Controlling Problem Weeds in Roundup-Ready Soybean with Soil-applied Herbicides

Controlling Problem Weeds in Roundup-Ready Soybean with Soil-applied Herbicides

May 11, 2007

Many practitioners are reporting two concerning issues of weed control.

  1. glyphosate used alone does not work as well today as it did five to six years ago; and
  2. it appears that Nebraska's fields are experiencing a slow shift in weed species.

In the last three years, UNL extension weed specialists have been receiving phone calls and complaints on glyphosate failing to control certain weed species, including some "new weeds." The list of such species 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 increased weed control costs due to additional herbicide applications or increased glyphosate rates.

The purpose of this article is to summarize preliminary data from studies conducted at Concord and North Platte in 2004 to test six soil-applied herbicides for control of the weed species listed above. The six broadleaf herbicides were: Authority 75DG (5 oz/ac), Sencor 75DF (8 oz/ac), Canopy XL 56DG (6.5 oz/ac), Commit 3ME (1.5 pt/ac), Pursuit Plus 2.9EC (2.5 pt/ac), Scepter 70DG (2.8 oz/ac) and Steel 2.6 EC (3pt/ac). These herbicides were applied to the soil after planting weed seeds.

The level of weed control at 40 days after planting varied by the weed species and herbicide. For example, Sencor provided excellent control (100%) of kochia, velveltleaf and Venice mallow but lower control (37%) of ivy leaf morningglory (Table 1). For control levels for each weed species and herbicides see Table 1.

The results indicate a potential to effectively control most of those weed species with various pre-emergence herbicides applied to the soil after soybean planting. Soil-applied herbicides also would provide an additional mode of action for weed control, reducing a chance for weed resistance. Soil-applied herbicides also provide a longer "comfort zone" for weed control early in the season by delaying the critical time for weed removal and reducing the need for multiple glyphosate applications later in the season.

Integrated Weed Management

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 elements of an Integrated Weed Management (IWM) program that integrates several tools for weed control, especially to combat weed resistance/tolerance issues. I believe that Roundup-Ready technology has a fit under the umbrella of an IWM system, and the value of this technology can be preserved only by proper management, and reduced overuse.

In essence, the development of an IWM program is based on a few general strategies that can be used on any farm. These rules are: 

  1. use agronomic practices that limit the introduction and spread of weeds, preventing weed problems before they start,
  2. help the crop compete with weeds , and
  3. use practices that do not allow weeds to adapt.

Combining agronomic practices based on these ideas will allow agronomists to design an IWM program for any field and tailored to a particular farming operation. The goal is to manage weeds, since eradication is not possible.

For more details on developing an IWM plan for your operation, see the UNL Extension Guide for Weed Management in Nebraska (EC130). 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. Proper use of this technology, as a component of an IWM program, is the key to preserving the long-term benefits of this technology while avoiding many of the concerns about its use or misuse.

Stevan Knezevic
Extension Weeds Specialist

Table 1. Weed species and their control (%) with various pre-emergence herbicides at 40 days after application at Concord in 2004 (preliminary data).

Weed species

Authority
5 oz/ac

Sencor 75DF 8 oz / ac

Canopy XL
6.5 oz /ac

Commit
1.5 pt/ac

Pursuit Plus 2.5 pt / ac

Scepter
2.8 oz /ac

Steel
3 pts/ac

Field bindweed

77

63

100

73

98

98

97

Ivyleaf morningglory

88

37

90

40

72

83

85

Kochia

100

100

100

100

100

98

100

Russian thistle

100

95

100

37

95

98

98

Yellow sweetclover

67

98

93

98

81

90

86

Velvetleaf

98

100

95

100

100

90

97

Venice mallow

92

100

100

100

97

98

97

Common waterhemp

100

100

100

96

100

95

100

Wild buckwheat

100

100

100

100

100

100

100

Lambsquarter

100

100

100

100

100

100

100

Roundup Ready corn

12

27

71

33

55

96

95

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