North Carolina cotton farmers who have discarded layby rigs and hooded sprayers might find luck with residual herbicides coated on granular fertilizers for late season control of tough weeds like Palmer amaranth.  

The idea is to reduce trips across the field by applying a residual herbicide and fertilizer simultaneously. North Carolina State University Extension weed specialist Charlie Cahoon likes the system and says it may fit where farmers have abandoned residual herbicides in a traditional layby, or hooded-sprayer, application.  

However, he emphasizes that herbicide-coated fertilizers only work as a part of a comprehensive herbicide program that starts with a good PRE and timely postemergence applications also including a residual. 

Cahoon and his colleagues have been conducting research on herbicide-coated fertilizer for several years now. He said it can be a cost-effective way for farmers to apply residual herbicides and granular fertilizers at the same time.  

At the CHROME AG Expo on Aug 21 at the Peanut Belt Research Station in Lewiston-Woodville, Cahoon highlighted research using herbicide coated fertilizer as a part of cotton weed management program.  

For the demo, cotton was planted in July and then kept clean with glyphosate plus Liberty until ready for herbicide-coated fertilizer treatments. Cahoon and his team then applied the residual herbicides coated on granular fertilizer. They applied 285 pounds per acre of ammonium sulfate which provided 60 pounds of nitrogen to meet the fertility needs of cotton. 

“Where we have abandoned traditional layby herbicide applications, this technique may have value for late season residual weed control while meeting our crop’s fertility needs. When we’ve controlled maybe one or two or three flushes of pigweed already, and we’ve got a cotton canopy established, I think they work really well in helping us with some late season weed control,” Cahoon said at the field day.  

Application timing and rates 

Cahoon went on to emphasize if applied too early in the season, farmers may not see the same results. “When we have put these herbicide-coated fertilizers out earlier in the season and they have to deal with multiple, and often larger, flushes of pigweed, this technique does not look as good as residual herbicide applied in a traditional liquid, broadcast applications.” 

“In the majority of our work, as comparison treatments, we will apply these fertilizers with residual herbicides on them and we will compare them to the same herbicides applied over the top and in a post-directed or hooded application,” he said. 

“Most of this has been primarily targeted at pigweed because it drives a lot of our decisions. When used as a part of a system including multiple postemergence applications prior to application of our herbicide coated fertilizers, we have seen similar control with this technique as our traditional liquid applications whether that’s over-the-top, post-directed, or through a hood,” he added. 

Cahoon and his team evaluated various fertilizer rates typically used for nitrogen fertility in cotton production. They studied ammonium sulfate rates ranging from 143 pounds per acre up to 429 pounds per acre. “The breakpoint seems to be if we get above 200 pounds of fertilizer, we’re getting the same level of weed control as our traditional liquid applications over the top or post-directed.” 

Cahoon said they examined both Zidua and Anthem Flex and a number of different products that you would never think about using in cotton. He said moving forward this application technique may open the toolbox for additional residuals herbicide for cotton that we cannot currently apply over-the-top in a liquid, broadcast application.  

“We’re using treatments like Valor, Zidua, and Anthem Flex coated on fertilizer, that we could never apply over-the-top of cotton in a liquid application because of injury concerns. They all performed similar and offered 90%-plus late-season control of pigweed,” Cahoon said.