ALBERT

Description: In New Zealand, pastoral farming for dairy and meat production is the major land use. As with any agricultural production system, weeds are a threat to efficient pasture production in New Zealand. In this review, we outline the problems caused by weeds in New Zealand pastures, and the management strategies being used to control them. There are currently 245 plant species from 40 plant families that are considered to be troublesome weeds in New Zealand pastures. The application of herbicides is an important approach to manage weeds in New Zealand pastures; however, a key to the success of these pastures is the use of clovers in combination with the grasses, so the challenge is to find herbicides that selectively control weeds without damaging these legumes. The use of spot spraying and weed wiping are often required to ensure selective control of some weed species in these pastures. Non-chemical agronomic approaches such as grazing management and using competitive pasture species often play a more important role than herbicides for weed management in many New Zealand pastures. Thus, integrated weed management using a combination of herbicides and good pasture management strategies leads to the most cost-effective and efficient control of pasture weeds in New Zealand.

Description: The alleles responsible for herbicide resistance in weeds can result in a fitness cost within affected plants. Over 200 cases of resistance to triazine herbicides have been confirmed in a wide range of weed species globally. In New Zealand, Chenopodium album L. was the first species reported as resistant to triazines. Several studies have already shown that triazine resistance in weeds is associated with fitness costs. Our current study provides further information about fitness penalties caused by triazine resistance during the vegetative growth phase of C. album. Triazine-resistant phenotypes produced less biomass and were shorter than susceptible ones prior to the onset of flowering. At an early stage of growth, triazine-resistant plants had lower photosynthetic efficacy and growth rates than susceptible plants, indicated by lower net assimilation rate (NAR) and relative growth rate (RGR), respectively. However, at a later stage of growth, the resistant plants had greater RGR values than susceptible phenotypes, though there were no significant differences in NAR between triazine-resistant and susceptible plants at this later stage. The triazine-resistant plants had less capacity for vegetative growth than susceptible plants during competition with wheat, indicating less ability to capture resources by triazine-resistant plants under competition. Overall, this study has revealed that the triazine resistance allele caused a substantial fitness cost to C. album only at the early phase of vegetative growth stage; thus, the use of crop competition to try managing triazine-resistant C. album plants should occur during this early phase.