As more weed species show resistance to glyphosate in orchards and vineyards, almond growers should focus on a resistance management weed control program that blends pre- and postemergent control for winter weeds and, where necessary, consider sequential applications to extend pre-emergent control to summer grasses.
UC Cooperative Extension weed specialist Dr. Brad Hanson said that managing for herbicide-resistant weeds requires monitoring and record-keeping to understand weed history in the orchard, rotating among multiple modes of action and, where necessary, thinking differently about applications to get residual control with pre-emergents through the summer.
Junglerice Resistance
“A lot of the issues with glyphosate resistance in almonds and other tree crops the last few years have been focused on winter weeds like horseweed, fleabane and ryegrass,” Dr. Hanson said. “But in recent years, we are also seeing more issues with summer grasses like junglerice.
“Lately, the discussion has revolved around using existing weed management tools in a different way without increasing herbicide load and cost to address this new reality.”
Almond growers have excellent broad-spectrum pre-emergent tools to control the winter weed spectrum. Hanson said tank-mixing different classes of chemistry for winter weeds is a very good strategy to broaden the weed control spectrum, but does not necessarily enhance duration of the application’s efficacy. Applying higher rates of broad-spectrum pre-emergents in fall can enhance residual duration, but is not necessarily the most cost-effective and environmentally sustainable approach.
Sequential Applications
Instead, Hanson said, growers with a history of summer grasses might want to consider extending residual control by using sequential applications of pre-emergent grass herbicides added to the winter pre-/postemergent tank mix and again at spring burndown. Growers with a history of summer grass problems should consider sequential applications that move some or possibly even all of the grass pre-emergents to spring burndown treatments to stretch residual activity into May, June and even August.
Hanson acknowledged incorporation can be an issue in drip-irrigated orchards, and suggested growers in that situation work to time applications in advance of a rain in early March.
“I think the cost should be nominal because we are moving the same herbicide to a time when growers are already making a pass across the field,” Hanson said. “We are not necessarily increasing the herbicide load with this strategy, but instead are using our knowledge about the specific weed problems present in an orchard and careful management to efficiently control weeds in a more environmental and economically sustainable approach.”
For more information, see Hanson’s presentation from last year’s Almond Conference here.
