Considerations for finishing crops with limited water and identifying last effective flower

16 January, 2019


At the time of writing it remains hot and dry across most regions with some growers now contemplating decisions on fields with regard to the prospect of being limited or short on water to finish. As a consequence, decisions to cut some fields out early are being considered to maximise yield potential from remaining irrigations. This article aims to summarise key influences that work with and against the crop and outlines a decision making process under limited water scenarios to assess the remaining water budget and if need be, to aid in identifying the Last Effective Flower (LEF) to cut out the crop.

Growth habit and environmental influences that will work against the crop:

Excessive vegetative growth: this can be problematic in two ways. Firstly, crops that shut down prematurely are likely to have a higher nutrient availability than required due to original yield target. This may lead to wasted resources going into nodes rather than bolls. The second is having bolls that are not able to be grown to maturity due to lack of time and resources

Hot weather: increases daily water use contributing to higher crop water requirements therefore reducing the number of days of moisture available to the crop for maturing bolls. Given the hot weather to date compounded by the forecast heatwave this week for many valleys, the crop will be under a lot of pressure.

Premature plant stress: a potential risk particularly in combination with hot weather. Stress to the crop during late flowering has the greatest impact upon yield. Stretching irrigations at this time to prolong the life of the crop can be risky and may be compounded, as subsequent irrigations may take longer, use more water and potentially impact other fields in the irrigation cycle waiting their turn.

Table 1: Crop water use an yield loss (%) per day of water stress (extraction of > 60% plant available water content). Source: Yeates et al. 2010#; Hearn and Constable*.

Past Conventional* Bollgard II# Avg. Crop water use (mm/day)^
Squaring 0.8 1.1 2.5 to 5
Peak flowering 1.6 1.7 7
Late flowering 1.4 2.7 8
Boll maturation 0.3 0.69** 6 to 3.5

** 14 d post cut out. ^ Crop water use generated from CSD Ambassador Network.

If considering cutting the crop out early with growth regulant, it is important to identify the last effective flower (LEF) which will be the last harvestable boll. This can provide an indication of the date to apply Mepiquat Chloride. This can be calculated loosely by working backwards based upon remaining irrigation available, an estimate of the current plant available water capacity (PAWC) of the field, approximate average water use up until defoliation and day degrees (DD) required to mature the LEF Figure 1 is a back of the envelope example.

Figure 1

An important observation to make in the example provided is that the profile will be empty. This is not ideal for defoliation purposes but does exploit one of the cotton plant’s attributes – its ability to extract moisture from the profile, maximising chances to finish the crop as best as possible. It goes without saying that the calculation for LEF can be adjusted to allow more moisture to be present in the profile at defoliation, it just means an earlier LEF date.

Table 2: Summary of crop response to water stress.

Leaf growth Most sensitive
Stem Growth (height)/ Fibre Length

Root Growth
Rate of squaring
Node production
Boll growth Least sensitive

It is highly probable that the crop will likely be suffering from water stress at this time. Table 2 gives a summary of the sensitivity of the cotton plant to water stress, late in the crop’s life, post cut out. This period is when the crop is primarily filling bolls and past the more sensitive aspects of crop development such as leaf growth, rate of squaring or crop height determination. The least sensitive development phase to water stress is boll growth, and this is the one factor which is still relevant at this time of the crop’s growth.

No doubt this is a scenario that some growers will be faced with in yet another challenging season; the CSD Extension & Development team may be able to assist with regional estimates of day degree requirements based upon current growth stage of the crop.

By Chris Teague – Extension & Development Agronomist, Border Rivers & Balonne.

Disclaimer: General guide only, not comprehensive or specific technical advice. Circumstances vary from farm to farm. To the fullest extent permitted by law, CSD expressly disclaims all liability for any loss or damage arising from reliance upon any information, statement or opinion, or from any errors or omissions in this document.