Knowledge about how the type, timing, and arrangement of cultural practices influence weed species composition is important for understanding the ecological results of control strategies and designing alternative crop management systems. We evaluated weed seed density, diversity, and community composition from 1997 to 1999 in a 35-yr-old study comparing cropping sequences (continuous corn, corn–soybean, corn–oat–hay) and tillage systems (conventional, minimum, and no-tillage) in Wooster, OH. Weed seedbank diversity, as measured by species richness (S), evenness (J), and the Shannon–Weiner index (H′), was influenced by crop diversity; mean values for each of the indices were generally higher for all combinations of the three-crop sequence than for the corn monoculture or the corn–soybean rotation. Except for 1998, mean seed density (to a depth of 10 cm) was higher in continuous cornthan in corn and soybean rotationsSpecies richness and seed density were also affected by tillage. Mean values forand mean germinable seedswere greatest in the no-tillage system, where the soil was disturbed only by the coulter units of the planter. Differences in weed seedbank community composition among tillage and rotation treatments were examined using two multivariate analyses. Using a multiresponse permutation procedure and canonical discriminant analysis, results suggest that the weed seed community in a corn–oat–hay rotational system differs in structure and composition from communities associated with continuous corn and corn–soybean systems. Additionally, germinable weed seed communities in no-tillage differed in composition from those in conventional and minimum tillage. Crop sequence and tillage system influenced weed species density and diversity and therefore community structure. Manipulation of these factors could help reduce the negative impact of weeds on crop production.

In addition to being a strong competitor with cotton and other row crops, Palmer amaranth has developed resistance to numerous important agricultural herbicides, including glyphosate. The objective of this study was to determine if the glyphosate-resistance trait can be transferred via pollen movement from a glyphosate-resistant Palmer amaranth source to a glyphosate-susceptible sink. In 2006 and 2007 glyphosate-resistant Palmer amaranth plants were transplanted in the center of a 30-ha cotton field. Susceptible Palmer amaranth plants were transplanted into plots located at distances up to 300 m from the edge of the resistant pollen source in each of the four cardinal and ordinal directions. Except for the study plots, the interior of the field and surrounding acreage were kept free of Palmer amaranth by chemical and physical means. Seed was harvested from 249 and 292 mature females in October 2006 and 2007, respectively. Offspring, 14,037 in 2006 and 13,685 in 2007, from glyphosate-susceptible mother plants were treated with glyphosate when the plants were 5 to 7 cm tall. The proportion of glyphosate-resistant progeny decreased with increased distance from the pollen source; approximately 50 to 60% of the offspring at the 1- and 5-m distances were resistant to glyphosate, whereas 20 to 40% of the offspring were resistant at the furthest distances. The development of resistance was not affected by direction; winds were variable with respect to both speed and direction during the peak pollination hours throughout the growing season. Results from this study indicate that the glyphosate-resistance trait can be transferred via pollen movement in Palmer amaranth.

Introduction of glyphosate resistance into crops through genetic modification has revolutionized crop protection. Glyphosate is a broad-spectrum herbicide with favorable environmental characteristics and effective broad-spectrum weed control that has greatly improved crop protection efficiency. However, in less than a decade, the utility of this technology is threatened by the occurrence of glyphosate-tolerant and glyphosate-resistant weed species. Factors that have contributed to this shift in weed species composition in Georgia cotton production are reviewed, along with the implications of continued overreliance on this technology. Potential scenarios for managing glyphosate-resistant populations, as well as implications on the role of various sectors for dealing with this purportedtragedy of the commons, are presented. Benghal dayflower, a glyphosate-tolerant species, continues to spread through Georgia and surrounding states, whereas glyphosate susceptibility in Palmer amaranth is endangered in Georgia and other cotton-producing states in the southern United States. Improved understanding of how glyphosate susceptibility in our weed species spectrum was compromised (either through occurrence of herbicide-tolerant or -resistant weed species) may allow us to avoid repeating these mistakes with the next herbicide-resistant technology.

Palmer amaranth is resistant to several herbicides, including glyphosate, and there is concern that the resistance traits can be transferred between spatially segregated populations via pollen movement. The objective of this study was to describe the physical properties of Palmer amaranth pollen, specifically size, density, and settling velocity (V_s), that influence pollen flight. The mean diameter for Palmer amaranth pollen, as determined by light microscopy, was 31 µm (range of 21 to 38 µm); mean pollen diameter as measured with the use of an electronic particle sizer was 27 µm (range of 21 to 35 µm). The mean density of the solid portion of the pollen grain was 1,435 kg m⁻³. Accounting for the density of the aqueous fraction, the mean density of a fully hydrated pollen grain was 1,218 kg m⁻³. By Stokes's law, the estimated mean theoreticalV_sfor individual Palmer amaranth pollen grains was 3.4 cm s⁻¹for the range of pollen diameters with a mean of 31 µm and 2.6 cm s⁻¹for the range of pollen diameters with a mean of 27 µm. Results from laboratory studies indicated the majority of single pollen grains settled at a rate of 5.0 cm s⁻¹. The difference between the theoretical and empirical estimates ofV_swas likely due to changes in pollen density and shape postanthesis, which are not accounted for using Stokes's law, as well as the presence pollen clusters.

In 2006, Palmer amaranth with confirmed resistance to glyphosate (GLY-R) was not controlled effectively in cotton with pyrithiobac, an acetolactate synthase (ALS)-inhibiting herbicide. Glyphosate at 870 g ae ha⁻¹or pyrithiobac at 70 g ai ha⁻¹applied postemergence provided 5 to 28% control of a putative GLY/ALS-R Palmer amaranth biotype in the field. Glyphosate at 6,930 g ha⁻¹and pyrithiobac at 420 g ha⁻¹applied alone provided no more than 89 and 65% control 1 to 8 wk after treatment (WAT), respectively. When applied as a tank mixture, glyphosate plus pyrithiobac at 870 + 70 g ha⁻¹provided between 16 and 41% control; glyphosate plus pyrithiobac at 6,930 + 420 g ha⁻¹controlled the Palmer amaranth in the field 89 to 95%. Dose-response analyses developed from greenhouse data indicated that the estimated glyphosate rates required to produce 50% injury and reduce plant fresh weights by 50% relative to the nontreated control in a suspected GLY/ALS-R Palmer amaranth biotype were 12 and 14 times greater, respectively, than the estimated values for the susceptible (S) biotype. The predicted pyrithiobac rates required to produce the same responses in the putative resistant population were 151 (50% injury) and 563 times (50% fresh weight reduction) greater than the estimated rates for the S biotype. Field and greenhouse analyses confirm that the Palmer amaranth biotype evaluated in both studies is resistant to glyphosate and an ALS-inhibiting herbicide.

A greater understanding of the factors that regulate weed seed return to and persistence in the soil seedbank is needed for the management of difficult-to-control herbicide-resistant weeds. Studies were conducted in Tifton, GA to (1) evaluate whether glyphosate resistance, burial depth, and burial duration affect the longevity of Palmer amaranth seeds and (2) estimate the potential postdispersal herbivory of seeds. Palmer amaranth seeds from glyphosate-resistant and glyphosate-susceptible populations were buried in nylon bags at four depths ranging from 1 to 40 cm for intervals ranging between 0 and 36 mo, after which the bags were exhumed and seeds evaluated for viability. There were no detectable differences in seed viability between glyphosate-resistant and glyphosate-susceptible Palmer amaranth seeds, but there was a significant burial time by burial depth interaction. Palmer amaranth seed viability for each of the burial depths declined over time and was described by exponential decay regression models. Seed viability at the initiation of the study was ≥ 96%; after 6 mo of burial, viability declined to 65 to 78%. As burial depth increased, so did Palmer amaranth seed viability. By 36 mo, seed viability ranged from 9% (1-cm depth) to 22% (40-cm depth). To evaluate potential herbivory, seed traps with three levels of exclusion were constructed: (1) no exclusion, (2) rodent exclusion, and (3) rodent and large arthropod exclusion. Each seed trap contained 100 Palmer amaranth seeds and were deployed for 7 d at irregular intervals throughout the year, totaling 27 sample times. There were seasonal differences in seed recovery and differences among type of seed trap exclusion, but no interactions. Seed recovery was lower in the summer and early autumn and higher in the late winter and early spring, which may reflect the seasonal fluctuations in herbivore populations or the availability of other food sources. Seed recovery was greatest (44%) from the most restrictive traps, which only allowed access by small arthropods, such as fire ants. Traps that excluded rodents, but allowed access by small and large arthropods, had 34% seed recovery. In the nonexclusion traps, only 25% of seed were recovered, with evidence of rodent activity around these traps. Despite the physically small seed size, Palmer amaranth is targeted for removal from seed traps by seed herbivores, which could signify a reduction in the overall seed density. To be successful, Palmer amaranth management programs will need to reduce soil seedbank population densities. Future studies need to address factors that enhance the depletion of the soil seedbank and evaluate how these interact with other weed control practices.