Determining Last Effective Flower

WHAT IS LAST EFFECTIVE FLOWER?

Last effective flower (LEF) is a term used to describe the estimated last flower set on the main stem branch that will likely contribute to mature, harvestable yield. It is important to remember that the dates represent on average, the last date that a flower can be set and reach maturity before it will likely experience a frost. These dates will vary year to year. Last effective flower can be an important tool for helping to determine water use management under limited water; and end of season management
decisions, such as an estimated cut out date, when to apply a cut-out rate of mepiquat chloride, estimate boll maturity, and when to start defoliation.

HOW IS LEF DETERMINED?

The last effective flower is determined by day degrees (heat units). As a general rule under the day degree base 12 model it takes at least 750 heat units for a flower to mature to an open boll, ready for picking. For the experimental DD 1532 this is estimated to be 500 heat units. Knowing this and by utilising average temperature data for the local region in the coming months, it is possible to calculate the date when the flower will have accumulated enough heat units to be a mature, harvestable boll. If the
grower has a date when they know they must defoliate by, they can work backwards from this date to determine when the last effective flower will be set.

WHERE DOES IT MATTER?

The importance of last effective flower varies greatly depending on the location the crop is growing in, water availability and season length. It can help a grower understand the importance and avoid getting caught out chasing yield when their area doesn’t get enough heat units to mature late set bolls. It is important to remember, however, at this point the crop would be experiencing cold shocks and growth will be significantly slower.

For example, on average for Deniliquin NSW, (Table 1) based on data from 1957 to 2020 LEF will have its last effective flower set on the 9th of February. It will then take an average to the 19th of May to accumulate enough heat units to mature the flower to a harvestable boll without the risk of contributing low micronaire to overall lint quality. In contrast, regions further north have more opportunity to extend the flowering period to chase viable bolls in comparison to southern regions.

Overall, it is important to have a yield goal, keep harvest date in mind and work back to determine when your last effective flowers should be set. Another factor to  consider – particularly in southern areas where winter rain is more likely to set in is that delaying a harvest date for late bolls may result in harvesting in the wet, delaying winter crop sowing behind cotton and potentially leading to compaction in the field.

Table 1: Last effective flower dates for various towns in cotton growing regions of Australia. Using 1957-2020 average day degree accumulation data from CottAssist.

Source: CottAssist day degree accumulations for the period 1 January to 31 July in the years 1957 to 2020. LEF date, mature boll date and days was sourced from CottAssist last effective flower tool.

WHY DOES IT MATTER?

Determining last effective flower can be an important tool in managing crops with limited water. Under limited water it is important to time irrigations to when it will have the greatest impact on a crop, which is during peak flowering. If it is known when the last effective flower will be and by association when it will be fully mature, irrigation can be scheduled during this period and minimise wasting resources.

In addition to this, LEF can help to work out the balance between cotton fibre qualities with setting late bolls to maximise yield. For example, late position bolls that are established as a flower in late February are potentially the highest contributors to low micronaire. If boll load is a majority of ‘late crop’ then you are at risk of contributing low micronaire bolls to your final quality.

This also applies to low weight bolls. The more late fruit there is making up yield, the lower each boll contributes to overall yield. That is, you would need more small late bolls to equate to less, larger bolls developed and matured earlier in the season.

© 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.