity et al. (1982b), who found that maximum grain yields occurred when 80% of the soil water deficit was replaced, rather than 100%. However, they could not offer a convincing reason for the yield decrease under full irrigation. In our studies, the application of extra SI resulted in progressively increasing the forage yields by 37% and 46% as compared with only one SI in the SY season, mainly as a result of the production of more plants (tillers) per unit area. It is highly probable that the newly initiated shoots during the grain filling period have affected the shift in the source-sink relationship by competing with the heads for the available assimilates and hence there was no appreciable increase in the seed weights. In contrast, a decrease in number of seeds per square metre was found (Table 1). It is also possible that some of the seeds,

SM. Farah et al./Agricultural

Water Management

33 (1997) 31-41

37

which were formed during the seed filling stage, being not quite filled and thus light seeds, were lost in the threshing process (Eck and Musick, 1979a). The differences in the grain yields as a result of SI were due to differences in seed numbers and seed weight (Table 1). Differences in seed numbers were apparent between treatments with no SI and those with one SI in the FY or between those with one and two SIs in the SY. Additional SI resulted in a very minor increase in the FY and a tendency to reduction in the SY. Regarding varieties, there was no significant variation between the three varieties in the number of seeds per square metre in both seasons. The seed weight of variety IN was significantly heavier than WA in the FY season, whereas no significant differences were evident between the varieties in the SY. Effects of SI on the weight of seeds followed a similar trend to those on grain yield. Lack of effect of additional SI on grain yields during the maturation period was reported by many workers. Maximum grain yields in irrigated sorghum were obtained in the second irrigation (Shiply et al., 1971) and by a single 10 cm irrigation, applied at heading or milk stage (Musick and Dusek, 1972). Imposing moisture stress periods at these stages caused similar yield reductions, whereas a later stress period caused smaller yield reductions (Eck and Musick, 1979a; Bakheit, 1994). The insensitivity of grain sorghum to moisture stress at the grain filling stage could be explained by the observation of Garrity et al. (1984) that stomata1 resistance in sorghum was sensitive to stress conditions only during the vegetative period, whereas stomata became nearly insensitive and remained opened during the reproductive period, and hence photosynthesis per unit leaf area was not decreased by water stress during the reproductive and grain filling periods. The interaction between varieties and irrigation was not significant in all the parameters tested, which suggests a lack of important differences in overall response to the irrigation treatments among the varieties tested, although the differences in grain yield as well as other parameters, for example, numbers of seeds and plants per unit area and forage yield, were pronounced. When grain yield was correlated with the yield components, namely, numbers of plants and seeds per unit area and lOOO-seed weight (SY season), the analysis revealed that variations among treatments were mostly associated with variations in the numbers of seeds per square metre (r2 = 0.86) and lOOO-seed weight (r2 = 0.75). 3.3. Water supply and yield The relationship between grain and forage yields to supplied water (rainfall + irrigation) for the three varieties, in the SY, is illustrated in Fig. 2. The data indicate large differences in response to similar seasonal water supply. Whereas forage yields tended to be linearly related to the supplied water, grain yields attained a maximum level with water supplied around 5885 m3 ha-i’, beyond which more water resulted in lower yield. A very similar trend was found with wheat grain yield when supplied water exceeded 9500m3 ha- ’ (Farah et al., 1993). However, Garrity et al. (1982a) found a linear relationship between evapotranspiration and both grain and total dry matter for sorghum, but their study did not include treatments which would cause reductions in

SM. Farcrh et al./Agricultural

38

0 4500

Wuter Muna~emrnt

5000 Supplied

Fig. 2. Responses

5500 Wakr

33 (1997) 31-41

6CCCl

6 xl

(m/ha)

of grain and forage yields as a function of supplied water in 1994 (FY) season.

evapotranspiration without reductions in yield. Plaut et al. (1969) and Inuyama et al. (1976) also reported a linear relationship between evapotranspiration and dry matter in grain sorghum under a variety of irrigation treatments, and concluded that the linear relationship seems to be a valid and general representation, but does not necessarily hold when other growth factors, such as fertility or temperature, are not optimum, and that the relationship between grain yield and transpiration is more complex than that of dry matter because grain yield is more sensitive to moisture reductions during certain growth stages. The IPE for grain and total dry matter yields for the FY and the SY are presented in Table 2. It is shown that for grain yields IPE generally decreased with addition of the last SI in both years, resulting in 9% and 12% reductions in the FY and the SY, respectively. For TDM, however, whereas the IPE progressively decreased with supplied water in the FY, the opposite was observed in the SY. The main reason for this was

Table 2 Total volume of water supplied (rainfall + irrigation) and irrigation production efficiency and total dry matter (TDM) above-ground yields in 1993 (FY) and 1994 (SY) seasons Treatment

A B C

Volume of water (m3 ha- ‘)

No. of irrigations

(IPE) values for grain

IPE (kgm-3) Grain yield

TDM yield

FY

SY

FY

SY

FY

SY

FY

SY

2

I

3 2

_

I

5488 4638 3788

6275 5885 4975

0.61 0.67 0.77

0.69 0.78 0.53

2.60 3.05 3.69

3.05 2.95 2.21

Volume of rainfall: FY, 2519 m3 haa’;

SY, 2462 m3ha-‘.

S.M. Farah et al./Agriculturul Table 3 Partial budget, dominance

and marginal

Water Management

33 (1997) 31-41

39

analysis of the grain and forage yields for 1994 (SY) season

Partial budget analysis: Item

One SI

Two Ski

Three SIs

Costs that vary (LS ha- ’) Grain yield (kg ha- ‘1 Forage yield (kg ha- ’) Value of gram (LS ha- ’) Value of forage (LS ha- ’) Gross benefits (LS ha- ‘) Net benefits (LS ha- ’)

686 265 1 6994 132550 17485 150035 149349

1238 4563 11108 228 150 27770 255920 254682

1475 4317 13099 215850 32748 248598 247123

Dominance

analysis:

Treatment

Variable cost (LS ha-

One SI Two SIs Three SIs

686 1238 1475

Marginal

’)

Net benefits (LS ha- ‘) 149349 254682 247123 ‘D’

analysis:

Treatment

Variable cost

One SI

686

Two SI

1238

Marginal cost

Net benefits

Marginal

net benefits

MRR (%I

149349 552

105333

19082

254682

LS, Sudanese pound; MRR, marginal rate of return; ‘D’ stands for the treatment or equal to those of the treatment with lower cost (dominated).

with net benefits lower than

probably the production of a higher number of plants (tillers) which were initiated by the supplied water in the SY.

3.4. Economic

evaluation

Only the results of the SY are presented in this paper (Table 3). The analysis showed that application of two and three SIs produced significantly higher net benefits as compared with only one SI. However, the three irrigations required 19% higher cost than two irrigations, with 2% lower net benefits, which suggests that, under the conditions of this study, two irrigations were more profitable than three irrigations as compared with only one irrigation. The marginal rate of return amounted to 19082%, indicating that every unit of investment in supplying two SIs produced a benefit of about 192 units. In conclusion, the data for both seasons strongly suggest that when using SI on sorghum, an optimum limit of irrigation strategy should be implemented to produce profitable yields. Much of the water supplied to the crop and incurring extra cost, namely the second and third SI in the FY and the SY seasons, respectively, was wasteful water, and ultimately resulted in uneconomical and/or reduced yields, thus undermining the very objective of supplementary irrigation for the sorghum crop.

40

S.M. Furuh

et al./Agrirulturul

Water Munqement

33 ~1997131-41

Acknowledgements The authors would like to express their appreciation to Professor S. Inanaga, the Director of the Arid Land Research Center, Tottori University, Japan, for provision of the facilities which made preparation of this paper possible. The research was carried out within the framework of the ARC, SudanUNDP programme on Supplementary Irrigation Under Rainfed Conditions and Improved Water Management at Farm Level, RAB/90/005.

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Olsson, K. and Rapp, A., 1991. Dryland degradation in central Sudan and conservation for survival. Ambio, 20(5): 192-195. Plaut, Z., Blum, A. and Amon, I., 1969. Effect of soil moisture regimes and row spacing on gram sorghum production. Agron. J., 61: 344-347. Salter, P.J. and Goode, J.E., 1967. Crop Responses to Water at Different Stages of Growth. Commonw. Agric. Bur., Famham Royal, UK, 246 pp. Shiply, J., Unger, P. and Reiger, C., 1971. Consumptive water use, harvestable dry matter production and nitrogen uptake by irrigated grain sorghum. Tex. Agric. Exp. Stn. Prog. Rep. 2951.