Herbicide Resistance: Evolution, Prevention and Control Jamshid Ashigh Extension Weed Specialist/ Assistant Professor

Chemical weed control Herbicides are the most efficient technology for large-scale weed control. The introduction of new herbicides to replace those herbicides failing due to resistance is essential for weed management.

Herbicide resistance world wide

Herbicides development and use The rate of introduction of new herbicides has slowed dramatically. Due to the high cost Ten multi-national corporations In 2004 there were four remaining There is a strong imperative to use the currently available herbicide resources in more sustainable ways. Sustainable agriculture IWM

Herbicide Resistance Herbicide resistance is the inherited ability of a plant to survive and reproduce following exposure to a dose of herbicide normally lethal to the wild type (WSSA). Similar to resistance to: Insecticides Fungicides Antibiotics

Evolution of herbicide resistance Weed populations are diverse Most individuals are susceptible to herbicides A few individuals are naturally resistant (1X10-8 to 1X10-10) After the herbicide is applied they are the only ones to survive

Evolution of Herbicide resistance Survivors put seeds in the seed bank The following year A few more survivors Not noticeable Several years (3-10 years) of selection before it becomes visible 30% of the total population

                                                                                               Resistance selection Source: J.L. Gunsolus. Herbicide Resistant Weeds. 1998. North Central Region Extension Publication 468.

Major factors influencing the evolution of resistance                                       Major factors influencing the evolution of resistance Selection pressure Initial frequency of herbicide-resistant individuals 1X10-8 to 1X10-10 Gene flow Pollen and seed movement Resistance controlled by single gene (e.g., ALS-inhibitors resistance)

Selection pressure Efficiency of the herbicide Frequency of use Sensitive weeds Frequency of use Applied alone Duration of effect

Weed characteristics that influence the evolution of resistance Annual life cycle Produce large numbers of seed Genetic variability Outcross

What makes the weeds resistant? Altered site of action Change in target enzyme Enhanced metabolism Ability to degrade herbicide Decreased absorption and translocation Herbicide does not get to its site of action Sequestration Herbicide not available to the plant (stored in vacuoles)

Altered site of action Alterations in the site of action that prevent the herbicide from binding are the most common mechanism of resistance Caused by mutation(s) in the gene(s) Change in the amino acids

Triazine mechanism of action Thylacoid membrane Chloroplast Source: Dr. J. Christopher Hall

Plastoquinone Binding Source: Dr. J. Christopher Hall

Triazine Binding Source: Dr. J. Christopher Hall

Triazine Resistance Source: Dr. J. Christopher Hall

Confirmation of resistance in the field Other causes of herbicide failure have been ruled out (e.g., plant size, time of application etc). The same herbicide or herbicides from the same family have been used year after year. One weed that is normally controlled is not controlled while other weeds are. Single weed species in patches and they are spreading. Mix of dead and healthy weeds from the same species are seen in the field.

Source: Dr. Earl Creech

Confirmation of resistance in the lab Seedlings. Pre emergence Post emergence DNA sequencing. Absorption, translocation and metabolism S R R S Pre- emergence Post- emergence

Herbicide resistant weeds Worldwide: 323 Resistant Biotypes, 187 Species (112 dicots and 75 monocots) United States: Resistant weeds have been reported from 46 states. In New Mexico: One confirmed case (Kochia) Suspected glyphosate-resistant Palmer amaranth. Source: Dr. I. Heap. www.weedscience.org

Glyphosate-resistance Glyphosate-resistant (Roundup ready) crops are the most widely used transgenic crops. 98% of cotton in US. Glyphosate is also used extensively in other crops. Nine weed species including Palmer amaranth have developed resistance to Glyphosate. Resistance is threatening the ongoing sustainability of Glyphosate.

Glyphosate-resistant palmer amaranth Glyphosate-resistant Palmer amaranth in Georgia, Tennessee, North Carolina, South Carolina and Arkansas. In late 2007, a Palmer amaranth population was reported to have survived Glyphosate applications in New Mexico. Palmer amaranth is an out-crossing plant: Spread faster than self-pollinated species.

Preliminary Results Resistant Susceptible 2X 1X

It is important to adopt proactive weed management strategies to prevent/control resistance.

Prevention and management Prevent seed production introduction Monitor the field Follow label direction Rotate herbicide Rotate crop Use less herbicide Integrate other methods of weed control with herbicides Use herbicide mixtures

Problem with rotating herbicides Herbicide rotation has not precluded the development of some types of resistance (e.g., multiple- or cross-resistant wild oat in Canada). Especially if the label rates are not followed Due to stacking of resistance genes Triazine resistance + ALS resistance

Criteria for good herbicide mixtures Same residual (persistence) Same efficacy (control the same spectrum of weeds) Different mode of action

Example of a bad mixture Herbicide Mode of action Residual Pursuit ALS Long Basagran Photosystem II None Follow label direction for mixture options

Label Legally binding Contains critical info: How to mix & apply Restrictions Use rates Spill info/contact Soil activity Species Time of application Mixture options