The toxicity, adsorption, and leaching of four herbicides in different soils were determined by a method based on a sensitive root bio-assay which appeared suitable for herbicides affecting root growth. The rate of leaching of the herbicides was determined by measuring the herbicidal effect of the leachate obtained from three heights of soil columns. The same bio-assay was utilized for testing the extent of inactivation of herbicides due to adsorption by soil. In this case, mixtures of various rates of soil with silica sand were used as cultures for the bio-assay.

Tridiphane [2-(3,5-dichlorophenyl)-2-(2,2,2-trichloroethyl)oxirane] is a potent synergist of atrazine for postemergence control of weeds such as giant foxtail (Setaria faberi Herrm. ♯ SETFA) and large crabgrass [Digitaria sanguinalis (L.) Scop. ♯ DIGSA]. We determined whether: a) tridiphane could syngergize the activity of other herbicides known to be detoxified via enzymatic conjugation to glutathione, and b) differential toxicities of the synergized herbicides could provide control of proso millet (Panicum miliaceum L. ♯ PANMI) in corn (Zea mays L. “PAG SXIII”). Four herbicides were used in this study: EPTC (S-ethyl dipropylthiocarbamate), atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine], CDAA (N-N-diallyl-2-chloroacetamide), and alachlor [2-chloro-2’,6’-diethyl-N-(methoxymethyl) acetanilide]. Herbicides and herbicide-synergist combinations were applied postemergence. In corn, tridiphane synergized the activity of EPTC at 2 and 4 kg ai/ha, causing 22 to 25% growth reduction. Atrazine was ineffective and CDAA only slightly effective in combination with tridiphane in causing growth reduction. However, alachlor was markedly synergized by tridiphane at both 2 and 4 kg ai/ha, resulting in 36 to 54% growth reduction compared with plants treated with herbicides alone. Proso millet followed a similar trend to corn: EPTC and alachlor were effectively synergized by tridiphane, while atrazine and CDAA were ineffective. Importantly, however, the rates of herbicide used for proso millet control were much lower than those used for corn. Our data suggest that tridiphane used in combination with EPTC or alachlor could provide improved control of proso millet in corn.

Waterhemp resistance to foliar applications of protoporphyrinogen oxidase (PPO)–inhibiting herbicides has become increasingly disconcerting given the widespread distribution of glyphosate resistance. Fortunately, soil-residual PPO-inhibiting herbicides remain efficacious in waterhemp populations resistant to PPO-inhibiting herbicides; however, these herbicides should theoretically select for the resistant biotype as herbicide concentrations diminish in the soil. Accordingly, the objectives of this research were twofold: (1) evaluate the efficacy of three PPO-inhibiting herbicides, foliar- and soil-applied, on PPO-resistant (PPO-R) and PPO-susceptible (PPO-S) waterhemp, and (2) investigate the differential effects of PPO-inhibiting herbicides on an R biotype and an S biotype during several discrete developmental events relevant to soil–residual herbicide activity (i.e., radicle protrusion, radicle elongation, and waterhemp emergence). Greenhouse and growth chamber experiments indicated that the R biotype was least sensitive to the diphenylether herbicide fomesafen, followed by sulfentrazone and flumioxazin; however, fomesafen plus s-metolachlor improved soil-residual efficacy over fomesafen alone. Growth stage considerably influenced the R : S ratio, decreasing from 38× to 3.4×, when comparing ratios generated from foliar applications and soil-residual applications measuring radicle protrusion, respectively. Overall, this research supports the use of full soil-residual herbicide rates, reinforcing the importance of best management practices to manage the spread of herbicide resistance.

The widespread, rapid evolution of herbicide-resistant weeds is a serious and escalating agronomic problem worldwide. During China’s economic boom, the country became one of the most important herbicide producers and consumers in the world, and herbicide resistance has dramatically increased in the past decade and has become a serious threat to agriculture. Here, following an evidence-based PRISMA (preferred reporting items for systematic reviews and meta-analyses) approach, we carried out a systematic review to quantitatively assess herbicide resistance in China. Multiple weed species, including 26, 18, 11, 9, 5, 5, 4, and 3 species in rice (Oryza sativa L.), wheat (Triticum aestivum L.), soybean [Glycine max (L.) Merr.], corn (Zea mays L.), canola (Brassica napus L.), cotton (Gossypium hirsutum L.)., orchards, and peanut (Arachis hypogaea L.) fields, respectively, have developed herbicide resistance. Acetolactate synthase inhibitors, acetyl-CoA carboxylase inhibitors, and synthetic auxin herbicides are the most resistance-prone herbicides and are the most frequently used mechanisms of action, followed by 5-enolpyruvylshikimate-3-phosphate synthase inhibitors and protoporphyrinogen oxidase inhibitors. The lack of alternative herbicides to manage weeds that exhibit cross-resistance or multiple resistance (or both) is an emerging issue and poses one of the greatest threats challenging the crop production and food safety both in China and globally.

Greenhouse studies were conducted to evaluate the growth response of itchgrass to water stress. Itchgrass plants produced the greatest aboveground biomass and seeds at 75% of field capacity and these parameters at 50 and 100% of field capacity were similar. With further increase in water stress, seed production was sharply reduced, but itchgrass was still able to produce an average of 63 and 9 seeds plant−1 at 25 and 12.5% of field capacity, respectively. Itchgrass plants responded to increasing water stress with increased leaf weight ratio; it was 2.5 times greater at 12.5% of field capacity than at 100% of field capacity. In another study, compared with daily irrigation, intervals of 9 d between irrigations reduced aboveground biomass of itchgrass by 27% and 12-d intervals reduced aboveground biomass by 67%. Compared with the daily irrigation regime, itchgrass seed production was reduced by 61% at intervals of 12 d between irrigations; however, the weed plants produced a considerable number of seeds (153 seeds plant−1) at the 12-d intervals. The ability of itchgrass to produce biomass and seeds under water stressed conditions necessitates strategies that minimize weed survival while maximizing irrigation efficiency for the crop at the same time.

Winter annual weeds are becoming prolific in agricultural fields in the midwestern United States. The objectives of this research were to understand the roles of soil temperature (daily average and fluctuation) and moisture on the emergence of nine winter annual weed species and dandelion and to develop predictive models for weed emergence based on the accumulation of modified thermal/hydrothermal time (mHTT). Experiments were established at Lincoln, NE; Mead, NE; and at two sites (irrigated and rainfed) near Clay Center, NE, in 2010 and 2011. In July of each year, 1,000 seeds of each species were planted in 15 by 20 by 6-cm mesh baskets installed between soybean rows. Soil temperature and water content were recorded at the 2-cm depth. Emerged seedlings were counted and removed from the baskets on a weekly basis until no additional emergence was observed in the fall, resumed in late winter, and continued until emergence ceased in late spring. Weather data were used to accumulate mHTT beginning on August 1. A Weibull function was selected to fit cumulative emergence (%) on cumulative mHTT (seven base temperature [Tbase ] by six base water potential [Ψbase ] by three base temperature fluctuation [Fbase ] candidate threshold values = 126 models); it was also fit to days after August 1 (DAA1), for a total of 127 candidate models per species. The search for optimal base thresholds was based on the theoretic-model comparison approach (Akaike information criterion [AIC]). All three components (Tbase , Ψbase , and Fbase ) were only important for Virginia pepperweed. For downy brome and purslane speedwell, including Tbase and Ψbase resulted in the best fit, whereas for dandelion including Tbase and Fbase resulted in the best fit. A model including only Tbase resulted in the best fit for most species included in this study (Carolina foxtail, shepherd's-purse, pinnate tansymustard, henbit, and field pansy). For field pennycress, the model based on DAA1 resulted in the best fit. Threshold values were species specific. Soil temperature was the major environmental factor influencing winter annual weed emergence. Even though soil moisture and often temperature fluctuation are essential for seed germination, Ψbase and Fbase were not as critical in the predictive models as initially expected. Most seedlings (> 90%) of downy brome, pinnate tansymustard, Carolina foxtail, henbit, and field pansy emerged during the fall. Virginia pepperweed, purslane speedwell, dandelion, shepherd's-purse, and field pennycress seedlings emerged during both fall and spring. The results of this research provide robust information on the prediction of the time of winter annual weed emergence, which can help growers make better management decisions.

The bleaching effects of norflurazon [4-chloro-5-(methylamino)-2-(α,α,α-trifluoro-m-tolyl)-3 (2H)-pyridazinone] and of oxyfluorfen [2-chloro-1-(3-ethoxy-4-nitrophenoxy)-4-(trifluoromethyl) benzene] were compared using the intact microalga Scenedesmus acutus and the fungus Phycomyces blakesleeanus. Under the influence of oxyfluorfen, but not of norflurazon, pigments and membranes were degraded. This activity is typical for oxyfluorfen. Norflurazon prevented carotene synthesis, but did not cause degradation of carotenoids, chlorophylls, or 35S-sulfolipid, a marker of photosynthetic membranes. Furthermore, light-induced ethane evolution by Scenedesmus is substantial with oxyfluorfen, but not with norflurazon present. Oxyfluorfen apparently causes radical-initiated peroxidation of polyunsaturated fatty acids.

Sufficient fertility is important for crop growth and yield but supplying a balanced amount of N, P, and K with compost and manure is a challenge and nutrient imbalances can benefit weeds more than crops. The goal of this study was to compare the aboveground growth responses of common northeastern U.S. crops and weeds to addition of composted poultry manure (CPM). A secondary goal was to test whether the observed growth responses to CPM could be attributed to the three macronutrients—N, P, and K—supplied in the CPM. Two field experiments were conducted over 2 yr. Species grown were corn, lettuce, kale, Powell amaranth, common lambsquarters, giant foxtail, and velvetleaf. Plants were grown in soil amended with CPM or single-nutrient organic N, P, and K fertility amendments. Single-nutrient P treatments with bone char did not adequately mimic P supply from CPM. In both years, biomass of all weeds studied increased with CPM amendment rate. Powell amaranth was the most responsive to CPM addition, increasing 1,775 and 159% from the control to the highest CPM rate in 2010 and 2011, respectively. Corn biomass increased by 22% with CPM rate in 2010 but did not increase with CPM rate in 2011. Lettuce biomass increased with CPM amendment rate (175% in 2010 and 109% in 2011), but due to the increased weed biomass at high CPM amendment rates, good weed control will be necessary to maintain this yield benefit. The increase in growth of Powell amaranth, common lambsquarters, and giant foxtail with CPM amendment was not due to N or K, and may have been influenced by P or another factor in the CPM. Velvetleaf was the only species that exhibited increased biomass with N addition (as blood meal), similarly to the increase with added CPM, suggesting the velvetleaf growth response to CPM was due to N in the CPM. The results show that nutrient ratios should be considered when applying organic amendments, because application rates of organic amendments that meet the crop's needs for one nutrient may oversupply other nutrients. Overfertilization will not benefit crop yield, but the results of this study show that high organic fertility application rates are likely to increase weed growth.

The exotic plant, swallow-wort, a twining perennial of the Milkweed family, has become increasingly invasive in natural areas, successional old fields, and tree plantations on calcareous soils of the lower Great Lakes basin. Because mechanical control strategies are inadequate, swallow-wort's response in natural areas to foliar spray and cut-stem applications of glyphosate and triclopyr was evaluated. Foliar spray applications of glyphosate (3.1 and 7.8 kg ae ha–1) and triclopyr (1.9 kg ae ha–1) were more effective than cut-stem applications of the herbicides (3.1 kg ae ha–1 glyphosate, 1.4 kg ae ha–1 triclopyr). There were no statistical differences in effect among the foliar spray applications. Response to cut-stem application of glyphosate (3.1 and 6.2 kg ha–1) and triclopyr (1.4, 2.8, and 5.6 kg ha–1) in the following year indicated glyphosate to be more effective than triclopyr at all concentrations tested. To effect long-lasting control, all treatments require additional herbicide application.

The adoption of precision technologies that spatially register measurements using global positioning systems (GPS) greatly facilitates conducting large-scale on-farm research by farmers. On-farm experiments that utilize producer equipment include variations in agronomic practices that occur in situations where we want to predict the effect of inputs on yield. The domain of inference for such on-farm studies therefore more closely matches that desired by researchers. To investigate the feasibility of on-farm research using GPS, a study was conducted to evaluate the potential benefit of site-specific weed management. The study utilized producer-maintained field-scale equipment on four Montana farms in dryland spring wheat production. Paired site-specific and whole-field herbicide treatment areas were established in 0.9 to 1.9-ha blocks using consultant weed maps and a geographic information system (GIS). Yield was unaffected by herbicide treatment strategy (site-specific or broadcast). Minimal detectable yield differences were evaluated for the experimental design (0.2 T ha−1). Net returns increased when the percentage of field infested by wild oat decreased. Visual ratings of wild oat density taken at harvest indicated no difference in wild oat control between treatments in two of four site-years. This research suggests that producer-owned equipment can be used to compare treatments, but the accuracy and subsequent power of such comparisons are likely to be low.

A reduction in soybean (Glycine max (L.) Merr.) yield of 25% (1968) to 30% (1969) resulted from one smooth pigweed (Amaranthus hybridus L.) per ft in 30-inch rows. A giant foxtail (Setaria faberii Herrm.) infestation of one plant per ft in 30-inch rows reduced yield 13% in 1969. Harvesting before weeds were desiccated resulted in significant threshing and separating losses as speed was increased from 1 to 2 and 3 mph. Stubble, lodging, and stalk losses were more than double in the pigweed and foxtail plots when compared to the weed-free plots after weeds were desiccated by frost.

Seed of 17 weed species that were buried at 2 and 15 cm at Fairbanks, AK, in 1984 were exhumed at 1-yr intervals and tested for germination and viability. Burial depth had a significant effect on seed viability of wild oats, bluejoint reedgrass, quackgrass, prostrate knotweed, and corn spurry with viability remaining higher at 15 than at 2 cm. The percent of viable foxtail barley and quackgrass seed that were dormant was greater at 15 than at 2 cm. Eleven of 17 species had greater than 6% seed viability 4.7 yr after burial. American dragonhead seed viability in 1989 was not significantly lower than in 1984. In contrast, viability of common hempnettle and quackgrass had dropped to zero by 2.7 and 3.7 years after burial, respectively. Foxtail barley and wild oats had less than 1% viable seed at the end of 4.7 yr. For most of the 17 species, a rectangular hyperbola model accounted for more of the variability in seed viability decline with time than did a negative logarithmic model.

Imazethapyr is a selective herbicide used to control a wide spectrum of weeds in several legume crops, including pea. The effect of imazethapyr on pea-Rhizobium symbiosis was studied through its effect on microorganism growth, microorganism nodulation ability and plant growth. Symbiotic plants were damaged by imazethapyr concentrations higher than 1.73 μM applied preemergence. The number of nodules per plant was affected more than nodule size, suggesting a direct imazethapyr effect on the nodule initiation rather than on nodule development. However, imazethapyr did not directly affect Rhizobium because doses higher than 0.34 mM were required to cause slight effects on Rhizobium growth in a defined medium. Also, nodulation ability of bacteria treated with imazethapyr was not affected. These results suggest that imazethapyr inhibits the growth of the symbiotic plant rather than having a direct effect on the bacteria. Accordingly, symbiotic plants were less affected than the corresponding nitrate-reducing pea plants.

Chlorsulfuron {2-chloro-N-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino] carbonyl]-benzenesulfonamide} applied preemergence or postemergence as a soil drench retards the growth of four corn (Zea mays L.) genotypes in soil with 50% inhibition in each case at a rate of approximately 5 g ai/ha. R-25788 (N,N-diallyl-2,2-dichloroacetamide), applied at 1.0 kg ai/ha to the soil preemergence with chlorsulfuron or postemergence 2 days prior to chlorsulfuron treatment, reverses shoot growth inhibition induced by the herbicide in varying degree in ‘XL25A’ and ‘XL72B’ corn hybrids and ‘FRM017’ and ‘FR619’ inbreds, but not in soybean [Glycine max (L.) Merr., var. ‘Amsoy-71’]. Under similar conditions, flurazole [2-chloro-4-(trifluoromethyl)-5-thiazolecarboxylic acid (phenylmethyl ester)] and CGA-92194 [α-(1,3-dioxolan-2-yl-methoxy)-imino-benzeneacetonitrile] are less effective than R-25788 in protecting corn genotypes from chlorsulfuron injury. R-25788 also partially reverses chlorsulfuron-induced root growth inhibition in XL25A and FR619 corn but not in pea (Pisum sativum L. var. ‘Dwarf Gray Sugar’). Acetohydroxyacid synthase (AHAS) is higher in both activity and the proportion that is very sensitive to in vitro chlorsulfuron inhibition when prepared from XL25A shoots than from the roots. R-25788 does not reverse the in vitro inhibition of corn acetohydroxyacid synthase (AHAS) by chlorsulfuron. Pretreatment of 5-day-old XL25A corn with R-25788 at 24 or 48 μM significantly increases AHAS activity by approximately 25% in both shoots and roots but does not change its in vitro sensitivity to chlorsulfuron. This antidote also increases the glutathione (GSH) content in roots and etiolated shoots. The R-25788-induced elevation of AHAS activity may contribute to its antidotal effect in corn.

The nature and mode of action of surfactants is reviewed. Surfactants are useful additives for formulated herbicides because they contribute to wetting, spreading, emulsification, and solubilization of the herbicide. The use of extra surfactant to increase herbicidal activity is a desirable practice. The factors in selecting a suitable agent are discussed in terms of surfactant properties.

Four cultivations of cotton [Gossypium hirsutum L. ‘Stoneville 213′] alone failed to reduce green weed biomass or increase seed cotton yields above that of the no weed control treatment, and thus resulted in a negative net return (−200 to −450 $/ha) all 4 yr. Two cultivations plus two hand-hoeings reduced green weed biomass and increased seed cotton yields 3 out of 4 yr, thus a positive net return was produced for 3 yr ranging from 300 to 640 $/ha. Fluometuron [1,1-dimethyl-3-(α,α,α-trifluoro-m-tolyl)urea] alone was the most effective treatment in reducing green weed biomass and increasing seed cotton yields, and produced the highest net return, 280 to 420 $/ha for the 4-yr period. The addition of cultivation did not improve the fluometuron treatment. However, diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea)] required three supplementary cultivations to equal the single fluometuron treatment. Trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine) plus three cultivations did not equal the single fluometuron treatment, although a positive net return was obtained every year. The addition of trifluralin to fluometuron also failed to improve weed control or net return over that of fluometuron alone.

Combinations of MSMA (Monosodium methanearsonate) with either methazole [2-(3,4-dichlorophenyl)-4-methyl-1,2,4-oxadiazolidine-3,5-dione] or metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazin-5(4H)-one] were applied at various rates for postemergence control of fully developed goosegrass [Eleusine indica (L.) Gaertn.], a major weed grass in bermudagrass [Cynodon dactylon (L.) Pers.] turf. Two applications of MSMA at 2.24 kg/ha applied in combination with either methazole or metribuzin at 0.14 kg/ha controlled 96 to 98% of the goosegrass and it was as effective as methazole (1.12 kg/ha) or metribuzin (0.56 kg/ha) applied alone. Two applications of methazole (1.12 kg/ha) or metribuzin (0.56 kg/ha) had lower quality ratings (28 and 39, respectively) on bermudagrass putting green at 2 weeks after treatment than combinations of MSMA (2.24 kg/ha) with either methazole (0.14 kg/ha) or metribuzin (0.14 kg/ha) (54 and 59, respectively). The initial injury from combination treatments was not severe and turf recovered within 3 to 4 weeks.

Field studies of herbicidal antagonism were conducted in corn (Zea mays L.), soybeans (Glycine max L. Merr.), and on industrial sites. The addition of chlorbromuron [3-(4-bromo-3-chlorophenyl)-1-methoxy-1-methylurea], cyanazine {2-[[4-(chloro-6-(ethylamino)-s-triazin-2-yl] amino]-2-methylpropionitrile}, bifenox [methyl 5-(2,4-dichlorophenoxy)-2-nitrobenzoate], or atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] wettable powder plus dicamba (3,6-dichloro-o-anisic acid) to glyphosate [N-(phosphonomethyl)glycine]-alachlor[2-chloro-2′,6′-diethyl-N-(methoxymethyl)acetanilide] combinations reduced the activity of glyphosate on quackgrass [Agropyron repens (L.) Beauv.], common dandelion (Taraxacum officinale Weber), and Canada thistle [Cirsium arvense (L.) Scop.] in no-till corn. Reduced weed control from antagonism resulted in decreased corn yields. Linuron [3-(3,4-dichlorophenyl)-1-methoxy-1-methylurea], chlorbromuron, or metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazin-5(4H)-one], when added to glyphosate, reduced the control of quackgrass but not that of Canada thistle, alfalfa, or common dandelion in soybeans. Antagonsim was not evident in annual weed species. In perennial weeds, the degree of antagonism was often reduced with increased dosages of glyphosate. The inclusion of terbacil (3-tert-butyl-5-chloro-6-methyluracil), bromacil (5-bromo-3-sec-butyl-6-methyluracil), and simazine [2-chloro-4,6-bis(ethylamino)-s-triazine] with glyphosate in mixes interfered with the control of smooth brome (Bromus inermis Leyss.), but not when applications of the residual herbicides were delayed. Quackgrass control was reduced when amitrole (3-amino-s-triazole) was mixed with glyphosate, or when applied separately.

Soybean and annual weed competition studies were conducted over 2 yr in Balcarce, Argentina, to define the critical period of weed control and to compare yield losses in soybean, when weeds competed in the total cropping area (row plus furrow) or only in the furrow. In the first year, broad-leaf weeds were more abundant than annual grasses, but in the second year, grasses were more abundant. Critical weed-free periods required to prevent yield losses of 2.5 and 10% were long and consistent for both years and for both types of competition, lasting between 50 and 61 d after crop emergence (DAE) or the V8 to V9 and R2 to R3 soybean growth stages, respectively. No differences in predicted yield for both places of competition was found when weeds grew after the V4 or later crop growth stages (35 DAE). The critical periods of weed removal to prevent equivalent yield losses were different in both years. Varying between 1 DAE in 1986–1987 and 30 DAE in 1988–1989, for 2.5% yield losses, corresponding to growth stages between V0 and V3 to V4, respectively. For yield losses lower than 10%, the critical periods of weed removal varied between 15 DAE in 1986–1987 and 35 DAE in 1988–1989, corresponding to the V2 and V4 crop growth stages, respectively. Nonsignificant differences in predicted yield were found between places of competition, if weeds were removed before V4 (35 DAE), maintaining yield losses lower than 20%.

Marked differences in the response of field bindweed (Convolvulus arvensis L.) to 2,4-dichlorophenoxyacetic acid (2,4-D) were found among plants grown from seed obtained from 19 states in the U. S. and one province in Canada. These differences were not correlated with the origin nor with plant vigor. Inherent physiological differences seemed to be responsible.