Although stand-alone and integrated management techniques have been cited as viable approaches to managing Canada thistle [Cirsium arvense (L.) Scop.], it continues to impact annual cropping and perennial systems worldwide. We conducted meta-analyses assessing effectiveness of management techniques and herbicide mechanism of action groups for controlling C. arvense using 55 studies conducted in annual cropping systems and 45 studies in perennial systems. Herbicide was the most studied technique in both types of systems and was effective at reducing C. arvense. However, integrated multitactic techniques, with or without herbicides, were more effective than sole reliance on herbicides for long-term control in both annual cropping and perennial systems. A variety of management techniques such as biocontrol, crop diversification, mowing, and soil disturbance provided control similar to that of herbicide. Our results suggest that many management techniques aimed at reducing C. arvense can also improve crop yield or abundance of desired plants. This study highlights the need to devote more research to nonchemical and integrated management approaches for C. arvense control.

Common beggar's-tick is an annual weed commonly found in citrus groves in Florida. A series of laboratory and greenhouse experiments were conducted to determine the germination response of common beggar's-tick to various environmental factors that influence seed survival, germination, and dormancy. The results suggest that common beggar's-tick germinated over a wide range of temperatures (15 to 40 C) and in both alternating light and dark and dark conditions. New seeds (collected in 2010) germinated better than the old seeds (collected in 2007) at 15/10 C; however, at temperatures above 35 C, the old seeds germinated better. The highest germination was 95% at 25 to 30 C with old seeds compared to 78 to 86% at 20 to 30 C with new seeds. Germination of common beggar's-tick was inhibited at osmotic potential above −0.6 MPa and salt concentrations of 320 mM. Highest germination in common beggar's-tick was found under neutral conditions (pH 7); germination decreased sharply under increasing acidity and alkalinity. Emergence decreased as depth of sowing increased, with greatest germination (89 to 91%) occurring when sown at the surface (0 cm) regardless of seed age. No germination was observed when seeds were buried at 10 cm. Results of this study suggest that favorable temperature and soil pH, and adequate moisture in Florida ensures the germination and continued presence of common beggar's-tick.

Italian ryegrass has invaded wheat fields in China and is becoming a predominant, troublesome weed. Fenoxaprop-P-ethyl has been widely used for weed control on Chinese farms since the 1990s. However, overuse has led to fenoxaprop-P-ethyl resistance in Italian ryegrass in Chinese wheat fields. In this study, we identified a putative fenoxaprop-P-ethyl–resistant population of Italian ryegrass, HZYC-6, from Henan province, China. Mutations involving Asp-2078-Gly and Ile-1781-Leu substitutions were identified in the carboxyl-transferase domain of acetyl-coenzyme A carboxylase in this population, and these mutations are the likely cause of the target site–based resistance to fenoxaprop-P-ethyl. In addition, we identified cytochrome P450–mediated metabolism of herbicides (non–target site based resistance) in the HZYC-6 population, indicating that multiple mechanisms of resistance may be segregating in this population. Furthermore, HZYC-6 was also highly resistant to haloxyfop-R-methyl and quizalofop-P-ethyl, moderately resistant to clodinafop-propargyl and sethoxydim, and had low resistance to clethodim and pinoxaden.

The potential extensive use of certain new herbicides has made it desirable to study their effects on soil microorganisms. If these compounds have any retarding or stimulating influences on the general soil microflora, this could be reflected in carbon dioxide evolution during soil respiration experiments. Microorganisms are important to soil fertility because they transform potential plant nutrients into active or available forms, and carbon dioxide is a significant index of their activity. The effects on carbon dioxide evolution of four new herbicides that have shown promising results in field trials at this station are presented in this report.

Overuse of acetolactate synthase (ALS)–inhibiting herbicides in rice has led to the evolution of halosulfuron-resistant rice flatsedge in Arkansas and Mississippi. Resistant accessions were cross-resistant to labeled field rates of ALS-inhibiting herbicides from four different families, in comparison to a susceptible (SUS) biotype. Resistance index of Arkansas and Mississippi accessions based on an R/S ratio of the lethal dose required for 50% plant mortality (LD50) to bispyribac-sodium, halosulfuron, imazamox, and penoxsulam was ≥ 21-fold. Control of Arkansas, Mississippi, and SUS accessions with labeled field rates of 2,4-D, bentazon, and propanil was ≥ 93%. An enzyme assay revealed that an R/S ratio for 50% inhibition (I50) of ALS for halosulfuron was 2,600 and 200 in Arkansas and Mississippi, respectively. Malathion studies did not reveal enhanced herbicide metabolism in resistant plants. The ALS enzyme assay and cross-resistance studies point toward altered a target site as the potential mechanism of resistance. Trp574–Leu amino acid substitution within the ALS gene was found in both Arkansas and Mississippi rice flatsedge accessions using the Illumina HiSeq platform, which corresponds to the mechanism of resistance found in many weed species. Field-rate applications of 2,4-D, bentazon, and propanil can be used to control these ALS-resistant rice flatsedge accessions.

The effect of ultraviolet (UV) light (240–260 mμ range) on 6, 7-dihydrodipyrido (1, 2-a:2’,1′-c)-pyrazidiinium salt (diquat) and 1,1′-dimethyl-4,4′-bipyridinium salt (paraquat) was studied. Both dipyridyls were very stable in dark but were altered rapidly in the presence of UV light. Radioassay data collected from planchets containing solidified diquat or paraquat indicated a loss of more than 50% of the C14 from both compounds after 48 hr and more than 75% after 96 hr. When radioactive aqueous solutions were placed under UV lights, little change in the radioactivity occurred during an 8-day period as determined by liquid scintillation. When the concentration of diquat or paraquat in these solutions was determined by chemical analysis, degradation of the original molecules was observed.

Information obtained by autoradiography of chromatograms, spotted with radioactive aqueous solutions exposed to UV light, indicated that two or possibly more C14-containing degradation products were formed from both diquat and paraquat. Co-chromatography indicated that one degradation product from paraquat was 1-methyl-4-carboxy-pyridinium ion.

Intraspecific interference of sicklepod and interspecific interference between sicklepod and soybeans were investigated in field experiments in 1982 and 1983. Sicklepod growth and reproductive parameters were reduced as intraspecific spacing between plants decreased from 90 to 2.5 cm. Sicklepod plants growing intraspecifically were competing at spacings below 90 cm. Soybean plant heights were affected only slightly by sicklepod interference. Soybeans grew little in height from 35 to 84 days after emergence, while sicklepod plants continued to grow until they were 30 to 45 cm taller than the soybeans. Sicklepod plants spaced 10 and 30 cm apart in soybeans reduced soybean yield (25 to 35%) more than sicklepod spaced 90 cm apart. Sicklepod produced 900 to 2000 seeds per plant even when growth was restricted by interference from soybeans.

The activity of soil-applied chlorimuron in yellow and purple nutsedge was studied in greenhouse and laboratory experiments. Soil-applied chlorimuron decreased tuber sprouting by 80% in yellow nutsedge and by 30% in purple nutsedge at 60 g ai/ha. Chlorimuron decreased shoot emergence by 53 to 83% and shoot growth by 85 to 99% in both species at rates as low as 10 g/ha. Previous exposure of tubers to chlorimuron-treated soil reduced tuber resprouting by 20 to 25% in herbicide-free soil at 60 g/ha in both species. There was no rate response in shoot emergence from tubers previously exposed to chlorimuron, but shoot dry weight decreased by 60 to 81% in both species at 60 g/ha. At 12 h after application, 47% of the total 14C applied to the shoot in yellow nutsedge and 32% of that applied in purple nutsedge were absorbed. However, less than 1% of the total 14C applied was translocated out of the shoot and into the roots and tuber in either species. In both species, 1.3% of the 14C applied to the roots and tuber was absorbed and 0.1% was translocated out of the roots and tuber into the shoot at 12 h after application. The pattern of root- and tuber-absorbed 14C distribution indicated that the 14C absorbed by the tuber remained in the tuber and that absorbed by the roots was translocated to the shoots.

Corn (Zea mays L.) in soybeans [Glycine max (L.) Merr.] currently can be controlled with either glyphosate [N-(phosphonomethyl)glycine] applied to corn that is taller than soybeans, or diclofop {2-[4-(2,4-dichlorophenoxy)phenoxy] propanoic acid} sprayed over-the-top of both corn and soybeans. We found a wide range in tolerance to over-the-top spray applications of glyphosate among 240 F2 hybrids of corn. Glyphosate was then applied with a rope-wick to one tolerant and one susceptible F2 corn hybrid growing in soybeans. In only two instances out of six (three stages of growth times 2 yr), the susceptible corn was controlled better than the tolerant corn; but, in these two instances, the better control was not reflected in greater soybean yields. In another experiment, one F2 corn hybrid identified (in a previous study) as tolerant of diclofop and one identified as susceptible were sprayed in soybeans with over-the-top diclofop treatments. In 11 instances out of 12 (two stages of growth times three rates times 2 yr), the susceptible corn was controlled better than the tolerant corn, and in nine of these instances, the better control was reflected in greater soybean yields. Corn parentage could be important in determining the degree of control obtained with either herbicide, but this is more likely with diclofop than with glyphosate.

Field experiments were conducted to determine the effects of various densities of four fall-seeded small grain mulches and diphenamid (N,N-dimethyl-α-phenyl benzeneacetamide) on weed control, yield, and quality in no-till flue-cured tobacco (Nicotiana tabacum L. ‘McNair 944’). A greenhouse study investigated the effects of non-soil-incorporated mulch from the same small grains plus alfalfa (Medicago sativa L.) on various growth parameters of tobacco (‘Speight G-70’). None of the mulches used in the greenhouse study adversely affected growth of the tobacco. Mulch from rye (Secale cereale L. ‘Abruzzi’) killed about 2 weeks before transplanting plus diphenamid provided better annual broadleaf weed control (85%) than wheat (Triticum aestivum L. ‘McNair’), barley (Hordeum vulgare L. ‘Keowee’), and no mulch. Oat (Avena sativa L. ‘Brooks’) mulch resulted in 80% broadleaf weed control. There were no differences in annual grass control (which was short lived) among mulches but control was lower in the no-mulch treatment. Rye mulch resulted in a 22% increase in the control of broadleaf weeds compared to no-mulch. Yield of the no-till tobacco did not differ among mulches and averaged 82% of that conventionally grown. The quality was not affected. The rye mulch did not affect the yield or quality of tobacco when compared to a nonmulch, noncultivated treatment. The 18% decrease in the no-till yield was apparently the result of the lack of tillage and increased weed interference and was not due to adverse effects from the rye.

Growth response of five species of soil algae to 4-amino-6-tert-butyl-3-(methylthio)-as-triazin-5 (4H)one (metribuzin) and its 6-isopropyl and 6-cyclohexyl analogs were studied in vitro. Species of Chlorella, Chlorococcum, Chlamydomonas, and Anabaena were exposed to 0.05, 0.1, 0.5, and 1.0 ppm (w/v) of the herbicides in liquid nutrient, while a species of Schizothrix was studied at the same levels in soil culture. With one exception, the growth and numbers of all of the algae were significantly reduced by increasing herbicide concentrations. Anabaena was not affected by 0.05 ppm of the cyclohexyl analog. No growth was permitted by any of the herbicides at 1.0 ppm. Chlorella and Chlamydomonas could not grow in the presence of 0.5 ppm of metribuzin or the cyclohexyl analog. Differences in response to the triazinones were noted between species. Chlamydomonas was the most sensitive and Anabaena and Chlorella the most resistant.

Croftonweed is an invasive plant in southwest China. We examined the relationships between its invasion patterns and native plant diversity at different spatio-temporal scales. At the 25 m2 scale, invasion success was negatively correlated with native plant diversity, indicating that resource availability might be the dominant factor regulating community invasibility. At the 400-m2 scale, both negative and positive relationships were detected, possibly identifying a spatial scale threshold where extrinsic environmental factors became more important to community invasibility. At the vegetation province scale, variations in physical environment outweighed the importance of intrinsic biotic factors and positive relationships between diversity and invader success were found. Native plant diversity also inhibited croftonweed over the course of community succession and at the early stages of invasion at local spatial scales. However, the changing relationship might be an artifact of sampling at different spatial scales.

Seed of johnsongrass [Sorghum halepense (L.) Pers.] were planted during 1976 and 1977 at monthly intervals from March through October to study the growth and reproductive habits of this weed at Shafter, California. Plants were grown for 12 weeks and harvested at 3-week intervals. Emergence and development of plants increased with temperature. April through August plantings yielded 10-to 20-fold as much total fresh weight as March and September plantings. After planting, rhizome production normally began in 3 to 6 weeks and seed production in 6 to 9 weeks. From the April through August plantings, rhizomes accounted for averages of 13, 25, and 40% of the total fresh weight of plants 6, 9, and 12 weeks after planting, respectively. This compared, respectively, with 16, 9, and 6% for roots and 70, 66, and 54% for shoots. Seedlings emerging in March produced rhizomes in early June and viable seed in late June. Seedlings emerging as late as September produced rhizomes and some viable seed before killing frosts in November.

Of 17 thiocarbamate herbicides tested in the greenhouse in repeat soil applications made 4 to 16 weeks apart, 9 showed definite losses in herbicidal activity after the second application. Those showing reduced activity included EPTC (S-ethyl dipropylthiocarbamate), vernolate (S-propyl dipropylthiocarbamate), and butylate (S-ethyl diisobutylthiocarbamate), which have been reported previously to develop accelerated breakdown, plus R-15574 (S-benzyl dipropylthiocarbamate and the sulfoxides of EPTC, vernolate, butylate, SC-7829 (S-propyl diisobutylthiocarbamate), and SC-8149 (S-butyl diisobutylthiocarbamate). Thiocarbamates that showed no significant reduction in activity after the second application were pebulate (S-propyl butylethylthiocarbamate), cycloate (S-ethyl N-ethylthiocyclohexanecarbamate), molinate (S-ethyl hexahydro-1H-azepine-1-carbothioate), R-1880 (S-ethyl dibutylthiocarbamate), R-1856 (S-tertiarybutyl dipropylthiocarbamate), R-1853 (S-isobutyl diethylthiocarbamate), R-1906 (S-butyl diisopropylthiocarbamate), and R-12001 [S-isopropyl 1-(5-ethyl-2-methyl)piperidine carbothioate]. Of 16 nonthiocarbamate herbicides tested, a reduction in control of weeds occurred after the second soil application with 2,4-D [(2,4-dichlorophenoxy)acetic acid], dalapon (2,2-dichloropropionic acid), chlorpropham (isopropyl m-chlorocarbanilate), propham (isopropyl carbanilate), TCA (trichloroacetic acid), pronamide [3,5-dichloro (N-1,1-dimethyl-2-propynyl)benzamide], napropamide [2-(α-naphthoxy)-N,N-diethylpropionamide], bensulide [O,O-diisopropyl phosphorodithioate S-ester with N-(2-mercaptoethyl)benzenesulfonamide], alachlor [2-chloro-2’,6’-diethyl-N-(methoxymethyl)acetanilide], and diethatyl [N-(chloroacetyl)-N-(2,6-diethylphenyl)glycine]. Losses with the latter five herbicides after repeat application have not been reported previously.

Formation of 14C-atrazine degradation products and their distribution in the top 90 cm of soil was determined over 16 mo in a Webster clay loam in the field. After 16 mo, 64% of the applied 14C could still be accounted for in the 90-cm soil profile. At 1 mo after treatment (MAT), 14C moved to the 70- to 80-cm depth. Rapid movement of radioactivity could be attributed in part to preferential movement through vertical macropores. Atrazine accounted for 32% of the 14C applied 16 MAT and was the predominant 14C-compound in soil below 10 cm through 12 MAT. Hydroxyatrazine (HA) was the major degradation product in the top 10 cm of soil accounting for 9% of the 14C present 1 MAT and increasing to 24% within 6 MAT. Deethylatrazine (BEA) was the predominant degradation product at depths greater than 10 cm, accounting for 26% of the 14C in the 10- to 20-cm depth 16 MAT. Deisopropylatrazine (DIA) accounted for less than 10% of the 14C recovered at any soil depth. Deethyldeisopropylatrazine (DEDIA) and an unidentified product were detected in soil extracts 1 MAT indicating further degradation past primary metabolites. The proportion of DEA and DIA increased while the proportion of HA decreased as soil depth increased indicating that DEA and DIA are more mobile in soil than HA. The large amount of radioactivity remaining in the soil 16 MAT suggests that a large pool of atrazine and its degradation products are present in the soil for a long period of time, having the potential to move deeper in the soil and ultimately contaminate ground water.

Yellow and purple nutsedge and southern root-knot nematode are common pests in intensively managed chile pepper production. Greenhouse studies were conducted to identify relationships among nutsedge species, chile peppers, and root-knot nematode. All practical combinations of the plant species and nematodes were grown together in pots for 14 wk. Both nutsedges and root-knot nematode reduced chile pepper height over time with no interaction. Competition from purple nutsedge or either nutsedge species plus nematodes reduced chile pepper top weight more than yellow nutsedge or nematodes alone. All pest combinations reduced chile pepper root weight. Chile pepper and nematodes individually reduced rates of shoot production, as well as top and belowground weights of yellow and purple nutsedge. Chile pepper and purple and yellow nutsedge hosted root-knot nematode host race 3. Nematode egg production was greater on chile pepper roots than on either nutsedge species. Purple nutsedge growing with chile pepper increased nematode production g−1 of chile pepper root. Purple nutsedge grown with chile pepper was the most detrimental for chile pepper growth, but most efficient for maintaining the root-knot nematode population on the least amount of root biomass. Since root-knot nematode infests yellow and purple nutsedge, and few nematicides are available for nematode control, root-knot nematode cannot be managed with crop rotation if nutsedge is not controlled.

Field and column experiments were conducted to determine the effect of controlled release formulations on weed control and leaching of alachlor and metribuzin on a Plainfield sand. Controlled release formulations including two starch encapsulations of both herbicides and a microencapsulation of alachlor were compared to emulsifiable concentrate and dry flowable formulations of alachlor and metribuzin, respectively. Herbicide movement was measured in laboratory columns and in the field throughout two growing seasons to a soil depth of 91 cm. Soybean injury and weed control were monitored. No significant differences in herbicide movement between starch-encapsulated and emulsifiable concentrate formulations were observed in either field or column experiments. Microencapsulation resulted in the greatest retention of alachlor in the soil surface in field and columns. Compared to the dry flowable formulation, starch encapsulation did not affect metribuzin distribution in the field but reduced leaching in columns. Controlled release formulations did not result in significant differences in weed control and soybean injury compared to the emulsifiable concentrate alachlor and dry flowable metribuzin formulations. Starch encapsulations had a limited effect on alachlor and metribuzin movement. Degradation appeared to be the primary mechanism for herbicide dissipation while leaching losses were minor.

A comparative scanning electron microscopy (SEM) study was performed on the surfaces of seeds and coleoptiles of several conspecific rice (Oryza sativa L.) varieties including the weed red rice and several cultivars: Mochi-Gommi and LA-110 (medium grained); Starbonnet, Labelle, and Bluebelle (long grained). Seed surfaces of red rice, LA-110 and Mochi-Gommi possessed trichomes that were more numerous at the seed apex than on other seed parts. Red rice also exhibited a large central awn extending from the seed apex. This awn had spines along its axis and numerous spines at its apical end. These characteristics were absent in the other varieties. No significant differences were noted in the seed-surface tubercles and in the quantity or structure of wax on the seed surface in all six samples. The coleoptiles of all samples had several similarities: the stomata were arranged in parallel rows on both the adaxial and abaxial surfaces, the epicuticular wax structure on both surfaces was rodlet, prickle hairs were found on the adaxial coleoptile surface (but absent on abaxial), spines were present on the edge of the apical end (older portion) and decreased in number toward the basal end (younger portion), and two types of papillae (based on size and shape) were present on both the adaxial and abaxial surfaces. Although there were varietal differences among developing coleoptiles on a macroscopic scale, significant morphological differences were not discernible at the SEM level.

Sixteen experiments have investigated alternative methods of predicting the competitive effects of a simulated weed (oats) on the yields of spring barley, spring oilseed rape (canola), peas, spring field (faba) beans and flax. The experiments were designed to discover whether early postemergence assessments of crop and weed vigor would achieve more reliable prediction of yield loss than weed density. Weed density (plants m−2) was a very variable predictor of yield loss. The standardized ranges (range/mean) of values over 3 to 4 years of data for the five crops, in the densities causing 5% yield loss, were between 1.14 and 2.59. Predictions based on the relative dry weight of crop and oats (oat dwt/(oat dwt + crop dwt)), assessed while the plants were still small, achieved more reliable predictions, as the standardized ranges were between 0.10 and 1.86. In three of the experiments, predictions based on relative dry weights were compared to similarly timed predictions based on measurements of relative leaf area and of ground cover, assessed subjectively (by eye) and photographically. Subjective and objective (photographic) assessments of cover achieved similar predictions of yield loss, indicating that visual assessments could be a viable tool to assess the potential competitive effects of weeds.

Knowledge of the soil nitrogen (N) supply and the N mineralization potential of the soil combined with an understanding of weed-crop competition in response to soil nutrient levels may be used to optimize N fertilizer rates to increase the competitive advantage of crop species. A greenhouse study (2006) and field studies (2007 to 2008) in Illinois and Nebraska were conducted to quantify the growth and interference of maize and velvetleaf in response to varying synthetic N fertilizer rates in soils with high and low N mineralization potential. Natural soils were classified as having “low mineralization potential” (LMP), while soils amended with composted manure were classified as having “high mineralization potential” (HMP). Maize and velvetleaf were grown in monoculture or in mixture in both LMP and HMP soils and fertilized with zero, medium, or full locally recommended N rate. In the greenhouse, velvetleaf interference in maize with respect to plant biomass increased as N rate increased in the HMP soil, whereas increasing N rate in the LMP soil reduced velvetleaf interference. In contrast, velvetleaf interference in maize decreased as N rate increased regardless of soil class in the field experiment. With respect to grain yield, velvetleaf interference in maize was unaffected by N rate or soil class. In both greenhouse and field experiments, velvetleaf biomass was greater in the HMP soil class, whereas maize interference in velvetleaf was generally greater in the LMP soil class. While soil N levels influenced weed-crop interference in the greenhouse, the results of the field study demonstrate the difficulty of controlling soil nutrient dynamics in the field and support a maize fertilization strategy independent of weed N use considerations.