It's been well-documented how herbicide resistance spreads from one field to another — in some cases one state to another — through seed movement, whether it's transferred via combine, birds, pets or people. But how about pollen-mediated gene flow? Recent research conducted by Amit Jhala, University of Nebraska Extension weed management specialist, and doctoral student Debalin Sarangi shows just how well resistance can be transferred via pollen.

SPREADING THROUGH POLLEN: While it's been well-documented how herbicide resistance can spread from one field to another through seed movement, recent research conducted at UNL's South Central Ag Lab shows how well resistance can be transferred via pollen-mediated gene flow.

"We rarely think about resistance moving through pollen activity because it's not very visible. Seed movement is very visible. When we combine that field and it has glyphosate-resistant common waterhemp, or any other resistant weed, and we don't clean the combine, we're spreading seeds from one field to another field," says Jhala. "Pollen movement is happening all the time. It's not something we can prevent very easily. It's not something we play a lot of attention to. But with these dioecious species [meaning male and female plants are separate — the case with common waterhemp and Palmer amaranth] it's very important."

This study, conducted over the last two years at UNL's South Central Ag Lab near Clay Center, involved growing glyphosate-resistant common waterhemp and susceptible common waterhemp in greenhouses until plants were 2 to 3 inches tall, and then transplanting them into the field.

The resistant plants were transplanted in a 20-meter square in the field. In the surrounding area, susceptible plants were planted at varying distances from 1 to 200 feet in eight different directions, making a kind of giant wheel with spokes of susceptible female waterhemp, and a hub of resistant male waterhemp at different distances from the resistant plants. They also removed all susceptible male common waterhemp plants from the field to make sure they didn't outcross with female plants.

"We wanted to know how far the pollen can move from resistant plants to susceptible plants, and what the chances of outcrossing are and up to what distance," Jhala explains.

After harvesting seeds from the susceptible population at the end of the season, they planted those seeds in a greenhouse to test the plants with glyphosate and determine the percentage of successful outcrossing. Not surprisingly, even at distances up to 200 feet, plants saw 5% to 8% outcrossing during both years of the study. However, at closer distances, with more pollen movement from resistant plants nearby, plants saw 60% to 70% outcrossing.

"Common waterhemp is a dioecious species. The male and female plants are separate. The only way for common waterhemp to reproduce is gene flow or outcrossing. Additionally, a molecular test also confirmed presence of glyphosate-resistant alleles in a susceptible common waterhemp plants that transferred through pollen movement. When they get pollen and fertilizer they produce seeds. That's why they're more prone to be pollinated by wind and spread the resistant gene by pollen," Jhala explains. "Glyphosate resistance is a dominant trait. If a male waterhemp plant outcrosses with a female waterhemp plant, whatever the seeds the females produces they will also be resistant."

That's why it's important to not only adopt a herbicide program with multiple modes of action, but also understand the biology of the weed species to prevent the spread of resistance.

"Zero tolerance is something that's very important" in weed control, says Jhala. "If you have some plants surviving your herbicide application, you need to remove them as soon as possible before they start flowering." Once they start flowering, it doesn't take long for resistance to spread via pollen.