As most crops are coming into cut out and bolls start to open, growers are beginning to think about yield potential. Boll factors are one way to estimate yield potential, but they can also be notoriously misleading.
Boll counts are often inaccurate yield indicators. As a rule of thumb, 12-14 bolls/m equates to 1.0 b/ha in irrigated, and 16-20 bolls/m equates to 1.0b/ha in dryland situations.
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 probably the biggest reasons. With the advent of satellite imagery, drones and yield monitors on pickers, growers have become very aware of how much yield can vary within a field. While boll counts made on 2-3 metres of randomly selected plots in fields provides a good yield prediction for the area sampled, it may not be a representative measurement of the whole field.
Growers and agronomists have addressed this by using precision agriculture tools such as EM surveys and yield maps from previous crops to identify areas that are representative of the majority of the field. These techniques are also used for locating moisture probes, tissue testing and in-season plant monitoring.
Boll weights can vary greatly, and hence will impact on how these numbers convert to yield. No greater example of this variability has been witnessed during the past few seasons, where excellent growing and boll filling conditions have produced very heavy bolls – in some cases, up to 25% heavier.
Boll factors, as displayed on the CSD website, provide an indication of how many bolls/m are required to produce 1.0 bale/ha. For each boll factor, there may be up to 33% variability from the upper and lower limits in the original data.
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.
Final boll weight is influenced from the time a square is initiated to the time the picker reaches it.
More seeds/boll = heavier bolls
The time between square initiation and flowering is 3-4 weeks. The number of ovules (that may become seeds) is determined just after the point of square initiation. The ovule number is largely influenced by the genetics of the variety, crop stress and nutrition. The square is particularly sensitive to environmental stress (e.g. heat/moisture) at this stage. As a rule of thumb, healthy crops have between 30-35 seeds per boll with some bolls having up to 40.
White flowers open at dawn and are usually pollinated in less than 8 hours. Fertilised ovules go on to become seeds, while those not fertilised are known as motes and these may produce a short fibre that is removed through the ginning process. The number of ovules fertilised is strongly influenced by environmental factors especially night time temperature, with high (>25C) values as seen throughout the months of December and January this season being detrimental.
More lint per seed = 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 vegetative 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 plants photosynthetic resources.
First position bolls are usually heavier than second position bolls and those on vegetative branches. On the main stem, 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, this obviously being worse in rank crops. The size of a boll is proportional to the size of the subtending leaf.