Produced by the CSD Extension and Development Team
The cotton plant grows over a long period of time and a significant part of its life is in a vegetative state growing leaves, stems and roots. The next phase of the cotton plants life is the reproductive stage where fruit competes for resources that are would have otherwise been used for vegetative growth. While it is vitally important that the resouces are available through the reproductive stage, the plant still needs to grow stem (nodes) and leaves to such allow more fruiting sites to develop which overall leads to more potential yield over the course of the season.
Too much demand from the fruit can cause the crop to cutout and stop producing new fruit. Too much vegetative growth can lead to crops self-shading and fruit loss which can in fact encourage more vegetative growth at the expense of fruit. Therefore it is a critical balancing act between vegetative and reproductive growth to make the most out of the growing season. We want the plant to optimise fruit production while continuing to grow more shoot (nodes) and leaves for encourage photosynthesis and overall carbohydrate supply to bolls.
The following are examples of when it might be time to consider using a growth regulator such as Mepiquat Chloride (an anti-gibberellin) to balance the vegetative/reproduction equation. Mepiquat Chloride prevents the production of the hormone Gibberellin which is responsible for cell expansion in plants.
Physiological Response
Insects or stress can cause early season fruit loss, which can lead to excessive vegetative growth when growing conditions are good. This is especially the case when flowering has occurred, and growth rates are already high. With reasonable fruit retention, boll load on the crop can normally restrict excessive vegetative growth, but in the instance that fruit are lost, the plant can become very vegetative. This can lead to the unbalancing of the vegetative/reproduction equation resulting with long stretched out internodes and plant that gets too tall. Consequences of this are shading of developing bolls, fruit abortion and boll rot later if conditions become wet and humid. As mentioned above another symptom is the crop continues to become even more vegetative (rank).
In this example, the most powerful information is the recording of plant growth and node development over time. In other words, ‘Feet in the field’. Get out and measure total nodes and height of the plant this week compared to next week. By doing this we can use the Vegetative Growth Rate (VGR) to help with the decision-making process.
VGR (cm/node) = This week’s height (cm) – Last week’s height (cm) / This week’s node number- Last week’s node number
VGR’s over 5.5 would probably require some form of growth management. It is very important to also consider any possible stress events that may impact on the crop in the near to distant future. Growth regulation during a plant stress eg. late irrigation or waterlogged soils, can impact on eventual yield outcomes.
Often the question is asked why use VGR and not just measure the distance between nodes. The concept of VGR is predict potential changes in growth rate. The use of measuring nodes accounts for the age of the crop. Sometimes a crop will grow tall/larger simply because it is developing rapidly or getting ‘older’. Rapidly developing crops (eg ones putting on fruit rapidly) sometimes need more grow to support the fruit as they develop. Also measuring distance between nodes after they are extended can mean that it is too late to manage the excessive growth easily.
Varietal Response
Varieties do grow at different rates in different environments. A cotton grower has two varieties and notices that one variety seems totally different to the other in terms of its growth. It is taller, leafier and has less fruit on the plant. They can be unsure why this is happening.
Cotton varieties differ from each other in terms of their indeterminacy (it ability to grow fruit) )) and the way present their leaves (leaf area) and the rate of fruit growth. Generally a more indeterminant variety will put on fruit for longer as it naturally maintains the vegetative/reproduction in favour of the vegetative part. Therefore, the greater the indeterminacy the more likely a variety will require growth regulant throughout the season. The less indeterminate, the less likely a variety would require growth regulant. Knowing the growth characteristics of the variety is very important in understanding the likelihood of growth regulant being applied, and when it might be applied. Contact your local CSD extension agronomist to find out what the lilkelyhood of applying a growth regulant is with the variety you have selected. It is important thought that all varieties should be monitored for VGR and fruit growth (counting fruit) throughout the season as sometimes when conditions are right, varieties can behave more or less indeterminate. If bolls numbers are not adequate or VGR remains high undertaking some plant mapping for retention may assist in understanding the reasons for these issues. The use of CottonTracka can assist with these decisions.
Plant Growth Habit NAWF
Figure: An example for Nodes Above White Flower (NAWF) for varieties with different determinacies. In this example the aim would be to get the crop to approach cutout (4 NAWF) at the end of January or early February (depending on the variety).
NAWF and Cutout Response
Using a growth regulant to reduce NAWF leading into cutout can be a very important decision. One key aim in managing a crop post flowering is to reduce NAWF until it gets to 4 NAWF ( cutout) at an appropriate time such that flowers have time to mature before harvest. After this point the fruit will not have time to effectively produce significant increases in yield’ and can substantially lower fibre quality. But what happens if flowering is late due to insect damage causing loss of fruit or regional weather patterns have influenced delayed first flower?
Use VGR to get a good understanding of where the crop is up to and how the vegetative/reproductive balance is travelling optimally, but if NAWF is maintaining but if the crop maintains 8-9 NAWF throughout the first half of January the crop will also probably need some growth regulant to start pushing the crop towards cutout (4 NAWF). Sometimes NAWF will remain high under good vegetative conditions such as good nutrition, high humidity, short irrigation interval or low fruit retention. In these circumstances growth regulation may be required to support an eventual cutout rate is applied in at the time when you want your crop to cutout (for some locations this can be the second half of January).
Cutout rates at the end of the season are also applied to even out the crop. Plants in a field grow can grow at slightly different rates. To assist defoliation and harvest it is important for bolls are maturing at a similar rate.
Summary
The balancing of the vegetative/reproductive fruit equation is an important management decision for realising yield potential. There is no better way of ensuring appropriate growth regulator management by having ‘Feet in the field’. Use the VGR (or a similar concept) to work out where how the crop is responding to its environment at important times during its growth. Understand that many factors influence how the crop grows and reacts to plant stress (regardless of variety). One must not assume that a crop will require growth regulators unless the circumstances of the crop are assessed. It is especially important when using growth regulants that that the plant is not under or about to be placed in a stress situation as it will potentially reduce yield.
Other Indicators at first that may indicate Growth Regulation based on CSD data.
| Parameter | Mepiquat® Indicated |
| Height | >70cm |
| Growth Rate | >2.5cm per day |
| Node Growth rate | <3 days per node |
| Max. Internode Distance | >7.5cm |
| Vegetative Nodes | >7 |
| Bottom Five Retention | <60% |
| Top Five Retention | <80% |
| NAWF @ 1st Flower | >8.5 |
| Weather | Cloudy/Rain |
| Water Relations | Excessive |
*based on CSD data