May 18, 2007

With more than 90% of Nebraska's soybean fields now planted to glyphosate-tolerant varieties, soybean producers clearly realize the benefits of this technology. However, widespread and repeated use of glyphosate-based herbicides raises several concerns, including:

1. the potential for weed resistance and
2. shifts in weed species.

In 2006 glyphosate-resistant marestail was confirmed in Nebraska - the first glyphosate-resistant weed species in the state . In the last three years, UNL extension weed specialists have received reports of glyphosate failing to control certain weed species, including some "new weeds," leading to a shift in weed species.

Weed species shift is not a new thing. Weedy and invasive species can easily adapt to changes in production practices. Species that do not adapt become "less frequent" and those that do adapt become "more frequent." Glyphosate controls many weed species, especially grasses, however some broadleaf species are naturally tolerant to label rates of glyphosate. Repeated use of glyphosate in Nebraska has led to a slow shift in weed species from those easily controlled by glyphosate to those more tolerant of this herbicide.

Based on our extension phone calls and questions from producers, crop consultants and agronomists, we compiled a list of such species: 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. If these weeds are not controlled, their seeds will be a major problem in the future, especially in no-till systems, due to lack of tillage as a tool for weed control. Such shifts in weed populations to more tolerant weeds is already resulting in increased weed control costs due to additional herbicide applications or increased glyphosate rates.

Research Tests Effect of Height on Product Effectiveness

This article summarizes preliminary data from studies conducted at Concord and North Platte in 2004, with the objective to determine the appropriate dose of glyphosate used alone to control these "more tolerant" weed species.

We tested seven rates of Roundup WeatherMax ranging from 4.6 oz to 60 oz with 2% v/v AMS. Each glyphosate rate was applied at three growth stages:

1. 2- to 5-inch tall weeds (early POST),
2. 6- to 12-inch weeds (mid POST) and
3. 12- to 20-inch weeds (late POST).

Visual ratings of percent weed control were conducted about 21 days after glyphosate treatment, based on a scale from 0 to 100 (where 0 = no injury and 100 = plant death). (Higher than labeled rates for individual applications were used for the purpose of this research. Please check your product label for recommendations for individual and season-long application rates.)

Most weeds survived label rates of Roundup WeatherMax (22 oz/ac). Weed size was the most important factor determining level of control for each species (Table 1). Ivyleaf morningglory and sweet clover were the hardest species to control. For example, the 22 oz rate provided only 50% control of 4-inch tall ivyleaf morningglory. Control was further reduced with increases in plant size, resulting in 30% and 21% control for 8- and 12-inch tall plants, respectively (Table 1). A similar trend was observed for other weed species (Table 1). Label rate of Roundup WeatherMax provided good control (>85%) of kochia and Russian thistle regardless of plant height (Table 1).

Since the label rate of WeatherMax was not able to provide adequate control of most species over 3 inches tall, we developed dose response curves to determine how much glyphosate is needed to achieve at least 90% control of taller plants (6-20 inches). This information also will help determine rates for those late applications of glyphosate in Roundup-Ready soybeans. Based on our data from the dose response curves, in order to achieve at least 90% control of taller weeds, Roundup WeatherMax at 1.5-4 times the label rate would be required. About 1.5-2 times the label rate was needed to control 3- to 6-inch tall wild buckwheat, Venice mallow, velvetleaf, waterhemp, sweet clover, ivyleaf mornigglory and field bindweed. About 3-4 times the rate was needed to control ivyleaf morningglory and yellow sweetclover 12-15 inches tall. For weed sizes and respective rates, see Table 1.

Data presented in this article reaffirms what many practitioners were observing, which is that glyphosate used alone does not work well today compared to five to six years ago. The label rate of glyphosate did not provide adequate control of most species we tested. If weed species continue to shift, glyphosate used alone will no longer be 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.

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 one of the basic ideas in an Integrated Weed Management (IWM) program which integrates several tools for weed control, especially to combat weed resistance/tolerance issues. I believe that Roundup-Ready technology has a fit in an integrated manager system. The value of this technology can be preserved only by proper management and reduced overuse. 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 an IWM program, is the key to preserving its long-term benefits while avoiding many of the concerns about its use or misuse.

Stevan Knezevic
Extension Weeds Specialist