FAR has received a report of a suspected population of glyphosate-resistant ryegrass on an arable farm in the South Island.
After repeatedly spraying a fence-line using only glyphosate over many years, weeds now appear to be surviving applications and spreading into the adjacent crop.
This is a good reminder that herbicide resistance should be considered when planning fence-line maintenance, says Charles Merfield, more commonly known as Merf.
The viticulture industry has widespread glyphosate resistance in ryegrass, and some other species, after years of repeated applications along grapevine rows.
Herbicide resistance is now an issue in 30% of vineyards nationally and in 50% of Marlborough vineyards.
As glyphosate (Group 9) is a key herbicide in arable crop production, it is vital that growers manage its use to prevent resistance occurring, Merfield says.
“Repeated herbicide sprays of the same Mode of Action are the highest risk factor for developing resistance.
“Once resistant weeds get into fence-lines, the lack of competition means they reproduce and spread very quickly.”
Fence-lines are also an under-utilised resource for ecosystem services, such as supporting beneficial insects like pollinators and natural enemies.
Alternative approaches to fence-line management include physical techniques such as mowing and cultivation/tillage; biological approaches of using beneficial plants to out-compete weeds; and chemical/herbicide options that have a lower resistance risk. These can be mixed and matched.
To encourage growers to re-evaluate their existing practices, a three-year FAR trial is underway at its Kowhai Farm research site near Lincoln to investigate the effectiveness of alternative strategies for fence-line weed control.
“Biological approaches such as using good plants to force out weeds are viewed as the best overall tactic in terms of simplicity, cost and co-benefits such as beneficial insect habitat. The big challenge is to find plant species that do the job well,” Merfield says.
Considerations when choosing suitable fence-line species include:
• it must not be a seed crop,
• must not be harmful to stock,
• needs to be highly competitive to outcompete weeds,
• should remain competitive in winter, and
• grow lower than fence height, or be suitable for mechanical trimming.
Three contrasting species have been chosen for the Kowhai Farm trial: alyssum (Lobularia maritima), browntop (Agrostis capillaris) and broom (Cytisus scoparius).
A potential controversial option, to provoke debate, was the woody weed, gorse (Ulex europaeus).
“In some areas gorse is considered a weed, but it is still common in fence-lines and a valuable early nectar source.”
Physical fence-line management techniques include using vineyard equipment designed to work around posts and vine trunks, such as mowers and cultivators.
Mowing can be undertaken on a regular basis, or used strategically to stop pollen and seed production. Grazing is a substitute for mowing.
Physical techniques, particularly mowing, can integrate with biological approaches, such as browntop.
In terms of a chemical approach, using pre-emergence residual herbicides as alternatives to post-emergence herbicides is an important overall resistance management strategy. Using residuals also applies to fence-lines, with the additional benefit that herbicides that cannot be used in crops are options, allowing more Mode of Action groups to be used.
Avoid using Groups 1 and 2 and to a lesser extent Group 15 in fence-lines, as resistance to these groups already exists on arable farms, Merfield says.
A range of herbicide options is being demonstrated in the trial.
“There is no one perfect solution. Every farm needs to work out their best approach.”