Boll Weight and Boll Factors

Throughout the season this year, we are seeing a diverse range of crops with climatic impacts. Added to this variability is the different forms of planting, from fully irrigated with varying levels of row configurations and water stress. As bolls begin to open, thoughts should start to turn towards yield potential. Every season there is variability in the relationships between boll number and final yield, and there are many reasons why this occurs.

• In-field variability and representative sampling are the biggest reasons.
• Boll weights can vary greatly, and hence will impact on how these numbers convert to yield.

Table 1 illustrates how 50% variation in boll weight equates to 50% variability in yield with the same boll numbers. These levels of boll weight variability have been observed within the same variety in CSD’s segmented picking work over many seasons.

Table 1: Impact of boll weight and boll number on yield potential.

Boll factors provide a good indication of the comparative boll weights between varieties. If everything else is equal, a variety with a lower boll factor will achieve a higher yield than a variety with a larger boll factor with the same boll counts. Additionally, there are regional, seasonal and stress related impacts which can alter the relative boll factors. Late set bolls will often be smaller than early set fruit as they do not have the amount of time to mature. The net effect of a higher proportion of late fruit with lower
weight is an increase in the number of bolls required to make a bale.

WHY BOLL WEIGHTS VARY

Final boll weight is influenced from the time a square is initiated to the time the picker reaches it.

MORE SEEDS/BOLL = HEAVIER BOLLS

Larger seeds will have more lint per seed. Other than genetics, the causes of this are not well understood, but competition between seeds in a boll is likely. The weight of each fibre is determined by the growing conditions from flowering onwards. Things that can influence this include nutrition, climate, boll load and location on the plant.

• Moisture stress during boll fill needs to be severe before it will limit boll weight. Bolls are less sensitive to stress than leaves, hence boll growth can still occur after  negetative development (e.g. new nodes) has stopped.
• Crops with low boll numbers can have higher boll weights as there is less fruit to draw on the plant’s photosynthetic resources.
• First position bolls are usually heavier than second position bolls and those on vegetative branches. On the main stem, the heaviest bolls usually occur around fruiting branches 4-7, and decrease towards the top of the plant as there is more competition for assimilates. Bolls at the bottom of the plant often suffer from shading, obviously this being worse in rank crops. The size of a boll is proportionate to the size of the subtending leaf.

Boll factors provide an indication of how many bolls/m are required to produce 1.0 bale/ha. Average boll factors from the 2020/21 can be found in table 2 below and the past season can be found online.

Table 2: CSD variety trial boll factors.

* Dryland results shown are per linear metre, a row configuration multiplication factor is needed to estimate yield potential.

** This variety data is limited being in 6 trials.

© 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.