Controlled traffic for vegetable production: challenges, benefits and opportunities

Soil degradation is an issue of global concern, given current projections of the need to increase food production, on diminishing soil and water resources, for an expanding population. Compaction is a key component of soil degradation; a key driver of compaction is mechanised agriculture. Early steam technology featured machines that were heavy for the power they delivered. Significant reductions in machine weight came with the introduction of the internal combustion engine in the late 19th century. That was the last time there was a marked decrease in machine weight. Farm machinery has been increasing in size, power and weight ever since, leading to significant increases in soil loading in the past few decades. The attraction of larger machinery is labour productivity and efficiency. Its major disadvantage is increased severity and depth of soil compaction, leading to a range of issues reflective of reduced soil function: reduced infiltration, aeration, soil biology, soil water storage, drainage, root growth and crop yield, and increased runoff, soilborne disease pressure and nitrous oxide emissions. Tillage has been used to remediate the impacts of soil compaction for the entire history of mechanised agriculture. This has traditionally been through the use of mechanical implements, although interest is increasing in the role of plant roots (i.e. biological tillage) to relieve compaction and improve soil function. As technology has improved, more attention has been given to traction and load support systems that reduce the soil impacts of machine loads. This approach has seen the evolution of rubber tracks, and pneumatic radial and hi-flex low ground pressure tyres. Despite these advances, tyre loads have increased to such an extent that it is difficult to reduce soil stresses below the limits that impact soil function and crop performance. Controlled Traffic Farming (CTF) presents an alternative approach to the management of soil compaction. It is an approach that not only avoids the negative consequences of traffic-induced soil compaction on plant growth and production, but also make use of the positive aspects of compaction to improve traction and trafficability. The characteristics that define CTF are: 1. All machinery has the same or modular working and track gauge width allowing establishment of permanent traffic lanes. 2. All machinery is capable of precise guidance along the permanent traffic lanes. 3. Farm, paddock and permanent traffic lane layout is arranged to optimise drainage and logistics. Ideally, all three of these principles would be in place, although because controlled traffic is often retro-fitted to existing farms, layout is often compromised. The first two points above are of prime importance regarding soil compaction management. Ideally, all wheel tracks for all machinery would be coincident, resulting in the minimum possible area of wheel tracks. There are many situations in which this is difficult to achieve, making compromise inevitable with existing mechanisation systems. The benefits of controlled traffic are numerous, and regardless of the industry sector considered, extend to the entire cropping system. Benefits include: improved soil structure, biology, infiltration, soil aeration, soil water storage, internal drainage, water and fertiliser use efficiency, crop growth, yield, timeliness, root crop harvest and economics and reduced runoff, erosion, soil borne diseases, tillage energy use, GHG emissions, number of tillage operations, tillage equipment inventory, tractor size, capital and operating costs and operating hours. Implementation of a controlled traffic system is not without its challenges. Barriers to adoption may include: machinery modification costs, erosion and rutting of compacted traffic lanes, machine tracking on compacted wheel tracks, reduced field efficiency due to traffic constraints, loss of cropped area if wheel tracks are left bare and reduced yield from wheel tracks if they are cropped. CTF has had its biggest commercial impact in the Australian grain industry over the past 25 years. Uptake in other industries, including the vegetable industry, has been much slower, with many of the reasons related to machinery. There are a number of soil management reasons as to why the vegetable industry would benefit from the introduction of controlled traffic. Characteristics of the vegetable industry that create particular challenges for sustainable soil management include: ‚Äö constantly moist-to-wet soils, particularly in temperate climates, as winter dominant rainfall and summer irrigation frequently create conditions conducive to soil compaction ‚Äö high input/high value per hectare (compared to rain-fed grain production) such that in-season productivity is usually prioritised over long-term sustainability ‚Äö high levels of soil disturbance in some harvest operations (e.g. for roots and tubers) ‚Äö high materials handling rates at harvest ‚Äö high traffic intensity in terms of load and tracked area ‚Äö limited flexibility to delay harvest in order to wait for improved soil conditions ‚Äö intensive tillage ‚Äö low levels of soil cover for protection of the soil resource ‚Äö diverse crop rotations requiring a range of machinery designs and functionality. In relation to this last point, the industry is constrained in its adoption of controlled traffic due to the complexities of achieving dimensional integration across a range of machines (particularly harvesters) used in the production of a variety of crops with differing characteristics, such as: ‚Äö growth habit (e.g. fruiting cf root and tuber crops) ‚Äö spatial arrangement (e.g. rows cf broadacre) ‚Äö harvest processing requirement (e.g. separation of harvested parts from the soil cf removal of harvested parts from the plant). This thesis brings together a body of research related to controlled traffic as it applies to the Tasmanian vegetable industry, although the findings would be relevant to many vegetable production areas that feature mixed crop rotations, mechanised harvest and undulating topography, either singly or in combination. It begins with an analysis of the diversity of machinery used and the impacts posed by the undulating topography of the main vegetable production regions. These circumstances are in significant contrast to the limited equipment suite and flat to mildly sloping topography that are features of the Australian grain industry, which has experienced successful adoption of CTF. The analysis showed that integration of machinery is a very significant challenge. Few machines used in the industry are suited to modification to enable CTF operation. Seasonal CTF provides a starting place for adoption. Mapping analysis showed that steeply undulating topography may not necessarily present significant challenges. Many fields already feature working layouts that are consistent with slope direction, which has advantages for both machinery operation and soil conservation measures under a controlled traffic system. The thesis then moves on to the response of soil to the implementation of controlled traffic. Field trials of controlled traffic in different production environments in Tasmania demonstrated improvements in soil physical properties, and a reduction in tillage operations (20 - 60%), compared to conventional production systems. Some measures of soil properties varied over the course of the research due to the limitations of machinery used, leading to compromises in the integrity of the controlled traffic system. Accurate machinery tracking in undulating topography proved challenging. An investigation of the impact of controlled traffic on the soil arthropod assemblage was undertaken. Spring sampling showed improvements in arthropod abundance (p<0.01) and richness (p<0.1), and collembolan abundance (p<0.01) under controlled traffic. Arthropod abundance was also greater in winter (p<0.1). While improvements in arthropod abundance and richness do not necessarily create an advantage for vegetable production, the higher populations and diversity suggest improvements in the soil environment which imply benefits for soil biology in general. The potential economic benefit of controlled traffic systems is an important consideration. Without fully integrated controlled traffic systems to serve as economic case studies, modelling was used to determine the difference in returns of three different vegetable farming systems. Data from conventionally operated farms and controlled traffic research helped inform the modelling. With many uncertainties to be resolved regarding vegetable industry conversion to controlled traffic, a conservative approach was taken to estimate the changes likely to occur due to adoption. Modelling indicated median increases in average returns of up to 29%. Economic modelling was based on the assumption that existing machinery could be modified to enable implementation of controlled traffic, an approach that remains unproven. A possible solution to the constraint of machinery incompatibility is the gantry or wide span (WS) tractor, which would provide a mechanisation platform suited to the adaptation and mounting of a wide range of implements and harvest technologies. Wide span tractors are not commercially available, although a Danish prototype used in field trials on a commercial farm in 2014 was used as the basis of the modelling. Once again, conservative estimates of costs and returns were applied to case study farm scenarios. Median increases in average returns of up to 59% were indicated. Sensitivity analysis showed that the most important cost factors were machinery capital and potential reductions in harvest efficiency, while predicted improvements in crop yield offered the most significant benefits. Wide span tractors offer a ‚ÄövÑv=controlled traffic friendly'approach to mechanisation for the vegetable industry. A range of conventional implements could be mounted ...