Rotation allows no-till farmers to diversify crops and chemistry and may be critical to success in no-till row-crop and wheat production.

“A monoculture allows specific weed species to develop,” said Curtis Thompson, Kansas State University Extension weed specialist. “Continuous wheat allows development of winter annual grasses.” Thompson told participants at the No-till Oklahoma Conference in Oklahoma City that no-till cropping systems allow diversity in crop selection and in weed control options, including herbicide choices and application timing.

“Timing for burndown herbicide application, for instance, varies with different crop options,” he said. “But to stay in business, farmers need to diversify.” Using old crop residue helps with weed control,” Thompson said. “Residue is our friend and helps with weed control.”

He cautioned farmers about herbicide selection in wheat that would be followed by a summer double-crop. “Consider the length of residual,” he said. “Maverick and Glean, for instance, have long residual. Finesse, Amber, Rave and Ally can have residual activity. Check labels for plant-back restrictions for specific crops and be careful with sulfonylureas.”

He said weeds may compete in a wheat crop but that pre-harvest burndown treatments in wheat “are often disasters. They delay harvest and they are expensive. Don’t go there if you can avoid it. This is a salvage operation.”

Thompson recommends no-till planting into clean wheat stubble. Weed control timing in stubble and stubble height may affect yields on the following crop. He said controlling weeds in harvested wheat in July is better than waiting until August. “Don’t let weeds head and produce seed,” he said. “Also, weeds use moisture and nutrients, so early control preserves nutrient and moisture for other crops. It’s also easier to market crops with less weed infestation.”

Thompson said removing weeds also means less residue to plant through.

No-till and crop diversity also may mean a shift in weed species, he said. “Use a soil active herbicide as a base for summer crops, even with Roundup Ready varieties.” He said the soil active herbicide allows farmers to get a handle on early weed infestations and reduces initial concentrations of weeds.

“For post-emergence herbicide applications, farmers have the same options they have in conventional tillage, but post selections may be affected by the crop intensity increase. Residual could affect a rotation crop.”

He said weed species shifts are possible. “We may see an increase in Roundup tolerant weeds, the ones that Roundup never did a good job controlling: yellow nutsedge, tumbleweeds and others.”

He said potential also exists for weeds such as marestail, ragweed, waterhemp and kochia developing resistance. “Tank mix herbicides with alternative modes of action. Do not use reduced rates of Roundup and use a soil active herbicide to reduce potential for resistance.”

He said no-till farmers tend to see more small-seeded grasses and broadleaf weeds, but may see a decrease in large-seeded weed species. “The larger-seeded weeds need more moisture and lay on the surface and have a tough time establishing.”

Timely weed control is the key to success, Thompson said. Owning a sprayer may be an important consideration to get herbicides out on time. He said glyphosate activity will increase with lower spray volume.

“Activity also decreases as weed size increases. It’s harder to get good coverage on large weeds. Avoid weed competition.”

Thompson said time of day also affects application efficacy. “Many herbicides have less activity early in the morning or late in the afternoon."

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