Late season irrigation management

Cotton crops reach peak flowering period in January – a critical time which can significantly influence final yield and fibre quality. Flowering for longer will lead to more bolls and generally higher yields. Cut-out generally occurs late January to mid February when the plant reaches 4-5 nodes above white flower (NAWF).

James Quinn, CSD says in non-limiting fully irrigated crops you should aim for 8+ NAWF at first flower or 777 Day Degrees and maintain at this level for as long as possible within the seasonal constraints for a region.  Crops that approach cut-out too rapidly are experiencing stress (mostly carbon stress) and will not support new fruit development.

During this period its important to maintain crop monitoring. In addition to the usual moisture, nutrient and insect scouting, make sure you continue to monitor crop growth including NAWF and vegetative growth rate (VGR) along with boll numbers and retention to determine fruit development and load.

A crop's NAWF will generally always decrease to a point where it ‘cuts out’ (4-5 NAWF).  Figure 1 shows data collected by CSD showing the average NAWF decline for the Gwydir Valley over five seasons. The aim of a crop manager is to generally avoid the too rapid decline of NAWF reducing yield potential, or delaying it too much causing issues with delayed crops at the end of the season.

Figure 1: Average NAWF decline for Gwydir Valley (5 seasons) (CSD 2015)

Cut-out generally occurs late January to mid February. At cut-out the plants demands for assimilate (products of photosynthesis) finally exceeds supply so that further growth and production of new squares ceases. Therefore no more harvestable fruit is set and the earlier bolls set start to open.

Mild moisture stress post cut-out has minimal impact on final yield (Table 1) and fibre quality (Table 2) however the impact on yield will depend on the length of the stress period and the proportion of bolls effected.

The date of the Last Effective Flower can be used to guide target cut-out dates based on season length.

The Last Effective Flower Tool (LEFT), available on CottASSIST website (www.cottassist.com.au), is a decision support tool that can be used to determine the predicted or desired date of last effective flower (cut-out). It is also useful for budgeting water requirements in advance. You can optimise nutrition, irrigation and use of growth regulators to guide the crop towards a desired date for Last Effective Flower.

The LEFT uses temperature data and day degree targets for boll period (flower to open boll) and square period (square to flower) to estimate the date of the last effective flower that will contribute to a harvestable boll. It works on the principle that it takes 430 day degrees for a square to become a flower and 750 day degrees for a flower to become a mature open boll.

To use the LEFT, you simply select your nearest weather station, and define the date of the last harvestable boll and click Run Simulation.

The date of last harvestable boll can be defined in one of two ways:

1. By Temperature - the time of the first frost can be defined by setting a daily minimum temperature. A minimum temperature of 2°C in a weather station equates approximately to a frost on the ground surface. The LEFT will scan the historical dataset to determine the first day (from 1st Jan through to 30th Jun) which reached this minimum temperature; OR
2. By Date - a calendar date can be entered to define when the last harvestable boll will open. The output provided by the LEFT includes the earliest, the latest, and the average date, on which the last effective square, flower and harvestable boll occurs.

Example: The following table shows the LEFT results for the Gwydir Valley by a selected date. 

For a crop to be ready to harvest by 15 April, on average in the Gwydir the last effective flower will occur on 11 Feb and the latest this will occur is 22 February. Changing the date of harvest (late harvestable boll) will result in changes to the last effective flower date.

Practices that can be used to delay cut-out include provision of additional water and nutrition. Limiting water and use of cut-out rates of growth regulators can be used to bring in cut-out more quickly.

End of season water requirements can be determined by estimating the number of days until defoliation and predicting the amount of water likely to be used over this period.

Depending on sowing date and temperature the last effective white flower will take 50 to 65 days to reach maturity (about 5 days after 1st defoliation). This means for early sown crops there are at least 50 days left in the growing season in which you need to manage your irrigations and preferably not stress the plant so as to enable this last flower to reach maturity.

At the time of first open boll, crop water use may be 5-7mm/day, but this can decline to only 3-4mm/day during the last 2-4 weeks prior to defoliation. 

If roots are extracting to a good depth (at least 1m) at cut-out, plants can easily extract 70 percent of the available water prior to last boll maturity. In cracking clay soils, plants can extract 125 to 150mm soil moisture, which is equivalent to 25 to 30 days water use (5mm/day) with little effect on yield or quality.

Therefore on most cotton soils unless water use is above 5mm/day there is `no need to irrigate in the 20 to 25 days before defoliation. Any new flowers that develop in that last 25 days will not have time to mature with the last bolls making up a small contribution to yield. Hence, you only have only 25 to 30 days in which to schedule irrigations. Assuming an irrigation is made at cut-out and the final irrigation will occur 25 to 30 days later.

You can plan to apply one irrigation or two irrigations between the cut-out irrigation and the final irrigation depending on soil type, the deficit you prefer, rooting depth and plant water use.

If water is becoming limiting you can stretch irrigations after cut-out because the water use drops off significantly in the second half of February and early March. Stretching irrigations prior to cut-out results in significant yield losses, as previously discussed.

Example: Final water requirements calculations (source WATERpak, chapter 3.2).

Crop A needs 356mm of water over the next 62 days. If the profile is currently full and the plant available water capacity is about 200mm, the deficit could reach 140mm (70% of 200mm) by defoliation without detrimental effect.

Therefore another 216mm of water needs to be supplied by irrigation and rainfall. With a typical irrigation deficit of 90mm for this example, at least two irrigations are required as well as 36mm in rainfall. If no rainfall eventuated, crop progress would need to be monitored to see if a third irrigation was required.

For similar soil moisture conditions, Crop B requires 75 mm of water to finish it off. Therefore irrigation is not required as there is 140 mm of soil moisture available. Only irrigate if the rooting depth is constrained or evaporation is higher than that predicted when estimating daily water use. On the other hand, Crop C will most likely require one irrigation because the crop requires more than the allowable depletion of 140mm (70% of PAWC). Likely rainfall would need to be considered in any such decisions.

The objective of the last irrigation is to ensure that boll maturity is completed without any stress, says James Quinn.

Final irrigation should provide:

• Sufficient water to optimise yield and fibre quality ie enough moisture to allow the growth and maturity of harvestable bolls.
• Adequate soil moisture for efficient defoliation ie aim to have soil moisture levels at or close to refill point at time of defoliation. It is important not to have the crop too stressed as this time as it will reduce the efficacy of crop defoliation practices.
• Soil profile dry enough to enable harvest without causing soil compaction
• Measuring Nodes Above (last) Cracked Boll (NACB) is most commonly used to accurately time final irrigation and defoliation. A detailed description of the NACB method is outlined in WaterPak, Chapter 3.2.