Palmer amaranth has emerged as the No. 1 herbicide-resistant weed in many areas of the South and Southeast. The same weed is showing up more often in California. However, it is not resistance that is causing the spread. It is weed shifts, said Wright. As herbicides control one species of weed, another is likely to become problematic.

Palmer amaranth is a nasty weed, Wright warns. “A couple of years ago we were concerned about horseweed. Horseweed is easy to control compared to Palmer amaranth, which produces double or triple the seed of horseweed. ” Wind can carry Palmer amaranth seed a quarter of a mile. It is creeping into fields, orchards and vineyards from ditch banks and roadsides.

Wright is warning growers that left uncontrolled outside of fields invites an in-field problem with not only Palmer amaranth, but other weeds as well.

Herbicide-resistant Palmer amaranth now consumes 1.6 million acres of cropland, according to Wright. It grows at the rate of 2 inches per day. It is forcing no-till farmers to return to tillage systems and hand-weeding and putting residual herbicides back into cotton copping systems. Fortunately, Wright said California cotton growers only briefly abandoned preplant herbicides. They’re now back in the vegetation management strategy.

Wright also identified annual morningglory, pigweed, lambsquarter, sprangletop and barnyardgrass as becoming more problematic to control with Roundup over the past few years.

There are viable alternatives to glyphosate, said Wright. Most are tank mix compatible with glyphosate.

There are also herbicide-resistant traits like Liberty Link in some varieties and in development are cotton varieties resistant to 2, 4-D and dicamba.

“There are a ton of herbicides that will work in cotton weed control. You do not have to rely on Roundup,” he said.

hcline@farmpress.com