Reducing chemical inputs in vegetable production systems using crop diversification strategies

Vegetable cropping systems are becoming larger, more specialised and increasingly reliant on agro-chemicals to manage pests, diseases and weeds. These trends in vegetable production have resulted in increased efficiencies and allowed producers to maintain profitability in a marketplace with greater competition and declining gross margins. However, concern is growing among consumers about the impacts of chemicals on human health and the environment. This research program explores the benefits and costs of alternative vegetable production systems with increased plant species diversity and their potential to reduce chemical inputs. The first trial conducted in this study focused on strip cropping with the view of adding additional layers of diversity in subsequent experiments. The trial used large plots with mixtures and monocultures of three vegetables: onions (Allium cepa), broccoli (Brassica oleracea var. italica) and potatoes (Solanum tuberosum). These vegetables were chosen to maximise diversity as they all have very different harvested products and do not share any major pests or diseases. This initial trial found that most vegetable diseases were too virulent to control with diversity alone and that onions were very poor competitors and hence not suited to mixed cropping systems. Furthermore, production benefits were found to occur at the zone of interaction, meaning that smaller plots with increased replication could be used in subsequent experiments. There were also trends indicating that the insect pest of broccoli Plutella xylostella was restricted by the mixed cropping system. A cover crop of cereal rye (Secale cereale) was chosen as an additional layer of diversity in the second trial conducted in 04/05, due its ability to be easily killed and rolled to form a thick mat of plant material for suppressing weeds. Results from this experiment found that the numbers of P. xylostella and the aphid Brevicoryne brassicae in broccoli were significantly reduced by the cover crop but not by the broccoli/potato strip crop. Another pest of broccoli, Pieris rapae, was not affected by either treatment. The experiments also showed that there were no significant differences in yield or quality of both potatoes or broccoli, in spite of the fact that broccoli grown in a cover crop matured one week later than broccoli grown in conventionally prepared soil (i.e. a bare soil background). Experiments in 05/06 showed that reductions in the numbers of P. xylostella and B. brassicae in broccoli grown in the cover crop were primarily due to interference with host location and not predation or reduced host plant attractiveness. The reductions in P. xylostella numbers are of particular significance to Brassica producers as this insect has the proven ability to become resistant to every known insecticide, therefore any non-chemical control method could result in substantial reductions in insecticide use and insecticide resistance. However, P. rapae was not affected by the rye cover crop presumably due to superior host location ability and egg spreading behaviour. These results were supported by data from a semi-commercial trial. In contrast to the previous years results, rye cover crop was shown to have significant effects on broccoli growth, reducing the number of leaves, plant biomass and yield as well as again delaying harvest by approximately one week. However, the rye cover crop improved the quality parameters, reduced the severity of hollow stem, eliminated excessive branching and removed the need for mechanical weeding. An economic analysis based on the experimental outcomes of this thesis indicated that using the rye cover crop in a broccoli production system reduced the total variable costs by $323/ha (6.7%) but also reduced the gross margin by $151/ha (5.9%) when compared to conventional practice. However, only a 2% increase in yield, or a 7% price premium due to the reduced chemical use, would be required to eliminate this deficit. The study also showed that mechanical challenges stemming from increasing plant species diversity in existing vegetable cropping systems, could be readily overcome through the modification of existing, commercially available farm machinery/equipment. In summary, introducing plant species diversity into the conventional vegetable cropping system, in the form of a cover crop, showed considerable benefits to broccoli production in terms of reduced insect pest pressure and quality improvements. Strip cropping as a diversification strategy did not result in increased yields or quality and had no significant effect on insect behaviour in the crops studied. Furthermore, this approach would be more difficult to implement commercially than the rye cover crop due to increased management complexity and incompatibility of chemical weed management strategies. Therefore future research efforts should focus on increasing plant species diversity in the vertical plane (above and below) using cover crops, rather than the horizontal plane (side by side) using strip cropping.