FACTORS WHICH LEAD TO INCREASED SEEDLING MORTALITY

Field and Bed Conditions at Planting

Through research funded by the FastStart Research & Development Fund (a collaboration between CSD and Syngenta) and in partnership with the CSIRO, CSD have been studying what the ideal planting conditions are for dryland and irrigated cotton. By examining soil strength, dispersion potential, stubble and trash levels, clod size and distribution, moisture content and bed consolidation over the past few seasons, the CSD Extension and Development team have witnessed a considerable variation in
the establishment achieved by growers within the CSD Ambassador Network and trial programs.

Figure 1 shows the establishment results from 362 crops across 7 planting years ranging from 22% to 100% based on planting date. The same seed lot can produce variable establishment depending on the field and planting conditions. Year on year, CSD sees a lot of variability in the trial program and Ambassador Network in terms of establishment percentage. These are the same seed lots with similar germination percentage, but the result is not the same. It highlights that there are a number of different aspects which contribute to successful establishment.

Fields within the CSD Ambassador Network were scored on a scale of 1-5 for seed bed conditions with one being ideal conditions, good consolidated beds with consistent small clod size, free of weeds and stubble. A score of five was for back to back conditions with large clods and heavy loads of trash on the surface.

When the scores were plotted against establishment percentages, it highlighted a relationship with the higher establishment in better seed bed conditions (as seen in Figure 2). However, it should be noted although the overall trend is negative, there is a lot of variability in the establishment results especially at level two and three, inferring further influences on establishment. Table 1 provides a simplified example the average establishment based on their field score.

Uneven or cloddy conditions can result in uneven seed placement, poor seed/soil and therefore seed/moisture contact, resulting in staggered germination and gappy stands. In watered up situations, seed movement deeper into the soil profile has also been recorded. Uneven planting depth can be exacerbated by planting at too fast a speed for the conditions, causing excessive bounce and vibration in the planter unit and an uneven traverse across the field.

The amounts of stubble cover, cotton trash and its fate post the planter passing will also have an impact on seed establishment. Worst case scenarios are when trash falls into the planting slot. Growers should ensure options such as trash wipers are benefiting the planting operation as required.

It should also be noted that less than ideal field  conditions can be overcome to achieve good establishment. Good establishment results have been witnessed when planting speed is reduced to maintain accurate seed depth, the traffic light temperature recommendation is followed and care is taken in irrigating to establish the crop.

EFFECT OF TEMPERATURE ON GERMINATION

Temperature plays a vital role in the rate of development and germination of a cotton seedling. Cotton is a temperature sensitive crop and the way the crop deals with the extremes of temperature is by shutting down or slowing physiological processes in the plant. Below 12°C, the growth of a cotton plant is severely retarded and enzymatic activity within the cotton plant does not function properly until temperatures are above 15°C.

Temperature experienced post planting will also have an impact on the time taken for the plant to emerge. The slower the plant grows, the greater the chance of seedling death occurring through disease and insect damage. Figure 3 illustrates the critical time of an early seedling’s life in terms of sensitivity to cold temperatures.

This is why it is so important to monitor soil and air temperatures to find the appropriate window to plant the crop. It has been an Australian cotton industry guideline for many years that cotton planting should not begin before soil temperatures reach 14°C or above at 10 cm depth, at 8.00am. Planting at temperatures below this will diminish root and shoot growth, reduce water and nutrient uptake and make plants much more susceptible to attack from seedling diseases and insects. 

As the cotton plant emerges faster and starts to generate its own energy from sunlight. Root growth is rapid, minimising the influence of pest and disease pathogens and allowing for the developing root to be firmly footed in soil moisture. Table 2 shows the results of a seedling survival trial conducted in 1988, and clearly demonstrates the effect of the minimum soil temperature on the rate of development and the survival of cotton seedlings. This trial shows the relationship temperature has on cotton
growth and development and this relationship has not changed in the past 30 years.

Table 2: Effect of temperature on cotton seedling survival and growth rate (Constable and Shaw 1988).

In 2018, CSD laboratory team conducted a study to research the effect of low temperatures on rates of germination. A seven-day germination test was conducted at a range of different temperatures on a single seedlot of cottonseed. Figure 4 clearly illustrates the increased rate of germination with increased temperatures within the tested range. While radicle emergence did occur at lower temperatures germination to a length of 40 mm, which is required for the seedling to be considered germinated under the assessment protocol, did not occur below 18°C. Slow development at establishment in field conditions opens the seedling up to insect and disease attack and expends seed energy reserves. CSD recommends utilising the traffic light tool for planting as a guide to planting time decisions, and to ensure that the cotton seed has the best opportunity to germinate and emerge healthily.