Following figure shows growth periods of cotton (after P.T. Walker)
The relationships between relative yield decrease (1 - Ya/Ym) and relative evapotranspiration deficit for the total growing period of cotton are shown in the figure below.
This figure shows the relationships between relative yield decrease (1 - Ya/Ym) and relative evapotranspiration deficit for the individual growth periods.
The crop growth periods for cotton are shown in Figure 14. The relationships between relative yield decrease and relative evapotranspiration deficit are shown in Figure 15. For calculation examples see p. 40 and Chapter V1.
Adequate water supply is needed for vigorous growth, good budding and fruiting and for the formation of healthy bolls. Excess water early in the growing period will restrict root and crop development. Cotton requires adequate water supply particularly just prior and during bud formation (2a). Continued water supply during flower opening (2b) and yield formation (3) periods results in prolonged and excessive growth and yield. Abrupt changes in water supply will adversely affect growth and cause flower and boll shedding. Severe water deficits during flowering may fully halt growth, but with subsequent water supply crop growth recovers and flower formation is resumed.
When the growing season is short such conditions lead to smaller yield. Water stress on cotton can be observed by discolouring of the stem and appearance of a bluish-green colour on the leaves.
Water supply for high production must be adjusted to the specific requirements of each growth period (see Scheduling). Optimum use of available water supply can be made by fully wetting the entire root zone up to 1.80 m at sowing and with subsequent wetting of the upper part (0. 50. to 1 m) of, the root zone only. Root activity may be increased' and full utilization of the availble soil water over the entire root zone is made with little or no soil water left at the end of the growing season. Other savings can be made by utilizing the available water in the entire root depth by timely discon-tinuing irrigation applications at the end of the total growing period. Also savings can be achieved by withholding supply during the flowering period (2) until some 70 percent of the total available soil water has been taken up by the crop. A combination of the above practices may save up to 20 percent water without greatly impairing yields.
At sowing, adequate soil water should be available for germination and establishment (0). During the vegetative period (1), soil water content over the root depth of some 0.75 m should not fall below 50 percent depletion; greater depletion of available soil water (up to 75 percent) will restrict vegetative growth but when followed by ample supply, vegetative growth will be somewhat excessive, which may cause late flowering, boll shedding and reduced yield when the growing season is short. At flowering (2), supply will need to be scheduled to control vegetative growth in relation -to productive growth. Water deficits from onset of flowering to peak flowering may cause a more negative effect on yield as compared to when occurring after peak flowering. With severe water deficits during late flowering and early boll formation, boll shedding can be excessive. Moderate water deficit occurring during flowering (2) but high enough to restrict vegetative growth, will lead to good boll-set and higher yields, despite a reduction in number of flowers.
Adequate water supply should be available during the yield formation period (3). Depending on water holding capacity of the soil, development of the root system and evaporative demand, the water supply during the yield formation period (3) should be discontinued at a certain time before the ripening period (4). Excessive water supply during the yield formation period (3) may cause delay in boll opening and greater susceptibility to lodging and boll-rot. Under conditions of a long and warm growing season, an irrigation after the first harvest is sometimes practised to obtain a second yield. When water supply is limited, a higher total production is obtained by extending the area and partially meeting crop water requirements rather than by meeting full crop water requirements over a limited area.