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Chapter 5 - Weed Control, No. 5, Fall 1986

New Wild Oat and Broadleaf Herbicide

Roger Veseth

The registration of a new herbicide for control of wild oat and certain broadleaf weeds in wheat, barley and sunflowers is expected in 1987. The experimental herbicide AC-222,293 from American Cyanamid will be marketed under the trade name Assert.

Assert will control both wild oat and broadleaf weeds including wild mustard, tansy mustard, flixweed and field pennycress. It also gives suppression of wild buckwheat, kochia and Russian thistle by stopping growth, but not killing the weeds.

Wild Oat Herbicide Comparisons

The herbicide has been tested extensively for use in small grains throughout the Northwest since 1982. In Idaho, STEEP researcher Dorm Thin, a University of Idaho weed scientist, and other researchers have been involved in testing Assert throughout the cereal growing regions of the state. Preliminary results on wild oat control and crop yields have been encouraging.

Wild oat control comparisons between Assert and three commercial wild oat herbicides, applied postemergence, were conducted by Thin and other researchers. In a summary of results from over 12 locations, wild oat control was highest with Assert, averaging 94 percent. The research was conducted on spring wheat, winter wheat and spring barley in Idaho during 1983 through 1985. Assert also had the lowest degree of variation (standard deviation) of wild oat control of any of the herbicides. Thin points out that the consistency of wild oat control indicates that Assert may be slightly more tolerant of adverse conditions than the other herbicides. For example, drought conditions occurred at several experimental sites across the state in 1985 before or immediately after applications of the herbicides. Still, wild oat control with Assert remained consistent with 1983 and 1984 results. Average experimental yields with Assert were equal to or greater than yields with the other herbicides in the 3 years of research.

Potential Advantage for No-Till

Wild oat herbicides that could be applied preplant or pre-emergence would have the advantage of avoiding adverse weather conditions which can prevent a postemergence herbicide application during critical leaf stages of wild oat. Currently available, soil-active wild oat herbicides require some soil incorporation for effective weed control. The availability of a soil-active wild oat herbicide that did not require incorporation into the soil would be particularly adapted to no-till farming, as well as minimum tillage and conventional tillage.

Preliminary results of a field study on no-till spring barley by Thin and research associate Joan Lish indicate that Assert may have some potential for wild oat control when applied preplant without soil incorporation. The study was conducted during 1986 in Caribou County in southeastern Idaho. A mid-July evaluation of percent wild oat control showed that Assert, applied postemergence at the 2 to 3 leaf stage of wild oat, controlled 99 percent. The preplant surface application without incorporation controlled 84 percent. Because of variability within the trial, differences in wild oat control between the two application methods were, however, not statistically significant. In a late evaluation of wild oat control near harvest, the researchers found that control decreased to 70 percent for the preplant application but was unchanged for the postemergence application. Thin and Lish plan to continue their evaluation of preplant applications of Assert under several tillage systems.

Mode of Action

Weeds take up Assert through the roots or the leaves and translocate the herbicide to the plant growing point. Once at the growing point, Assert inhibits production of three key amino acids (valine, leucine and ioleucine) that are essential to cell formation. This inhibition causes disruption of plant metabolism, including synthesis of protein and DNA, which results in cessation of cell division and eventual death of the plant. The mechanism for Assert herbicide action is believed to be similar to that of sulfonylurea herbicides such as Glean, Harmony and others.

Soil Persistence and Rotation Limitations

The persistence of Assert in the soil appears to be more of a potential problem in acid soils of intermediate to high precipitation areas than in alkaline soils of the drier regions. Laboratory research by American Cyanamid indicates that Assert persists longer at pH 5 and pH 7 than at pH 9. This is just the opposite of the soil persistence of sulfonylurea herbicides such as Glean. Broadleaf crops such as sugarbeets, red beets, peas, lentils, rapeseed, mustard, tomatoes and broccoli have been injured in field plant-back studies 1 year or less after application of Assert. Preliminary research indicates that these crops should not be planted within a minimum of 15 months after application of Assert.

Tank Mixing Herbicides

For additional broadleaf weed control, preliminary research indicates that Assert can be effectively tank-mixed with Glean and other sulfonylurea herbicides. Tank-mixing with bromoxynil, MCPA and 2,4-D has reduced wild oat control in some instances. Sharp reductions in wild oat control commonly results from tank mixes with dicamba.

Remaining Research

Thin concludes that Assert can effectively control wild oats and selected broadleaf weeds in small grains under the varied Idaho cropping conditions. More research is needed, however. Additional information must be available on the persistence in the soil as affected by soil type and pH, rotational crop safety, preplant and preemergence nonincorporated applications, tank-mix compatibility and varietal tolerance of wheat and barley.

Use of Trade Names

Research results are given for information only and are not to be construed as a recommendation for an unregistered use of a pesticide. Always read and follow label instructions carefully. To simplify the information, trade names have been used. Neither endorsement of named products is intended nor criticism implied of similar products not mentioned.


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