Farmers and horticulturists in New Zealand were dealt a blow just before Christmas when the Foundation for Arable Research (FAR) announced it confirmed an instance of glyphosate-resistance in annual ryegrass on a Marlborough vineyard.
This was discovered during a monitoring project led by FAR, part-funded by the Ministry for Primary Industries' Sustainable Farming Fund.
Glyphosate is widely used on farms and also by horticulturists, local authorities and home gardeners due to its effectiveness, low cost and relative safety to operator and environment.
Glyphosate has become the weed killer of choice to control vegetation and, in particular, grass weeds. Due to the fast breakdown of glyphosate, repeated applications are often needed between rows of fruit plants, on road sides, along fence lines and in crop margins. It is by far the most used herbicide in New Zealand.
This resistance means some individual plants in the target species have naturally developed a means of avoiding the effects of glyphosate. This resistance can then be passed through to later generations.
AgResearch scientist Trevor James said the project team was alerted to a suspected case of resistance in autumn, 2011.
"We received a call from a chemical company representative stating that glyphosate was not killing all the weeds, specifically some grasses, on a vineyard in Marlborough," he said.
"We obtained some of the surviving plants and grew them on in the glasshouse until some of them set seed in autumn 2012. The seed we collected was then grown in the spring of 2012, and these plants treated with various rates of glyphosate, we found that nearly half the tested plants showed symptoms of glyphosate resistance."
Recent years have seen an increase in farming techniques depending heavily on the 'chemical plough' to control weeds. Techniques such as direct drilling, reduced cultivation and pasture renewal allow farmers to increase productivity and farm more sustainably.
"Environmental repercussions would include the increase use of, and dependence on, less environmentally-friendly herbicide options; greater dependence on more intensive cultivation leading to greater degradation of soil structure and soil health; and the risk of some weeds spreading as the cost of controlling them would increase," FAR chief executive Nick Pyke said.
"On-farm, the impacts would include reduced income, due to increased chemical costs and reduced crop yields. Removing glyphosate from the suite of available chemicals would also increase the resistance pressure on other herbicides."
Glyphosate-resistant grass and broadleaf weeds are well established as an economically significant problem for Australian farmers, whose experiences are being used to better inform New Zealand farmers. FAR's Avoiding Glyphosate Resistance project recently funded a workshop with experts from the University of Adelaide.
Project manager Mike Parker said the project will continue to bring together representatives from a range of agricultural and horticultural industries, chemical companies and regional authorities to highlight the problem and formulate both national and sector-specific strategies for avoidance.
While work continues overseas and in New Zealand, initial advice is to ensure glyphosate is not the only chemical used on the same paddock year after year.
Instead, the group recommends mixing it with a different mode-of-action herbicide group, every three or four years. This will kill any weeds that may be building up resistance and help break the cycle.