A few weeks ago, I had the opportunity to moderate a panel discussion about emerging threats in alfalfa at the Western Alfalfa & Forage Symposium in Sparks, Nevada. The panel included perspectives from a university researcher and extension specialist, extension agronomists and an alfalfa grower to offer insights into the newer pests that alfalfa growers should keep an eye on in 2024.
Palmer amaranth is a primary threat
Of all the emerging threats in alfalfa, panelists agreed that Palmer amaranth is the most concerning. This pigweed is well known for being difficult to control and is spreading across the major alfalfa-growing regions of the U.S. As Michael Rethwisch, farm adviser for the University of California Cooperative Extension, noted during the panel discussion, this difficulty comes from its ability to grow 2 to 3 inches per day, its high seed production ranging from 100,000 to 500,000 seeds per plant and its resistance to most herbicides.
To manage Palmer amaranth, Rethwisch recommended the use of preemergence herbicides. “In this situation, that ounce of prevention is definitely worth that pound of cure,” Rethwisch said. He also noted that without early intervention, growers may need to apply a burndown midseason. This is unfortunate but may be necessary if Palmer amaranth takes over an alfalfa field.
The first step in controlling weeds like Palmer amaranth is proper identification. Earl Creech, professor and extension agronomist at Utah State University, shared that Palmer amaranth is fairly easy to identify compared to other pigweeds. “It’s odd because it’s very distinctive,” he said.
If you are concerned about a pigweed on your farm, reach out to an expert to get clarification and identify what it is. Contact an extension specialist from your state university, the crop advisers that you work with or even a technical sales representative at BASF. Do not ignore it. If it is Palmer amaranth, it’s not going to just go away. Taking action sooner rather than later will give you a better chance at controlling it.
Additional weeds of concern
While Palmer amaranth should be a top concern for alfalfa growers, there are other weeds to be aware of heading into the 2024 season. As part of the panel discussion, Giuliano C. Galdi, agronomy and crops adviser for the UC Cooperative Extension, shared that growers in his area of the Central Valley of California are also keeping an eye on fleabane and purslane, as they have developed some tolerance to glyphosate. While resistance is certainly a concern, he did advise that glyphosate is still an important tool that alfalfa growers should keep using, just not for every single application throughout the entire season.
Rulon Fowers, an alfalfa grower at Circle F Farms in Utah, reiterated the importance of spraying weeds while they’re young. In his area, kochia is a weed of particular concern. “If you don’t spray kochia when it’s young enough, you’re never going to kill it. From there, it can grow into a more resistant variety, and pretty soon, you won’t be able to kill it.”
Insects are still an issue
In addition to weeds, the panelists have several insect pests on their list of potential threats for 2024. According to Rethwisch, every year is slightly different when it comes to problematic insects in alfalfa.
One factor affecting insect pests is changes in insecticide use. Rethwisch noted that growers are moving away from the use of pyrethroids for weevil control. This has resulted in an influx of other pests, such as potato leafhoppers.
Changes in climate patterns also contribute to changes in insect patterns. Rethwisch shared that last year, the Southwest desert areas of the U.S. had a lot of wind as well as some rain. This led to an uncommon pest in the region’s alfalfa production areas. “We had a lot of fields turn yellow in May and June. This was very unusual for us, and the problem turned out to be spider mites.”
According to the panelists, aphids will once again be a pest to worry about in the upcoming alfalfa growing season.
Rethwisch shared that in Australia, alfalfa growers are dealing with insecticide-resistant blue alfalfa aphids. This is something that U.S. growers will need to keep a watch out for in the future, as California pest control advisers (PCAs) noticed that insecticides did not work quite as well in 2023. While resistance has not been proven, it is certainly something to be concerned about.
Fowers agreed that the blue alfalfa aphid is one of the most problematic pests he’s encountered and is something alfalfa growers need to keep an eye out for. He also noted that stem nematodes have been increasingly problematic. “Unfortunately, there’s no treatment for it,” he said. “You just have to plant the highly resistant varieties and hope that it doesn’t come back.”
In looking ahead to the 2024 growing season, the panelists provided some helpful guidance on which pests alfalfa growers should be aware of. Their insights are greatly appreciated and give growers some important things to consider as they plan for the upcoming season.
A recording of the full panel discussion, as well as the other presentations from the Western Alfalfa & Forage Symposium, is available on demand. I encourage you to watch and learn more about the emerging pest threats in alfalfa ahead of the upcoming alfalfa growing season.
