Method of sowing
Flood irrigation is a common practice in flat-bed-sown crops, whereas in raised bed or ridge-sown crops irrigation is applied in furrows. Furrow irrigation has a higher WUE than flood irrigation in soybean. Furrow opening after two rows of soybean provide a significantly higher seed yield and WUE compared with flat sowing without furrow opening (Autkar et al., 2006).
Soybean has a higher WUE in a broad-bed and furrow system with irrigation at 0.6 IW:CPE ratio than in a flat-bed system (Bharambe et al, 1999). The width of the raised bed may influence the crop yields. Although less water is used for irrigation with wider raised beds, the central rows in the bed are not able to benefit from the furrow-applied irrigation. On the other hand, with a narrower width of the raised bed, and consequently more frequent furrows, drainage of excess water is easier and fast in the case of heavy rains. Among 6, 9, 12 and 15 m-wide raised beds, the highest seed yields of soybean have been obtained in the 6 m raised bed and the lowest in the 15 m raised bed (Tomar et al., 1999). A broad-bed and furrow system helps in decreasing run-off and increasing infiltration of rainfall (Singh et al., 1999b). In some areas the plant population and crop growth are adversely affected due to water accumulation in the case of flat-bed-sown crops, whereas the ridge and furrow system of planting helps to avoid these problems (Lakpale et al., 2009), which may ultimately result in high seed yields and WUE.
No-till sowing of crops, including soybean, is gaining in popularity among farmers due to various advantages associated with this method of sowing. The water intake in a no-tilled field is lower than that in a tilled field. In a no-tilled field, less irrigation water is applied at each irrigation, especially in early irrigations, than in a conventionally tilled field, although irrigation may be required more frequently. Mechanical impedance of the surface soil is much higher in no-tillage than in conventional tillage fields; because of this, the root length density and root branching index of soybean are higher in the surface soil layer in no-tilled field and, therefore, more dependent on irrigation (Iijima et al., 2007).
Soybean varieties differ in their WUE (Al-Assily and Mohamed, 2002; Hufstetler et al., 2007). Some varieties may be more tolerant to water stress (or drought) than others. Indeterminate cultivars are known to be better able to recover from water stress than determinate types (Villalobos-Rodriquez and Shibles, 1985). Some studies have reported drought tolerance and high WUE in some genotypes of soybean (Noureldin et al., 2002b). Moisture-efficient genotypes have root and shoot characteristics that allow the plant to use moisture in an effective manner.
Irrigation demands depend upon the prevailing weather conditions. Seasonal rainfall and the prevailing temperature are important weather parameters that affect the WUE in soybean. Seed yield as well as WUE in soybean is higher during the kharif (rainy) season than during the summer season (Elamathi and Singh, 2000). High temperatures during the summer result in greater losses of moisture.
The plant population should be the optimum for realizing high seed yields of soybean. Too low or too high plant populations not only result in poor seed yields, but WUE is also lowered. In the case of suboptimal plant populations, water loss is more through evaporation, while with too high plant population there is more water loss through transpiration and there may be a shortage of water during the reproductive phase.
Root length and the pattern of root growth influence WUE. Deep-rooted genotypes extract water from deeper layers; this is not taken up by shallow-rooted genotypes and is 'lost'. Deep-rooted genotypes can also withstand water stress far better than those with shallow roots.
Genotypes may differ in the pattern of root growth. Some genotypes have a more horizontal root growth, whereas others have a more vertical root growth. Vertical root-growth patterns are expected to provide a higher WUE than horizontal root-growth patterns, along with greater tolerance under drought conditions.
Long-duration genotypes stand in the field for a longer period and, therefore, may require a greater number of irrigations than short-duration genotypes. When grown during the same season, long-duration genotypes are expected to require greater amounts of irrigation water than short-duration ones. Terminal water deficits reduce soybean yields more in the case of long-season cultivars than in short-season ones (Muchow and Sinclair, 1986), as short-duration cultivars mature before the occurrence of water stress or experience water stress for a shorter period.
The sowing time greatly influences crop growth, crop yield and WUE. In Indonesia, December-sown soybean has been found to produce almost twice the yield of January-sown crop (van Cooten and Borrell, 1999), as the early sown crop was able not only to match growth with water supply, but also avoided end-of-season drought.
The irrigation method used for raising a soybean crop greatly influences WUE. In flood irrigation, water is applied to the whole area of the field and is many cases the surface is not uniform, resulting in a great deal of wastage of irrigation water. In the case of furrow irrigation, water is applied only in the furrows, covering less area than is normally covered under flood irrigation and, therefore, WUE is normally higher with furrow irrigation.
Alternate furrow irrigation can further enhance WUE by using less water. In sprinkler and drip irrigation systems, WUE is further improved as water losses are checked to a large extent. Sprinkler and drip irrigation systems are normally used in fields with uneven surfaces and in areas where there are severe water shortages. With surface and subsurface drip irrigation systems, the latter has higher irrigation WUE due to lower evaporation loss (Bhattarai et al., 2008). However, the initial high costs involved in establishing these systems is a major factor in these systems remaining unpopular among farmers.
Straw mulches check evaporation losses and thereby improve moisture availability for crop plants, ultimately leading to high seed yields. The surface application of straw mulch and straw incorporation may both have beneficial effects on improving WUE. For example, sugarcane (Saccharum officinarum) trash incorporation has been found to increase WUE in soybean (Bharambe et al., 2002).
Depending upon availability, the straw of wheat (Triticum aestivum), paddy (Oryza sativa) or any other crop can be used for mulching. However, the rate of application should be such that it does not have any adverse effect on the emergence of soybean plants when it is applied immediately after sowing. Alternatively, straw mulch may be applied between crop rows after emergence. Straw mulch application is often not be feasible due to the costs involved in its application. However, if the previous crop is harvested with a combine harvester (e.g. wheat) then soybean may be sown with no-tillage (using Happy Seeder) and wheat straw can serve as mulch.
Irrigation at the critical growth stage may save the crop and results in higher WUE over its application at other crop stages. WUE is higher with irrigation only at R5 than at other stages (Kim et al., 1999). During the early stages, there is little crop cover and consequently evaporation losses are high. Reducing irrigations during the vegetative stage helps to improve WUE (Neyshabouri and Hatfield, 1986) by avoiding large evaporation losses.
Weeds, aside from competing for other resources, also compete with crop plants for moisture. Some weed species are more vigorous than soybean plants and thus have greater ability to extract water. Therefore, the removal of weeds at appropriate times is a must for checking crop-weed competition for various resources, including moisture, and thereby improves WUE of soybean.
An optimally fertilized crop exhibits proper growth and development, high yield and high WUE. The highest ET and WUE in soybean have been reported with the application of 100% of recommended nitrogen, phosphorus and potassium (NPK) levels plus 10 t farmyard manure ha-1 compared to with 100% of recommended NPK levels alone or no fertilizer (Hati et al., 2000).
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