COLUMBIA, Mo. – There is still time to manage resistance to HPPD-inhibiting herbicides in corn this year, but the window is shortening, according to University of Missouri Extension weed scientist Kevin Bradley.

Bradley heard complaints last year from Missouri growers unable to control waterhemp in corn after applying post-emergence Group 27 herbicides (mesotrione-containing products such as Callisto and Halex GT).

“Reports of failures to control waterhemp with pretty much any herbicide are nothing new,” said Bradley. “But in at least some of the reported cases it appeared growers did everything correctly, so the possibility of resistance came to the forefront.”

In the past few months, the MU Weed Science team screened 10 waterhemp populations in their greenhouses from seeds collected at the end of the last crop season. Three of the populations were considered “officially resistant,” another three showed some early signs of resistance and the other four were not resistant at all.

Most of Missouri’s corn is planted already, Bradley said, but if growers haven’t yet put on a pre-emergence residual that contains multiple effective modes of action for waterhemp, there is time to do so in most places. “Waterhemp germination likely hasn’t occurred yet in many areas, but that is going to change very soon.”

Bradley said the most common causes he’s seen for poor waterhemp control following application of a Group 27 herbicide include some combination of these factors:

• Waterhemp plants too big at the time of the application.
• Adjuvants weren’t optimal for the herbicide package and/or the Group 27 herbicide that was applied.
• Rate of the Group 27 herbicide was too low.
• Atrazine not included as a tank mix partner and/or poor environmental conditions immediately before or after the application.

“These are mostly solvable issues and factors that can be corrected, so I think the lesson we can learn here is to make sure you pay attention to all the details when making applications of Group 27 herbicides on waterhemp this year,” Bradley said.

Resistance in waterhemp to Group 27 HPPD-inhibiting herbicides is not a new phenomenon in Missouri, he said. “We screened nearly 200 waterhemp populations collected from Missouri fields back in 2012 and found three populations with resistance then.”

Since 2012, HPPD resistance in waterhemp hasn’t gotten a lot of attention, Bradley said. That is likely because there is nearly no corn-on-corn production in Missouri and there’s greater diversity of herbicides and herbicide mixtures typically applied to corn compared to soybeans.

“We have got to broaden our thinking on weed management and not just keep relying on herbicides alone,” urged Bradley.

Bradley said the results from this recent screening “clearly show there are populations with HPPD resistance out there in Missouri, and as long as we keep spraying these products in corn and placing the selection pressure on waterhemp, the one thing I know about waterhemp is that it will find a way.”