Herbicide-Resistant Weeds

Transcription

1 Herbicide-Resistant Weeds Herbicide Resistance Defined. Response to Herbicide: Plants can respond to the application of a herbicide in one of three ways: Susceptible: A susceptible plant is killed at normal use rates of a particular herbicide. Tolerant: A tolerant plant is naturally immune to a particular herbicide when used at its normal use rate. For example, atrazine herbicide will kill many annual grass weeds, thus they are susceptible. However, atrazine applied to grain sorghum or corn causes no injury, as these crops are naturally tolerant to this particular herbicide because of rapid metabolism and breakdown within these plants. Resistant: A resistant plant once was susceptible to a particular herbicide but, because of selection for a genetically different biotype, it is not injured by the herbicide, even by rates up to 50X the normal use rate. Cross-resistance is the result of resistance to two or more herbicides due to the presence of a single resistance mechanism. For example, one herbicide selected for the resistant biotype, but this biotype is also resistant to other herbicides in the same mode of action. Multiple resistances occur when resistance to several herbicides results from two or more distinct resistant mechanisms in the same plant All Star Training Inc. 1

2 A Note on Convention: For purposes of discussion, susceptible and resistant biotypes, plants, populations, etc, of a given species will be designated S- and R- respectively. First Occurrences: The first herbicide-resistant biotype was identified in Washington State in Common groundsel, Senecio vulgaris, resistant biotype was found to be resistant to simazine herbicide. Since then, numerous cases of herbicide resistant biotypes, approximately 275 in 165 weed species to more than 16 different herbicide families, have been identified. Varying Time-Tables: The time required for a weed population to be identified as resistant varies. Resistance to an herbicide may occur in as little as 2-5 years or it may never develop. Resistance occurs with a wide variety of herbicide chemistries, weed species, and agricultural systems. For example, resistance to chlorsulfuron (Glean) developed after five years of continuous use in wheat. Triazine-resistant weeds occurred after 10 years of continuous use. Resistance to alachlor (Lasso) has never been reported despite 20 years of use. How Weed Resistance Occurs. Mode and Site of Action Mode of Action (MOA): Mode of action generically refers to how an herbicide kills a plant. Site of Action (SOA): Site of action refers to the exact plant function that is disrupted by an herbicide. Distinction: Two herbicides may both stop the production of amino acids in plants by attacking and inhibiting enzymes. Both of these herbicides have the same mode of action (called amino 2014 All Star Training Inc. 2

3 acid synthesis inhibition); however, they may inhibit different enzymes, so they have different sites of action. Herbicide Families: A plant that is resistant to an herbicide using a given site of action will likely be resistant to another herbicide using the same site of action. For this reason, herbicides using the same site of action are grouped together into herbicide families. Example: See the following page: Herbicide Mode & Site of Action. Here, the mode of action is amino acid synthesis inhibition. The site of action is the particular enzyme it inhibits. HERBICIDE MODE & SITE OF ACTION 1. Normal activity of an enzyme substrate complex. Amino Acids: Plant Lives 2. Herbicide binding to the enzyme and preventing normal action. Substrate cannot bind No Amino Acids: Plant Dies 2014 All Star Training Inc. 3

4 How Weed-Resistance Begins Biotype: The term biotype is often used to describe a group of plants within a species that has biological traits uncommon to the population as a whole. The plants can be weeds or desired plants, where herbicide resistance has been achieved through selection or genetic transformation techniques. Selection Intensity: Many of today s herbicides are effective enough to eliminate entire populations of a given weed species infesting a field. When this happens, a competitive advantage can be gained by any biotype of this species that may not be susceptible to the herbicide. Once you remove the R-biotype s competition, by controlling it with an effective herbicide, it will then flourish. This concept of competitive advantage is often referred to as selection intensity. Selection intensity is helped by herbicides with long soil residual where susceptible plants are controlled for months. Why Resistance Occurs: In situations where highly effective herbicides have been used, particularly year after year at the same location, there is a possibility of weed resistance showing up. Selection for an R-biotype may or may not happen, depending upon many factors such as cropping sequence, herbicide use intensity, and herbicide selection. There is not a clear explanation as to where the genetic differences in weed biotypes come from; however, herbicide use itself has not been shown to cause this All Star Training Inc. 4

5 Likelihood of Resistance: It is the general assumption that small percentages (probably less than 1%) of R-biotype weeds are present in natural populations. If the selection pressure remains intense, and no other herbicides are used, then these small populations will eventually reproduce and thrive. The result of this is a total lack of control. Process by Which Resistance Occurs: Repeated use of a highly effective and long residual herbicide leads to weed biotypes tolerant to the herbicide that survive and go to seed. Continued herbicide use eliminates competition for R-biotype population, and therefore the S-biotype is killed off while the R-biotype thrives. (See next page: Development of Herbicidal Resistance.) DEVELOPMENT OF HERBICIDAL RESISTANCE 1. Development begins with one resistant plant in a field. When sprayed with the herbicide, the plant lives and reproduces while most of the susceptible plants die. 2. The next year, more resistant plants are present in the weed population. Spraying the same herbicide site of action kills most of the susceptible plants, but the resistant plants survive and 3. Several years of this practice, and the resistant plants finally dominate the weed population and the field All Star Training Inc. 5

6 Population Dynamics Purpose: The population dynamics of a weed species is to understand the change in population size (# plants / unit area) over time in response to weed biology, environmental factors, and weed management. Calculation of Weed Population: The weed population observed in the second year after planting (N t+1 ) is a function of changes that have occurred since the beginning of the first year (N 1 ). During that year, additions to the population have occurred through births (B), such as seed production, and immigration (I) through dispersal of seeds from outside the population. One can also observe losses from population through deaths (D), such as seed decay and predation, and emigration (E) when seeds are dispersed away from the population. N t+1 = N t + B D + I E The population of a weed species is also very dynamic within the growing season. One can take measurements of the weed population at various growth stages during its lifecycle (identified as states) and then examine how each state responds to environmental or management factors. Observable States: For the lifecycle of a summer annual weed species, you can observe the number of weed seed found in the soil seedbank through soil sampling. A little later into the spring, you can observe the number of seedlings that are present. Towards the middle of the summer, the number of flowering adults can be counted and then the amount of new seed that is produced by each of these flowering adults can be determined. (See next page: Annual Weed Life Cycle) 2014 All Star Training Inc. 6

7 ANNUAL WEED LIFE CYCLE Observable states in Population Dynamics: Weeds drop seeds into the seedbank from the previous year. Given proper conditions, some of those seeds will germinate to become seedlings. Seedlings must then compete with both one another and other plants in order to become flowering adults. Adults disperse seeds, some of which are deposited back into the seedbank All Star Training Inc. 7

8 Transitions Between States. Germination & Emergence: There are three fates of weed seed in the seedbank. First, seed are lost through a diversity of means, such as decay and predation. Second, seed can germinate and emerge to become seedlings once appropriate environmental conditions, such as moisture and temperature, are reached. Third, seed can remain in the seedbank until next year or even longer. These seeds are dormant, meaning that they are viable but in a state of suspended animation, waiting for the right environmental conditions. The germination and emergence of a population can be measured as a proportion of the number of seedlings to the number of seeds found in a given area. Survival: Survival is the transition of seedlings into flowering adults. Seedlings must survive through a number of environmental factors and management practices to become a population of adults at a certain density and of a certain size. The survival ratio of a population can be measured as a proportion of the number of flowering adults to the number of seedlings found in a given area. Reproduction: Individual adult plants reproduce seed depending on their population density and size. The reproduction of a population is a factor of the number of flowering adults multiplied 2014 All Star Training Inc. 8

9 by the number of seeds produced by each adult. Dispersal is the shedding of the new seed back into the soil seedbank at the end of its growing season. Application of Population Dynamics. Rates of Transition: Rates for germination and emergence, survival, and reproduction can range widely depending on the weed species, environmental conditions, and on choice of weed control practices. Scenario: In March, a soil sampling for fertility measurements determined that, on average, 150 shattercane seed and 285 waterhemp seed per square yard were present in a field. In mid-april, in scouting for weed problems, 3 shattercane and 15 waterhemp seedlings were found per square yard. Question: What is the rate of germination and emergence for shattercane and for waterhemp? Answer: a. Shattercane: 3 seedlings / 150 seeds = 0.02, or 2%. b. Waterhemp: 15 seedlings / 285 seeds = 0.053, or 5.3%. Population Containing Resistant Weeds: If a portion of the seedbank population were resistant to the particular herbicide that was applied, germination and emergence and/or survival 2014 All Star Training Inc. 9

10 of that R-biotype portion would likely be higher than the germination and emergence and/or survival of the S-biotype. A soil-applied herbicide would impact the germination and emergence success of the weed species, and a postemergence herbicide would greatly impact the survival of the weed species. Scenario: It s suspected that the shattercane population is resistant to the herbicide we are using due to its repeated application during the previous 4 years and poor shattercane control last year. Of the 150 shattercane seed per square yard measured in the soil seedbank in March, its expected that 30% are the R-biotype and 70% are the S-biotype. Question: How many R-biotypes will the seedling population contain if the proportion of germination and emergence is 2%? How many S-biotypes? Answer: R-biotypes: S-biotypes 2014 All Star Training Inc. 10

11 Ecological Fitness: A comparison of the growth and competitiveness of S- and R-biotypes for several weed species has revealed that the ecological fitness is higher for the S-biotype than the R-biotype when grown together before being subjected to the herbicide. Ecological fitness differences among R- and S- biotypes mean that, on an individual plant basis, differences exist in vegetative growth and reproductive success. Differences in other lifecycle states and transitions, such as dormancy, germination, survival, and seed production could influence the outcome of population dynamics with mixed R- and S- biotype populations. Factors Influencing Selection Intensity. The intensity of the selection pressure (for example, how effective is the weed control practice chosen) can be measured by the rate of germination and emergence of seedlings and/or the survival of seedlings that become flowering adults. Intensity of the selection pressure is influenced by the interaction of the herbicide itself and weed biology, among others. Herbicide Factors: When herbicide is applied, it exerts selective pressure on a weed species by killing all S-biotype plants and not injuring the R-biotype plants. Intensity of selection pressure can be increased in the following ways: Site of Action: If the herbicide acts on a single mechanism/site of action. For example, ALSinhibiting herbicides act on a specific single enzyme in the plant. In a S-plant, the herbicide binds to the enzyme and does not allow it to function. In an R-plant, the structure of this enzyme has changed so that the herbicide can no longer bind and the enzyme functions normally. Effectiveness of Herbicide: If the herbicide is highly effective (>95% control), the S-biotype will be completely eliminated allowing the R-biotype to flourish without competition from any missed S-biotypes All Star Training Inc. 11

12 Persistence of Herbicide in Soil: If the herbicide provides soil actively for a long period of time (persistent), it can control weed species that typically emerge later in the season and thus, continue to remove S-biotypes and allow for R-biotypes to emerge and survive. Weed Biology Factors. Characteristics of the weed species and its population structure will influence the rate of appearance of herbicide resistance in a given population. Mutations: The frequency of mutations conferring herbicide resistance that occur within the plant population depends on several genetic factors. These include gene number, inheritance mechanisms, gene type, and nature of the modification of the gene. Little is known about the mutation rate and frequency of R-genes in field populations of weed species. It appears that there are some rare individuals in which a naturally occurring mutation has occurred that confers resistance even before herbicide use and selection has occurred. Weed species with a diverse genetic background may have 1 R-biotype in every 1 million or 1 billion seed. Adaptability: Weeds have the ability to adapt and prosper in diverse environments. For example, waterhemp plants look similar to each other, but some plants germinate earlier in the spring while others produce more seed, flower early, or perhaps prefer more moisture. In a field that is well-irrigated for four years, the selection is most likely for waterhemp that grows well under irrigation. Similarly, in a field that uses the same site of action year after year selects for waterhemp plants that are not controlled by that herbicide. Seedbank: Weeds have the capability of producing large numbers of seeds. If an R-biotype survives and reproduces, many new R-biotypes are returned to the seedbank for next year s 2014 All Star Training Inc. 12

13 population. The weed seedbank is a collection of old and new seed, and R- and S- biotypes. This can act as a buffer to the rate of appearance of herbicide resistant weed populations. Ecological Fitness: Ecological fitness of a given biotype indicates that it will leave a greater proportion of its genes in the future gene pool of the population, such that the most fit will leave the greatest number of offspring. Differences in ecological fitness between R- and S- biotypes will influence the rate of appearance of resistance, as well as development of resistant populations when not treated by the herbicide. Triazine R-biotypes often have reduced fitness and thus, may slow down the development of resistance and allow reversion of populations to susceptible levels if herbicide use is changed. However, in many cases, fitness is unchanged in R-biotypes. Detecting Resistant Weeds. Rate of Appearance: (See next page: Rate of Resistance in Weed Population). The rate of appearance of resistant weed populations occurs exponentially. It is not a gradual one that you can see. Once you do see it, it is already too late to prevent. The first observation is usually weed control failure, such as a large number of surviving plants (escapes) found in the middle of a group of dead plants. Not Always the Case: Sometimes when a weed control failure is observed, the presence of a herbicide resistant weed population is not always to blame. Other factors can be mistaken for herbicide failure. Unfavorable environmental conditions at the time of application. Inadequate spray coverage (spray skips) All Star Training Inc. 13

14 Oversized weed plants at the time of application. Weeds that have emerged after the time of application. An Ounce of Prevention: An effective weed control program is critical here. The best solution for the problem of weed resistance is to prevent their development and appearance in the first place. The key to resistance management is to reduce the intensity of selection pressure by using a combination of the following techniques: Scout fields on a regular basis throughout the growing season to identify weeds and observe escapes. Use a decision-support tool, such as WeedSOFT, to determine whether a weed control practice is necessary based on economics, not for cosmetic or aesthetic reasons. Rotate herbicides with different modes of action. Apply herbicides in tank-mixed, pre-packaged, or sequential mixtures that include multiple modes of action. Combine mechanical control practices such as rotary hoeing and cultivation with herbicide treatments wherever possible. Clean tillage and harvest equipment before moving from fields infested with resistant weeds to those fields that are not infested. Attempt to keep total weed population numbers down, especially post harvest and in fallow years All Star Training Inc. 14

15 Use crop rotations that allow for different herbicide and/or cultivation techniques and that may also provide different competitive environments to shift weed populations. Read the Label: Always remember to read and follow the instructions when applying an herbicide All Star Training Inc. 15