June 8, 2007

Widespread and repeated use of glyphosate-based herbicides in Roundup-Ready crops raises several concerns such as the potential for weed resistance and shifts in weed species. Currently, there are not any 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 extension weed specialists have received phone calls and complaints on glyphosate failing to control certain weed species, including some “new weeds,” including: 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 data from 2004 studies at Concord and North Platte to test glyphosate tank-mix with other herbicides to control these weed species.

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

1. 2- to 5-inch weeds (early POST),
2. 6- to 12-inch weeds (mid POST) and
3. 12- to 20-inch 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 tank-mix used. Weed size also was an important factor affecting the overall level of weed control (Table 1). Most species that were 2-5 inches 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 provided 90% control of sweet clover 4 inches tall compared to a much lower control level of 40% and 10% for 8- and 14-inch plants, respectively (Table 1), indicating the importance of the plant size. Few species were controlled well regardless of their size. For example, a tank-mix of Roundup and Classic provided excellent control (greater than 90%) of Russian thistle and kochia regardless of the plant size. For weed specific control levels and tank-mixes, see Table 1.

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

Tank mixing glyphosate with various herbicides also would provide additional mode 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 (eg. integrating several tools for weed control), especially to combat weed resistance/tolerance issues.

I believe that Roundup-Ready technology fits under the umbrella of an IWM system. The value of this technology can be preserved only by proper management, and reduced overuse. The concepts of IWM become even more important when 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. Therefore, proper use of this technology, as a component of a full integrated weed management program, is the key to preserving the long-term benefits of this technology while avoiding many concerns about its use or misuse (eg. overuse).

Stevan Knezevic
Extension Integrated Weed Management Specialist