The impact of herbicide management on long-term changes in the diversity and species composition of weed populations

Field vegetable systems face challenges to maintain sustainable weed management, including a reduction in available herbicides and encouragement towards reduced tillage. In a 9-year study, six herbicide products were compared, each at three rates, with a single product per plot. These were assessed in a minimal cultivation system designed to exert maximum pressure for change in weed populations, to assess for predictable changes in these populations. Weed density and species number declined with increasing herbicide rate, confirming that some species are able to survive in reduced rates. Pre-emergence herbicides resulted in a larger number of species, greater species diversity and lower species dominance, compared with post-emergence products. Species numbers increased over the first 6 years, with emergence periodicity coinciding with springtime soil disturbance. The number of species with ecological functional value increased in response to repeated use of single herbicide products. Observed species shifts illustrated complex responses to the combination of three separate drivers: changes in the dominant periodicity associated with tillage timing; a response to herbicide product and rate related to species susceptibility; and changes in community dynamics caused by variability in weather and the interaction with herbicide efficacy. Improved understanding of the effects on weed communities of the interactions between these drivers and the cropping system is essential in achieving a balance between sustainable weed management and the provision of ecological function.