Research Highlights

Weed Seedbank Management Aids Resistant Weed Control

By Laura Temple

The development of weed resistance to herbicide modes of action challenges soybean farmers to protect their crops from weed competition. New herbicide tolerance technologies are providing more tools to manage herbicide-resistant weeds.

“In addition to ensuring new herbicide technologies control tough weeds in soybeans, we wanted to see if they help reduce weed seed production,” says Dr. Michael Flessner, associate professor and Extension weed science specialist for Virginia Tech. “We looked at the effectiveness of new herbicide systems on weed control at different sizes, as well as weed seed production and viability.”

The Virginia Soybean Board supported this research on herbicide programs with multiple modes of action. Flessner focused on control of Palmer amaranth and common ragweed, two key problems for Virginia farmers. 

“In Virginia, we currently just have ALS and glyphosate resistance in Palmar amaranth and common ragweed,” he explains. “We know these weeds can develop resistance to other modes of action, like PPOs, and we want to reduce the chances of additional resistance becoming a long-term problem.”

Based on three years of data, Flessner believes the best way to do that is to control weeds when they are small and prevent additions to the weed seedbank. And this research found multiple modes of action most effective for both goals. 

Reducing Palmer Amaranth Seed Production

Even when competing with a soybean crop, Palmer amaranth can produce hundreds or thousands of seeds, which survive in the seedbank for years. Reducing the number of viable seeds replenishing that seedbank over multiple seasons supports long-term weed control and limits development of resistance.

Flessner’s research trials included an early application of glyphosate to ensure that glyphosate-resistant Palmer amaranth was studied. Herbicides were applied to weeds at three different sizes: 2 to 4 inches, 4 to 8 inches, and 8 to 12 inches. The results show that seed production increased with size at herbicide application. 

But regardless of size, glyphosate-resistant Palmer amaranth treated with glyphosate produced more than twice the viable seeds of the untreated weeds per plant, due to eliminating other weed competition for the weeds. Although adding just one effective mode of action to glypohsate, in this case fomesafen, a PPO inhibitor, prevented seed production in smaller weeds, it wasn’t effective on larger weeds. 

 “Treatments with at least two effective modes of action, such as 2,4-D, dicamba or glufosinate with fomesafen, very effectively eliminated seed production in smaller weeds,” Flessner says. “The same treatments provided adequate seed reduction on the 8- to 12-inch weeds. But it is clearly best to control weeds when they are small.”

However, the prolific nature of Palmer amaranth means that weed escapes are inevitable. The research also looked at seed production following simulated rescue treatments, made when female Palmer amaranth reached first inflorescence, or the formation of its first flower. 

At this stage of weed growth, treatments with multiple modes of action were most effective in both weed control and reducing seed production in the herbicide-tolerant soybean systems studied. Most treatments with two or more modes of action reduced seed production to below 100 seeds per plant. However, herbicide treatments did not reduce the germination rate or viability of those seeds compared to the untreated check. 

Common Ragweed Trends Similar

The trends in control of common ragweed mirrored the results for Palmer amaranth, though seed production is much lower. Again, results showed that applications of glyphosate to glyphosate-resistant common ragweed actually increased weed seed production due to lack of competition.

Fortunately, herbicide treatments with multiple effective modes of action, including 2,4-D, dicamba, or glufosinate with fomesafen, nearly eliminated common ragweed seed production. 

These results reinforce the importance of herbicide timing and the need for multiple, effective herbicide modes of action. Farmers need to think critically about including glyphosate in weed control programs for fields with known glyphosate-resistant weeds, especially if they hope to reduce the weed seedbank over time.

Published: Jul 19, 2021