Weed species shift is not a new thing, it has happened since man started cultivating crops. Weedy and invasive species can easily adapt to any changes in production practices in order to take advantage of the available change. Species that do not adapt to those changes become “less frequent” compared to those that do adapt to the system and become “more frequent”. Despite the fact that glyphosate controls many weed species, especially grasses, there are broadleaf species that are naturally tolerant to the label rates of glyphosate. It appears that as a result of repeated use of glyphosate in Nebraska, there has been a slow shift in weed species from those easily controlled by glyphosate to those more tolerant of it.
Based on our extension phone calls and questions from producers, crop consultants and agronomists, we compiled a list of such species, 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. 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 an increased weed control costs due to additional herbicide applications or increased glyphosate rates.
Therefore, the purpose of this article is to summarize 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 above mentioned 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, targeting:
Visual ratings of percent weed control were conducted at approximately 21 days after glyphosate treatment and were based on a scale from 0 to 100 (where 0 = no injury and 100 = plant death).
Most weeds were able to survive the label rates of Roundup WeatherMax (22 oz/acre), while the weed size was the most important factor that determined the 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 ivyleaf morningglory that was 4 inches tall. The control level was further reduced with the increase in morningglory size, resulting in 30% and 21% control for 8-inch and 12-inch 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 the plant heights (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 (not shown) to determine how much glyphosate is needed to achieve at least 90% control of taller plants (6-20 inches). 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 to 4 times the label rate. About 1.5-2 times the rate was needed to control 3- to 6-inch tall wild buckwheat, Venice mallow, velvetleaf, waterhemp, sweet clover, ivyleaf mornigglory and field bindweed. About three to four times the rate was needed to control 12- to -15-inch ivyleaf morningglory and yellow sweetclover. 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 as well today as it did five or six years ago. The label rate of glyphosate did not provide adequate control of most species tested. If the trends in weed shifts continue, 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, may effectively control most 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 (eg. integrating several tools for weed control), especially to combat weed resistance/tolerance issues. I believe that Roundup Ready technology has a place in an integraed weed management 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. Proper use of this technology, as a component of an integrated weed management program, is the key to preserving the long-term benefits of this technology while avoiding many of the concerns about its use or misuse (eg. overuse).
Stevan Knezevic
Extension Integrated Weed Management Specialist
| Table 1. 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). | |||
Weed species |
Weed heights |
Level of weed control with 22 oz rate of WeatherMax |
Rate of WeatherMax needed to achieve 90% control (in fl. oz.) |
| Field bindweed | 3 6 9 |
95% 70% 60% |
22 oz 35 oz 35 oz |
| Ivylead morningglory | 4 8 12 |
50% |
37 oz 50 oz >60 oz |
| Kochia | 4 12 20 |
100% |
15 oz 20 oz 22 oz |
| Russian thistle | 5 10 15 |
100% 90% 90% |
12 oz 22 oz 22 oz |
| Yellow sweetclover | 4 8 14 |
50% 35% 10% |
35 oz 48 oz >60 oz |
| Velvetleaf | 5 10 21 |
85% 65% 60% |
28 oz 34 oz 40 oz |
| Venice mallow | 6 10 18 |
70% 50% 45% |
32 oz 40 oz 58 oz |
| Common waterhemp | 4 9 14 |
98% |
16 oz 28 oz 30 oz |
| Wild buckwheat | 3 |
70% 60% 45% |
28 oz 33 oz 40 oz |
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| Published by University of Nebraska-Lincoln Extension in the Institute of Agriculture and Natural Resources Cooperating with the counties and the U.S. Department of Agriculture. | ||
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