Late season irrigation management

The date of the Last Effective Flower can be used to determine target cutout dates.

The date of the last effective flower can be used to match the time when a manager may choose to cutout the crop based on Nodes Above White Flower (NAWF). Cutout occurs when NAWF equals 4 or 5.  Physiological cutout is described as the time when production of new fruiting site/square ceases and can be estimated using the date that represents the time of last effective square.

Sandra Williams, CSIRO:

The Last Effective Flower Tool (LEFT) is available on the CottASSIST website. It’s a decision support to that can be used to determine the predicted or desired date of last effective flower (cut-out).

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.

For 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 the 10 Feb and the latest this will occur is the 18 Feb. Changing the date of harvest (late harvestable boll) will result in changes to the last effective flower date.

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 to 4 weeks prior to defoliation.

If roots are extracting to a good depth (at least 1m) at cutout, 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 cutout and the final irrigation will occur 25 to 30 days later.

You can plan to apply one or two irrigations between the cutout 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 cutout because the water use drops off significantly in the second half of February and early March. Stretching irrigations prior to cutout results in significant yield losses, as discussed here.

From 2006-08, I investigated the impacts of stretching irrigation interval after cutout on yield and fibre quality of Bollgard II®. Trials at Narrabri showed that by having one rather than two irrigations after cut out and timing this irrigation to occur between the normal two irrigations reduced yield by 10 to 18 per cent if there was no rainfall and as little as 0 to 9 percent if good timely rain fell after this final irrigation (>40mm). In this trial the one irrigation applied after cutout was stretched to 21 days. I would suggest that if the irrigation interval was only 14 days, then the yield reductions would have been even less.

In the same trial I also found little impact on fibre quality as a result of stretching irrigations after cutout (Table 1). Where there was no irrigation and no rainfall after cutout fibre length decreased 0.02", strength decreased 1.2 g/tex and micronaire increased 0.28. Table 1 shows the effect of no irrigation and only one irrigation after cutout on fibre quality, compared to a fully irrigated crop.

Table 1: Effect of water stress after cut out on fibre quality. Comparison with fully irrigated* (From Steve Yeates' 2008 Cotton Conference paper: Towards better water management of Bollgard II cotton).

* Data for hand picked areas where plastic sheet prevented rainfall on dry treatments. These experiments were conducted at Narrabri.

• Use CottASSIST's Last Effective Flower Tool to determine the predicted or desired date of last effective flower (cut-out).
• Determine the water requirements of your crop from cutout to defoliation by estimating the number of days until defoliation and predicting the amount of water likely to be used over this period. Assessing the water requirements and knowing the amount of soil moisture remaining will allow calculation of the best strategy with the remaining water, options to consider include stretching the second last irrigation, bring the last irrigation forward (smaller deficit) so that less water is applied in the last irrigation or skipping the last irrigation.
• If water is becoming limiting you can stretch irrigations after cutout because the water use drops off significantly in the second half of February and early March. Stretching irrigations prior to cutout results in significant yield losses, so where water is limited the impact will be less at the end of the season.
• Where retention of first position bolls is high monitor Nodes Above (last) Cracked Boll (NACB) to accurately time final irrigation and defoliation.