crop showing hail damage

This article applies what we have learnt about cotton growth and development in the CQ environment and look at what to expect once the decision is made to grow the crop on.

(Whether or not a field should be retained vs abandoned will not be covered here, as crop insurance and a range of other factors will affect that decision-making process when significant damage has been sustained.)

Hail damage will vary from minor leaf and terminal shoot damage through to extensive canopy destruction, including stem and fruit loss. The highest impact of this damage is likely to occur either:

  1. Straight after emergence when the growing points are irrecoverably damaged, or
  2. Much later in the crop cycle between mid-flowering and early boll opening.

The first will create a scenario similar to poor establishment with impacts largely linked to the extent and uniformity of remaining viable plants across the field area. The second scenario’s impact will depend on the loss of leaf area and stem terminals at a time when the crop is highly dependent on photosynthate production (to support growing bolls and continued canopy expansion or a large boll load post cut-out). Crop recovery in these instances will depend on the extent of leaf loss, terminal damage, remaining boll load and the time of season.

Where moderate canopy leaf area loss coincides with a mid to late flowering boll load, canopy expansion is likely to stall and scope for rapid compensation will be stymied. Physical injury to the bolls may also predispose the crop to an increased incidence of boll rots and tight lock should further wet weather coincide with boll opening, potentially compounding losses.

With fruit requiring 24-28 per cent more energy to produce on a gram for gram basis compared to stem and leaf, the impact of partial canopy loss during early flowering is likely to be less as the smaller fruit load will exert less competition for assimilates, allowing canopy expansion and compensation to occur more rapidly.

For crops that stall due to reduced canopy leaf area coinciding with higher boll loads, compensation may be significantly delayed. With the boll period (flower to open boll) averaging 45-55 days in CQ, crops may not initiate any substantial re-growth of stem, leaf and squares for a month or more until mature bolls cease competition for existing assimilates.

The capacity for compensation once a crop begins to grow away has been well demonstrated by grown-on crops and more recently the test plantings of double picked cotton. The yield potential for crops that re-grow following a hail-related setback will depend on the number and size of bolls that remain after the event together with the length of season remaining once regrowth is underway (following any delays).

Detailed analysis of crop benchmarking data from 44 separate crops over a 7 year period has demonstrated that crop biomass production (grams of stem, leaf & fruit dry-matter per day) decreases between November and April. This relationship can be extrapolated to calculate boll accumulation rates during the season. Whilst actual growth rates will vary due to differences in hail damage, field characteristics and crop management, the relativity of changes in boll accumulation over time provide some insights for the potential to recover yield.

The highest boll accumulation rates occur for crops flowering and filling bolls during November and December (Fig 1) with the rate of boll biomass accumulation per day declining precipitously from January onwards. Growth rates in individual fields will vary, but in relative terms this data shows that the rate of compensation will be nearly 40 per cent slower during March compared to November. With a faster growth rate and greater season length the capacity for recovery is infinitely better for a crop recovering from hail in early November compared to late January.

The extent of recovery for later damage events will depend on the length of time that boll production can be extended. If a crop damaged by hail in January was able to retain 5-6 bales of yield potential with compensation commencing in March, rates of biomass accumulation recorded in local crops suggest an additional 4-6 bales of cotton could be produced, depending on Autumn conditions.

This potential gain needs to be considered alongside the cost of inputs required grow additional crop biomass (water, nutrition & crop protection) and subsequent farming system opportunity costs that would arise from a delayed harvest. Another factor to consider is that hail damage can vary considerably from one end of a field to the other which can complicate post-recovery agronomic decision making.

Hail recovery decisions are rarely clear cut. However, CQ enjoys a natural advantage in having a longer season compared to southern valleys, and experience with grown-on crops together with recent crop benchmarking data hopefully provides people with a clearer perspective of the opportunity and limitations for crop compensation following deleterious weather events at different times during the season.

Fig 1. Average boll biomass production rates per month calculated from crop biomass benchmarking data 2013-2019 for the Central Highlands.