All irrigations during a season are not equal in terms of providing economic return on the money spent to irrigate. This section provides a brief description of the effect of water stress on cotton during the different growth stages of the plant and the relative benefit of irrigating to relieve stress.
Irrigation alleviates the detrimental impact of soil water deficit stress on two diverse physiological processes in plants that occur when they cannot get enough water. The most sensitive physiological process in plants to water deficit stress is cell growth. From root tips expanding through the soil to fibers elongating on seedcoats, the ability of individual cells within a plant to expand is largely determined by the availability of soil water. Along with reducing growth, soil water deficit stress triggers hormonal changes in reproductive growth that results in the shedding of fruiting structures (squares and bolls). Irrigation management should be aimed at reducing stress at critical times so the plants are provided the greatest ability to initiate, retain, and mature bolls.
Water use by cotton – low. Water is critical for germination and irrigating at this stage is primarily for stand establishment. If the seedbed is dry and irrigation is needed to establish a stand, it is preferable to irrigate before planting. Pre-irrigation reduces the possibility of seedling disease compared to irrigating shortly after planting. In addition, irrigating after planting will cool the soil and may reduce seedling growth rates. Once the seeds germinate, sufficient moisture must be in close proximity of the seedling until sufficient roots are developed to increase the area of water uptake. Establishment of the root system is quite fast, with taproots growing up to 2.4 inches per day after they emerge from the seed.
Water use by cotton – from <0.1 to 0.1 inches of water per day. Early season water deficit after stand establishment is often not an issue if there is adequate water for emergence and early seedling development. Water demand at this time is low and young cotton plants partition significant resources to the roots. Unless soil water deficit is extremely severe, irrigation at this time contributes relatively little to yield.
In fact, a mild water deficit early in the season can stimulate root production, especially encouraging deeper root systems. Primed Acclimation (PA) is an irrigation concept that uses intentional mild drought stress during early vegetative development to induce physiological changes in the plant that make it more drought tolerant during mid-season, when detrimental effects of water stress are maximal. As the name implies, a time of mild, controlled water deficit acclimates plants to water scarce conditions; thereby beginning (or priming) a cascade of plant responses that increases water-use efficiency. Some of these changes include increased root growth, decreased wateruse, changes in fruiting patterns, and elicited molecular/enzymatic responses. PA can maintain yield with significant water reduction. For cotton, the PA period lasts about 35 days, starting at full stand establishment (~14 days after planting) to late squaring/first bloom. During this time period, water may be reduced by as much as 30% with no yield loss in some southern production regions. An additional benefit to properly applied PA is a reduction in plant growth regulator needed later in the growing season and a more uniform maturity.
Note that figures 5.1 to 5.4 illustrate the general time period the growth stages occur relative to the number of days after emergence. The red line in the charts represents the leaf area index – a measure of how many leaves are present on the plant. For most cotton planted on 38 to 40 inch row spacing, the gaps between plant rows usually closes as the leaf area index approaches 3.
Water use by cotton – increases from 0.1 to 0.2 inches of water per day as plants grow. The approximate 21 days from first square to first bloom is a critical time for avoiding severe waterdeficit stress. During this period, cotton vegetative growth is very rapid and the number of potential fruiting sites for the crop is determined, especially in short season environments. This is also the period when plants are most rapidly taking up phosphorus and potassium from the soil because of rapid root growth. There is evidence from field-based imaging and measurements of cotton root systems that the maximum depth of the rooting system can be achieved relatively quickly and often exceeds 36 inches in depth. Maximum depths may be reached within 40 to 60 days after planting. Severe water deficit stress during this period is especially damaging to the cotton crop in short-season environments.
Water use by cotton – increases from 0.2 to 0.28 inches of water per day as plants grow. Water deficit stress early in this growth stage reduces plant growth which reduces the number of fruiting sites that are initiated. In addition, severe water deficit stress can also reduce boll number through shedding of young bolls and results in substantial yield loss. During early bloom, squares are generally not lost due to water deficit stress, so if square shedding is observed, other causes should be investigated. Water deficit stress at this time also impacts yield by reducing the size of surviving bolls. Severe stress reduces fiber quality through shorter staple and higher micronaire. At this growth stage, maximum rooting depth is achieved but lateral roots continue to grow throughout the rooting profile so that the final size of the root system may not be reached until 90 days after planting.
Water use by cotton – decreases from 0.28 inches of water per day as plants age. Water deficit stress during this growth stage is less critical than during squaring and early flowering. Water stress during this period can result in square and young boll shedding. However, these losses of late fruit have less impact on yield than loss of early season bolls. Fiber quality parameters affected by stress at this time are fiber length and micronaire, particularly in the young bolls.
After bolls start opening, plants should be allowed to become water stressed to allow for better harvest conditions. Stress at this time hastens boll opening, makes defoliation easier, and reduces regrowth.