Experimental investigation and modelling of the impacts of cotton picker traffic on vertosol soil compaction and potential yield under random and controlled traffic farming systems

Experimental investigation and modelling of the impacts of cotton picker traffic on vertosol soil compaction and potential yield under random and controlled traffic farming systems

The John Deere 7760 (JD7760) cotton picker is used worldwide on mechanised cotton farms. More than 80% of Australian cotton farmers use it. A modified version, called CTF7760, was also adapted to controlled traffic farming (CTF) systems. The JD7760 has improved operational safety and efficiency, and requires less operating and labour costs. However, its weight (≈32 tonnes) is about twice that of previous models which raises concerns about soil compaction.

Vertosols are widely used for cotton production globally. It constitutes about 75% of soils under cotton production in Australia. However, Vertosols are highly susceptible to compaction even with one pass of machinery, especially under wet soil conditions. Soil compaction could translate into significant losses in crop yield and farm returns. The magnitude and distribution of soil compaction is dependent on factors such as wheel load, soil-tyre contact area, tyre inflation pressure and soil conditions. Controlled traffic farming, recently adopted by some Australian cotton farmers, is one of the effective solutions for reducing soil compaction. Nevertheless, the majority of Australian cotton farmers continue to use the conventional random traffic farming system (RTF).

Previous studies on soil compaction due to JD7760 traffic focused on cotton crop response across the overall field. None appears to have investigated compaction and cotton response at the single row level. Thus, the aim of this study was to investigate the influence of soil compaction due to JD7760 and CTF7760 traffic on a row by row basis. This study involved field trials in 2016 and 2017. These trials were also used to validate the soil compaction model (SoilFlex) and cotton yield model (OZCOT- APSIM).

Three farms with different traffic systems located at Koarlo (RTF), Undabri (RTF) and Yambacully (CTF) in Queensland, Australia were examined as study sites. Soil water content (Swc), dry bulk density (Pb) and soil penetration resistance (SPR) were measured before and after harvester traffic to a depth of 80 cm to assess the degree of soil compaction. These parameters were measured in cotton rows numbered Row 1, Row 2 and Row 3. At the RTF sites, Row 2 was located between the front dual-wheels of the JD7760 while Row 1 and Row 3 were located on the outer and inner sides of the wheels, respectively. At the CTF site, CTF7760 wheel traffic was between Row 2 and Row 3. Row 1 was separated from the wheel by Row 2 and a furrow due to harvester modification.

Vertosol response to rainfall and seasonal climatic variability was also monitored from October 2016 to May 2017 after harvest. Two novel approaches were introduced for row by row yield data collection from the JD7760: (1) harvesting of a single row at a time and (2) use of harvester CAN-BUS to extract yield data for each row. Harvester efficiencies based on yield losses were also determined.

It was found that increasing Swc due to rainfall in early October 2016 resulted in Vertosol swelling in the topsoil under both RTF and CTF. This led to a slight decrease in Pb and SPR and provided some compaction alleviation. High temperature in January 2017 resulted in Vertosol shrinkage which led to a significant increase in both Pb and SPR in the topsoil at all sites. The site under CTF exhibited lower sensitivity to seasonal variability with a lower rate of moisture loss (7%) in the topsoil for the period between January 2017 and May 2107, as compared to the RTF sites (18%). Significant compaction was observed after one pass of the JD7760 in the depth of 0–30 cm under both RTF and CTF. Compaction due to CTF7760 traffic in the cultivated area was however significantly lower than that of the JD7760.

Traffic over the furrows led to significant compaction which spread to neighbouring cotton rows and directly affected cotton yield. At the RTF sites, the 0–20 cm soil layer of Row 2 was the most affected by harvester traffic as it showed the highest Pb and SPR compared to Row 1 and Row 3. There was no impact on Row 1 after one pass of the CTF7760 harvester throughout the 0–80 cm depth. Traffic from the CTF7760 harvester covered 33% of the farm compared to 66% for the RTF sites. This means that CTF provided protection to about two-thirds of the farm in terms of soil structure preservation and reduced compaction effects compared to RTF.

Furthermore, it was found that Row 1 produced a higher yield than Row 2 and Row 3 with both the CAN-BUS and hand-picking under RTF and CTF. Row 2 at the RTF sites was the most sensitive to harvester traffic, leading to 21% and 14% lower cotton yields with machine and hand-picked methods, respectively, than at the CTF site.

Cotton yield under CTF was up to about 33% higher than under RTF. The CTF7760 harvester had a superior performance to the JD7760 harvester with 47%, 72% and 74% lower losses in Row 1, Row 2, and Row 3, respectively. The findings obtained with the soil compaction (SoilFlex) and yield (OZCOT-APSIM) models agreed well with field experimental results.

Overall, both field experiments and computer simulation models were employed to achieve the aim of this study. It was found that harvester traffic caused significant compaction in cotton rows and furrows located between, adjacent to, and in wheel tracks under both RTF and CTF in both the topsoil and subsoil, which consequently led to decline in cotton yield. However, this impact was less under CTF. The main original contributions of this study are that it has provided new knowledge and a deeper understanding of the impact of the JD7760 on both soil compaction and cotton yield at the single row level. This information is crucial, not only to Australian farmers, but for improvements in management practices in cotton and other crop farming systems globally. This study has also introduced two new approaches for measuring row by row cotton yield. The findings presented in this thesis represent an important scholarly contribution to the growing body of knowledge related to soil compaction and cotton yield.