Palmer Amaranth Populations Research Articles

Recurrent Sublethal-Dose Selection for Reduced Susceptibility of Palmer Amaranth (Amaranthus palmeri) to Dicamba

The management of glyphosate-resistant Palmer amaranth has been a challenge in southern United States cropping systems. Registration of dicamba-resistant crops will provide an alternative management option to control herbicide-resistant Palmer amaranth populations, particularly those having resistance to herbicide Groups 2, 3, 5, 9, 14, and 27. However, repeated use of sublethal doses of dicamba may lead to rapid evolution of herbicide resistance, especially in Palmer amaranth—a species with a strong tendency to evolve resistance. Therefore, selection experiments with dicamba were conducted on Palmer amaranth using sublethal doses. In the greenhouse, a known susceptible Palmer amaranth population was subjected to sublethal dicamba doses for three generations (P1–P3). Susceptibility of the individuals to dicamba was evaluated, and its susceptibility to 2,4-D was characterized. Based on the greenhouse study, following three generations of dicamba selection, the dose required to cause 50% mortality increased from 111 g ae ha−1for parental individuals (P0) to 309 g ae ha−1for the P3. Furthermore, reduced susceptibility of the P3to 2,4-D was also evident. This research presents the first evidence that recurrent use of sublethal dicamba doses can lead to reduced susceptibility of Palmer amaranth to dicamba as well as 2,4-D. Here, we show that selection from sublethal dicamba doses has an important role in rapid evolution of Palmer amaranth with reduced susceptibility to auxin-type herbicides.

Influence of Nitrogen Status on the Sensitivity of Glyphosate-Resistant and -Susceptible Tall Waterhemp (Amaranthus tuberculatus) and Palmer Amaranth (Amaranthus palmeri)

Anecdotal observations of improved glyphosate efficacy on glyphosate-resistant (GR) tall waterhemp populations in corn production compared with soybean suggested the presence of nitrogen (N) fertilizer may influence the expression of glyphosate resistance. Greenhouse and field experiments were conducted to determine the influence of soil-applied nitrogen fertilizer on the growth rate and sensitivity of glyphosate-susceptible (GS) and GR tall waterhemp and Palmer amaranth. The addition of supplemental fertilizer increased the relative growth rate (plant height and shoot volume), number of nodes, and percentage of shoot nodes with axillary branches on GS and GR biotypes of both weed species. The axillary bud activity was increased 52 and 8% with increasing N for the GR and GS biotypes of tall waterhemp and Palmer amaranth, respectively. The GS populations of tall waterhemp and Palmer amaranth were more sensitive to glyphosate in the greenhouse under increased fertilizer levels compared with no fertilizer. Additionally, GR tall waterhemp was more sensitive to glyphosate under the higher fertilizer treatments, which resulted in a reduction in the calculated resistance factor (RF) from 27.8 under no fertilizer to 4.7 for the high fertilizer treatment. The RF for GR Palmer amaranth was not influenced by the fertilizer treatments in the greenhouse. Field experiments demonstrated that glyphosate efficacy may be greater on GR populations of tall waterhemp and Palmer amaranth under high N conditions, but these results were not consistent and most likely were influenced by soil moisture in 2012, which was more limiting than N supply. This research implies that soil fertility can influence the sensitivity of some GR weed species to glyphosate and the RF. Therefore, the evolution and management of GR weed species in commercial crop production may be influenced by the nutrient status of the soil and the use of supplemental fertilizers.

Integrating Herbicide Programs with Harvest Weed Seed Control and Other Fall Management Practices for the Control of Glyphosate-Resistant Palmer Amaranth (Amaranthus palmeri)

A large-plot field experiment was conducted at Keiser, AR, from fall of 2010 through fall of 2013 to understand to what extent soybean in-crop herbicide programs and postharvest fall management practices impact Palmer amaranth population density and seed production over three growing seasons. The effect of POST-only (glyphosate-only) or PRE followed by (fb) POST (glyphosate or glufosinate) + residual herbicide treatments were evaluated alone and in combination with postharvest management options of soybean residue spreading or soil incorporation, use of cover crops, windrowing with/without burning, and residue removal. Significant differences were observed between fall management practices on Palmer amaranth population density each fall. The use of cover crops and residue collection and removal fb the incorporation of crop residues into soil during the formation of beds were the most effective practices in reducing Palmer amaranth population. In contrast, the effects of fall management practices on Palmer amaranth seed production were inconsistent among years. The inclusion of a PRE herbicide application into the herbicide program significantly reduced Palmer amaranth population density and subsequent seed production each year when compared to the glyphosate-only program. Additionally, the glufosinate-containing residual program was superior to the glyphosate-containing residual program in reducing Palmer amaranth seed production. PRE fb POST herbicides resulted in significant decreases in the Palmer amaranth population density and seed production compared to POST application of glyphosate alone for all fall management practices, including the no-till practice. This study demonstrated that crop residue management such as chaff removal from the field, the use of cover crops, or seed incorporation during bed formation in combination with an effective PRE plus POST residual herbicide program is important for optimizing in-season management of Palmer amaranth and subsequently reducing the population density, which has a profound impact on lessening the risk for herbicide resistance and the consistency and effectiveness of future weed management efforts.

Baseline Sensitivity of Nebraska Waterhemp and Palmer Amaranth to Dicamba and 2,4‐D

The development of dicamba (3,6‐dichloro‐2‐methoxybenzoic acid)‐ and 2,4‐D [(2,4‐dichlorophenoxy) acetic acid]‐resistant soybean [Glycine max (L.) Merr.] will increase the frequency of applying these herbicides to control broadleaf weeds in soybean production systems. We evaluated the sensitivity of randomly selected waterhemp [Amaranthus tuberculatus (Moq.) Sauer var. rudis (Sauer) Costea and Tardif] and Palmer amaranth (Amaranthuspalmeri S. Wats) populations to a single dose of dicamba and 2,4‐D. Selected populations were then used in dicamba and 2,4‐D dose‐response experiments to evaluate: (i) visual injury at 28 d after treatment; and (ii) plant survival, biomass accumulation, and flowering at 84 DAT. Across selected populations, the estimated dicamba dose for 50% visual injury (I50) was below the recommended field rate (560 g ha−1) and ranged between 59 and 117 g ha−1 for waterhemp and 43 and 81 g ha−1 for Palmer amaranth populations. The estimated 2,4‐D dose for I50 was also below the recommended field rate (800 g ha−1) and ranged between 127 and 242 g ha−1 for waterhemp and 127 and 196 g ha−1 for Palmer amaranth. Waterhemp plants flowered with up to 560 g ha−1 dicamba and 2240 g ha−1 2,4‐D, while Palmer amaranth plants flowered with up to 280 g ha−1 dicamba, and 1120 g ha−1 2,4‐D. Individual plants will likely survive and flower following recommended field rates of dicamba (waterhemp) and 2,4‐D (both species). Close monitoring of weed populations are needed for the stewardship of available weed management technologies.Core Ideas We conducted a state‐wide baseline study of auxinic herbicide sensitivity in randomly selected waterhemp and Palmer amaranth populations. Individual plants of both species will likely survive and flower after field rates of dicamba and/or 2,4‐D. Close monitoring of weed populations and continuous update of best management practices are needed for the stewardship of available weed management technologies. This study sets a benchmark to detect shifting in sensitivities to dicamba or 2,4‐D after resistant crops are released.

Palmer Amaranth (Amaranthus palmeri) Identification and Documentation of ALS-Resistance in Argentina

Palmer amaranth has greatly disrupted agricultural practices in the United States with its rapid growth and rapid evolution of herbicide resistance. This weed species is now suspected in Argentina. To document whether the suspected plant populations are indeed Palmer amaranth, molecular comparisons to known standards were conducted. Additionally, these same plant populations were screened for possible herbicide resistance to several acetolactate synthase (ALS)-inhibiting herbicides. Sequencing data confirmed that suspected populations (A2, A3, A4) were indeed Palmer amaranth. Another population (A1) was tested to determine whether hybridization had occurred between Palmer amaranth and mucronate amaranth the native amaranth species of the region. Tests confirmed that no hybridization had occurred and that A1 was simply a unique phenotype of mucronate amaranth. Each population was screened for resistance to imazapic, nicosulfuron, and diclosulam. All Palmer amaranth populations from Argentina were shown to be resistant to at least one ALS-inhibiting herbicide. The populations were then subjected to further testing to identify the mutation responsible for the observed ALS resistance. All mucronate amaranth populations exhibited a mutation previously documented to confer ALS resistance (S653N). No known resistance-conferring mutations were found in Palmer amaranth.

Tolerance of Elevated Ozone and Water Stress in a California Population of Palmer Amaranth (Amaranthus palmeri)

Palmer amaranth is a highly invasive weed species causing huge economic losses in agricultural cropping systems under a broad range of environmental conditions. Sensitivity of this species to ozone (O3) air pollution and to soil water deficit, relative to native species or competing crops, may affect its competitiveness and invasive potential. In recent years, both high tropospheric O3and soil water deficiency have become common in the San Joaquin Valley of California. Responses to these environmental parameters may help predict the invasiveness of this species and have implications for landscape hydrology. We assessed the impact of O3and soil water deficit on Palmer amaranth. Five- to seven-leaf–stage potted plants were placed in continuous stirred tank reactor chambers and maintained for 30 to 35 d under 12-h mean daylight O3exposures (0700–1900 hours) of 4, 59, or 114 ppb O3. Within the chambers the plants were either well-watered (WW) or exposed to regulated deficit irrigation (RDI) and grown for about 7 wk. Dry weights of the leaves, stems, roots, and leaf area were determined. Day- and nighttime stomatal conductances (gs) were measured at 1.5-h intervals. Nocturnalgswas about 16 to 29% of daytimegs; this suggests that the species could have substantial nighttime water loss, uncoupled from carbon gain in the weed, and could affect water availability for crops and reduce irrigation efficiency. Nocturnalgswas lower in the RDI than in the WW, but daytimegswas not affected by O3or irrigation regime. Neither O3nor irrigation regime affected root or shoot parameters. As O3and drought are two key stressors in the San Joaquin Valley, to which potential competing species have been found to be sensitive, Palmer amaranth may proliferate and become more invasive in the future, potentially altering landscape hydrology and reducing irrigation efficiency.

Multiple-Herbicide Resistance Is Widespread in Roadside Palmer Amaranth Populations.

Herbicide-resistant Palmer amaranth is a widespread issue in row-crop production in the Midsouthern US. Palmer amaranth is commonly found on roadside habitats in this region, but little is known on the degree of herbicide resistance in these populations. Herbicide resistance in roadside Palmer amaranth populations can represent the spread of an adaptive trait across a selective landscape. A large-scale survey was carried out in the Mississippi Delta region of eastern Arkansas to document the level of resistance in roadside Palmer amaranth populations to pyrithiobac and glyphosate, two important herbicides with broad history of use in the region. A total of 215 Palmer amaranth populations collected across 500 random survey sites were used in the evaluations. About 89 and 73% of the surveyed populations showed >90% survival to pyrithiobac and glyphosate, respectively. Further, only 3% of the populations were completely susceptible to glyphosate, while none of the populations was completely controlled by pyrithiobac. Among the 215 populations evaluated, 209 populations showed multiple resistance to both pyrithiobac and glyphosate at varying degrees. Dose-response assays confirmed the presence of high levels of herbicide resistance in the five selected populations (≥ 25-fold compared to a susceptible standard). Results demonstrate the prevalence of multiple-herbicide resistance in roadside Palmer amaranth populations in this region. Growers should be vigilant of Palmer amaranth infestation in roadsides adjacent to their fields and implement appropriate control measures to prevent likely spread of herbicide resistance into their fields.

Long-Term Management of Palmer Amaranth (Amaranthus palmeri) in Dicamba-Tolerant Cotton

Research was conducted from 2011 to 2014 to determine weed population dynamics and frequency of glyphosate-resistant (GR) Palmer amaranth with herbicide programs consisting of glyphosate, dicamba, and residual herbicides in dicamba-tolerant cotton. Five treatments were maintained in the same plots over the duration of the experiment: three sequential POST applications of glyphosate with or without pendimethalin plus diuron PRE; three sequential POST applications of glyphosate plus dicamba with and without the PRE herbicides; and a POST application of glyphosate plus dicamba plus acetochlor followed by one or two POST applications of glyphosate plus dicamba without PRE herbicides. Additional treatments included alternating years with three sequential POST applications of glyphosate only and glyphosate plus dicamba POST with and without PRE herbicides. The greatest population of Palmer amaranth was observed when glyphosate was the only POST herbicide throughout the experiment. Although diuron plus pendimethalin PRE in a program with only glyphosate POST improved control during the first 2 yr, these herbicides were ineffective by the final 2 yr on the basis of weed counts from soil cores. The lowest population of Palmer amaranth was observed when glyphosate plus dicamba were applied regardless of PRE herbicides or inclusion of acetochlor POST. Frequency of GR Palmer amaranth was 8% or less when the experiment was initiated. Frequency of GR Palmer amaranth varied by herbicide program during 2012 but was similar among all herbicide programs in 2013 and 2014. Similar frequency of GR Palmer amaranth across all treatments at the end of the experiment most likely resulted from pollen movement from Palmer amaranth treated with glyphosate only to any surviving female plants regardless of PRE or POST treatment. These data suggest that GR Palmer amaranth can be controlled by dicamba and that dicamba is an effective alternative mode of action to glyphosate in fields where GR Palmer amaranth exists.

Assessing the Economic Impact of Inversion Tillage, Cover Crops, and Herbicide Regimes in Palmer Amaranth(Amaranthus palmeri)Infested Cotton

Cotton (Gossypium hirsutumL.) producers in Alabama are faced with a rapidly expanding problem that decreases yields and increases production costs: herbicide-resistant weeds. Producers increasingly rely on integrated weed management strategies that raise production costs. This analysis evaluated how tillage, cover crops, and herbicide regime affected net returns above variable treatment costs (net returns) for cotton production in Alabama from 2009 to 2011 under pressure from Palmer amaranth (Amaranthus palmeriS. Wats.). Annual net returns were compared for two tillage treatments (inversion and noninversion tillage), three cover crops (crimson clover [Trifolium incarnatumL.], cereal rye [Secale cerealL.], and winter fallow), and three herbicide regimes (PRE, POST, and PRE+POST). Results indicate that under heavy Palmer amaranth population densities one year of inversion tillage followed by two years of noninversion tillage, along with a POST or PRE+POST herbicide application had the highest net returns in the first year; however, the economic benefit of inversion tillage, across all herbicide treatments, was nonexistent in 2010 and 2011. Cotton producers with Palmer amaranth infestations would likely benefit from cultural controls, in conjunction with herbicide applications, as part of their weed management system to increase net returns.

Survey of Glyphosate‐ and Imazapic‐Resistant Palmer Amaranth (Amaranthus palmeri) in Florida

Herbicide‐resistant Palmer amaranth (Amaranthus palmeri S. Wats.) is the most troublesome weed in Florida cotton (Gossypium hirsutum L.) and peanut (Arachis hypogaea L.) production. In 2012, Palmer amaranth populations were surveyed to document the extent and level of resistance to glyphosate and imazapic. Mature seedheads were sampled at 31 locations in the northwest region of Florida where cotton and peanut are predominantly produced, and in the north‐central region of Florida that mainly consists of peanut production. Palmer amaranth plants were grown from field‐collected seed in the greenhouse and treated with glyphosate and imazapic to establish a dose response for each population. Glyphosate‐resistant Palmer amaranth was found in 11 populations that were located primarily in the northwest region, with some populations having up to 30‐fold resistance. Imazapic resistance was much more widespread, being found in 29 of the 30 populations tested. Since peanut production in Florida has historically relied on successive imazapic applications, it is not surprising that widespread resistance to imazapic has developed. Resistance to both herbicides was found in 10 populations. Results from this survey reveal the magnitude of herbicide resistance in Florida and the need for growers to diversify management strategies in the future.

Sequential and Co-Application of Glyphosate and Glufosinate in Cotton

Glufosinate controls GR Palmer amaranth (Amaranthus palmeri S. Wats.), but might be less effective than glyphosate on certain weeds. Glyphosate and glufosinate applications in tolerant cotton (Gossypium hirsutum L.) cultivars can broaden the spectrum of control and aid in resistance management. Research investigating sequential application and potential interactions with co-applications of glyphosate and glufosinate is limited. Field research at six locations evaluated Palmer amaranth and annual grass control with glyphosate and glufosinate co-applied or sequentially applied. Herbicides were applied to two-leaf cotton and repeated 14 d later. A GR biotype comprised 10 to 90% of the Palmer amaranth populations. Greater Palmer amaranth control was achieved following sequential applications of glufosinate compared with glyphosate. Co-application of glufosinate plus glyphosate was more effective than glyphosate alone at most locations but never more effective than glufosinate alone. Glyphosate controlled goosegrass (Eleusine indica [L.] Gaertn.) more than glufosinate and a similar response was observed for large crabgrass (Digitaria sanguinalis [L.] Scop.). Glufosinate and glyphosate co-applied were less effective than glyphosate alone on both grasses, but not more effective than glufosinate alone. Glufosinate followed by (fb) glyphosate was the preferred order for sequential application to control Palmer amaranth at most locations. With high percentages of the GR biotype, glufosinate applied sequentially was more effective than any other sequential applications. Glyphosate fb glufosinate and glufosinate fb glyphosate controlled large crabgrass similar to glyphosate applied sequentially and greater than glufosinate applied sequentially. For goosegrass, glyphosate fb glufosinate was more effective than the reverse. Seven days after the second postemergence application, sequential application of glyphosate fb glufosinate controlled goosegrass as well as glyphosate applied sequentially and better than glufosinate applied sequentially.

Glyphosate-Resistant Palmer Amaranth (Amaranthus palmeri) Morphology, Growth, and Seed Production in Georgia

Herbicide-resistant Palmer amaranth has become the most economically detrimental weed of cotton in the southeast United States. With the continual marginalization of potential herbicide tools, research has expanded to include alternative means of affecting future Palmer amaranth populations by altering safe sites and reducing inputs to the seedbank population. The influence of delayed Palmer amaranth establishment on seed production potential has not been investigated in the southeast United States. Studies were conducted to evaluate the influence of time of Palmer amaranth establishment on morphology, growth, and seed production. The experiment was a factorial, with five levels of Palmer amaranth transplanting (0 to 12 wk after cotton planting) and two levels of crop type (cotton and noncrop). In the absence of crop competition, the first cohort of Palmer amaranth produced 446,000 seeds per plant. This potential seed production was reduced 50% when Palmer amaranth plants were established nearly 6 wk later. In contrast, the first Palmer amaranth cohort growing in competition with cotton produced 312,000 seeds, 30% less than was produced in the absence of competition. Interference from cotton shifted time to 50% seed production to nearly 4 wk earlier in the growing season, relative to plants without crop competition. Delayed establishment of Palmer amaranth simulates premature loss of herbicide efficacy and alters the competitive balance between cotton and Palmer amaranth. Although the planting time that resulted in 50% reduction in Palmer amaranth plant height at the end of the season was not affected by the presence of cotton, 50% reduction in plant width and biomass occurred within the first 3.8 wk of the growing season when growing with cotton, more than 4 wk earlier than Palmer amaranth growing without a crop. This suggests that early season weed control programs will have a large effect on Palmer amaranth potential growth and seed production, as well as crop yield. To implement soil seedbank reduction strategies effectively, information on mechanisms that regulate Palmer amaranth persistence in the soil seedbank is needed.

Impact of Genetic Background in Fitness Cost Studies: An Example from Glyphosate-Resistant Palmer Amaranth

Since its discovery in 2005, glyphosate-resistant Palmer amaranth has become a major problem for many farmers in the southern United States. One mechanism of resistance found in a Georgia population of glyphosate-resistant Palmer amaranth is amplification of the 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene throughout the genome, with some resistant plants containing and expressing more than 100EPSPSgenes. Such high numbers ofEPSPSgenes and protein production could result in a fitness cost to resistant plants due to (1) metabolic cost of overproduction of this enzyme and (2) disruption of other genes after insertion of theEPSPSgene. A greenhouse experiment was set up to investigate differences in growth and reproduction between glyphosate-susceptible and -resistant Palmer amaranth plants. Measurements included growth rate, plant height/volume ratio, final biomass, photosynthetic rate, inflorescence length, pollen viability, and seed set. This study found no significant fitness costs for plants with the resistance trait. This study also provided a clear example of how controlling for genetic background is important in fitness cost studies and how potentially misleading results can be obtained if only a few fitness traits are measured. These results indicate that glyphosate-resistant Palmer amaranth plants with highEPSPSgene copy numbers are likely to persist in field populations, even in the absence of glyphosate, potentially leading to long-term loss of glyphosate as a control option for Palmer amaranth.

Mechanism of Resistance and Inheritance in Glyphosate Resistant Palmer amaranth (Amaranthus palmeri) Populations from New Mexico, USA

Two populations of Palmer amaranth from New Mexico have been confirmed to be resistant to glyphosate. In the present study, the molecular basis of resistance and the mode of inheritance of resistance in those populations were investigated. Quantitative real-time polymerase chain reaction analysis indicated up to an eightfold increase in genomicEPSPScopy number in glyphosate resistant plants compared with susceptible plants. The relative genomicEPSPScopy number of resistant plants was positively correlated with the relativeEPSPScDNA expression levels. Eight hours after treatment with glyphosate, the shikimate accumulation levels in resistant plants were negatively correlated with the genomicEPSPScopy numbers. Multiple sequencing of theEPSPScDNA of resistant plants did not reveal any glyphosate resistance-conferring mutations. The evaluation of F1, reciprocal F1, and F2Palmer amaranth families indicated that resistance to glyphosate does not follow a single-gene segregation pattern. Results suggest that theEPSPSamplification is the primary molecular basis of resistance in glyphosate resistant populations of Palmer amaranth from New Mexico.

Interference and Control of Glyphosate-Resistant and -Susceptible Palmer Amaranth (Amaranthus palmeri) Populations under Greenhouse Conditions

Interference for 40 d after emergence (DAE) of corn, cotton, peanut, and snap bean by four glyphosate-resistant (GR) and four glyphosate-susceptible (GS) Palmer amaranth populations from Georgia and North Carolina was compared in the greenhouse. Greater interference from Palmer amaranth, measured as crop height and fresh weight reduction, was noted in cotton and peanut compared with corn or snap bean. Crop height 15 to 40 DAE was reduced similarly by GR and GS populations. Crop fresh weight, however, was reduced 25 and 19% in the presence of GS and GR populations, respectively. Measured as percent reduction in fresh weight, GR and GS populations of Palmer amaranth were controlled similarly by glufosinate, lactofen, paraquat, and trifloxysulfuron applied POST. Atrazine and dicamba controlled GR populations more effectively than GS populations.

Use of AFLP Markers to Assess Genetic Diversity in Palmer Amaranth (Amaranthus palmeri) Populations from North Carolina and Georgia

Glyphosate-resistant Palmer amaranth is a serious problem in southern cropping systems. Much phenotypic variation is observed in Palmer amaranth populations with respect to plant growth and development and susceptibility to herbicides. This may be related to levels of genetic diversity existing in populations. Knowledge of genetic diversity in populations of Palmer amaranth may be useful in understanding distribution and development of herbicide resistance. Research was conducted to assess genetic diversity among and within eight Palmer amaranth populations collected from North Carolina and Georgia using amplified fragment length polymorphism (AFLP) markers. Pair-wise genetic similarity (GS) values were found to be relatively low, averaging 0.34. The highest and the lowest GS between populations were 0.49 and 0.24, respectively, while the highest and the lowest GS within populations were 0.56 and 0.36, respectively. Cluster and principal coordinate (PCO) analyses grouped individuals mostly by population (localized geographic region) irrespective of response to glyphosate or gender of individuals. Analysis of molecular variance (AMOVA) results when populations were nested within states revealed significant variation among and within populations within states while variation among states was not significant. Variation among and within populations within state accounted for 19 and 77% of the total variation, respectively, while variation among states accounted for only 3% of the total variation. The within population contribution towards total variation was always higher than among states and among populations within states irrespective of response to glyphosate or gender of individuals. These results are significant in terms of efficacy of similar management approaches both in terms of chemical and biological control in different areas infested with Palmer amaranth.

Resistance to Glyphosate in Palmer Amaranth (Amaranthus palmeri) Populations from New Mexico Pecan Orchards

Two populations of Palmer amaranth suspected of being resistant to glyphosate have been reported since 2007 in pecan orchards in Doña Ana County, New Mexico. The objectives of the study were to confirm and evaluate the level of resistance, to evaluate the effectiveness of alternative herbicide mechanisms of action, and to compare the cost of effective alternative herbicides for weed management in pecan orchards. Greenhouse experiments indicated that the resistant populations were able to survive glyphosate at 736 g ae ha−1. Compared with a susceptible (S) population, one of the resistant (R) populations had sevenfold resistance to glyphosate. POST application of 12 herbicides, with five different mechanisms of action, all provided at least 88% control of both R and S populations when applied at their recommended field rates. PRE application of trifluralin and metolachlor also provided more than 99% control of R and S populations. The results of field studies indicated that the financial benefit of season-long weed management with glyphosate, in pecan orchards, was comparable with some of the tested alternative herbicides.

Varying Tolerance to Glyphosate in a Population of Palmer Amaranth with Low EPSPS Gene Copy Number

A Palmer amaranth population (seeds collected in the year 2000; Washington Co., MS) suspected to be susceptible to glyphosate was examined as a population and as individual plants and found to exhibit varying tolerance or resistance to glyphosate. Whole plant spraying of glyphosate (0.84 kg·ha?1) to the population revealed that approximately 40% of this population were resistant to glyphosate and an LD50 of 0.75 kg·ha?1 was determined. Spray application of glyphosate indicated that some plants displayed varying degrees of resistance 14 days after treatment. Initial tests using leaf disc bioassays on 10 individual plants selected randomly from the population, allowed characterization of glyphosate resistance using both visual ratings of injury and quantitative measurement via chlorophyll content analysis. After initial bioassays and spray application, five plants with a range of tolerance to glyphosate were selected for cloning so that further studies could be accomplished on these individuals. Q-PCR analysis of these clones showed that resistance was not due to elevated EPSPS gene copy number. Shikimate levels were lower in the resistant and higher in the susceptible clones which correlated with varying degrees of resistance demonstrated in bioassays and spray application of glyphosate of these clones. Results demonstrate that individuals in a population can vary widely with respect to herbicide resistance and suggest that uptake, translocation, sequestration, metabolism or altered target site may contribute to the resistance in some individuals of this population.

Bioassay and Characterization of Several Palmer Amaranth (<i>Amaranthus palmeri</i>) Biotypes with Varying Tolerances to Glyphosate

The wide distribution of Palmer amaranth (Amaranthus palmeri) in the southern US became a serious weed control problem prior to the extensive use of glyphosate-resistant crops. Currently glyphosate-resistant populations of Palmer amaranth occur in many areas of this geographic region creating an even more serious threat to crop production. Investigations were undertaken using four biotypes (one glyphosate-sensitive, one resistant from Georgia and two of unknown tolerance from Mississippi) of Palmer amaranth to assess bioassay techniques for the rapid detection and level of resistance in populations of this weed. These plants were characterized with respect to chlorophyll, betalain, and protein levels and immunological responses to an antibody of 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) the target site of glyphosate. Only slight differences were found in four biotypes grown under greenhouse conditions regarding extractable soluble protein and chlorophyll content, but one biotype was found to be devoid of the red pigment, betalain. Measurement of early growth (seedling shoot elongation) of seedlings was a useful detection tool to determine glyphosate resistance. A leaf disc bioassay (using visual ratings and/or chlorophyll analysis) and an assay for shikimate accumulation were effective methods for determining herbicide resistance levels. The two unknown biotypes were found to be resistant to this herbicide. Some differences were found in the protein profiles of the biotypes, and western blots demonstrated a weak labeling of antibody in the glyphosate-sensitive biotype, whereas strong labeling occurred in the resistant plants. This latter point supports research by others, that increased copy number of the EPSPS gene (and increased EPSPS protein levels) is the resistance mechanism in this species. Results indicate the utility of certain bioassays for the determination of resistance and provide useful comparative information on the levels of inherent constituents among closely related plants.

Herbicide Programs for Enhanced Glyphosate-Resistant and Glufosinate-Resistant Cotton (Gossypium hirsutum)

Research was conducted at experimental research stations near Keiser and Marianna (Marianna-A), AR, in 2007, and in a grower's field near Marianna (Marianna-B), AR, in 2008, to compare herbicide programs, including POST application(s) of glyphosate/glufosinate alone or in combination with residual herbicides applied as PRE, mid-POST (MPOST), or layby POST-directed (PD) in enhanced glyphosate- and glufosinate-resistant cotton. Weed species evaluated included Palmer amaranth, pitted morningglory, hemp sesbania, barnyardgrass, and a mixture of large crabgrass and goosegrass. At Marianna-B, AR, the Palmer amaranth population was a mixture of glyphosate-resistant and -susceptible plants. For both cotton cultivars and at all locations, inclusion ofS-metolachlor plus fluometuron PRE increased weed control and/or decreased the number of glufosinate or glyphosate applications needed in-season. At Marianna-B, AR, PRE residual herbicides and/or glufosinate were required to control glyphosate-resistant Palmer amaranth. Addition of pyrithiobac to glufosinate or glyphosate did not increase weed control. A layby PD application of flumioxazin plus MSMA was required to increase late-season control of all weed species in POST glufosinate-only programs, but not in POST glyphosate-only programs. None of the programs caused > 5% injury to either cotton cultivar. Seed-cotton yield was similar in all herbicide programs at Keiser, AR, and Marianna-A, AR, except for the POST glyphosate-only program, which yielded less than the PRE followed by POST programs in glyphosate-resistant cotton at Keiser, AR. In general, PRE herbicides did not increase cotton yield but did improve early and late-season control of glyphosate-susceptible and -resistant weeds in both cotton cultivars.