Water running short? How do we manage our irrigations?

When water is limited growers may need to change from their normal irrigation practice to optimise yield, quality and water use efficiency. As with fully irrigated production, the aim is to limit or minimise the amount of stress on the crop. Cotton’s response to water stress depends on the stage of growth that stress occurs, the degree of stress and the length of time the stress is present.

In order to determine when to irrigate under limited water conditions it is important to monitor both crop water use and crop development as the timing of stress can have significant impacts on yield and water use efficiency. 

A cotton plant, when not stressed, grows in a predictable way, which allows its crop development to be predicted using daily temperature data (day degrees). Monitoring of squaring nodes, fruit retention and nodes above white flower will help keep track of how a crop is progressing compared to potential development when under stress.

Knowing what stage the crop is at will help in predicting crop water use, peak water demand occurs during peak flowering. Growers can use the crop development tool on the CottASSIST website to track their crop. Using day degrees is a better indicator for comparing crops  than simply days after sowing, for example in many regions this season crops are more advanced and hence have higher water use earlier than in an average year due to warmer growing conditions early in the season.

Monitoring nodes after white flower (NAWF) will assist in deciding which crops need irrigating when water is limited. When fruit retention is high, crops with more NAWF generally have more vigour. Where there is sufficient water available the aim is to extend the flowering period as long as possible to match the season length.

Once the crop has reached cut-out (NAWF <5), the most critical period for minimising water stress has past. Stressed crops may reach cut-out earlier as leaf expansion and the development of new nodes slows in response to water stress. When irrigation water is limited stress has less of an impact if it occurs late or early in the season but stress during the flowering period can lead to significant yield loss as this is the period when the crop is most susceptible to stress (see Table 1). Whilst yield loss can still occur after cut-out the reduction in yield is lower.

Stress during peak flowering is likely to result in double the yield loss compared to stress during squaring and late boll maturation as shown in Table 1.

Table 1: Yield loss (%) per day of water stress (extraction of > 60% plant available water)

* Hearn and Constable 1984
** Yeates et al. 2010
*** 14 days post cut out

Visual signs of crop stress such as leaf colour and wilting can be indicators of stress however, many of these occur after stress has occurred so are not useful in anticipating crop requirements but rather an indicator that stress has or is occurring.

Stretching the time between irrigations beyond the target deficit can lead to significant yield losses, therefore in most seasons it is better to skip the last irrigation rather than stretching irrigations during flowering . With very severe shortages delaying the first irrigation is preferable to lengthening the irrigation between flowering. Soil moisture monitoring is invaluable for timely irrigations and when water is limited predicting how much water will be needed to refill the profile.  The short term forecast can help refine scheduling  in predicting future crop water use.

The dynamic deficits research has shown that when the forecast is for low evaporative demand irrigations can be safely delayed past the normal target deficit without a negative impact on yield and quality and if rainfall occurs during the period of low evaporative demand then there is an opportunity to capture this rainfall in the crop and save water.

Many of the current measurements of crop stress will tell you that a stress has occurred in the past. Monitoring of crop stress in real time might offer some valuable insight for irrigation decision making. My current research project is testing the feasibility of using canopy temperature for irrigation scheduling in a fully irrigated system by developing a time temperature threshold for irrigation scheduling. The aim of this research is to establish and test a threshold for the length of time the crop can tolerate temperatures exceeding its biological optimum  before there are significant impacts on yield.

In my first year of irrigating cotton I was able to closely match a grower’s schedule in Emerald and Dr Rose Brodrick’s scheduling at ACRI using only canopy temperature indicating that the temperature threshold approach could be a robust yet simple scheduling approach to support growers decision making when unsure of when to irrigate.

I am using canopy temperature measurements as a measure of crop stress and investigating whether the time temperature approach could be adapt to get maximum benefit for a limited number of irrigations. Recent experiments monitoring crop growth, stress, yield and maturity grown under different row configurations and varying degrees of water stress aim to identify key growth stages and crop stress trigger points to get maximum benefit for a limited number of irrigations.

• Current recommendations for limited water situations are to aim to concentrate water applications during flowering (first flower to cutout) and minimise stress during this period. This is the period that cotton is most sensitive to water stress and loss of early fruit will require further growth and water to support growth later in the season.
• Monitoring of crop development and using CottASSIST's crop development tool to determine how a crop is performing in comparison to the expected growth of a well watered crop.
• Continue to use a variety of tools to schedule irrigations including soil moisture and weather forecasts.