The herbicide glyphosate became widely used in the United States and other parts of the world after the commercialization of glyphosate-resistant crops. These crops have constitutive overexpression of a glyphosate-insensitive form of the herbicide target site gene, 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS). Increased use of glyphosate over multiple years imposes selective genetic pressure on weed populations. We investigated recently discovered glyphosate-resistant Amaranthus palmeri populations from Georgia, in comparison with normally sensitive populations. EPSPS enzyme activity from resistant and susceptible plants was equally inhibited by glyphosate, which led us to use quantitative PCR to measure relative copy numbers of the EPSPS gene. Genomes of resistant plants contained from 5-fold to more than 160-fold more copies of the EPSPS gene than did genomes of susceptible plants. Quantitative RT-PCR on cDNA revealed that EPSPS expression was positively correlated with genomic EPSPS relative copy number. Immunoblot analyses showed that increased EPSPS protein level also correlated with EPSPS genomic copy number. EPSPS gene amplification was heritable, correlated with resistance in pseudo-F 2 populations, and is proposed to be the molecular basis of glyphosate resistance. FISH revealed that EPSPS genes were present on every chromosome and, therefore, gene amplification was likely not caused by unequal chromosome crossing over. This occurrence of gene amplification as an herbicide resistance mechanism in a naturally occurring weed population is particularly significant because it could threaten the sustainable use of glyphosate-resistant crop technology.5-enolpyruvylshikimate-3-phosphate synthase | herbicide resistance | mobile genetic element | evolution | Palmer amaranth

A glyphosate-resistant Palmer amaranth biotype was confirmed in central Georgia. In the field, glyphosate applied to 5- to 13-cm-tall Palmer amaranth at three times the normal use rate of 0.84 kg ae ha−1controlled this biotype only 17%. The biotype was controlled 82% by glyphosate at 12 times the normal use rate. In the greenhouse,I50values (rate necessary for 50% inhibition) for visual control and shoot fresh weight, expressed as percentage of the nontreated, were 8 and 6.2 times greater, respectively, with the resistant biotype compared with a known glyphosate-susceptible biotype. Glyphosate absorption and translocation and the number of chromosomes did not differ between biotypes. Shikimate was detected in leaf tissue of the susceptible biotype treated with glyphosate but not in the resistant biotype.

An experiment was conducted at four locations in North Carolina during 1994 and 1995 to evaluate weed control, cotton yield, fiber quality, and net returns in no-tillage bromoxynil-tolerant cotton. The experiment focused on using bromoxynil or pyrithiobac sodium applied early POST over-the-top as alternatives to fluometuron plus MSMA applied early POST directed. Fluometuron plus MSMA was more effective than bromoxynil or pyrithiobac sodium on tall morningglory, large crabgrass, goosegrass, and broadleaf signalgrass. Bromoxynil and fluometuron plus MSMA were similarly effective on common lambsquarters, common ragweed, and eclipta and more effective than pyrithiobac sodium. Pyrithiobac sodium and fluometuron plus MSMA were similarly effective on smooth pigweed and Palmer amaranth and more effective than bromoxynil. Prickly sida control by bromoxynil and pyrithiobac sodium was equal to or greater than control by fluometuron plus MSMA. All early POST herbicides controlled pitted morningglory similarly. Regardless of the early POST herbicides used, fluometuron applied PRE and cyanazine plus MSMA applied late POST directed increased control of most weeds and increased cotton yield and net returns. Bromoxynil and pyrithiobac sodium effectively substituted for fluometuron plus MSMA only in systems that included fluometuron applied PRE and cyanazine plus MSMA applied late POST directed. Effects of herbicide systems on cotton fiber quality were minor.

Composition and abundance of weed populations often change in response to new or extensively used weed management practices. Glyphosate-resistant (GR) technology is one such weed management practice now used extensively. A recent survey of weed scientists was conducted to address weed shifts in GR corn, cotton, and soybean. Twelve scientists in 11 states responded to the survey. Averaged over estimates from scientists, GR corn, cotton, and soybean were planted on 15, 90, and 88% of the hectarage in 2003, respectively. Acreage of GR corn is expected to rise, whereas only minor changes in acreage of GR cotton or soybean are expected. Weed shifts have not been observed in GR corn but have occurred in GR cotton and soybean. In GR cotton,Amaranthus, Commelina, Ipomoea, andCyperusspecies as well as annual grasses were noted as becoming more problematic. Similar to cotton,IpomoeaandCommelinaspecies are becoming more troublesome in GR soybean. In addition, in GR soybean, various winter annuals, lambsquarters species, and waterhemp species were noted as becoming more problematic. All scientists felt that weeds shifts were occurring, and two-thirds of these scientists noted that weed shifts are currently of economic concern. The scientists recommend the following to help manage weed shifts: additional herbicides in mixture with glyphosate, rotation to herbicides other than glyphosate, rotation to non–GR crops, and greater use of soil-applied herbicides.

An experiment was conducted at six locations in North Carolina to compare weed-management treatments using glufosinate postemergence (POST) in glufosinate-resistant soybean, glyphosate POST in glyphosate-resistant soybean, and imazaquin plus SAN 582 preemergence (PRE) followed by chlorimuron POST in nontransgenic soybean. Prickly sida and sicklepod were controlled similarly and 84 to 100% by glufosinate and glyphosate. Glyphosate controlled broadleaf signalgrass, fall panicum, goosegrass, rhizomatous johnsongrass, common lambsquarters, and smooth pigweed at least 90%. Control of these weeds by glyphosate often was greater than control by glufosinate. Mixing fomesafen with glufosinate increased control of these species except johnsongrass. Glufosinate often was more effective than glyphosate on entireleaf and tall morningglories. Fomesafen mixed with glyphosate increased morningglory control but reduced smooth pigweed control. Glufosinate or glyphosate applied sequentially or early postemergence (EPOST) following imazaquin plus SAN 582 PRE often were more effective than glufosinate or glyphosate applied only EPOST. Only rhizomatous johnsongrass was controlled more effectively by glufosinate or glyphosate treatments than by imazaquin plus SAN 582 PRE followed by chlorimuron POST. Yields and net returns with soil-applied herbicides only were often lower than total POST herbicide treatments. Sequential POST herbicide applications or soil-applied herbicides followed by POST herbicides were usually more effective economically than single POST herbicide applications.

Field experiments were conducted in Georgia to evaluate weed control and crop tolerance with glufosinate applied to ‘PHY 485 WRF®’ cotton. This glyphosate-resistant cotton also contains a gene, used as a selectable marker, for glufosinate resistance. Three experiments were maintained weed-free and focused on crop tolerance; a fourth experiment focused on control of pitted morningglory and glyphosate-resistant Palmer amaranth. In two experiments, PHY 485 WRF cotton was visibly injured 15 and 20% or less by glufosinate ammonium salt at 430 and 860 g ae/ha, respectively, applied POST two or three times. In a third experiment, glufosinate at 550 g/ha injured cotton up to 36%. Pyrithiobac or glyphosate mixed with glufosinate did not increase injury compared to glufosinate applied alone;S-metolachlor mixed with glufosinate increased injury by six to seven percentage points. Cotton injury was not detectable 14 to 21 d after glufosinate application, and cotton yields were not reduced by glufosinate or glufosinate mixtures. A program of pendimethalin PRE, glyphosate applied POST twice, and diuron plus MSMA POST-directed controlled glyphosate-resistant Palmer amaranth only 17% late in the season.S-metolachlor included with the initial glyphosate application did not increase control, and pyrithiobac increased late-season control by only 13 percentage points. Palmer amaranth was controlled 90% or more when glufosinate replaced glyphosate in the aforementioned system. Pitted morningglory was controlled 99% by all glufosinate programs and mixtures of glyphosate plus pyrithiobac. Seed cotton yields with glufosinate-based systems were at least 3.3 times greater than yields with glyphosate-based systems because of differences in control of glyphosate-resistant Palmer amaranth.

In 2005, the existence of glyphosate-resistance in Palmer amaranth was confirmed at a single 250 ha field site in Macon County, Georgia. Currently, all cotton producing counties in Georgia are infested, to some degree, with glyphosate-resistant Palmer amaranth. In 2010 and 2011, surveys were administered to Georgia growers and extension agents to determine how the development of glyphosate-resistance has affected weed management in cotton. According to respondents, the numbers of cotton acres that were treated with paraquat, glufosinate and residual herbicides effective against Palmer amaranth more than doubled between 2000 to 2005 and 2006 to 2010. Glyphosate use declined between 2000 to 2005 and 2006 to 2010 although, on average, the active ingredient was still applied to a majority of cotton acres. Although grower herbicide input costs have more than doubled following the evolution and spread of glyphosate resistance, chemically-based control of Palmer amaranth is still not adequate. As a consequence, Georgia cotton growers hand weeded 52% of the crop at an average cost of $57 per hand-weeded ha; this represents a cost increase of at least 475% as compared to the years prior to resistance. In addition to increased herbicide use and hand weeding, growers in Georgia are also using mechanical, in-crop cultivation (44% of acres), tillage for the incorporation of preplant herbicides (20% of the acres), and post-harvest deep-turning (19% of the acres every three years) for weed control. Current weed management systems are more diverse, complex and expensive than those employed only a decade ago, but are effective at controlling glyphosate-resistant Palmer amaranth in glyphosate-resistant cotton. The success of these programs may be related to producers improved knowledge about herbicide resistance, and the biological attributes that make Palmer amaranth so challenging, as well as their ability to implement their management programs in a timely manner.

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