Maximizing weed seed exposure to seed predators by delaying post-harvest tillage has been suggested as a way to increase weed seed loss to predation in arable fields. However, in some areas of northeastern Spain, fields are still tilled promptly after cereal harvest. Tillage usually places seeds in a safer environment compared to the soil surface, but it can also increase seed mortality through seed decay and fatal germination. By burying the seeds, tillage also prevents weed seed predation. Weed seed fate in a tilled vs. a no-till environment was investigated during the summer fallow months in three cereal fields in semi-arid northeastern Spain. Rigid ryegrass and catchweed bedstraw seeds were used. Predation rates were measured in a no-till area within each field in 48-h periods every 3 wk, and long-term predation rates were estimated. Fate of buried seeds was measured by burying 20 nylon bags with 30 seeds of each weed species from July to September at a depth of 6 cm in a tilled area contiguous to the no-till area. Predation rates over the entire summer were 62% and 49% for rigid ryegrass and catchweed bedstraw, respectively. High availability of crop seeds (preferred by ants) on the soil surface may have decreased predation of weed seeds early in the season. Seed loss due to burial was 54% and 33% for rigid ryegrass and catchweed bedstraw, respectively. Unusual above-average precipitation probably prompted higher than normal weed germination rates (fatal germination) in some fields, and thus led to higher seed mortality rates compared with an average year. These results suggest that leaving the fields untilled after harvest may be the optimum strategy to reduce inputs to the weed seedbank during the summer fallow period in semi-arid systems.

Phytase activity increased 2 to 3 fold in barley (Hordeum vulgare L.), 6 fold in corn (Zea mays L.), and 12 fold in squash (Cucurbita maxima Duchesne) seedlings 2 days after the initiation of germination in the dark. Most of the phytase was found in the embryo of the seed, presumably in the cotyledon(s). Gibberellic acid and benzyladenine supplied during germination exerted only limited influence on the development of phytase in these seedlings. Of the herbicides studied, only sodium arsenite exerted an influence on phytase activity per se that was of physiological significance. The presence of 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine), 3-amino-2,5-dichlorobenzoic acid (amiben), 2-chloro-N-isopropylacetanilide (propachlor), 2,6-dichlorobenzonitrile (dichlobenil), or α,α,α-trifluoro-2,6-dinitro-N-N-dipropyl-p-toluidine (trifluralin) in the culture solution during germination partially inhibited the development of phytase activity. The herbicidal inhibition was enhanced if potassium phosphate at a non-inhibitory level also was present in the culture solution. The interaction of atrazine and amiben with phosphorus was not due to increased internal concentrations of inorganic phosphorus to a level inhibitory to the development of phytase activity.

Control of field bindweed (Convolvulus arvensis L.) on irrigated land was studied by application of herbicides once in the fall and then only in the spring for the next 4 yr. Control of field bindweed 8 months after a fall application of 2.2 kg/ha of dicamba (3,6-dichloro-o-anisic acid) or 3.4 kg/ha of 2,4-D [(2,4-dichlorophenoxy)acetic acid] was 90 and 83%, respectively. Spring applications of 0.28 kg/ha of dicamba, 0.56 kg of 2,4-D, or the mixture of these two herbicides suppressed the growth of field bindweed similarly each year. By the fall of the fourth year, field bindweed covered an average of 9% of the soil surface in the plots that received both fall- and spring-applied herbicide treatments, 72% in plots that received only fall-applied herbicide treatments, and 80% in the untreated plots. Yield of corn (Zea mays L. ‘Pioneer 3306’) was significantly higher in all treated plots than in the untreated check plots in 1 out of 2 yr. Yield of sorghum [Sorghum bicolor (L.) Moench ‘Pioneer 833’] was not increased significantly in any treated plots, but in 1 yr the mixture of 0.28 kg/ha of dicamba plus 0.56 kg/ha of 2,4-D reduced yield significantly when this mixture was applied twice at these same rates in the spring.

A plant response study was conducted in the greenhouse to evaluate interactions between imazamox and acifluorfen, fomesafen, or lactofen. Antagonistic interactions between imazamox and acifluorfen or fomesafen affected control of I. hederacea and A. theophrasti 21 d after treatment. Combinations of imazamox plus lactofen also antagonized control of I. hederacea. I. hederacea absorbed less 14C-imazamox when applied with nonlabeled acifluorfen, but absorption by A. theophrasti was not affected by the combination. Addition of nonlabeled lactofen did not affect absorption of imazamox in A. theophrasti or I. hederacea. Addition of acifluorfen or lactofen reduced translocation of 14C-imazamox to foliage above and below the treated leaf and the root in both A. theophrasti and I. hederacea. Antagonism of A. theophrasti control was due to reduced translocation of imazamox. Antagonism of I. hederacea control was due to reduced translocation of imazamox when applied with lactofen or reduced absorption and translocation when applied with acifluorfen.

In glasshouse experiments, Panicum repens rhizomes with more than two nodes emerged from soil depths of 1 to 20 cm. Emergence from one-node rhizomes was greatly reduced at depths more than 5 cm. In contrast, culm emergence from ginger like rhizomes decreased when burial depth (BD) was greater than 20 to 30 cm. P. repens emerged from deeper soil levels with an increasing number of nodes per cutting. Culms emerged from 36% of total underground regenerative organs. About 75 and 95% of the emerged culms were recorded 30 and 60 d after burial, respectively, and 91% emergence was recorded from 20-cm BD. In field studies, only 9% of nodes produced culms following cross-plowing. About 61 and 67% of the emerged culms were recorded 50 and 60 d after land preparation, respectively. Results indicate that mechanical and chemical control methods could be most effective for P. repens when applied approximately 50 d after land preparation. Burying deeper than 30 cm by deep plowing and reducing rhizome length by cross-plowing might effectively reduce emergence of P. repens.

The techniques of cryostat sectioning, freeze-drying, and microradioautography were adapted to study the absorption and translocation of radio-labeled 3-(3,4-dichlorophenyl)-1,1-dimethylurea (14C-diuron) in root-treated cotton (Gossypium hirsutum L., var. Coker 201) plants. Diuron appeared to move primarily in the apoplast and to be transported acropetally and laterally in the transpiration stream. Radioactivity was observed to accumulate in striking concentrations in the lysigenous, or pigment, glands and the trichomes of the cotton plant. This accumulation is postulated to be a major factor in lowering the effective concentration of the herbicide in the leaves of cotton as compared to leaves of more susceptible plants, and thus may be a significant factor in the tolerance of cotton to diuron.

Mechanical wounding broke dormancy and promoted germination of primary dormant pure-line wild oat (Avena fatua L. # AVEFA) caryopses that were in the first physiological stage of dormancy. Caryopses incubated in petri dishes responded to wounding by germinating more rapidly than wounded caryopses incubated in soil. There was no difference in germination between wounded and lanolin-covered wounded caryopses when incubated in soil. The less dormant AN265 caryopses responded to the wounding treatment more rapidly than the more dormant caryopses of M73 in both petri dish and soil incubation systems. Chlorocholine chloride (CCC), an inhibitor of gibberellic acid (GA) biosynthesis, reduced the germination rate of wounded AN265 and M73 caryopses above and at 1 mM, respectively. Application of 25 mM CCC to wounded dormant AN265 caryopses decreased the level of hexose in the embryo to 47% of the control level. There was no significant effect of CCC on the endosperm hexose, embryo and endosperm imbibed fresh weight, and sucrose and starch content. The results suggest GA or GA biosynthesis is the limiting factor to germination.

The reduction in root yield (Y) of sugarbeets (Beta vulgaris L.) caused by specific densities (X) of kochia [Kochia scoparia (L.) Schrad.] was predicted by using the linear equation Y = 4.30 + 1.81 X. This equation was most accurate when the kochia density was 20 plants or less per 30.5 m of row. In 28 of 30 weed-control systems, where kochia densities ranged from 0.2 to 16.5 plants per 30.5 m of row, the predicted yield reductions were within 5% of the actual yield reductions.

Field and laboratory studies were conducted at Stoneville, MS, from 1996 to 1998 to determine the influence of subsoiling (SS) and conventional tillage (CT) of a Sharkey clay soil on microbial characteristics and herbicide degradation. Soil samples obtained from imazaquin-treated and nontreated plots from the soybean row and interrow position were analyzed. Because only the row position is actually disturbed by SS, a comparison of row and interrow position on the parameter was conducted. Imazaquin (preemergence, 140 g ai ha−1) had no effect on microbial populations, microbial enzyme activity (fluorescein diacetate [FDA] hydrolysis and triphenyl-tetrazolium chloride [TTC] dehydrogenase), and organic carbon content. Estimates of microbial activity based on FDA hydrolysis and TTC dehydrogenase activity indicated greater activity under CT; however, microbial biomass and organic carbon were not affected by tillage or row position. A laboratory study assessed the degradation of carboxyl- and ring-labeled 2,4-D as influenced by tillage and row position. Soils from CT plots had an initially higher mineralization rate of 14C carboxyl-labeled 2,4-D compared to soils from SS plots; however, no effect of tillage or row position was observed on the cumulative amount of 14CO2 evolved 14 d after treatment (DAT) in 1996 and 18 DAT in 1998. In studies with ring-labeled 2,4-D, a higher 14CO2 evolution was detected in soils obtained from SS plots, regardless of row position, whereas a greater amount of radioactivity was observed in the unextractable fraction from CT soils. Because differences in 2,4-D mineralization between tillage regimes were minimal, adoption of SS as a tillage practice for heavy clay soils in the Mississippi Delta may have a limited effect on microbial characteristics and biodegradation of soil-applied herbicides.

Various chemical treatments were evaluated over two growing seasons for control of triazine-resistant common lambsquarters (Chenopodium album L. # CHEAL) and for control of a triazine-resistant infestation containing both redroot pigweed (Amaranthus retroflexus L. # AMARE) and Powell amaranth (A. powellii S. Wats. # AMAPO). Atrazine [6-chloro-N-ethyl-N′-(1-methylethyl)-1,3,5-triazine-2,4-diamine], cyanazine {2-[[4-chloro-6-(ethylamino)-1,3,5-triazin-2-yl] amino]-2-methylpropanenitrile}, and metribuzin [4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one] provided unsatisfactory control of these biotypes. Satisfactory control of common lambsquarters was obtained with preemergence applications of pendimethalin [N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine] or dicamba (3,6-dichloro-2-methoxybenzoic acid), or postemergence applications of dicamba, bromoxynil (3,5-dibromo-4-hydroxybenzonitrile), or bentazon [3-(1-methylethyl)-(1H)-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide]. Satisfactory control of pigweed was obtained with preemergence applications of alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide] or postemergence treatments of dicamba, bromoxynil, or 2,4-D [(2,4-dichlorophenoxy) acetic acid].

Several regimes of supplying exogenous aromatic amino acids to intact, 3-day-old, soybean [Glycine max (L.) Merr. ‘Hill’] seedlings by root uptake were tested to determine if growth retardation caused by root-fed, 0.5 mM glyphosate [N-(phosphonomethyl) glycine] could be reversed. Generally, root-fed levels of aromatic amino acids just below growth-retarding levels (e.g. 1 mM phenylalanine + 0.1 mM tyrosine) reversed root growth inhibition caused by glyphosate to a small (ca. 10%) but significant extent. Feeding aromatic amino acids for 1 to 3 days before glyphosate exposure did not enhance the reversal. Uptake and metabolism of root-fed, aromatic amino acids in control and glyphosate-treated plants were verified by increased levels of hydroxyphenolic compounds (end products of aromatic amino acid metabolism) and by uptake and incorporation of 14C-labeled phenylalanine and tyrosine. On a fresh weight basis, glyphosate had no inhibitory effect on uptake or incorporation of these amino acids into protein or secondary phenolic compounds. After 3 days of exposure, glyphosate had no substantial effects on shikimate dehydrogenase activity in control or aromatic amino acid-fed seedlings. These data suggest that either root-fed aromatic amino acids are compartmentalized differently than the endogenous pools affected by glyphosate or that root-fed glyphosate exerts most of its effect on growth of soybean seedlings through means other than inhibition of aromatic amino acid synthesis.

Little effort has been directed toward integrating biological control; that is, the use of living natural enemies to control weeds, with more conventional weed control practices. The usual focus of biological control has been on the introduction of weed insects and pathogens from foreign areas to control naturalized range, pasture, and/or aquatic weeds, principally single species that had become dominant over a plant community. The released natural enemies were allowed to reach their own balance with the target weed without added assistance. As economic and environmental perspectives change along with our knowledge and technological capability, however, it becomes worthwhile to review periodically if and how biological control might be integrated with other farming practices. Because my biological control experience has centered on the use of weed-feeding insects, this discussion will focus on methods of enhancing and integrating insect impact on weeds with other weed-control activities.

The effect of shoot feeding by the biocontrol agents, Galerucella calmariensis and Galerucella pusilla (Coleoptera: Chrysomelidae) on purple loosestrife (Lythrum salicaria) seed production and seed germination was quantified in two Minnesota wetlands. In a wet meadow where Galerucella spp. were present on isolated plants, feeding by adults and larvae during shoot elongation resulted in stunting and malformation of shoot tips. There was a subsequent reduction in purple loosestrife inflorescence length and number of flower buds and seed capsules. As Galerucella spp. larvae preferentially fed on shoot meristems, even low levels of feeding on a whole-plant basis (approximately 10% defoliation) reduced seed production. In a sedge meadow wetland with severe feeding damage (a minimum of 70% leaf defoliation), few to no flower buds formed on plants, and subsequently, few to no seed capsules were produced on purple loosestrife plants. Of the few capsules that were produced, number of seeds per capsule and percent germination of seeds did not differ from control plants. In both wetlands, feeding on a main shoot of purple loosestrife did not result in a compensatory increase in the number of axillary inflorescences. Feeding by Galerucella spp. and the subsequent reduction in number of seeds produced on purple loosestrife plants will decrease the number of seeds available for dissemination to new sites. Fewer seeds will enter the seedbank, and over time, feeding by Galerucella spp. will decrease the number of seeds available for seedling recruitment. The benefit of leaf defoliation on purple loosestrife plants caused by Galerucella spp. feeding has been reported. In this study, we have quantified the additional benefits of reduced seed production from Galerucella spp. feeding on purple loosestrife in North America.

Additive experiments were performed to determine optimum densities for nematode-suppressive cover crops to extend the benefit from the cover crops by also using them for weed suppression. In a preliminary experiment in 2002, a range of cover-crop densities was evaluated in mixtures with smooth pigweed at 5 plants m−2. Smooth pigweed biomass accumulation was suppressed by cowpea, sunn hemp, and velvetbean at the lowest cover-crop populations (38, 44, and 15 plants m−2, respectively). Based on these results, experiments were conducted in 2003 at two locations to examine the effects of lower cover-crop densities on a higher smooth pigweed population density of 15 plants m−2. Cowpea and velvetbean densities ranged from 10 to 50 plants m−2 and sunn hemp from 20 to 100 plants m−2. In 2003, cowpea density had no effect on smooth pigweed biomass. However, smooth pigweed biomass declined linearly by 51% as sunn hemp density increased to 100 plants m−2. Similarly, as velvetbean densities increased, smooth pigweed biomass decreased showing a linear response at one location and quadratic response at the second location. Maximum suppression of smooth pigweed biomass by velvetbean occurred at the highest cover-crop density (50 plants m−2). Excellent suppression of smooth pigweed at 5 plants m−2 or fewer will result in densities of 38, 44, and 15 plants m−2 of cowpea, sunn hemp, and velvetbean. However, with smooth pigweed at 15 plants m−2, optimum cover-crop densities were not obtained because no suppression was obtained with cowpea, and the lowest weed biomass with sunn hemp and velvetbean occurred with the highest densities used. Therefore, when high smooth pigweed densities are expected, sunn hemp and velvetbean should be used at densities greater than 100 and 50 plants m−2, respectively, and further study with higher densities will be needed to define optima.

The effect of formulation and adjuvants on absorption and translocation of 14C-clethodim was determined at 1, 4, 12, 24, 48, and 72 h after treatment (HAT) in bermudagrass under greenhouse conditions. Absorption of 14C-clethodim with the 0.12 kg L−1 (15 to 85%) formulation was higher than with the 0.24 kg L−1 (5 to 40%) formulation, regardless of presence or absence of adjuvant. There was considerable variation in the effect of adjuvant on 14C-clethodim absorption. When either ammonium sulfate (AMS) or AMS plus crop oil concentrate (COC) was added to the 0.12 kg L−1 formulation, 14C-clethodim absorption increased significantly at all harvest times except at 12 HAT compared with 0.12 kg L−1 formulation alone, whereas, 14C-clethodim absorption after addition of COC to the 0.12 kg L−1 formulation was similar to the 0.12 kg L−1 formulation alone up to 24 HAT. Conversely, COC enhanced 14C-absorption at all harvest times when added to 0.24 kg L−1 formulation. Most of 14C-clethodim (79 to 100% of absorbed) remained in the treated leaf, independent of formulation or adjuvant. Formulation did not have an impact on distribution of absorbed 14C-clethodim; however, presence of an adjuvant increased movement of 14C-clethodim out of treated leaf. Of the absorbed 14C-label, most remained in the treated leaf. 14C-clethodim that translocated out of the treated leaf remained in the shoot, and negligible amount of 14C-clethodim translocated to roots. These results demonstrated improved absorption of clethodim with formulations containing half the active ingredient (0.12 kg L−1) and inclusion of both AMS and COC.

Mefluidide {N-[2,4-dimethyl-5-[[(trifluoromethyl)sulfonyl] amino] phenyl] acetamide}+bentazon [3-isopropyl-1H-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide] at 0.14 + 0.84 kg ai/ha was the most injurious of five herbicide treatments applied once to one- to two-leaf red rice (Oryza sativa L. ♯3 ORYSA). The most injurious treatment to five-to six-leaf red rice was DPX Y6202 {ethyl [2-[4-[6-chloro-2-quinoxalinyl]oxy]phenoxy] propionate} at 0.56 kg ai/ha. Two applications, regardless of red rice growth stage, of all treatments except fluazifop {[±]-butyl-2-[4-[[5-trifluoromethyl]-2-pyridinyl] oxy] phenoxy] propanoate} resulted in 86 to 99% injury.

Giant ragweed is a highly competitive weed that continually threatens crop production systems due to evolved resistance to acetolactate synthase–inhibiting herbicides (ALS-R) and glyphosate (GR). Two biotypes of GR giant ragweed exist and are differentiated by their response to glyphosate, termed here as rapid response (RR) and non–rapid response (NRR). A comparison of data from surveys of Indiana crop fields done in 2006 and 2014 showed that GR giant ragweed has spread from 15% to 39% of Indiana counties and the NRR biotype is the most prevalent. A TaqMan® single-nucleotide polymorphism genotyping assay was developed to identify ALS-R populations and revealed 47% of GR populations to be ALS-R as well. The magnitude of glyphosate resistance for NRR populations was 4.6 and 5.9 based on GR50 and LD50 estimates, respectively. For RR populations, these values were 7.8 to 9.2 for GR50 estimates and 19.3 to 22.3 for LD50 estimates. A novel use of the Imaging-PAM fluorometer was developed to discriminate RR plants by assessing photosystem II quantum yield across the entire leaf surface. H2O2 generation in leaves of glyphosate-treated plants was also measured by 3,3′-diaminobenzidine staining and quantified using imagery analysis software. Results show photo-oxidative stress of mature leaves is far greater and occurs more rapidly following glyphosate treatment in RR plants compared with NRR and glyphosate-susceptible plants and is positively associated with glyphosate dose. These results suggest that under continued glyphosate selection pressure, the RR biotype may surpass the NRR biotype as the predominant form of GR giant ragweed in Indiana due to a higher level of glyphosate resistance. Moreover, the differential photo-oxidative stress patterns in response to glyphosate provide evidence of different mechanisms of resistance present in RR and NRR biotypes.

Tomato (Lycopersicon esculentum Mill. ‘Fireball’), white bean (Phaseolus vulgaris L. ‘Seaway’), and tobacco (Nicotiana tabacum L. ‘Delhi 34’ and ‘White Gold’) were pretreated with herbicides at one of three rates prior to fumigation with ozone at 0, 7.5, 15, or 30 pphm (parts per hundred million) for two 1.5-hr period. The plants were harvested 5 to 7 days after ozone fumigation, dried, and weighed. The natural logarithms of the dry weight data were subjected to multiple regression analysis to test for synergistic or antagonistic interactions between ozone and the various herbicides. Synergistic phytotoxicity was definitely observed for pebulate (S-propyl butylethylthiocarbamate) and possibly for chloramben (3-amino-2,5-dichlorobenzoic acid) in combination with ozone on tobacco ‘White Gold’ and ‘Delhi 34’, respectively. For most of the other combinations (chloramben, trifluralin α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine) or monolinuron (3-(p-chlorophenyl)-1-methoxy-1-methylurea) on white bean, diphenamid (N,N-dimethyl-2,2-diphenylacetamide) or trifluralin on tomato, chloramben on tobacco ‘White Gold’, and pebulate on tobacco ‘Delhi 34’) the phytotoxicity in the presence of ozone was additive and no interaction was indicated. An antagonistic interaction between ozone and benefin (N-butyl-N-ethyl-α,α,α-trifluoro-2,6-dinitro-p-toluidine) was indicated on the two cultivars of tobacco.

American sloughgrass [Beckmannia syzigachne (Steud.) Fernald] is one of the most predominant and troublesome weeds in wheat (Triticum aestivum L.) fields rotated with rice (Oryza sativa L.) in China. Mesosulfuron-methyl is one of the main herbicides used to selectively control B. syzigachne in winter wheat fields in China. After many years of application, mesosulfuron-methyl failed to control B. syzigachne in Yutai County. The objectives of this study were to determine the resistance level to mesosulfuron-methyl and other acetolactate synthase (ALS) inhibitors in the B. syzigachne population collected from Yutai County (R) and identify the mechanism of resistance. The results indicated that the R population was 4.1-fold resistant to mesosulfuron-methyl and was cross-resistant to pyroxsulam (600-fold), imazethapyr (4.1-fold), flucarbazone (12-fold), and bispyribac-sodium (12-fold). In vitro assays revealed that ALS in the R population was as sensitive as that in a susceptible (S) population. Gene sequence analysis identified no known resistant mutations in the ALS gene of the R population. Furthermore, real-time quantitative reverse transcriptase PCR experiments indicated that the expression level of the ALS gene in the R population was not different from that of the S population. However, the cytochrome P450 inhibitor malathion reversed the R population's resistance to mesosulfuron-methyl. The result of ultraperformance liquid chromatography–tandem mass spectrometry (UPLC-MS-MS) spectral analysis indicated that the metabolic rates of mesosulfuron-methyl in the R population were significantly faster than in the S population. Therefore, non-target resistance to mesosulfuron-methyl has been demonstrated in the R population. The resistance was very likely caused by enhanced herbicide metabolism.

Two field experiments were carried out from 1993 to 1995 to evaluate the critical period of weed control and to develop suitable weed management practices for jungle rice, horse purslane, and cockscomb in soybean. Horse purslane was more competitive during early growth stages (up to 45 days after sowing [DAS]) and cockscomb was more competitive during later growth stages, whereas jungle rice was competitive throughout the growing season. The critical period of weed control was found to be 30 to 45 DAS. Weed-free maintenance up to 45 DAS resulted in a 74% increase in grain yield of soybean over the unweeded control. Keeping soybean weed free for 45 d or allowing weeds to remain in the crop for less than 30 d resulted in no significant yield loss. Sequential application of a reduced rate of soil-applied trifluralin 1.0 kg ha–1 (0.67 ×) with postemergence fluazifop 0.75 kg ha–1 (0.75 ×) or a reduced rate of soil-applied trifluralin or pendimethalin at 1.0 kg ha–1 (0.67 ×) followed by hand hoeing 35 DAS provided better control of a broad spectrum of weeds than a single application of a postemergence herbicide applied at reduced or recommended rates. Integration of reduced rates of soil-applied herbicides with post-emergence herbicides or hand hoeing 35 DAS produced soybean yields similar to the hand-weeded treatment. Compared to the weed-free or integrated weed control, a single application of soil-applied or postemergence herbicide did not control a broad spectrum of weeds and reduced soybean yield.