Herbicide-Resistant Weeds and Herbicide-Tolerant Turfgrasses Tim R. Murphy The University of Georgia

Herbicide Resistance Tolerance Susceptible Mode-of-action Resistant

Herbicide Resistance Tolerance The ability of a turfgrass or weed species to withstand the effects of an applied herbicide naturally. For example, most turfgrasses are tolerant of broadleaf herbicides. Dandelions are tolerant of crabgrass herbicides (Barricade, Pre-M, etc…).

Herbicide Resistance Susceptibility The inability of either a desirable species or weed to withstand the effects of an applied herbicide. For example, common chickweed is controlled by (susceptible to) Confront. A tall fescue lawn is tolerant of Confront and is susceptible to Roundup.

Herbicide Resistance Mode-of-Action is a herbicide’s sequence of events, including absorption, movement in the plant (translocation) to the target site, and activity at the target site (e.g. simazine, atrazine blocks photosynthesis) which, together, result in the herbicide killing susceptible plants.

Herbicide Resistance Definition inherited ability of a weed or crop biotype to survive a herbicide application to which the original population was susceptible. example, goosegrass or annual bluegrass plants that survive herbicides that normally control goosegrass or annual bluegrass are considered resistant plants. Biotype = a group of plants within a species that has biological traits that are not common to the population as a whole.

Herbicide Resistance cross resistance - weed biotype that has gained resistance to more than 1 herbicide with the same mode of action. Same or different families. multiple resistance - weed biotype that has developed tolerance to more than one herbicide brought about by different selection pressures (different modes of action).

Why Are Plants Resistant to Herbicides? altered site of action enhanced metabolism sequestration

Altered Site of Action Source: J.L. Gunsolus. Herbicide Resistant Weeds. 1998. North Central Region Extension Publication 468.

Herbicide Resistance How does it occur?

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

Herbicide Resistance Around the World 304 Resistant Biotypes 182 Species (109 dicots and 73 monocots) over 270,000 fields Source: Heap, I. The International Survey of Herbicide Resistant Weeds. Online. Internet. October 20, 2005 .

The Beginning of Weed Resistance 1968 (Washington) nursery crops common groundsel atrazine simazine Photo: The Missouri Flora Web-Site

Weed Characteristics That Favor Resistance high reproductive capability seed dispersal mechanisms

Worldwide Source: Dr. Ian Heap (www.weedscience.com)

Herbicide Characteristics That Influence Weed Resistance herbicides with a single site of action herbicides used multiple times during the growing season herbicides used for consecutive growing seasons herbicides used without other control strategies

Herbicide Resistant Weeds By Mode of Action

Glyphosate Resistance Around the World (8 species) rigid ryegrass (1996) goosegrass (1997) horseweed (2000) Italian ryegrass (2001) hairy fleabane (2003) buckhorn plantain (2003) common ragweed (2004) Palmer amaranth (2005)

Glyphosate Resistant Horseweed in US (confirmed in 12 states) DE (2000) TN (2001) IN (2002) MD (2002) NJ (2002) OH (2002) AK (2003) MS (2003) NC (2003) OH (2003) PA (2003) CA (2005) Horseweed pictures from SWSS Weed ID Guide

Other Weeds in the U.S. that have Developed Resistance to Glyphosate Palmer amaranth (GA-2005) Italian ryegrass (OR-2004) SWSS SWSS Common ragweed (MO-2004) USDA L. Hall - UGA Rigid ryegrass (CA-1998)

U.S. Acres Treated with Glyphosate 1996-2003 Source: USDA - National Agricultural Statistics Service – Agricultural Chemical Usage Reports

Herbicide Resistance Why? Herbicide resistant weeds should be a concern to anyone that applies herbicides on a regular basis. This is true even where successful herbicide programs exist. Why?

Herbicide Resistance Example: Dinitroaniline herbicides provide excellent crabgrass and very good goosegrass control in turfgrasses (home lawns, golf courses, nonresidential turf, etc…). Doesn’t it make sense to use a DNA herbicide every single year since they are so effective?

Herbicide Resistance The problem with using any herbicide family without change is that weed populations are not identical (biotypes within a population). Resistance does not appear and become a problem because of genetic mutations or changes in plants. Resistant plants already exist within the population.

Herbicide Resistance Resistant plants become a problem when the susceptible members of the population are killed by herbicide applications. The resistant plants escape, reproduce, and over time, can become the majority of the population. The susceptible plants eventually are replaced by the resistant plants.

Herbicide Resistance - Turfgrasses Resistance is encouraged when the same herbicide or herbicide family is used year after year for the same weed control. Goosegrass – resistant to dinitroanilines and Illoxan. Annual bluegrass - resistant to triazines, ethofumesate, dinitroanilines.

Herbicide Resistance Goosegrass that is resistant to Barricade (DNA herbicide), is resistant to Pre-M, Balan, Surflan, etc…(other DNA herbicides), and Dimension because they all control weeds by the same mode of action. Remember, DNA herbicides are root inhibitors that affect cell division.

DNA Root inhibition Swollen, club shaped roots Surflan on St. Augustinegrass pendimethalin on zoysiagrass Dimension on bermudagrass Swollen, club shaped roots

Resistant Goosegrass Susceptible Goosegrass

Herbicide Resistance Annual bluegrass that is resistant to atrazine, is resistant to simazine and metribuzin. These herbicides are all photosynthetic inhibitors.

How long does resistance last in absence of further selection pressure? DNA resistant green foxtail % of resistant seeds after 7 years initial resistance was at least 90% Source: Weed Technology (1997): 11:369-372.

Herbicide Resistance Should Only Be Suspected When .……... other causes of herbicide failure have been ruled out. the same herbicide or herbicides with the same mode of action have been used year after year. one weed that is normally controlled is not controlled healthy weeds are mixed with controlled weeds (same species) a patch of uncontrolled weed is spreading.

Causes of Herbicide Failures weed size** moisture temperature humidity rate application method calibration others All possible reasons for poor performance should be investigated before considering the possibility of resistance!!!

Herbicide Resistance The way to avoid or delay herbicide resistance is to rotate not just herbicides from year to year, but to use herbicides that have different mode-of-action. To best avoid the proliferation of resistant weeds, a herbicide with a different mode-of-action should be used at least one year out of every four.

Herbicide Resistance Why would rotating herbicide families be a potential problem for turfgrass managers??? Several Reasons Turfgrasses are usually perennial plantings. They remain the same year after year. Depending on the turfgrass species, you may not have many effective available options. Some of your alternative options may not provide acceptable control, or cost may be an issue.

Herbicide Resistant Weeds Strategies for Control/Prevention proactive vs. reactive rotate herbicides with different MOA scout sites prevent seed production clean mowers and equipment

Herbicide Tolerant Turfgrasses Technology developed for glufosinate and glyphosate Creeping bentgrass is scheduled for first release for glyphosate Transgenic (GMO) cultivars Glufosinate-tolerant turfgrasses may be released

Herbicide Tolerant Turfgrasses May see stacked gene releases Ex. St. Augustinegrass with tolerance to glyphosate and with a reduced growth rate gene Unlike glyphosate, glufosinate tolerance can be inserted into an existing cultivar

Herbicide Tolerant Turfgrasses Advantages Nonselective herbicides Broad weed control spectrum Very favorable environmental characteristics with glyphosate and glufosinate Simplifies weed control May reduce total amount of herbicides applied to turfgrasses

Herbicide Tolerant Turfgrasses Potential Problems Requires federal review for registration Some are concerned about transgenic plants Some potential for outcrossing May act as weeds in some crops May be high in cost –annual fee vs. high price on seed

Herbicide Tolerant Turfgrasses Transgenic technology is actually leased One never owns technology User will pay a technology fee. User will not be able to sell transgenic turfgrasses or turfgrass seed without permission of registrant

Roundup Ready Crops The most significant item with glyphosate has been the introduction of glyphosate-resistant crops in 1996. Today there are Roundup Ready soybean, cotton, corn, and canola on the market with wheat, potatoes, and other small grains awaiting approval.

How does glyphosate resistance work How does glyphosate resistance work? Essentially, a resistant form of the EPSP synthase enzyme has been taken from bacteria and placed in plant so that the plant essentially bypasses the blockage caused by glyphosate.