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Widespread and repeated use of glyphosate-based herbicides in Roundup-Ready crops raise several concerns from the practical standpoint such as potential for weed resistance and shifts in weed species. Currently there are no known cases of glyphosate-resistant weeds in Nebraska; however, it appears that our fields are experiencing a slow shift in weed species. In the last three years, university weed extension specialists have been receiving phone calls and complaints on glyphosate failing to control certain weed species, including some “new weeds”, which includes: marestail (horseweed), morning-glory (common and ivyleaf), wild buckwheat, Pennsylvania smartweed, lady=s thumb, venice mallow, yellow sweetclover, field bindweed, waterhemp, kochia, Russian thistle, primrose species and volunteer Roundup-Ready corn.

The purpose of this article is to summarize preliminary data from studies conducted at Concord and North Platte in 2004, with two objectives: 1) to test glyphosate tank-mix with other herbicides to control above mentioned weed species, and 2) to determine the appropriate dose of glyphosate used alone to control above mentioned weed species.

For the first objective we used a labeled rate of glyphosate (Roundup WeatherMax @ 22 oz/acre) tank-mixed with “half rates” of the 7 most common broadleaf POST herbicides: Classic 25DF (0.3 oz/acre), Cobra/Phoenix 2EC (5 oz/acre), Raptor 1SC (3 oz/acre.), Pursuit (Extreme)(3 pt/a), Reflex/Flaxstar 2EC (8 oz/a) , Scepter 70DG (1.44 oz/acre), and Ultra Blazer 2 SC (12 oz/a). Each tank-mix contained appropriate amounts of additives such as AMS (2.5 lbs/acre), NIS (0.125% v/v) and/or COC (1% v/v) as indicated on the product label. Each tank mix was applied at 3 growth stages of the weed, targeting (1) 2 - 5" tall weeds (early POST), (2) 6-12" weeds (mid POST) and (3) 12-20" weeds (late POST).

The level of weed control at 21 days after herbicide treatment, varied from 10% to 100% depending on the weed species and a tank-mix used, while the weed size was still an important factor that determined the overall level of weed control (Table 1). Most species that were 2-5" tall (early POST applications) were controlled relatively well with a tank-mix of the label rate of Roundup WeatherMax with appropriate herbicides. For example, a tank-mix of Roundup and Classic proved 90% control of sweetclover 4" tall compared to a much lower control levels of 40% and 10% for 8" and 14" tall plants, respectively, indicating the importance of the plant size. A few species were controlled well regardless of size. For example, a tank-mix of Roundup and Classic provided excellent control (> 90%) of Russian thistle and kochia at all plant heights.

This data indicates the potential to effectively control these species with various tank mixes if applied early POST (for weeds up to 5" tall). Taller weeds may require higher rates of broadleaf herbicides, perhaps full recommended rates. A study is needed to confirm such hypothesis.

Tank mixing glyphosate with various herbicides would provide additional modes of action for weed control, thus reducing a chance for weed resistance. Furthermore, products like Extreme and Scepter could also provide additional soil residual activity for prolonged weed control, which is one of the goals of an integrated weed management program (eg. integrating several tools for weed control). Using various weed control tools is not a new thing, we only “forgot” about it since the introduction of Roundup-Ready crops. Changing modes of actions in your herbicide program is also one of the basic ideas in an Integrated Weed Management (IWM) program, especially to combat weed resistance/tolerance issues. Roundup-Ready technology has a fit under the umbrella of an IWM system only, and the value of this technology can be preserved only by proper management, and reduced overuse. The concepts of IWM become even more important as other Roundup-Ready crops become more common (eg. Roundup-Ready corn, Roundup-Ready alfalfa). It is easy to fall into a trap of overusing glyphosate when one glyphosate-tolerant crop is grown after another.

Table 1. Weed species and their heights at the time of herbicide application, and level of weed control (%) at 21 days after application with 22 oz rate of Roundup WeatherMax tank mixed with various herbicides at Concord in 2004 (preliminary data).

For the second objective we tested 7 rates of Roundup WeatherMax ranging from 4.6 oz - 60 oz with 2% v/v AMS. Each glyphosate rate was applied at 3 growth stages of the weed, targeting (1) 2 - 5" tall weeds (early POST), (2) 6-12" weeds (mid POST) and (3) 12-20" weeds (late POST). Visual ratings of percent weed control was conducted at approximately 21 days after glyphosate treatment, and it was based on a scale from 0 to 100 (where 0 = no injury and 100 = plant death).

Most weeds were able to survive the label rate of Roundup WeatherMax (22 oz/acre), while weed size was the most important factor in determining the level of control for each species (Table 2). Ivyleaf morningglory and sweet clover were the hardest species to control. For example, the 22 oz rate provided only 50% control of ivyleaf morningglory that was 4" tall. The control level was further reduced with the increase in morningglory size, resulting in 30% and 21% control for 8" and 12" tall plants, respectively. Similar trends were observed for other weed species. The labeled rate of Roundup WeatherMax provided good control (>85%) of kochia and Russian thistle regardless of the plant heights.

Since the label rate of WeatherMax was not able to provide adequate control of most species over 3" tall, we developed dose response curves (not shown) to determine how much glyphosate is needed to achieve at least 90% control of taller plants (6"-20" tall). This information will also help determine rates for those late applications of glyphosate in Roundup-Ready soybeans. Many soybean fields receive late applications of glyphosate for various reasons. Based on our data from the dose response curves, in order to achieve at least 90% control of taller weeds, it was required to use much higher rates of Roundup WeatherMax ranging from 1.5 - 4X of the label rate. About 1.5-2X rate was needed to control 3-6" tall wild buckwheat, Venice mallow, velvetleaf, waterhemp, sweet clover, ivyleaf mornigglory and field bindweed. About 3-4X rate was needed to control 12"-15" tall ivyleaf morningglory and yellow sweetclover.

Data presented in this article reaffirms what many practitioners were observing, which is that glyphosate used alone does not work well today compared to 5-6 years ago. The label rate of glyphosate did not provide adequate control of most species we tested. If the trends in weed shifts continue to occur, glyphosate used alone is no longer a viable tool for weed control in Roundup-Ready systems. Mixing glyphosate with other post-emergence broadleaf herbicides, or using soil applied herbicides after soybean planting, indicates a potential to effectively control most of these species. (SK)

Table 2. Weed species and their heights at the time of herbicide application, levels of weed control with 22 oz rate of Roundup WeatherMax (at 21 days after application), and the rate of Roundup WeatherMax rate needed to provide 90% control of respective species at Concord in 2004 (preliminary data).

David P. Shelton
Extension Agricultural Engineer

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