Prepared by Dr Greg Constable, CSIRO Cotton Research Unit, Myall Vale Research Station, Narrabri and the Cotton Seed Distributors (CSD) Extension & Development Team, Adam Kay, Robert Eveleigh, John Marshall and Craig McDonald

This article is aimed at presenting and discussing the merits of monitoring some aspects of fruiting dynamics.

That information is useful in cool seasons to determine whether crop development has been up to potential and may enable identification of sluggish crops which need to be pushed. It is important to make such a diagnosis early otherwise crop setback and delay may impact on yield and maturity. 

In the last 10 to 20 years there have been many developments based on new understanding of cotton’s morphology and physiology. This monitoring facilitates a better assessment of crop development status. When done correctly and at the right time this can enable corrective management to prevent or correct any problems.

Pictured above: New CSIRO breeding lines are assessed for agronomic management requirements. This picture shows monitoring squaring nodes (SN) and Vegetative Growth Rate (VGR). 

There are many measurements to monitor a cotton crop. Some of the more common and important data are:
1. Insect scouting for incidence, abundance and impact of pests.
2. Moisture status to accurately schedule irrigation.
3. Nutrient status to diagnose fertiliser requirements.
4. Plant mapping:
a) node numbers and plant height to estimate vegetative growth rate and possible growth regulator application
b) numbers of sites, squares and bolls
c) retention of fruit on important node positions,
d) nodes above white flower (NAWF) and nodes above cracked boll (NACB) to measure crop cutout or maturity.

Obviously many of these measurements can be done at the same time. For example, if insect scouting indicates a large number of mirids present, it may be important to assess whether fruit have been lost, ie: by measuring retention and/or total fruit numbers.

Theory, measurement and experience indicate a number of important key events should be expected in a cotton crop.
1. The first fruiting branch is expected on about the 7th mainstem node of most Australian varieties. About 500 day degrees will have elapsed by this stage.
2. Fruiting branches appear at regular intervals of about 45 day degrees.
3. On a well grown crop with little insect damage, by the time of first flower there are about 8 fruiting branches present ( NAWF = 8). These 8 branches will provide the bolls contributing to a large proportion of final yield, and these bolls generally will have the best fibre quality. If there are fewer than 8 nodes above white flower at the beginning of flowering, yield potential is often reduced and it maybe too late to recover.

The measurement of squaring nodes in a young cotton crop can be a useful tool for monitoring the early development of crops and can give an early indication of any stresses that may ultimately reduce the nodes above white flower (NAWF) when flowering commences. This method has been used successfully by the University of Arkansas in the development of the COTMANTMExpert System.

The squaring node measurement can therefore give an early indication of potential problems and allow early remedial action rather than waiting till the crop flowers before realising there maybe a problem.

The measurement can commence at squaring or 500 day degrees and be carried out through to flowering at 800 day degrees and then nodes above white flower can be monitored through to cut out (3-4 NAWF) at around 1100 day degrees.

The theoretical values for number of squaring nodes (SN) and nodes above white flower (NAWF) at diff e rent accumulated day degree levels is shown in illustration above.

The theoretical values of Figure 1 provides a framework by which to monitor the crops progress, by graphing day degrees and squaring nodes against it. A variation of monitoring squaring nodes is to count squares; This data is also very valuable and is analagous to the original SIRATAC Yield Development Thresholds.

The squaring node number should be used in conjunction with fruit retention when making decisions.

High or low square retention can greatly influence the significance and the interpretation of the squaring node curve.

Where a crop is consistently on or just above pink line it indicates the crop is on track. Measurements well above can indicate the need to evaluate the need for growth regulators.

Measurements well below the line indicate a problem with the crops development that may require a management solution ie: water, fertiliser etc.

Reference: COTMAN™Expert System 5.0

In the case (illustration above) node development is ahead of schedule, this is likely caused by optimum conditions after planting or a low first square position.

If associated with high fruit retention you need to be ready to meet the demands of fruit. If retention is low you may need to monitor VGR (Vegetative Growth Rate) to see if a growth regulator is required.

In the case (illustration right) node development has been delayed. This can be caused by factors such as seedling disease, herbicide damage, high first fruiting node, etc.

If associated with high retention, maintain inputs to get on track. If retention is low, correct those causes to avoid late maturity.