Temperature requirements and tolerances

6.3 Water use

Water use efficiency in cotton can be simply defined as the measure of total yield (lint) produced per unit of water supplied to the crop (Gibb & Constable 1995). Because of the reliance on rainfall that is highly variable, dryland production has the biggest potential to benefit through improved water use efficiency of cotton plants (Australian Cotton Cooperative Research Centre 2002a; Cooperative Research Centre for Sustainable Cotton Production 1995). However, irrigated cotton may also benefit from improved water use efficiency particularly in drought years where less water is available for irrigation (Cooperative Research Centre for Sustainable Cotton Production 1995), if water allocations were to be reduced because of environmental demands or the cost of water were to rise.

Excess of water, called waterlogging has negative impact on cotton plants and leads to yield loss. In Australia, waterlogging in cotton is estimated to cause annual yield losses of approximately 1 bale/ha or 11% (Dennis et al. 2000). Waterlogging occurs mainly when heavy rain follows a scheduled irrigation, especially when combined with poorly draining soils and inadequate field slope. The majority of the Australian crop is gorown under furrow irrigation on cracking clay soils (Silburn et al. 2013) and fields are commonly irrigated five or six times during the growing season between flowering and peak boll development (McLeod et al. 1998). In NSW, cotton production occurs mainly on cracking grey clay soils (vertisols) of the Namoi and Gwydir River Valleys which have inherently low drainage rates (Hodgson & Chan 1982).

Research in the early 1980’s showed that a 32 hour waterlogging treatment of cotton could lead to yield losses of 42% (Hodgson 1982; Hodgson & Chan 1982), although another study showed a recovery of plants following waterlogging stresses leading to no reduction in yield (Hocking et al. 1987). A more recent experiment following a similar protocol to the Hodgson study recorded approximately 40% yield loss but only when more severe waterlogging conditions were imposed (Bange et al. 2004b). The reduced yield loss due to waterlogging seems to be partly related to improvements in field design and soil structure. An increased awareness of soil management programs by cotton farmers has led to a reduction in soil compaction and there have been improvements in the furrow irrigation fields with more even water flow due to the use of laser guided levelling systems. The more even slope and hill heights have meant that water does not collect in low areas.

Waterlogging damages plants due to low oxygen concentrations (hypoxia) around the roots. It happened because water displaces the oxygen in the soil, and cannot be replaced by diffusion of atmospheric oxygen. The low oxygen conditions inhibit energy production in the plant roots and other oxygen-dependent pathways, including those involving cytochromes, oxidases and desaturases.

The visual symptoms of waterlogging are initially wilting (Hocking et al. 1985; Reicosky et al. 1985) then leaf chlorosis, premature senescence and reduced boll number, leading to lint yield loss (Hodgson & Chan 1982). Damage to crop yields has already occurred once leaf yellowing is observed (Constable 1995). The impact of waterlogging early in crop growth has a far greater influence on yield than waterlogging at mid-flowering or later (Bange et al. 2004a), although yield loss due to waterlogging can be sustained at all stages of crop growth (Hodgson & Chan 1982).

Uptake of potassium, phosphorus (Hocking et al. 1987) and nitrogen (Constable 1995; Hocking et al. 1985) is impaired in waterlogged cotton, especially in young plants just before flowering and can result in the plants becoming temporarily deficient in these nutrients. During the first three to four days of waterlogging most of the yield loss is due to less nitrogen being absorbed from the soil (Constable 1995).