Two rapid, nondestructive assays were developed and tested for their potential in differentiating glyphosate-resistant from glyphosate-susceptible biotypes of horseweed. In one assay, leaves of glyphosate-resistant and -susceptible corn, cotton, and soybean plants as well as glyphosate-resistant and -susceptible horseweed plants were dipped in solutions of 0, 300, 600, and 1200 mg ae L−1 glyphosate for 3 d and subsequent injury was evaluated. In the second assay, plant sensitivity to glyphosate was evaluated in vivo by incubating excised leaf disc tissue from the same plants used in the first assay in 0.7, 1.3, 2.6, 5.3, 10.6, 21.1, 42.3, and 84.5 mg ae L−1 glyphosate solutions for 16 h and measuring shikimate levels with a spectrophotometer. The leaf-dip assay differentiated between glyphosate-resistant and -susceptible crops and horseweed biotypes. The 600 mg L−1 rate of glyphosate was more consistent in differentiating resistant and susceptible plants compared with the 300 and 1,200 mg L−1 rates. The in vivo assay detected significant differences between susceptible and glyphosate-resistant plants of all species. Shikimate accumulated in a glyphosate dose-dependent manner in leaf discs from susceptible crops, but shikimate did not accumulate in leaf discs from resistant crops and levels were similar to nontreated leaf discs. Shikimate accumulated at high (≥ 21.1 mg ae L−1) concentrations of glyphosate in leaf discs from all horseweed biotypes. Shikimate accumulated at low glyphosate concentrations (≤ 10.6 mg L−1) in leaf discs from susceptible horseweed biotypes but not in resistant biotypes. Both assays were able to differentiate resistant from susceptible biotypes of horseweed and might have utility for screening other weed populations for resistance to glyphosate.

Knowledge of Palmer amaranth demographics and biology is essential for the development and implementation of weed management strategies. A field experiment was conducted to investigate the effects of Palmer amaranth density on seedling mortality, flowering initiation, and flowering progress throughout the growing season and biomass production and fecundity in wide-row soybean. The experimental design was a randomized complete block design with three levels of Palmer amaranth density-clusters: high, medium, and low. Palmer amaranth mortality rate was greater at high Palmer amaranth population density-cluster, reaching a peak within 30 to 40 d after Palmer amaranth emergence (DAE) (0.55 and 0.80 for 2014 and 2015, respectively), in comparison with mortality rate at medium and lower density-clusters. Likewise, as Palmer amaranth density increased, biomass and seed production per unit area of the weed also increased. Biomass production at the high density-cluster in 2014 was 664.7 g m−2 compared with 542.9 and 422.1 g m−2 at medium and low density-clusters, respectively. Similarly, biomass production at high density-cluster in 2015 was 100.6 g m−2 compared with 37.3 and 34.2 at medium and low density-clusters, respectively. In addition, seeds produced at high density-cluster were 1.5 million and 245,400 seeds m−2 for 2014 and 2015, respectively. Seed production was reduced by 29% and 54% in 2014 and by 65% and 75% in 2015 at medium and low density-clusters, respectively. Earlier flowering initiation (i.e., between 30 to 40 DAE) occurred in higher Palmer amaranth density-clusters, indicating a trade-off between reproduction and survival at high densities and more stressed environments for species survival. Palmer amaranth male-to-female sex ratio was greater at high densities, 1.3 and 1.9, compared with lower densities of 0.6 to 0.7 and 0.7 to 0.8 in 2014 and 2015, respectively. The plasticity of Palmer amaranth population and population-structure regulation, vegetative growth, and flowering shifts at various levels of intraspecific competition (i.e., high vs. low population density-clusters) and the trade-off between these biological transitions merits further investigation.

Analysis of reciprocal F1, F2, F3, and backcross progeny from crosses between atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine]-resistant and several susceptible genotypes of Brassica campestris L. indicated that triazine resistance was inherited cytoplasmically from the female parent. A partially dominant chlorotic cotyledon was used as a genetic marker to confirm true hybrids in the reciprocal F1 progenies. Phytotoxicity of atrazine was correlated with loss of the Hill reaction.

The integration of cover crops with selected postemergence herbicides was evaluated on the basis of weed control and grain yields in no-till soybean and corn. Soybean was planted into wheat residue, whereas corn was planted into hairy vetch residue. Full, half, and quarter rates and sequential herbicide applications were made. The wheat cover crop did not increase weed suppression but increased soybean grain yields. Half rates of thifensulfuron plus quizalofop-P as single or split applications were as effective as full rates in reducing weed weight in soybean. Soybean grain yields were similar in the half- and full-rate treatments in 1994, but yield was highest in the full-rate treatment in 1995. The hairy vetch cover crop did not increase weed suppression but lowered corn stands and grain yields in 1995 and enhanced corn grain yields in 1996. Full, half, and quarter rates (1996 only) of nicosulfuron plus primisulfuron were equally effective in reducing weed weight. Corn grain yields were similar at all herbicide rates in 1995 but were inversely related to herbicide rate in 1996. Split herbicide applications did not improve weed suppression over single applications of the same herbicide rate in either crop. Results indicate that cover crops can improve crop productivity and reduced rates of environmentally benign herbicides can minimize the herbicide requirements in no-till corn and soybean.

Greenhouse, growth chamber, and field studies were conducted to determine effects of temperature on the potential of cogongrass (Imperata cylindrica L. Beauv. # IMPCY) and torpedograss (Panicum repens L. # PANRE) to spread beyond present distribution in the Lower Coastal Plains of Southeastern United States. Johnsongrass [Sorghum halepense (L.) Pers. # SORHA] was included in the study for comparative purposes. Growth of each species was greater with day/night temperature regimes of 30/25 C than with 27/22 or 24/18 C. Apical six-node-long rhizome fragments of all three species were killed by a 24-h exposure to −4.5 C. Johnsongrass produced at least three to five times more dry matter than either cogongrass or torpedograss. Order of interspecific competitiveness under 30/25, 27/22, and 24/18 C day/night temperature regimes was johnsongrass > torpedograss > cogongrass. After 2 yr, johnsongrass and torpedograss competition reduced common bermudagrass [Cyndon dactylon (L.) Pers. # CYNDA] yield by 99 and 37%, respectively, while cogongrass competition did not affect common bermudagrass yield. It is concluded that cogongrass and torpedograss are unlikely to spread into more temperate regions of the United States. However, both species are apt to become even more widespread and troublesome in the Lower Coastal Plains of Southeastern United States.

Primary physical dormancy caused by seed coat impermeability to water is a major reason for the persistence of velvetleaf in soil seedbanks. Understanding temporal trends in seed dormancy status will help predict potential emergence in the spring. Experiments were begun in 1992 and 1993 to determine the effects of velvetleaf seed maturation time, storage environment, and storage duration on changes in seed dormancy and germination over 20 mo. Seeds buried 1 and 10 cm deep exhibited a 30 to 70% decline in physical dormancy from maturity until winter, little change in dormancy from winter through the following summer, and a further decline the next autumn. The loss of physical dormancy was more rapid for early than for late maturing seeds and more rapid in 1992 than in 1993. Physical dormancy of seeds held at 4 C declined steadily, at a rate of approximately 0.8% per day, over the course of the study. Germination of seeds buried 1 cm averaged 23 to 37% in the first spring after harvest, which was equivalent to 68 to 100% of seeds that had lost physical dormancy over autumn and winter. The percentage of seeds with enforced dormancy reflected the loss of physical dormancy during autumn and the loss of seeds to germination during spring and summer. Additional information on how autumn temperature and moisture conditions influence the pattern of dormancy decline could aid in explaining the variation in velvetleaf infestations over time.

In the northern region of the northern Great Plains of North America, the relative abundance of dandelion in field crops has increased over the past two decades, and farmers need information to help them to better manage this species and slow its spread. A study was conducted to determine the emergence timing of dandelion from both rootstock and seed, and to investigate the efficacy of preseeding (spring) versus postharvest (autumn) herbicide treatments on dandelion in spring wheat fields. Emergence of dandelion plants from rootstock was very early (mean time to 50% emergence [E50] of 430 growing degree days [GDD] Tbase 0 C), while seedling emergence was much later (mean E50 of 980 GDD). Dandelion does not have a persistent seed bank, and seedling emergence occurred only after dandelion plants arising from rootstock flowered and shed seed. Herbicide treatments that included glyphosate plus florasulam, glyphosate plus tribenuron, or higher rates of glyphosate alone (≥675 g ae ha−1), provided high levels of dandelion control. Autumn herbicide applications were more effective than spring applications for reducing dandelion infestation levels (both aboveground biomass and density). Autumn herbicide applications came after peak emergence timing for dandelion plants emerging both from rootstock and from seed. Because dandelion is a simple perennial, population spread must be limited by controlling seedlings. Autumn herbicide applications provide control of dandelion seedlings and therefore, should limit dandelion population spread.

The effect of chlorsulfuron {2-chloro-N-[[(4-methoxy-6-methyl-1,3,5 -triazin-2-yl)amino] carbonyl] benzenesulfonamide} on the phytotoxicity of postemergence applications of the methyl ester of diclofop ((±)-2-[4-(2,4-dichlorophenoxy)phenoxy] propanoic acid} to Italian ryegrass [Lolium multiflorum (Lam.) # LOLMU] was investigated in field and greenhouse experiments. The addition of chlorsulfuron (14.8 g ai/ha field, 12.4 g/ha greenhouse) to 0.90 kg/ha diclofop reduced Italian ryegrass control 27% in the field and increased Italian ryegrass fresh weights in the greenhouse more than twofold over diclofop alone. The antagonism was more evident when combinations were used to treat Italian ryegrass in the three-leaf stage than the two- or four-leaf stage. Sequential applications yielded results similar to tank mix combinations. Chlorsulfuron and diclofop were approximately 400 and 50%, respectively, more active on Italian ryegrass when applied preemergence than when applied postemergence. Combinations of chlorsulfuron and diclofop applied preemergence to Italian ryegrass resulted in additive responses.

Establishment of seedlings of big sagebrush subsequent to range weed control treatments limits the economic feasibility of such treatments for increased forage production. This study investigated the dynamics of seed production by this species as one step in understanding seedling establishment. Studies were conducted on five different ecological sites along the arid extremity of big sagebrush distribution in western Nevada. Demography, physical dimensional parameters, biomass, and community structure were determined for each site and related to seed production over a 4-yr period. In addition, seeds harvested from each site were used to grow plants that were transplanted to establish reciprocal gardens at all sites. Seeds were subsequently harvested from the plants in these gardens. The average stand studied was capable of producing about 50 million seeds/ha annually. The plant characteristics most closely correlated with seed production were leaf weight, canopy density, and root crown diameter. The stand that consistently had the highest average seed production had the lowest woody biomass accumulation rate. In reciprocal gardens, plants grown from this stand had the highest average seed production. Seed production of big sagebrush plants appears to be related to site characteristics, shrub biomass characteristics, and inherent genetic potential of the plants in a given collection.

Inhibitory effects of two sesquiterpene lactones, costunolide and parthenolide, isolated from dichloromethane extract of the stem bark of southern magnolia and a parthenolide derivative, 1,10-epoxyparthenolide, were evaluated on germination and seedling growth of wild oat. The sesquiterpene lactones effected a significant reduction of seed germination, particularly at the highest concentrations of 200, 400, and 600 mg L−1, with costunolide being the most active one. Furthermore, the three sesquiterpenes strongly inhibited root and shoot growth of the weed. However, the inhibition of root growth by all compounds was greater than that of shoot growth. Parthenolide inhibited growth of both root and shoot more strongly than the other compounds and a reference herbicide imazamethabenz. At a concentration of 100 mg L−1, parthenolide caused 87 and 41% growth inhibition of root and shoot, respectively. Parthenolide was tested for its effect on acetolactate synthase (ALS) activity. The compound inhibited the enzyme in a concentration-dependent manner, with 50% inhibition of 51.44 µM. The results of this study indicated that the herbicidal activity of the isolated sesquiterpene may be attributed to inhibition of ALS. The promising phytotoxic activity of sesquitepene lactones reported here could be considered a starting point for developing environmentally safer herbicides.

Eurasian watermilfoil, an aquatic flowering plant, was found to be a suitable bioassay plant for the detection of photosynthetic inhibitor herbicides in water and aqueous extracts of soil. Stock cultures of Eurasian watermilfoil were maintained in an algal-free medium under constant environmental conditions. Oxygen evolution from three lateral shoots 3 to 5 cm in length was measured before and after herbicide treatment. Inhibition of photosynthesis was detected within 10 min of treatment. Concentrations as low as 5.9 × 10–8M terbacil and 6.4 × 10–8M diuron were detected in water. Simazine, atrazine, and metribuzin were detected in water at a concentration of 10–7M. Eurasian watermilfoil bioassays in lake water spiked with various concentrations of terbacil, metribuzin, atrazine, and simazine indicated that the bioresidual activity of these chemicals can be estimated in samples of natural water without extraction or purification. Using aqueous extracts taken from herbicide-treated soil, the Eurasian watermilfoil bioassay detected minimum levels of 0.047 and 0.07 kg/ha terbacil and atrazine, respectively. Neither soybean nor oat showed visible injury symptoms at these concentrations. The Eurasian watermilfoil bioassay has the advantage of being more rapid and/or more sensitive than methods using algae, oat seedlings, or leaf discs.

Viability of enzymatically isolated and purified oat (Avena sativa L. ‘Markton’) mesophyll protoplasts was established by their ability to photosynthetically fix CO2 and to accumulate neutral red. When protoplasts were treated with 10-3 M alachlor [2-chloro-2′,6′-diethyl-N-(methoxymethyl)acetanilide], barban (4-chloro-2-butynyl m-chlorocarbanilate), dalapon (2,2-dichloropropionic acid), dicamba (3,6-dichloro-o-anisic acid), glyphosate [N-(phosphonomethyl)glycine], oryzalin (3,5-dinitro-N4,N4-dipropylsulfanilamide), paraquat (1,1′-dimethyl-4,4′-bipyridinium ion), picloram (4-amino-3,5,6-trichloropicolinic acid), or trifluralin [α,α,α-trifluroro-2,6-dinitro-N,N-dipropyl-p-toluidine), only alachlor and barban caused significant leakage of 14C-labeled material from the protoplasts as compared to controls. An abrupt increase in leakage occurred when protoplasts were treated with Tween 20 (polyoxyethylene 20 sorbitan monolaurate) at concentrations above 0.01% (v/v). The amount of leakage from protoplasts treated with dalapon or paraquat in combination with Tween 20 appears to be equivalent to the sum of the individual treatments. Treatment of the plasma membrane ATPase with the above named herbicides during the assay period shows that alachlor, barban, and dicamba reduces enzyme activity whereas glyphosate increases the activity. Pretreatment of the membrane ATPase with the herbicides in assay buffer [36mM Tris [tris-(hydroxymethyl)-aminomethane]-MES [2-(N-morpholino)ethanesulfonic acid], 3mM MgSO4, 50mM KC1] did not inhibit the enzyme activity during the assay period. Pretreatment with dicamba, glyphosate, and picloram in gradient buffer [1mM Tris-MES, 1mM MgSO4, 1mM dithiothreitol] did inhibit enzyme acitivty during the assay period. However, these herbicides cause the pH of the gradient buffer to be acidic.

One year after the last of six annual applications of 3-(p-chlorophenyl)-l,l-dimethylurea (monuron) at 2.4, 3-(3,4-dichlorophenyl)-l,l-dimethylurea (diuron) at 2.4 and 7.2, and 2-chloro-4,6-bis(ethylamino)-s-triazine (simazine) at 1.0 and 3.0 lb/A in a furrow-irrigated vineyard on Warden silt loam, a bioassay using oats (Avena sativa L., var. Kanota) as the indicator showed that 1.6, 2.8, 10.6, 0.6, and 2.0 lb/A, respectively, remained in the surface 12 in of soil. Of the total, 62 to 89% was located in the surface 2 in and 86 to 100% was in the surface 4 in. Three years after the last of four annual applications of monuron at 7.2 lb/A, 0.8 lb/A remained in the surface 12 in.

Homozygous, sethoxydim-tolerant corn was field tested at two locations in 1989 and 1990. Sethoxydim at 0.22, 0.44, and 0.88 kg ha−1 was applied to sethoxydimtolerant corn in the 3- and 7-leaf stages. None of the sethoxydim treatments caused visible injury to the sethoxydim-tolerant corn, but all treatments were lethal to a parental corn line used as a control. Sethoxydim applied at either stage of corn development had no effect on number of days to 50% silk emergence, plant height, or grain yield, compared to nontreated plants. Sethoxydim-tolerant corn was also tolerant to mixtures of sethoxydim plus other postemergence herbicides that control dicotyledonous weeds. Sethoxydim mixed with atrazine or sethoxydim applied in sequential applications with dicamba or 2,4-D gave annual grass control similar to sethoxydim applied alone. However, the sethoxydim plus bentazon treatment resulted in reduced grass control in comparison to sethoxydim alone. When the broadleaf herbicides were mixed with sethoxydim or applied as sequential treatments, broadleaf weed control was the same as when the broadleaf herbicides were applied alone. The high level of corn tolerance to sethoxydim and the broad spectrum of weed control resulting from combinations of sethoxydim plus other postemergence herbicides indicates that sethoxydim-tolerant corn hybrids could increase the options available for weed control in corn.

Studies were conducted under controlled growth conditions to determine whether differential absorption, translocation, or metabolism was the basis for the differential response of grain sorghum hybrids to mesotrione. Mesotrione-tolerant (‘Dekalb DKS35-70’) and mesotrione-susceptible (‘Pioneer 84G62’) grain sorghum hybrids were treated with 14C-labeled mesotrione. At 1 d after treatment (DAT), absorption was 7% in both hybrids; at 7 DAT, however, absorption remained nearly steady in Pioneer 84G62 but increased to 12% in Dekalb DKS35-70. Translocation of 14C-mesotrione in sorghum hybrids was similar with less than 30% of the absorbed herbicide translocated out of the treated leaf by 7 DAT. A distinct metabolite of 14C-mesotrione was separated in both hybrids at 3 DAT. The amount of mesotrione parent compound that remained in Pioneer 84G62 and DKS35-70 was 72 and 65%, respectively. Dekalb DKS35-70 had significantly less mesotrione at 3 DAT than Pioneer 84G62 did, but the amount of mesotrione was similar for both hybrids at 5 and 7 DAT. Rapid metabolism of mesotrione may help explain the differential response of grain sorghum hybrids.

Field and greenhouse studies were conducted to determine the degree to which annual bluegrass (Poa annua L.) could be controlled by inhibiting seedheads. The materials used were: MH (1,2-dihydro-3,6-pyridazinedione); chlorflurenol (methyl 2-chloro-9-hydroxyfluorene-9-carboxylate), plus methyl 9-hydroxyfluorene-9-carboxylate, and methyl 2,7-dichloro-9-hydroxyfluorene-9-carboxylate; and endothall [7-oxabicyclo (2.2.1) heptane-2,3-dicarboxylic acid]. The effects of these materials on pollen quality and the viability of seed produced by treated plants were also determined. For all chemicals used, multiple applications at low rates resulted in better seedhead inhibition than single treatments at higher rates and their effects lasted longer. However, treatments that inhibited seedheads by an amount predicted to reduce annual bluegrass (more than 75%) often caused objectionable foliar discoloration. Endothall, particularly the granular formulation, caused excessive injury at all rates. All growth regulators reduced the number of seed produced, which affected the number of seeds that germinated from soil that was taken from treated plots. The number of seed found in the soil was sufficient to allow the stand to be self-perpetuating. All treatments reduced the percentage of fertile pollen, however, this reduction was not significant because the germination of seed harvested from treated plants was not reduced significantly. Even though these treatments reduced seedheads significantly, the population of annual bluegrass the following year was not reduced.

Economic decision rules for postemergence herbicide control of barnyardgrass in corn were developed. Damage and control functions that formed the basis of an economic model were estimated. Barnyardgrass density and time of emergence relative to the crop were fundamental to calculate the damage function. The control function described barnyardgrass dry weight response to variable doses of two herbicides. Both the biologist's and economist's weed control decision rules, derived from the economic model, were influenced by time of weed emergence relative to the crop, corn yield, and price. Inclusion of time of weed emergence relative to the crop improved our interpretative ability of derived decision rules. The biologist's threshold weed density was more sensitive to changes in parameter values than the economist's optimal herbicide dose strategy. Herbicide use with recommended label dose was greater than either the economically optimal or the biologically effective doses. Use of the biologically effective dose for postemergence weed control decisions was cost efficient and could be of practical significance to corn growers.

Research was conducted to elucidate mechanism(s) of nicosulfuron and primisulfuron selectivity in corn, woolly cupgrass, and shattercane. Corn absorbed less than one half the nicosulfuron and primisulfuron that woolly cupgrass and shattercane absorbed. Acetolactate synthase (ALS) isolated from three species responded similarly to nicosulfuron and primisulfuron. Values for 50% (I50) inhibition of ALS for nicosulfuron were 36.9, 37.2 and 29.5 nM for corn, woolly cupgrass and shattercane, respectively. Primisulfuron I50 values were 13.8, 11.5 and 10.0 nM for corn, woolly cupgrass and shattercane, respectively. Shattercane's I50 was different from corn's but not from that of woolly cupgrass. Corn rapidly metabolized nicosulfuron and primisulfuron, with a half-life of less than 4 h. Shattercane metabolized the herbicides more slowly, with a half-life greater than 72 h for nicosulfuron and 36 h for primisulfuron. Nicosulfuron and primisulfuron half-lives were greater than 72 h and less than 4 h, respectively, in woolly cupgrass. Therefore, selectivity with nicosulfuron and primisulfuron is likely based on metabolism to nonphytotoxic compounds. Corn tolerance to nicosulfuron and primisulfuron was also attributed to reduced herbicide penetration and translocation below the treated leaf.

A need exists to manage or manipulate plant pathogens for biocontrol of weeds in row crops. Many weed species remain refractory to conventional weed-control technology. Some of these weeds occur in the same families as important crops and are resistant or tolerant to many of the herbicides that are applied for weed control within those crops. Preemergence treatments are needed for some weed species for which only postemergence treatments are now available. The cost of controlling some weeds is so prohibitive that many farmers will not spend the necessary money; therefore, more economical control is needed.

Glyphosate-resistant (GR) cropping systems are popular and used extensively by producers. However, the long-term impacts of heavy reliance of this technology on weed community structure are not known. Five fully phased field experiments (two no-tillage and three conventional tillage) were established at four locations in southwestern Ontario where the effects of herbicide system (glyphosate or conventional) in corn and soybean and crop rotation (corn–soybean or corn–soybean–winter wheat) on midseason weed communities were examined. Multivariate analysis on data over the last 3 yr of the 6-yr experiment showed that weed communities were distinctly different among the treatments within each experiment. At several locations, midseason weed communities were more similar in corn and soybean treated with glyphosate compared to the same crops treated with conventional herbicides, reflecting the continuous application of the same selection pressure in both crops. Analysis of trait-densities revealed an increase in species with late initiation of seedling recruitment at the expense of weed species with medium time of initiation of seedling recruitment rather than early recruiting species. Increases in perennial species, species with a short interval between recruitment and anthesis, and wind-dispersed species were also observed. Trait-density–based analysis of the weed community was an effective method for reducing the complexity of divergent weed communities that enabled direct quantitative comparison of the herbicide-induced effects on these weed communities.