In this article we will discuss about the water management for sorghum.
In India, sorghum is predominantly a rainfed crop both during monsoon and postmonsoon seasons. Irrigated sorghum grown from April to July, as summer crop, constitutes only 5 per cent of the total area under sorghum. Although, sorghum is considered drought tolerant and able to give good yields under dryland farming, it responds very well to irrigation.
Critical Stages for Irrigation:
Under ideal conditions, sorghum has an extensive and deep root system. Root system of most commercial cultivars extends to more than 2.0 m deep into the soil. To capitalise on deep moisture extraction possibilities during later part of growing season, it is important to maintain surface moisture during establishment stage, 2-4 weeks after sowing to encourage strong root system.
Severe soil moisture stress for 7 days at pre-boot, flowering and soft dough stage indicated drastic reduction in grain yield due to stress at pre-boot stage, relative to the other two stages.
Results of experiments at Coimbatore on summer sorghum (Table 5.8) indicated that omission of seedling stage irrigation reduced the grain yield from 3970 to 990 kg ha-1. The next most critical stage was flower primordial stage. The four treatments, which received seedling plus primordial stage irrigations, resulted in comparable yields.
TABLE 5.8: Grain yield of sorghum as influenced by soil moisture stress at different growth stages.
From these results and other studies, it is evident that the two most critical stages for irrigation are seedling and primordial stages. Contrary to other cereals, flowering and grain development stages of sorghum are relatively less critical stages for soil moisture stress. Deep and extensive root system of sorghum may be able to utilise the deep soil profile moisture during flowering and grain development stages.
Postrainy season sorghum on receding soil moisture is performing normally in spite of the fact that subsoil moisture is relatively low after primordial initiation compared with soil moisture level up to primordial stage.
The rate of water use by sorghum is at its peak at flowering stage and rises rapidly 20 days after seeding till 65 days, which remains steady till dough stage and later falls rapidly as maturity advance. It is estimated that a 100-115 days crop may have water requirement of 450-500 mm.
Irrigation Scheduling:
A large number of experiments have been conducted for formulating an ideal irrigation schedule to sorghum crop. At Siruguppa (Karnataka) one irrigation at 75 per cent DASM in surface 30 cm was adequate for kharif sorghum on heavy soils. However, summer sorghum gave highest grain yield when irrigations were scheduled at 50 per cent DASM, which required about 550 mm, water in 9 irrigations.
Irrigation at IW/CPE ratio of 0.4 was ideal at Parbhani and Bhavinsagar, while a ratio of 0.6 was optimum at Rahuri. At Dharwad and Navsari, rabi sorghum gave highest yield at IW/CPE ratio of 0.6 and 0.9 with 31 and 52 cm water, respectively.
Where rainfall is not sufficient and irrigation water is limited, irrigation should be based on avoiding water deficits during seedling (establishment) and primordial initiation stages and saving water by reducing water supply at vegetative and late maturity stages.
The following irrigation schedule appears to be ideal, depending on the available water supply:
1. If water supply is adequate for only two irrigations, they should be provided at seedling and primordial stages for near optimum yield.
2. In addition to the above two stages, flowering stage should receive priority if water is adequate for 3 irrigations.
3. Four irrigations, one each at seedling, primordial, flowering and grain development stages appears to be adequate for realising maximum grain yield even when water is not a limiting factor.
Irrigations at 50 per cent DASM or IW/CPE ratio of 0.6 at the above four stages is adequate for normal yield. If water supply is limited, seedling and primordial stages should receive irrigations at 50 per cent DASM or 0.6 IW/CPE ratio and the latter two stages at 75 per cent DASM or 0.7 IW/ CPE ratio. Sorghum crop is usually irrigated by check basin or border methods of surface irrigation. Soil moisture conservation assumes significance in dryland sorghum production.
In the rainy season, the chances of total failure of sorghum due to soil moisture stress from flowering to maturity are limited since rainfall is relatively assured during September and October. However, for postrainy season sorghum on receding soil moisture, terminal soil moisture stress, especially from flowering to physiological maturity may result in total crop failure. Under these circumstances, moisture conservation should receive priority for postrainy season sorghum.
Rapid loss of profile soil moisture should be minimised by preventing deep cracking of heavy soils. Creation of loose surface soil mulch by repeated intercultivations with blade harrows, a traditional practice is effective in this regard.
Thinning of plant population by removing every third row has been found advantageous to mitigate soil moisture stress at Bellary (Karnataka). Use of antitranspirants and reflectants may aid in reducing transpiration losses. However, their use under field conditions has not been accepted by farming community.
Drought Avoidance and Tolerance:
Drought resistance of plants in any environment is due to either avoidance or tolerance mechanism. Drought avoidance (evasion) includes any mechanism, which keeps the tissue water potential above that which would cause cellular injury. Avoidance is relative and it is difficult to place a value on it but it could be said that it is the number of bars water potential maintained in the field above that which would cause 50 per cent killing of the cells.
It is thus assumed that the greater this value, the less are the direct detrimental effects of the existing reduced water potential on cellular activity. Avoidance mechanisms include the ability to keep the water potential high by absorption of water and conduction to the shoot or ability to reduce water loss (stomatal closure). Completion of life cycle or some critical stage in the cycle before the damaging drought occurs would also be an avoidance mechanism.
Drought tolerance is defined as the equilibrium relative humidity (or water potential), which cause 50 per cent killing of the cells. That is, at slightly higher water potential the cells will survive or function metabolically although the tissues are desiccated. The greater the desiccation tolerance, the lesser the direct effects of reduced water potential will be on metabolic activity and cellular injury, other than that resulting in killing of the cells.