Kochia Resistance to Popular Herbicides Moving Westward

Weed resistance to popular herbicides is a growing concern amongst those of us involved in producing crops for the world’s growing population. I have not heard many crop growers in this region share resistance problems but kochia resistance to glyphosate (Roundup) and fluroxypr (Starane) is moving westward. Just a few years ago high levels of resistance were documented in southern Minnesota. Last year, there was widespread concern in eastern North Dakota that glyphosate was not killing kochia.

Last fall NDSU researchers collected around 80 seed samples of kochia from around the state. These were germinated and treated in the greenhouse with either glyphosate or fluroxypr. Results from a non-replicated run indicated that 12 percent of samples had at least one plant that regrew after 3x glyphosate (Roundup PowerMax) and 25 percent of samples had a survivor produce new branches after 2x fluroxypr (Starane Ultra). The number of surviving plants ranged from 1 to all plants within these samples.

Kirk Howatt, Research Weed Scientist, and Rich Zollinger, Extension Weed Specialist, are leading the research at NDSU designed to find ways to reduce weed resistance to herbicides. Their first run of three greenhouse studies to characterize kochia response to fluroxypr has been completed. Observations indicate about 8xthe level of resistance to fluroxypr is present in five or six of the 80 samples. Two other samples with survivors are responding quite similarly to the susceptibility check and may not be statistically different once a second run is completed.

Weeds typically are easier to kill when plants are smaller but many have observed that very small, or “puffball”, kochia are difficult to kill with any herbicide, including glyphosate. One greenhouse trial demonstrated that fluroxypr was most effective when applied to one to two inch tall kochia. Most kochia smaller than this survived and regrew, while larger kochia showed herbicide symptoms and also had less kill of plant foliage.

The research does show growers experiencing kochia resistance to glyphosate and fluroxypr still have some herbicide options. Sharpen at one fluid ounce/acre or Gramoxone/Paraquat with MSOadjuvant was very effective in treating kochia that was two to three inches tall. Only a few larger plants produced regrowth. Aim had good activity but again larger plants were not completely killed and regrew quickly. Application of fluroxypr with dicamba as in the premix Pulsar improved activity compared with fluroxypr alone. Plants were still green but with very little new shoot production.

Surprisingly, Atrazine at .38lb active ingredient per acre was variable in effectiveness as individual kochia plants were either completely dead or unaffected. Kochia has become resistant to Atrazine in other regions of the country but has not been documented in North Dakota. With low Atrazine rates used in North Dakota and use primarily in corn, it was thought that Atrazine resistance to kochia would be delayed for many years. Howatt and Zollinger encourage corn growers to watch for kochia escaping atrazine treatment and welcome notification.

HPPD herbicides used in corn (Callisto, Impact, and Landis) may not kill kochia if used alone and is the reason NDSU recommends adding atrazine to these and most other POST applied herbicides used in corn.

Howatt and Zollinger promise to continue their work in combating kochia. They feel that combinations of effective active ingredients (herbicides) will be necessary to control kochia, especially at locations where herbicide resistance has occurred and where kochia is taller than desired at application.