With a good arsenal of herbicides on-hand, walnut growers should be able to control weeds in a non-glyphosate system, but, according to Brad Hanson, UCCE weed specialist at UC Davis, it will take better management and be more expensive.

Speaking at the 2021 Walnut Conference, a virtual event sponsored by West Coast Nut magazine, Hanson said growers will face several challenges if they elect not to use glyphosate in their herbicide program.

“The biggest challenges will be related to timing,” Hanson said. “Glyphosate is forgiving of weed size for the ones that it works on, [while] large weeds can be difficult to control with contact herbicides like paraquat or glufosinate. And you are going to have a little bit more of a challenge with contact herbicides on grasses.”

Weed types that will be most challenging in a non-glyphosate system include perennials, such as bindweed, Hanson said. “Once established, those types of weeds are going to be really difficult to control with preemergence or contact herbicides,” he said. “In grasses, we can do okay with the ACCase herbicides. We are much more limited in terms of translocated activity with broadleaves, [and] timing is going to be a challenge.”

In a question-and-answer session after his presentation, Hanson said he doesn’t believe that EPA will ban glyphosate, the active ingredient in Roundup, anytime soon. “So far, there has been no indication that EPA thinks that it is carcinogenic, and there haven’t been any changes to the label at the federal level,” Hanson said. “In fact, in a 2020 registration review, EPA clearly stated that glyphosate is not carcinogenic.”

Market forces, however, could be another story.

The market factors seem more likely to drive changes in specialty crops like walnuts, he said. “So, if buyers or food companies demand something gets done, that is probably more likely to have a faster impact than a regulatory change on that particular herbicide.”

According to herbicide use data that Hanson displayed, glyphosate was used on 354,789 acres of California walnuts in 2017, or about 93% of the state’s 380,000 bearing and nonbearing walnut acres. Oxyfluorfen, or Goal, was applied on 197,471 acres that year, accounting for the second-most widely used herbicide in walnuts, followed by glufosinate, which was used on 118,698 acres.

More Preemergence Usage

Notably, Hanson said, the data showed significant increased use of preemergence materials, such as indaziflam (Alion) and rimsulfuron (Matrix) over previous years. Use of penosxulam (a component of Pindar GT) also was up dramatically.

“I definitely see a trend of increase in preemergence use over the last few years,” he said.

In general, Hanson said, compared to other specialty crop producers, walnut growers have access to a good supply of herbicides. “We have quite a few herbicides registered in walnuts,” he said, “both preemergent herbicides and postemergence materials.”

In terms of modes of action, however, the options are limited, he said, and no new modes of action have been added for the past several years.

Fortunately, he said, with some exceptions, herbicide resistance in walnut orchards is primarily isolated to glyphosate-resistance in broadleaves, like horseweed and fleabane, and winter grasses. “Winter grasses, and specifically ryegrass, are still far and wide the greatest problem in orchards from a resistance standpoint,” Hanson said.

Hanson noted that there have been reports of annual bluegrass, or Poa annua, showing resistance in both the Sacramento and San Joaquin valleys, but it is not believed to be as problematic as the ryegrass. There have also been reports of Palmer amaranth resistance in California, but he noted there has been little evidence of that in tree crops to date.

Stacked Resistance

Researchers also in recent years have seen evidence of multiple resistance, or stacked resistance, in isolated weeds. In most cases, that involves resistance to a second herbicide in addition to glyphosate, Hanson said.

“We have reports of small acreages with paraquat resistance in annual bluegrass, or Poa,” he said. “And then, if you start looking at ryegrass populations, you can find lots of different combinations of paraquat resistance, ACCase resistance and some resistant to glufosinate, in addition to resistance to glyphosate.

“So far, those haven’t been hugely problematic to my knowledge,” he continued, “but they are out there, and a species like ryegrass is certainly prone to develop resistance rapidly.”

In terms of summer grasses, Hanson said junglerice has shown resistance to glyphosate in recent years, and he has fielded reports of glyphosate resistance in sprangletop and witchgrass, although those reports have not been confirmed.

Threespike goosegrass is another weed worth keeping an eye on, Hanson said. A short-statured and short-lived perennial, the weed is a very prolific seed producer and very tolerant to glyphosate.

“It has a tendency to persist mostly in orchards that are glyphosate-only programs or in the orchard middles, where herbicide use is less intense,” Hanson said. “It seems like most of our preemergence herbicides can work very well on the seedling stage of this grass, so if you have a good preemergence program, you can keep it out of the strips. But once it is established, it can be fairly difficult to control because of its tolerance to glyphosate. This is one to be aware of if you see it in your orchards.

“Definitely keep an eye out on all these summer grasses because they emerge in May, June and July when our preemergent wintertime programs are starting to run out of steam,” he said.

Sequential Treatments

Hanson noted that he and a postdoctoral student recently launched a project where they are looking at sequential preemergent herbicide treatments for extended summer grass weed control. “The whole approach was in regard to how best to use preemergence herbicides,” he said.

Typically, he said, when looking for longer control, the common practice was to increase the rates applied in the winter. “That always seemed like an inefficient way to solve that problem. So, what we looked at was a second preemergent herbicide application, targeting the timing just ahead of summer grass control. So, this was basically a one-two punch – a winter program, and then a March or April preemergence application specifically targeting grasses.”

The researchers applied several combinations in the trial at two sites, such as Alion in the winter followed by Prowl in the spring; or Tuscany followed by Prowl; or Pindar GT followed by Prowl.

“Essentially, in each case, we got very good summer grass control where there was one of the followed-by treatments,” he said, “which makes sense. If you put the preemergence herbicide on shortly before the problem emerges, you are going to have much better control than if you put it on six months before that weed starts to emerge.”

Hanson added that he found it encouraging that, in most cases, whether the spring treatment was at a two-quart or four-quart rate, it usually resulted in control that was better or at least as good as the four-quart rate applied in the winter.

“So, just by using the herbicide more appropriately, based on the target weed’s biology, we actually save money, reduce the pesticide load in the environment and get better weed control,” he said.