Retention in CSD Varieties Containing Bollgard® 3

with assistance from Paul Grundy, CottonInfo IPM Technical Lead

The production of fruiting bodies or ‘squares’ indicates that the plant has transitioned into a reproductive growth phase. Squares can be initiated on both vegetative and reproductive branches. Squares can occur in multiple positions along a branch and they are accompanied by a sub-tendering leaf.

Many abiotic and biotic factors affect the ability for the plant to retain or to remove squares.

Square loss may have implications on how to manage the crop through the remainder of the season, particularly if losses continue past very early flowering. In contrast, square loss prior to first flower is unlikely to have major implications for crop management.

WHAT IS THE CAUSE?

Retention loss can be caused from a combination of factors:

• Overcast conditions
• Lower than average temperatures
• High insect pressure

A cotton crop’s growth is dependent on accumulated temperature and solar radiation. Overcast conditions or unfavourable temperatures (both low and high) can disrupt photosynthesis and plant respiration processes and induce shedding of squares or young bolls. Shedding caused by climatic factors, rather than insects, can be distinguished by examining the pattern of fruit loss. Weather induced shedding often has a defined pattern within the canopy, with the same fruiting branches being affected across multiple plants; or damage confined to fruit and squares of a specific size or age throughout the plant. It is common to have shedding occur when sunshine resumes after a period of cloudy weather. In comparison, insect damage will be more random in terms of location and size of fruit impacted.

Table 1: Comparison of retention percentage between seasons at mid-square and first flower based on CSD’s Ambassador Network Program’s data.

WHAT EFFECTS WILL THIS HAVE ON THE CROP?

CSD varieties containing Bollgard® 3 are adaptive and can readily compensate for square loss, particularly if it occurs prior to flowering. Compensation typically occurs within the canopy on positions adjacent to or above the location where fruit was lost. For example, first position fruit on the lower branches will often be compensated by the retention of a second or third position fruit on the same fruiting branch or greater retention of first or second position fruit on the fruiting branches directly above the branches from which positions are lost. Additional fruiting sites may also be retained on vegetative branches with the plant having the ability to relocate surplus assimilates
to different canopy areas, in order to support additional boll retention.

Fruit loss is more likely to become a problem for yield and fibre quality if losses occur over a prolonged period during flowering, which necessitates the production of additional mainstem nodes and fruiting branches to create compensatory bolls. The main concern with this scenario is whether sufficient day degrees remain within the season to mature these later set fruit. This is more of a concern in southern regions such as the Murrumbidgee with cooler temperatures and winter dominated rainfall
compared to locations in northern New South Wales or Queensland.

In the event of a cooler finish to the season where there is a high number of late set bolls due to prolonged fruit shedding, crops with a high proportion of late set fruit may have insufficient season length to adequately mature developing fibres, which in turn can result in a discount to fibre quality. This is particularly likely for micronaire. A penalty in the grade of micronaire may outweigh the benefits of the late set yield.

Another factor to consider in a season with lower retention is the impact on the crop’s canopy development. Surplus assimilates due to low retention can, in turn, increase the rate of vegetative (stem and leaf) expansion that in turn causes the crop to ‘self shade’ the lower leaves causing a negative feedback loop and predisposing the crop to further unproductive rank growth. Keep a check on main stem internode lengths in the upper canopy (see VGR method) to determine how quickly the plant is growing. Around first flower, aim for an internode length of 5 cm. Normally, as boll load increases, vegetative growth will begin to slow naturally (which is why white flowers appear to come to the top) but management with mepiquat chloride when boll load is insufficient may be necessary to slow excessive growth.

With appropriate management for possible excess vegetative growth, the final yields of CSD varieties containing Bollgard® 3 should not be impacted by low retention prior to flowering. However, if losses continue and depending on the region where the crop is grown, climate conditions may prevent extending the season to mature the later set bolls. Remember not to get caught chasing yield when your area doesn’t get enough heat units to mature late set bolls. Have a harvest date in mind and work back
to determine when your last effective flowers should be set. Chasing those top bolls beyond this point could also mean harvesting in the wet, leading to compaction.

© 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.