Resistance of Weeds to Herbicides

Abstract

Herbicides are the most widely used group of pesticides worldwide. The widespread use of herbicides has allowed tremendous gains in agricultural productivity worldwide. Since the 1950’s herbicides have progressively replaced mechanical weed control because herbicides are more cost effective (Gianessi & Reigner, 2007). In 2009 over 95% of all the major agronomic crops grown in the U. S. were treated with herbicides (USDA-NASS, 2009). Transgenic herbicide-resistant crops were commercially introduced in the U. S. in 1996 when glyphosate-resistant (Roundup Ready®) soybean was released. Use of the very broadspectrum herbicide, glyphosate, provided outstanding weed control (Dill, 2005; Dill et al. 2008). The most recent data indicates that the percent of the total acres of each of the following crops planted with glyphosate-resistant cultivars is soybeans 91%, canola 91%, cotton 71%, and corn 68% (Brookes & Barfoot 2009). Herbicides are used on >90% of arable farmland in the U.S. and herbicide-resistant crops has been used widely since the mid1990’s. Herbicide resistance in weeds was first discovered in 1968 (Ryan 1970) and there are currently 347 confirmed weed biotypes worldwide (Heap 2010). When discussing pest resistance, whether it is weeds, pathogens, or insects, it is important to define the resistance. Some of the basic differences in the definitions of pest resistance depend on the basic definitions. The most basic unit of biological classification is the species, defined as a group of individual organisms displaying common characteristics and having the ability to mate and produce fully viable progeny. A population is a group of organisms within a species that co-occur in time and space (Radosevich et al. 1997) and share a distinct range of genetic variation. A species is usually composed of several to many populations. A genotype is the sum of the genetic coding or the genome of an individual. A biotype may not be fully coincident with genotype, as an individual has many genes. Certain genes may be expressed or unexpressed and not pertain to the phenotype associated with the biotype. A biotype is a phenotype that consistently expresses or exhibits a specific trait or set of traits. Weed scientists tend to refer to a biotype as a group of individuals with distinctive biochemical or morphological traits (e.g. resistance to a specific herbicide mechanism of action; growth and morphological traits). A phenotype refers to the physiological and morphological profile of the expressed genes in an individual. A single genotype can produce different phenotypes in response to environmental conditions present. This fundamental property of organisms is known as phenotypic plasticity. The alteration of phenotype (morphological or biochemical) without change in either the coding sequence of a gene or the upstream promoter region is classified as epigenetic change (Rapp & Wendel 2005). There is some controversy over whether