Controlling Problem Weeds In Roundup-Ready Soybean With Soil-Applied Herbicides

If these weeds are not controlled, their seeds will be a major problem in the future, especially in systems not using tillage for weed control. Weed population shifts to more tolerant weeds already are resulting in increased weed control costs.

The purpose of this article is to summarize data from UNL studies at Concord and North Platte in 2004 testing six soil-applied herbicides for control of the weed species listed above. The six broadleaf herbicides were: Authority 75DG (5oz/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 (3 pt/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 less control (37%) of ivy leaf morningglory (Table 1).

The results indicate a potential to effectively control most of those weed species with pre-emergence herbicides applied to the soil after soybean planting. Soil-applied herbicides also would provide 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.

Integrating weed control tools is not a new thing, but has been less common since the introduction of Roundup Ready crops. Changing modes of actions in your herbicide program is a basic aspect of an Integrated Weed Management (IWM) program, especially when used to combat weed resistance and tolerance issues. When Roundup-Ready technology is used as part of an integrated weed management system, this technology can be preserved and its overuse can be avoided. IWM programs are based on a few general rules that can be used on any farm:

• use of agronomic practices that limit the introduction and spread of weeds (preventing weed problems before they start);
• help the crop compete with weeds; and
• use practices that do not allow weeds to adapt.

Combining agronomic practices based on these rules allows you to design a program for any field, changing and tailoring annually as your operation makes change. The goal is to manage weeds below an economic level, not eliminate them. . For more details on applying integrated weed management to your operation, see the Guide for Weed Management in Nebraska.

The concepts of IWM become even more important with the increased use of other Roundup-Ready crops (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, but proper use of this technology as part of an integrated weed management program, is key to preserving this technology while avoiding overuse.

Stevan Knezevic
Extension Weeds Specialist
Haskell Ag Lab, Concord, Northeast REC

Table 1.  Weed species and their control (%) with various pre-emergence herbicides at 40 days after application at Concord in 2004.
Weed species	Authority	Sencor 75DF	Canopy XL	Commit	Pursuit Plus	Scepter	Steel
	5 oz/ac	8 oz / ac	6.5 oz /ac	1.5 pt/ac	2.5 pt /ac	2.8 oz /ac	3 pt/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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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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