Horseweed: the impact of glyphosate resistance
Horseweed (marestail) was the first confirmed glyphosate-resistant weed species that caused headlines in the United States. Eight years later, glyphosate-resistant horseweed now exists in 17 different states. It threatens the yield potential of row, tree and vine crops.
What is impact of horseweed that is not glyphosate resistant? Usually found in no-till fields, horseweed is a prolific seed producer dispersing seed like a dandelion with an 80 percent germination rate. A single weed doesn’t do much yield damage; however, large, dense populations are extremely competitive. This can cause harvest issues, increasing competition for vital water and nutrients, and complicates management programs, resulting in increased the cost for growers.
Glyphosate-resistant horseweed can be fairly manageable, but it also indicates intense glyphosate use. In areas like Tennessee, resistant horseweed pressure is so problematic that nearly 100 percent of cotton and corn acres, and 20 percent of corn acres are infested with glyphosate-resistant marestail. Other, more detrimental and harder to manage weeds, may be harder to control with glyphosate. In the south, growers are struggling in the face of glyphosate-resistant Palmer pigweed. Developing a weed management program to manage weed pressure before problems arise can help reduce selection pressure and preserve glyphosate technology while protecting yield potential.
Simple management recommendations to keep in mind are:
· Scout fields
· Use effective herbicides with different modes of action
· Use full rates
· Apply when weeds are small (less than 6 inches)
· Control weed escapes
· Avoid spreading seed through machinery or people by thoroughly cleaning machinery and clothing between fields
· Control weeds in fence rows and ditches
For more information:
· Glyphosate-Resistant Horseweed: What’s Next?
· Glyphosate-Resistant Horseweed Bulletin for Southern Growers