The objectives of this research were to compare the weed control efficacy of liquid, granular, and microencapsulated formulations of preemergence herbicides in moldboard plow, chisel plow, ridge tillage, and no-tillage corn and soybean production systems, and to determine whether herbicide formulation can influence herbicide interception and retention on surface corn residue. Common lambsquarters populations were threefold higher in corn than in soybeans. A mixed population of giant foxtail and green foxtail was highest in the chisel plow and lowest in the ridge tillage system as were total weed numbers. Percent weed control was not influenced by tillage when considered across all herbicide treatments. Weed control was not influenced by herbicide formulation in the moldboard plow, chisel plow, or ridge tillage systems, but granular herbicide applications provided better weed control than liquid applications in the no-tillage system and across various rates of corn residue in an experiment with no tillage variables. Two- to threefold less granular-applied herbicide was intercepted by surface corn residue at the time of application compared to liquid-applied herbicide. Increasing amounts of postapplication rainfall decreased the difference among formulations with regard to both total soil reception of the herbicide and resultant weed control. There was no consistent advantage for the microencapsulated formulation over the other herbicide formulations. Surface corn residue controlled many weeds without the aid of a herbicide and actually contributed to overall weed control even where herbicides were applied. This suggests that the binding of preemergence herbicides on surface crop residue may not be the cause of weed control failures in reduced-tillage systems as is often assumed to be the case.

We conducted several experiments on linuron-resistant and -susceptible Portulaca oleracea and on atrazine-resistant and -susceptible Chenopodium album to determine their immediate and long-term responses to photosynthesis-inhibiting herbicides. Several photosynthesis-inhibiting herbicides were used, and O2 evolution was measured with a Clark-type O2 electrode. Resistance ratios (RRs) for P. oleracea, based on O2 evolution inhibition, were 8 and > 6 for linuron and diuron, respectively; > 800 for atrazine; and > 20 for terbacil. Linuron-resistant P. oleracea was negatively cross-resistant to bentazon and pyridate (RR = 0.5 and 0.75, respectively). Time-course measurements of fresh weight, photosynthetic CO2 assimilation, and photochemical efficiency indicated that linuron and atrazine inhibited electron transport in susceptible (S) P. oleracea and C. album, ultimately resulting in death. Measurements of photochemical efficiency and CO2 assimilation of linuron-resistant P. oleracea treated with linuron indicated a transient injury from which plants recovered within 14 d. Recovery of linuron-resistant P. oleracea from atrazine injury was more rapid than from linuron injury for all measured variables. Atrazine-resistant C. album had no cross-resistance to linuron. Sequence analysis of the D1 protein revealed that linuron-resistant P. oleracea had a serine-to-threonine substitution at position 264.

Cotton (Gossypium hirsutum L. ‘Westburn M’) was grown for 2 yr at two locations with full-season interference from buffalobur (Solanum rostratum Dunal ♯ SOLCU) at densities ranging from 0 to 64 plants/10 m of row. Dry weight of buffalobur harvested increased by 0.063 to 0.303 kg/plot for each additional weed/10 m of row. Intraspecific competition among buffalobur plants was observed at the higher weed densities. Cotton plant height was reduced by 16 and 32 buffalobur plants/10 m of row (or more) at Tipton and Perkins, OK, respectively, when compared with cotton grown under weed-free conditions. The threshold densities at which initial lint yield reductions occurred were 8 buffalobur plants/10 m of row in 1982 and 1983 at Tipton and 32 and 2 plants/10 m of row at Perkins in 1982 and 1983, respectively. Lint yields were reduced curvilinearly from 6 to 18 kg/ha for each additional buffalobur plant/10 m of row. Fiber quality was not significantly influenced by weed density when analyzed over all experiments; however, 50% span length, uniformity index, and micronaire were adversely affected in some environments.

Field experiments were carried out in northern Greece during 1994, 1995, and 1996 to study the effect of nitrogen fertilization on competition between sterile oat and wheat, barley, and triticale. Dry weight of all crops was not affected until early March by sterile oats (110 plants m−2), but wheat and triticale dry weight were significantly reduced by sterile oats competition after that time. Grain yield of both wheat and triticale was equally reduced by 61% due to the presence of sterile oats, whereas the reduction for barley grain yield was 9%. Nitrogen fertilization (150 kg N ha−1) slightly increased yield of all crops grown without weed competition compared to the control (0 kg N), whereas the same treatment increased sterile oats dry weight as well as its competitive ability against wheat and triticale. Split application of nitrogen (50 kg N ha−1 before planting and 100 kg N ha−1 in early March) caused a slightly higher increase in sterile oats dry weight compared to the control or one application (150 kg N ha−1) before planting, when grown with wheat and triticale. However, dry weight of sterile oats grown with barley was severely reduced by the interference of the crop. Total nitrogen content of all crop plants grown without sterile oats increased with nitrogen fertilization compared to the control. However, total nitrogen in crop plants grown with sterile oats was reduced compared to the weed-free control; percent reduction was greater in plants grown in plots treated with nitrogen than in the control. These results indicate that barley could be used for limiting sterile oats interference in areas where winter cereals are grown; time of nitrogen application could also be used for a slight reduction of sterile oats competitive ability against wheat or triticale.

Clear and black polyethylene mulches were placed over raised seed beds in four studies during a 3-yr period. At intervals during treatment times of up to 12 weeks, soil samples were taken and the recovered seed were tested for germination. The number of seed killed and depths to which they were killed varied with species. The study included natural seed populations of annual bluegrass (Poa annua L. ♯3 POAAN), barnyardgrass (Echinochloa crus-galli (L.) Beauv. ♯ ECHCG), annual species of Cyperus, dayflower (Commelina communis L. ♯ COMCO), and goosegrass (Eleusine indica L. Gaertn. ♯ ELEIN). After 40 days exposure under clear mulch, seed of dayflower were killed to a depth of 11 cm. Under the same conditions, seed of annual Cyperus sedges and barnyardgrass were killed only in the upper 3 to 4 cm. After 3 weeks under clear plastic, germinable seed of goosegrass were recovered only at depths greater than 5 cm. Seed in plots under clear polyethylene mulch were killed more quickly than those in plots covered with black polyethylene mulch.

Allelopathic effects of lantana (Lantana camara L. ♯3 LANCA) residues (root, shoot), foliar leachates, and the soil (where lantana was grown) on milkweedvine (Morrenia odorata Lindl. ♯3 MONOD) seed germination and growth over a 30-day period were examined. Foliar leachates or the soil collected from the field where lantana had been growing had no effect on the final germination percentage or the seedling growth of milkweedvine. Incorporation of dried lantana shoot or root material into soil had no effect on the final percentage germination but caused significant reductions in milkweedvine growth over a 30-day test period. Roots were more inhibitory than shoots. Fifty percent of milkweedvine seedlings died within 15 days after germination at 1% (w/w) dried lantana root incorporation into the soil, and higher concentrations increased seedling death. Lantana roots incorporated into the soil produced foliar symptoms such as wilting and desiccation, whereas lantana shoots incorporated into the soil produced yellowing of the foliage of milkweedvine. Allelopathic activity of lantana residues was still strong even after decomposition of lantana residues for 4 weeks prior to the planting of milkweedvine seeds.

Recent advances in sequencing technologies (next-generation sequencing) offer dramatically increased sequencing throughput at a lower cost than traditional Sanger sequencing. This technology is changing genomics research by allowing large scale sequencing experiments in nonmodel systems. Waterhemp is an important weed in the midwestern United States with characteristics that makes it an interesting ecological model. However, very few genomic resources are available for this species. One half of a 70 by 75 picotiter plate of 454-pyrosequencing was performed on total DNA isolated from waterhemp, generating 158,015 reads of an average length of 271 bp, or a total of nearly 43 Mbp of sequence. Included in this sequence was a nearly complete sequence of the chloroplast genome, sequences of several important herbicide resistance genes, leads for simple sequence repeat (SSR) markers, and a sampling of the repeated elements (e.g., transposons) present in this species. Here we present the waterhemp genomic data gleaned from this sequencing experiment and illustrate the value of next-generation sequencing technology to weed science research.

Nicosulfuron, mesotrione, dicamba plus diflufenzopyr, and carfentrazone are postemergence herbicides from different chemical families with different modes of action. An association between the sensitivity of sweet corn to these herbicides was observed when 143 F3 : 4 families (F4 plants) derived from of a cross between Cr1 (sensitive inbred) and Cr2 (tolerant inbred) were evaluated in greenhouse trials. The ratio of tolerant : segregating : sensitive families was not significantly different from a 3 : 2 : 3 ratio, which would be expected if a single gene conditioned herbicide response. Families cosegregated for responses to these herbicides. In field studies with 60 F3 : 5 families in 2005 and 120 F3 : 5 families in 2007, responses to these herbicides and foramsulfuron and primisulfuron were associated. Responses to bentazon in field trials were similar to the aforementioned herbicides for tolerant families, but differences were noted for families that were sensitive or segregated for responses to nicosulfuron, foramsulfuron, primisulfuron, mesotrione, dicamba plus diflufenzopyr, and carfentrazone. The gene(s) affecting herbicide sensitivity in Cr1 maps to the same region of chromosome 5S as a previously sequenced cytochrome P450 gene, where alleles previously designated nsf1 and ben1 were associated with sensitivity to nicosulfuron and bentazon and appear to be the result of a 392–base-pair insertion mutation. This work supports the hypothesis that a single recessive gene or closely linked genes in the sweet corn inbred Cr1 condition sensitivity to multiple cytochrome P450 enzyme-metabolized herbicides.

Stranglervine (Morrenia odorata Lindl. # MONOD) was identified in 1957 at a commerical citrus grove in Florida. It is believed that this plant pest was introduced into Florida as an ornamental plant from South America. In the early 1960's, it became recognized as a serious weed pest that competed with the citrus trees for sunlight, water, and nutrients; girdled tree limbs; and interfered with spraying, harvesting, and irrigation practices. By the 1970's the vine was distributed throughout most of the citrus-growing areas in Florida. Control measures were limited to herbicides and mechanical cultivation. It was estimated that the cost of controlling this pest was $124/ha/yr. In many instances, control measures were inadequate to keep the vines from becoming established in the trees (11, 12, 13). Studies reported by El-Gholl (4) further demonstrated the inherent ability of this weed to persist.

An increased rate of butylate (S-ethyl diisobutylthiocarbamate) degradation occurred on a Kennebec silt loam (sil) exhibiting rapid EPTC (S-ethyl dipropylthiocarbamate) breakdown in the field. Butylate degradation was not as rapid as EPTC degradation on field soils previously treated with EPTC. Addition of R-33865 (O, O-diethyl-O-phenol phosphorothioate) to butylate extended its persistence in field soils with and without prior EPTC application. Although microbial degradation of butylate, EPTC, and vernolate (S-propyl dipropylthiocarbamate) was increased on EPTC-pretreated soils, EPTC and vernolate degradation was much more rapid than butylate degradation. Addition of R-33865 to butylate, EPTC, or vernolate extended their soil persistence on soils pretreated with EPTC. In a laboratory experiment, a maximum rate of EPTC degradation resulted from one prior application. The rate of butylate degradation increased with each successive butylate application, but the degradation rate was not as high as those observed for EPTC on EPTC-pretreated soil.

Greenhouse studies were conducted to determine the basis for reduced johnsongrass control when glyphosate was applied in mixtures with 2,4-D or dicamba. Glyphosate was applied to johnsongrass at 0.28, 0.56, 0.84, and 1.12 kg/ha alone and in combination with 2,4-D or dicamba at 0.14, 0.28, 0.14, or 0.56 kg/ha. Johnsongrass shoot and root fresh weights measured 4 weeks after treatment were higher when glyphosate was applied with 2,4-D (0.28 kg/ha glyphosate) or dicamba (0.28 kg/ha or 0.56 kg/ha glyphosate) compared to glyphosate applied alone at these rates. The antagonism of johnsongrass control was not observed with combinations of some of the higher glyphosate rates with 2,4-D (0.56 or 0.84 kg/ha glyphosate) or dicamba (0.84 or 1.12 kg/ha glyphosate). The reduction of glyphosate activity on johnsongrass occurred when any of four forms of 2,4-D or two forms of dicamba were added to the glyphosate spray mixture. Glyphosate uptake into johnsongrass leaves and subsequent translocation to the roots was reduced by the presence of 2,4-D or dicamba. The reduced glyphosate uptake and translocation could account for the decreased toxicity of glyphosate to johnsongrass when applied with 2,4-D or dicamba.

The loss of 2-methoxy-3,6-dichlorobenzoic acid (dicamba), 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), and 4-amino-3,5,6-trichloropicolinic acid (picloram) in runoff water was determined with a gas chromatograph and bioassays. When determined 24 hr after application, losses of dicamba and picloram were greater from sod plots than from fallow plots while 2,4,5-T losses were about equal. Four months after application, losses of all herbicides averaged less than 1% of that lost 24 hr after application. The amount of picloram lost in a simulated rainfall of 0.5 inch varied with rate, but the percentage lost was the same. The slope of the plot and movement over untreated soil influenced the percentage of picloram lost. The maximum loss obtained for any herbicide was 5.5% and the average approximately 3%.

Palmer amaranth’s ability to evolve resistance to different herbicides has been studied extensively, but there is little information about how this weed species might be evolving other life-history traits that could potentially make it more aggressive and difficult to control. We characterized growth and morphological variation among 10 Palmer amaranth populations collected in Florida and Georgia from fields with different cropping histories, ranging from continuous short-statured crops (vegetables and peanut) to tall crops (corn and cotton) and from intensive herbicide use history to organic production. Palmer amaranth populations differed in multiple traits such as fresh and dry weight, days to flowering, plant height, and leaf and canopy shape. Differences between populations for these traits ranged from 36% up to 87%. Although glyphosate-resistant (GR) populations collected from cropping systems including GR crops exhibited higher values of the aforementioned variables than glyphosate-susceptible (GS) populations, variation in traits was not explained by glyphosate resistance or distance between populations. Cropping system components such as crop rotation and crop canopy structure better explained the differences among populations. The higher growth of GR populations compared with GS populations was likely the result of multiple selection forces present in the cropping systems in which they grow rather than a pleiotropic effect of the glyphosate resistance trait. Results suggest that Palmer amaranth can evolve life-history traits increasing its growth and reproduction potential in cropping systems, which explains its rapid spread throughout the United States. Furthermore, our findings highlight the need to consider the evolutionary consequences of crop rotation structure and the use of more competitive crops, which might promote the selection of more aggressive biotypes in weed species with high genetic variability.

Ten warm-season and seven cool-season weed species were grown in the glasshouse on Hartsells fine sandy loam taken from field plots that had been in long-term fertility studies. The soil pH was 5.9 and phosphorus (P) levels ranged from 8 to 95 kg/ha and the potassium (K) levels from 40 to 213 kg/ha. Growth response to P and K levels was measured by dry herbage yield. Redroot pigweed (Amaranthus retroflexus L.), jimsonweed (Datura stramonium L.), and Florida beggarweed [Desmodium tortuosum (Sw.) DC] were the most responsive warm-season weed species to P, whereas chickweed [Stellaria media (L.) Cyrillo] was the most responsive cool-season weed to P. Extreme P deficiency symptoms were evident on species giving a high response to P. P-deficient plants were severely stunted and exhibited a characteristic reddish-purple color. Showy crotalaria (Crotalaria spectabilis Roth), tall morningglory [Ipomoea purpurea (L.) Roth], sicklepod (Cassia obtusifolia L.), Carolina geranium (Geranium carolinianum L.), and coffee senna (Cassia occidentalis L.) were the most tolerant to low soil P. Redroot pigweed, jimsonweed, and Florida beggarweed were the most responsive warm-season weeds to K. Wild mustard [Brassica kaber (DC.) L.C. Wheeler var. Pinnatifida (Stokes) L.C. Wheeler] and annual bluegrass (Poa annua L.) were the most responsive cool-season weeds to K. Potassium deficiency symptoms were characterized primarily by severe stunting. Buckhorn plantain (Plantago lanceolata L.), Carolina geranium, and curly dock (Rumex crispus L.) were the most tolerant cool-season weeds to low soil K. Generally, weeds were more sensitive to low soil-test P than K.

The initial metabolism of acetochlor [2-chloro-N-(ethoxymethyl)-N-(2-ethyl-6-methylphenyl)acetamide] in tolerant and susceptible plants was examined to determine if metabolism is a factor in the selective phytotoxicity of this herbicide. Six crop and ten weed species were used. In every case acetochlor was converted initially to thioether conjugates. In thirteen of the species, initial conjugation was with glutathione (GSH). The homoglutathione (hGSH) conjugate was formed in three legumes. Tolerant plant seedlings metabolized acetochlor more readily than susceptible plant seedlings. Based on these results, acetochlor detoxification is a primary factor in the selective phytotoxicity of this herbicide.

The influence of soybean row width and glyphosate application timing was determined on survival, biomass, and seed production of cohorts from a mixed population of Palmer amaranth and pusley species (Florida and Brazil pusley) along with soybean seed yield. The first Palmer amaranth and pusley cohort comprised plants that emerged from soybean planting through the V3 (3 wk after soybean emergence [WAE]) soybean stage (cohort 1). The second cohort comprised plants that emerged between the V3 to V6 (5 WAE) soybean stages (cohort 2), and the third cohort emerged after the V6 through the R2 soybean stage (cohort 3). Glyphosate at 840 g ae ha−1 was applied at V3; V6; V3 and V6; and V3, V6, and R2 in rows either 19 or 97 cm wide. A nontreated control was included for comparison in each row width. Sequential glyphosate applications at V3 and V6 or at V3, V6, and R2 soybean stages resulted in 1 to 3% survival of cohort 1 compared with 23 to 28% survival after a single glyphosate application. Vegetative biomass production by cohort 1 accounted for 71% of the total pusley biomass produced in the nontreated plots. Cohort 1, 2, and 3 contributed 68, 31, and 1%, respectively, of the total 37,900 seeds m−2 produced by pusley plants in nontreated plots. Delaying a glyphosate application to the V6 stage resulted in higher biomass and more than twice the seed produced from cohort 1 when compared with cohort 2. Glyphosate applied at V3 and V6 stages prevented pusley seed production from cohort 1, and an additional glyphosate application at the R2 stage prevented seed production from cohorts 2 and 3. No Palmer amaranth emergence occurred after the V6 soybean stage in either row width. A single glyphosate application at the V3 or V6 stage eliminated cohort 1 of Palmer amaranth in narrow rows. Palmer amaranth plants from cohort 1 in wide rows that survived the V3 glyphosate application produced 3.3 g m−2 biomass and 600 seeds m−2. Averaged over years and row widths, soybean yields after sequential glyphosate applications were 2,490 to 2,640 kg ha−1 compared with 1,850 to 2,020 kg ha−1 after a single glyphosate application at the V3 or V6 stage. This research confirms that sequential glyphosate applications are superior to a single application for minimizing pusley and Palmer amaranth survival, biomass, and seed production along with an improvement in soybean yields.

Modern invasion biology is a new science, with the holy grail of being able to predict the trajectory of particular invasions. Although this goal has yet to be achieved, there has been much progress through experimental research and meticulous study of the scope and mechanisms of existing invasions. Several well-established patterns are relevant to potential biofuel feedstocks: (1) ca. half of all damaging plant invaders were deliberately introduced, not accidental hitchhikers or escapees; (2) some native plants have become invasive; coevolution with native community members was not proof against unexpected damage; (3) many introduced plants were innocuous for decades or even centuries in their new locations before suddenly exploding across the landscape; lack of current observed impact does not guarantee safety; and (4) control or even eradication of widespread invaders is sometimes possible, but it is far from certain and it is often very expensive. We cannot count on effectively managing an introduction gone awry. Because much invasion biology is targeted at developing methods of preventing anthropogenic movement or establishment of species, invasion biologists have occasionally been assailed as obstructionists by various interests who fear their livelihoods will be impeded: the seed and horticulture trades, foresters, the pet industry, fish and game biologists, etc. A fringe group of philosophers, sociologists, landscape architects, and others have even taken to calling invasion biology a thinly veiled form of xenophobia. Some biofuels advocates have joined this litany, accusing invasion biologists of playing on the emotions of an uneducated public by raising fears of a new kudzu. Invasion biologists need not be cast in this role. In collaboration with agronomists, geneticists, physiologists, and other scientists, they have much to offer in understanding the risks posed by particular feedstocks and developing approaches that would minimize these risks and mitigate unforeseen consequences.

Sorption and desorption of metribuzin [4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one] were measured in Dundee silty clay loam surface and subsurface soils. Soil samples were taken from a profile that had been divided into six sections from the surface to a depth of 175 cm. Metribuzin sorbed weakly to all soils from this profile with Freundlich distribution constants ranging from 0.78 to 1.34 μmole/kg. Soils from lower depths of the profile having higher pH (>7.0) and clay contents (>35%) sorbed significantly more metribuzin than the lower clay content, higher organic matter surface soils. Stepwise regression of the distribution constants against the variables pH, organic matter, clay content, and sand content showed that clay was the single best predictor, with sorption increasing as clay content increased (r2 =0.750). The combination of two variables most related to sorption was clay and pH (r2 =0.860, P= 0.15). Organic matter was not one of the primary variables related to sorption. Metribuzin was easily desorbed in all soils with less than 5% of the originally applied metribuzin remaining after three desorption treatments. This would indicate little tendency for irreversible sorption. This study demonstrates that soil properties within a profile determine sorption and, subsequently, movement of metribuzin.

Parasitic witchweeds inflict most of their damage while still underground and attached to crop roots. Most selective translocated herbicides are detoxified by crops such as corn and thus cannot reach the attached parasites. Corn with target site resistance to acetolactate synthase (ALS)-inhibiting herbicides was tested to ascertain whether these herbicides could control witchweeds, assuming that witchweeds do not obtain amino acids from the crop. Postemergence directed sprays of 27 g ae ha−1 imazapyr 54 d after planting (DAP) delayed Striga asiatica emergence on corn in South Carolina from 3 wk (control) to 7 wk and to 11 wk when mixed with 45 g ae ha−1 AC 263 222. Treatments with up to 71 g ae ha−1 imazamox, and up to 71 g ae ha−1 AC 263 222 only delayed Striga emergence by 1 wk, and 71 g ae ha−1 imazethapyr was ineffective. ALS-inhibiting herbicides were far more effective when applied in 1-ml drenches above the seed at planting. Chlorsulfuron (10 g ai ha−1) and sulfometuron (50 g ai ha−1) were somewhat phytotoxic to Pioneer 3245IR. Rimsulfuron (30 g ai ha−1), metsulfuron (10 g ai ha−1), halosulfuron (120 g ai ha−1), and imazethapyr (140 g ae ha−1) were marginally active in Kenya, with some mature Striga hermonthica seed-bearing capsules appearing at harvest (12 wk). Imazapyr at 15 g ae ha−1 gave 70 to 95% suppression of capsule formation, whereas no capsules appeared at 30 g ae ha−1. The use of imazapyr in Kenya increased the harvest index by 17% when corn plants in Striga-infested soils were kept insect and disease free by using insecticides and fungicides. Thus, complete control can be achieved at affordable cost by farmers in subsistence conditions.

The effect of ammonium sulfate (AMS) on efficacy, absorption, and translocation of glufosinate in various weed species was investigated in greenhouse and field experiments. Adding ammonium sulfate at 20 g L−1 increased the efficacy of glufosinate on Echinochloa crus-galli, Setaria faberi, and Abutilon theophrasti, but not on Amaranthus rudis or Chenopodium album. AMS increased foliar absorption of 14C from 14C-glufosinate to the greatest extent in A. theophrasti and S. faberi and to the least extent in C. album. AMS increased the translocation of 14C out of the treated leaf 24 h after treatment in A. theophrasti and S. faberi, but not in C. album. These results suggest that AMS can increase the efficacy of glufosinate on A. theophrasti and S. faberi by increasing foliar absorption and subsequent translocation of glufosinate.