Weed Resistance Strategies
(Excerpts from an article by Agronomist Dr. Daniel Davidson for the ILsoyadvisor.com, published by the Illinois Soybean Association) 

There are several important topics on farmers’ minds today including how to remain profitable during an era of low crop prices, how to farm and pay attention to the new nutrient loss reduction strategies and still optimize yield, and how to fight the battle with weed resistance and win when the enemy seems to be piling up all around you.

If you already haven’t made your resolutions, you might add paying more attention to managing weed resistance in 2015. Learning to master weed resistance will make controlling weeds easier in all your fields in the future. But it takes commitment, effort and surveillance to win the battle, keep fields clear and prevent resistance from spreading to other fields or worsening in individual fields.

Weed resistance is nothing new and has been around for a couple decades now. To make the case “Weeds have evolved resistance to 155 different herbicides. Herbicide-resistant weeds have been reported in 84 crops in 65 countries,” stated by the International Survey of Herbicide Resistant Weeds website. 

Resistance was a relatively small management challenge before the introduction of glyphosate. However, when the Roundup Ready trait became available in the mid-90s and glyphosate was used as a primary means of weed control, concern about resistance and weed control faded, but reprieve was short-lived. The overdependence on glyphosate for a decade from 1995 to 2005 created a lot of selection pressure, and eventually some populations appeared that were resistant to glyphosate. For corn and soybeans, it first seemed to appear in marestail. Of course now we can add additional weeds to that, including Palmer amaranth and waterhemp and glyphosate resistant ragweed species are in some states. What we can conclude is that weed resistance is here to stay, is expanding to more species and control is becoming more complex with resistance to multiple sites of action. At the same time there are no new herbicide chemistries or modes of action coming to market, making weed control even more challenging.

Managing weed resistance is like controlling weeds before the introduction of Roundup Ready soybeans nearly 20 years ago, where it took more work. Use good agronomy so the soybeans out compete weeds. Control weeds at multiple times including fall annual weed control, spring burndown, residual pre-emergence and a combination postherbicide mix. Learn to use herbicides with multiple modes of action each season and rotate modes of action as you rotate crops. CLICK HERE TO READ THE ENTIRE ARTICLE

Diagnosing & Managing Weed Resistance 

(Excerpts from the Illinois Cooperative Extension Service) 

Several criteria may be used to correctly diagnose a herbicide-resistant weed problem:
1. All other causes of herbicide failure have been eliminated. 
2. Other weeds on the herbicide label (other than the one in question) were controlled effectively.
3. The field has a history of continuous use of the same herbicide or herbicides with the same mode of action.
4. The weed species that now demonstrates potential resistance was controlled effectively in the past by the herbicide.

Why be concerned? If previously controlled weed species are no longer controlled by a particular herbicide, the return on the herbicide investment is greatly reduced. 

The best solution for the problem of weed resistance is to prevent their development. Incorporate as many of the following management strategies as possible to deter the development of herbicide-resistant weeds.
1. Scout fields on a regular basis to identify resistant weeds. Respond quickly to changes in the weed population to restrict the spread of plants that may have developed resistance.
2. Rotate herbicides with different modes of action. Do not make more than two consecutive applications of herbicides with the same mode of action against the same weed unless other effective control practices are included in the management system. Consecutive applications can be single applications in two years or two split-applications in one year. 
3. Apply herbicides in tank-mixed, prepackaged, or sequential mixtures that include multiple modes of action. Both herbicides in the mixture must have substantial activity against potentially resistant weeds, as well as similar persistence, if they possess soil activity. For example, if one is concerned about potentially ALS-resistant pigweed, a tank mixture of Basagran with an ALS inhibitor would be a poor choice because Basagran has very little activity on pigweed. A couple of guidelines may help with tank-mix or premix selection: (a) When applied alone at the rate that will be used in the tank-mixture, does the tank-mix or premix partner control the weed species that I am concerned may develop resistance?; and (b) If I apply the tank-mix or premix partner alone at the rate that will be used in the tank-mix, will it have similar residual activity to the other component?
4. As new herbicide-tolerant/-resistant crops become available, their use should still not result in more than two consecutive applications of herbicides with the same mode of action against the same weed species unless other effective practices are included in the management system.
5. Combine mechanical control practices such as rotary hoeing and cultivation with herbicide treatments whenever possible.
6. Clean tillage and harvest equipment before moving from fields infested with resistant weeds to those fields that are not infested. This approach may not be practical, but it can help prevent the spread of resistant weed seed that is present in soil which adheres to the equipment.