How to get Last Irrigation Right

Last irrigation is to ensure that boll maturity is completed. For Bollgard® 3 crops, late flowering is where water stress is going to be the most detrimental to final yield and correct irrigation timing is needed to ensure the crop does not suffer water stress.

^14 days post cut

Table 1: Crop water use and yield loss (%) per day of water stress (extraction of > 60% plant available water content) (Source: Yeates et al. 2010˜; Hearn and Constable 1984*) (crop water use generated from CSD Ambassador Network Program^).

Crops under stress prior to defoliation, four Nodes Above Cracked Boll (NACB), can suffer some fibre quality reduction, especially low micronaire. The degree of reduction increases the earlier the stress occurs. The last irrigation is also important to ensure that the later bolls are finished without stress but not impacting on picking and other farming operations.

There are several methods available to accurately time final irrigation measuring measuring four nodes above cracked boll is most commonly used.

Figure 1: Structure of a cotton plant demonstrating NACB.

MEASURING/ESTIMATING LATE CROP WATER USE

When the number of days to reach 4 NACB has been determined, the amount of water the crop will require during that time can be calculated.

This requires three key pieces of information:

• Soil water holding capacity (SWHC).
• The daily crop water use, daily crop water use will decline as bolls begin to open (Table 1).
• How dry you want the soil profile after this cotton crop.

EFFECT OF LATE WATER STRESS ON YIELD AND FIBRE QUALITY

In the event of water stress late in the season due to lack of rainfall and/or hot temperatures in January and February, fibre quality issues may occur;

• Yield: Can be reduced due to less cotton at the top of the plant and slightly lower boll weights.
• Micronaire: Can be affected due to lower micronaire in top bolls.
• Length: Minimal difference, as the fibre length has already been determined before late stress occurred.

The effects of the late stress on the fibre properties of the cotton crop are going to be reliant on length of stressed conditions, climatic conditions, variety, and boll composition throughout the plant.

WHEN NACB DOESN’T WORK

It is not an effective indicator in situations such as:

• There has been some ‘tipping out’, hail or a gappy plant stand, and vegetative branches with bolls younger than indicated by NACB.
• A crop has ‘stopped and started again’ because of a stress.
• There are thin stands with plants with high numbers of vegetative branches. Each vegetative branch should be treated as an individual plant when determining defoliation timing.
• Bolls have reached maturity when cut with a sharp knife, the seed embryo is developed and the seed coat has turned brown and black.

As with all determinations of crop maturity it is critical that only bolls which will be harvested are monitored.

© Cotton Seed Distributors Ltd 2022. 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 in this document or from any errors or omissions in this document. Roundup Ready Flex®, Roundup Ready®, Bollgard II® and Bollgard® 3 are registered trademarks of Monsanto Technologies LLC, used under licence by Monsanto Australia Ltd. Insect control technology incorporated into these seeds is commercialised under a licence from Syngenta Crop Protection AG. Sicot, Sicala, Siokra and Sipima cotton varieties are a result of a joint venture research program, Cotton Breeding Australia, conducted by CSIRO and Cotton Seed Distributors Ltd (CSD). CSD is a partner in the CottonInfo joint venture, in partnership with Cotton Research Development Corporation and Cotton Australia.