After wheat harvest, controlling weeds depends on timing

K-State weed specialist recommends growers pause before a post-harvest herbicide application in times of little moisture

Drought notoriously impacts many facets of wheat production. Growers deal with reduced yields and shortened plant heights while facing other issues that cloud a crop’s success.

Little moisture also influences the result of a post-wheat harvest herbicide application, said Kansas State University weed specialist Sarah Lancaster.

“After-harvest weeds can be particularly challenging in dry growing seasons because drought-stressed weeds and weeds that have been half-harvested are not functioning like we want them to for achieving good control,” she said.

A successful post-harvest herbicide program occurs when producers observe increased weed vigor, according to Lancaster.

“The weeds need to be actively growing for translocated herbicides (like glyphosate) to move through the plant and kill it,” Lancaster said. “Also, contact herbicides – like paraquat or Sharpen – need adequate leaf area to be properly absorbed.

She adds: “Generally, you want to wait until there is some regrowth of those weeds so that those herbicides work more effectively.”

To control weeds that have not sprouted after a wheat harvest, Lancaster encourages producers to choose a herbicide with residual activity.

“It is easy to see the weeds that have emerged through wheat stands because they rapidly grow after taking the (wheat) canopy off when harvesting,” she said. “However, for preventative planning, it is important to consider adding a product like atrazine to that wheat stubble for future weeds.”

Producers commonly use atrazine and another herbicide called metribuzin in their post-harvest weed control programs. Lancaster said both products possess similarities and differences that growers should examine before application.

“They’re both photosynthesis-inhibiting herbicides in Group 5 that have some synergy with paraquat,” she said. “However, there are some differences in their chemistries. Interestingly, some kochia and pigweeds have developed a post-emergence triazine resistance, so in some cases, metribuzin will show better control.”

Other herbicides available for after-wheat harvest include Sharpen, Reviton and Valor. Lancaster reminds growers that – like the two Group 5 herbicides – some pigweeds have developed resistance against these three Group 14 products.

“It’s critical to know the characteristics of a certain weed population,” she said. “Valor does provide some residual activity and can be a good way to control weeds like Palmer amaranth (popular pigweed).

“Sharpen and Reviton – even though they are in the same herbicide family – act differently than Valor. They have more burndown efficacy on some broadleaf weeds but do not have the same residual effect of Valor.”

Producers may realize a wide array of benefits from controlling weeds after harvest with the previously mentioned herbicides, according to Lancaster.

“In some cases – because every weed that grows may go to seed – thousands of plants can appear within a generation. After-harvest control allows growers to reduce a weed’s seed bank within a field.”

She added: “It also better manages water resources for subsequent crops and leads to more successful yields and crop performance.”

More information about after-harvest weed control in wheat fields is accessible in the 2024 Chemical Weed Control for Field Crops, Pastures, and Noncropland publication.

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