The cross-resistance of triazine-resistant biotypes of smooth pigweed (Amaranthus hybridus L. # AMACH), common lambsquarters (Chenopodium album L. # CHEAL), common groundsel (Senecio vulgaris L. # SENVU), and the crop canola (Brassica napus L. var. Atratower) to a selection of herbicides was evaluated at both the whole plant and chloroplast level. The triazine-resistant biotypes of all four species showed a similar pattern of cross-resistance, suggesting that a similar mutation had occurred in each species. The four triazine-resistant biotypes were resistant to injury from atrazine [6-chloro-N-ethyl-N′-(1-methylethyl)-1,3,5-triazine-2,4-diamine], bromacil [5-bromo-6-methyl-3-(1-methylpropyl)-2,4-(1H,3H)pyrimidinedione], and pyrazon [5-amino-4-chloro-2-phenyl-3(2H)-pyridazinone] and were slightly resistant to buthidazole {3-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-4-hydroxy-1-methyl-2-imidazolidinone}. The triazine-resistant biotypes were more sensitive to dinoseb [2-(1-methylpropyl)-4,6-dinitrophenol]. Triazine-resistant smooth pigweed showed resistance to 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] with slight resistance to linuron [N′-(3,4-dichlorophenyl)-N-methoxy-N-methylurea] and desmedipham {ethyl [3-[[(phenylamino)carbony] oxy] phenyl] carbamate}. There was little or no resistance to diuron [N′-(3,4-dichlorophenyl)-N,N-dimethylurea], bromoxynil (3,5-dibromo-4-hydroxybenzonitrile), bentazon [3-(1-methylethyl)-(1H)-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide], or dicamba (3,6-dichloro-2-methoxybenzoic acid). Parallel studies at the chloroplast level indicated that the degree of resistance to inhibition of photosynthetic electron transport was highly correlated with the degree of resistance to herbicidal injury. This correlation indicates that atrazine, cyanazine, metribuzin, pyrazon, bromacil, linuron, desmedipham, and buthidazole cause plant injury by inhibition of photosynthesis. This correlation also indicates that triazine resistance and cross-resistance at the whole plant level is due to decreased sensitivity at the level of photosynthetic electron transport. Cross-resistance to numerous additional herbicides was evaluated on isolated chloroplast thylakoid membranes and these results are discussed. 14C-atrazine was displaced from thylakoid membranes by several herbicides, indicating that these herbicides compete for a common binding site.

In western Canada, little is known about the seedbank ecology of volunteer canola. Therefore, integrated recommendations for the management of this weed are limited. In this study, we investigated the seedbank persistence and seedling recruitment of two spring canola genotype groups with different secondary seed dormancy potentials under contrasting tillage systems. The study was conducted at two locations with different soils in the Mixed Moist Grassland ecoregion of Saskatchewan. A single cohort seedbank was established in 1999 and was followed for 3 yr in successive wheat crops. In a separate laboratory study, the six canola genotypes examined were classified as those with high and those with medium potentials for the development of secondary seed dormancy (HD and MD, respectively). After one, two, and three winters, maximum persistence of 44, 1.4, and 0.2% of the original seedbank was observed among the treatments, respectively. In 2001, HD canola genotypes tended to exhibit 6- to 12-fold greater persistence than MD canola genotypes, indicating lower seedbank mortality in HD canola. Seedling recruitment of HD canola also was higher than MD canola when differences were observed between these genotype groups. Therefore, long-term seedbank persistence of canola can be reduced by growing genotypes with low inherent potential for the development of secondary seed dormancy. The proportion of persisting seeds tended to be higher under conventional tillage than under zero tillage because of lower seedbank mortality, but no clear distinction in seedbank persistence in terms of absolute time could be made between these two tillage systems. Volunteer canola seedling recruitment followed the pattern of a typical summer-annual weed, where seedling emergence was observed only during May and June.

Growth chamber experiments were conducted to evaluate the effects of relative humidity and temperature on the efficacy, absorption, and translocation of glufosinate at 205, 410, and 820 g ha−1 in Palmer amaranth, redroot pigweed, and common waterhemp. Low relative humidity decreased control of all three species by glufosinate. However, control increased as application rate increased at low relative humidity. Amaranth species grown under 21/16, 26/21, and 31/26 C day/night temperature regimes responded differently to glufosinate. At 26/21 C, glufosinate at 820 g ha−1 controlled redroot pigweed less effectively than it controlled Palmer amaranth and common waterhemp, whereas at 410 g ha−1, glufosinate controlled common waterhemp more effectively than it controlled the other two species. Neither temperature nor relative humidity altered the absorption of 14C-glufosinate in any of the three species. Most of the absorbed glufosinate remained in the treated leaves at all three temperature regimes and two relative humidity levels. However, glufosinate translocation was greater in plants grown at 90% than in those grown at 35% relative humidity, and this phenomenon coincided with greater control of the amaranth species at the high humidity level. The study showed that relative humidity had a greater effect than temperature on glufosinate toxicity to Palmer amaranth, redroot pigweed, and common waterhemp.

Field studies were conducted near Blackville, SC, in 2002 and 2003 and near Pendleton, SC, in 2003 to compare the critical period for weed control (CPWC) in narrow- (48 cm) and wide-row (97 cm) corn. A series of treatments of increasing duration of weed interference and length of weed-free period were imposed within each row spacing. Diversity and density of the weed spectrum were greater at Blackville than at Pendleton. Weed interference duration and weed-free period curves were similar between row widths for each of the 3 site-years. Averaged over row width, the CPWC was 36 to 40 d longer at Blackville than at Pendleton. The CPWC began 5 to 9 d after corn emergence (DAE) (one- to two-leaf stage) and ended 45 to 53 DAE (eight- to 10-leaf stage) at Blackville. At Pendleton, the CPWC was only 4 d, beginning 21 DAE (five-leaf stage) and ending 25 DAE (five- to six-leaf stage). Light interception by corn at Blackville at the end of the CPWC averaged 78%, but light interception averaged only 31% at Pendleton at the end of the CPWC, implying that the weed density or weed spectrum may be more of a determinate of the CPWC than canopy formation. Light interception was similar between row widths throughout the growing season, resulting in similar late-season weed biomass between row widths. The CPWC and crop competitiveness with late-emerging weeds was similar between wide- and narrow-row corn when corn light interception did not differ between row widths. Therefore, other strategies, such as increasing the population of narrow-row corn, are likely needed to provide a competitive advantage over wider rows.

Field experiments were conducted in 1996 and 1997 to determine the effect of the rate and time of glyphosate application on weed emergence, survival, biomass, and Glycine max yield in reduced-tillage (RT) and no-tillage (NT) glyphosate-resistant G. max planted in rows spaced 18 (narrow-row) and 76 cm (wide-row). Glyphosate was applied at 0.42, 0.63, and 0.84 kg ae ha−1 at V2, V4, R1, and R4 growth stages. On separate plots, 0.84 kg ha−1 glyphosate was applied at each growth stage with hand weeding. A weed-free check was maintained with preemergence imazethapyr plus metolachlor supplemented with hand weeding, and a nontreated check was included. Weed population density before glyphosate application ranged from 239 to 606 plants m−2 in RT and 33 to 500 plants m−2 in NT systems. Setaria faberi and Chenopodium album were the predominant species. Weed control efficacy and crop yield were influenced more by application time than by glyphosate rate. Glyphosate applied at V2, V4, and R1 gave season-long control of weeds in 18-cm rows. In 76-cm rows, glyphosate applied at V2, V4, and R1 gave almost complete control of weeds, but broadleaf weeds emerged after application at V2. The critical time of weed removal, the time beyond which weed competition reduced G. max yield by 3% or more compared to the weed-free check, was at R1 and V4 in 18-cm RT G. max in 1996 and 1997, respectively, and at V2 in 76-cm RT G. max in both years. The predicted critical time of weed removal in 18- and 76-cm NT G. max was R1 and V4, respectively, in 1996 and R1 in 1997. This research showed that there was variation in the onset of the critical time of weed removal between tillage systems, as well as within tillage systems across years. The results indicate a single glyphosate application can prevent yield loss in narrow-row, glyphosate-resistant G. max under favorable conditions, but application timing becomes more critical in wide rows because the critical period of weed removal occurs earlier. Late-emerging weeds may warrant a second glyphosate application in wide-row G. max.

The persistence of alachlor [2-chloro-2′,6′-diethyl-N-(methoxymethyl) acetanilide], metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide], and propachlor (2-chloro-N-isopropylacetanilide) in soil was examined under laboratory and field conditions using sorghum (Sorghum vulgare Pers. ‘NB280S′) or annual ryegrass (Lolium multiflorum Lam.) as bioassay species. In laboratory studies, degradation rate of alachlor and propachlor was greater at 50 and 80% than at 20% field capacity at 20 C. Degradation of metolachlor was greater at 80 than at 20% field capacity. Degradation rate of alachlor and metolachlor at 50% field capacity was greater at higher temperatures. Propachlor degradation rate varied with temperature. Under irrigated, cropped field conditions, the order of persistence was metolachlor > alachlor > > propachlor.

Field observations indicate that sulfonylurea-resistant kochia may germinate at lower soil temperatures and/or germinate more rapidly than susceptible kochia in the absence of herbicide. To investigate this possibility, seeds from three resistant and two susceptible kochia accessions were germinated at temperatures ranging from 4.6 to 13.2 C on thermal gradient plates. At 4.6 and 13.2 C, germination rates of all resistant accessions were higher than susceptible accessions, while germination rates of one resistant accession were higher than susceptible accessions at 7.2 and 10.5 C. Percent germination of all resistant accessions was significantly higher than susceptible accessions after 48 h at 4.6 C. At higher temperatures, percent germination of some resistant accessions was higher after 12 or 24 h, but germination of all accessions was similar at later times. HPLC analysis revealed that seeds from resistant accessions contained about 2-fold higher free levels of branched chain amino acids than seeds from susceptible accessions. The results indicate that mutations conferring resistance to sulfonylurea herbicides in these kochia accessions may concomitantly reduce or abolish acetolactate synthase sensitivity to normal feedback inhibition patterns, resulting in elevated levels of branched chain amino acids available for cell division and growth during early germination.

Kochia plants resistant (R) to field rates of dicamba were characterized for their frequency of occurrence and levels of resistance and for the physiological fate of applied 14C-dicamba. Of 167 randomly sampled fields and seven fields identified by producers to contain R kochia, 19 contained plants that produced 1% or more R progeny. The maximum percentage of R progeny produced by parental plants from any field was 13%. An inbred R line derived from a field collection was 4.6-fold more resistant to dicamba than an inbred susceptible (S) line. Rates of 14C-dicamba uptake and translocation were similar in R and susceptible (S) plants up to 168 h after treatment (HAT). Concentrations of the primary metabolite, 5-hydroxy dicamba, were similar in R and S tissues up to 60 HAT, although amounts were significantly greater in R treated leaves by 96 and 168 HAT. However, because there were negligible levels of dicamba metabolites in R shoots and because the rate of metabolism was relatively slow, the observed changes were inadequate to account for observed resistance levels. Thus, dicamba resistance in kochia cannot be attributed to differential herbicide absorption, translocation, or metabolism. These findings, together with our field observations on the unusually slow spread of resistance within and among fields may indicate that dicamba resistance is a quantitative trait.

Several grass and broadleaf weed species around the world have evolved multiple-herbicide resistance at alarmingly increasing rates. Research on the biochemical and molecular resistance mechanisms of multiple-resistant weed populations indicate a prevalence of herbicide metabolism catalyzed by enzyme systems such as cytochrome P450 monooxygenases and glutathione S-transferases and, to a lesser extent, by glucosyl transferases. A symposium was conducted to gain an understanding of the current state of research on metabolic resistance mechanisms in weed species that pose major management problems around the world. These topics, as well as future directions of investigations that were identified in the symposium, are summarized herein. In addition, the latest information on selected topics such as the role of safeners in inducing crop tolerance to herbicides, selectivity to clomazone, glyphosate metabolism in crops and weeds, and bioactivation of natural molecules is reviewed.

Common waterhemp has become a problem weed species in Midwest soybean production. Determining the critical interference period after soybean and common waterhemp emergence is necessary for the implementation of weed control practices before soybean seed yield loss occurs. Field experiments were conducted during 1996, 1997, and 1998 to determine the influence of duration of common waterhemp interference on soybean seed yield. Removal of common waterhemp 2 wk after soybean unifoliolate leaf expansion resulted in soybean seed yield equivalent to a season-long weed-free control. Delaying common waterhemp removal until 4 wk after soybean unifoliolate leaf expansion resulted in decreased soybean seed yield. Allowing common waterhemp interference to persist 10 wk after soybean unifoliolate leaf expansion reduced soybean seed yield by an average of 43% over 3 yr. These results suggest that soybean producers should implement common waterhemp management strategies earlier than 4 wk after soybean unifoliolate leaf expansion in order to reduce the potential loss of soybean seed yield.

Resistance to herbicides occurs in weeds as the result of evolutionary adaptation (Jasieniuk et al. 1996). Basically, two types of mechanisms are involved in resistance (Beckie and Tardif 2012; Délye 2013). Target-site resistance (TSR) is caused by changes in the tridimensional structure of the herbicide target protein that decrease herbicide binding, or by increased activity (e.g., due to increased expression or increased intrinsic activity) of the target protein. Nontarget-site resistance (NTSR) is endowed by any mechanism not belonging to TSR, e.g., reduction in herbicide uptake or translocation in the plant, or enhanced herbicide detoxification (reviewed in Délye 2013; Yuan et al. 2007).

Production systems based on reduced-tillage practices account for over 60% of the cropped land on the Canadian Prairies. Concerns have been expressed regarding potential shifts in weed communities as a result of changing tillage practices. Study objectives were to (1) determine the feasibility of combining and analyzing weed abundance data from 10 medium- to long-term studies on the Canadian Prairies that compared conventional-, reduced-, and zero-tillage systems, (2) identify species that are associated with specific tillage systems, and (3) place species into plant response groups according to the similarity of their tillage system response. Conventional-tillage systems were defined as including both a fall and spring sweep-plow operation before seeding spring crops, whereas reduced tillage consisted of only one sweep-plow operation shortly before seeding. Crops within zero-tillage systems were planted directly into the previous crop's stubble. The association between weed species and tillage systems was investigated using indicator species analysis. Species were assigned to tillage response groups on the basis of the results of the analysis and the expertise of the project scientists. Perennial species such as Canada thistle and perennial sowthistle were associated with reduced- and zero-tillage systems, but annual species were associated with a range of tillage systems. Field pennycress was placed in the conventional-tillage response group, Russian thistle in the zero-tillage group, and wild buckwheat and common lambsquarters were equally abundant in all tillage systems. The goal of classifying weed species based on common functional traits in relation to responses to tillage systems was not realized, in part, because the required information on species biology and ecology was either unavailable or not applicable to local conditions.

Available literature indicates that relatively few agricultural leaders and farmers became interested in weeds as a problem before 1200 A.D. or even 1500 A.D. For many centuries, weed control was mostly incidental to tillage for seedbed preparation and growing of crops and to growing and cutting or pasturing of thickly planted crops. Occasional references in literature previous to 1900 mentioned use of mechanical devices and a few inorganic herbicides specifically for weed control.

State weed laws directed at control of plant diseases were enacted during 1721 to 1766, but weed and seed laws involving weeds directly were not enacted until 100 to 200 years later. Only a few extension-type publications on weeds were issued in the United States and Canada between 1860 and 1900. There was a rapid increase in such publications after 1900. Research with inorganic chemicals as herbicides was begun in the 1890's in Europe and in a few states and provinces, and was increased at a rapid pace until the early 1940's. New developments in mechanical and biological control of weeds increased steadily during the same period. However, weed control remained a relatively minor phase of agronomy, botany, horticulture, agricultural engineering, and plant physiology until the early 1950's.

About 10 years after the discovery of (2,4-dichlorophenoxy)acetic acid (2,4-D) in 1942–1944, the much increased interest of scientists, federal and state governments, industrial companies, and the general public had begun to bear fruit. The word “weed” or “weeds” began to appear in the titles of college courses and extension specialists. Weed conferences had been organized in six regions of the United States and Canada and in 10 states.

The first meeting of the Weed Science Society of America was held in 1956 and Weed Science was adopted as its official journal. The number of herbicides in general use in the United States and Canada increased from 15 in 1940 to 25 in 1950, and to 100 in 1969. The total support for weed research in 1962 in the United States was six times that in 1950. The number of full-time research and extension workers or their equivalents in part-time workers had increased 20-fold and 13-fold, respectively, over the number in 1940.

The rate of advancement in the art and science of weed control has increased so rapidly that the progress in each of the recent brief periods 1941 to 1968, 1901 to 1940, and 1800 to 1900 is considered greater than that in all previous periods, beginning about 6000 B.C.

Dormant wild oat seed require afterripening under warm-dry conditions for conversion to a nondormant state capable of germination. Research was conducted to determine the relationship between temperature and seed moisture levels on afterripening of dormant wild oat line M73 seed, and to evaluate the status of water binding in the dormant seed. Conversion of dormant wild oat seed to a nondormant state at 20 to 40 C occurs primarily in the range of 5 to 20% seed moisture. There is an inverse relationship between temperature and seed moisture content for afterripening as measured by seed germination. As the afterripening temperature increases, the seed moisture content must decrease for maximum afterripening (germination) to occur. Moisture isotherms and derived enthalpy curves indicate three regions of water binding which reflect decreased binding of water to seed components as the moisture content in the dormant seed increases. Maximum afterripening, in the second region of water binding, corresponds to seed moisture contents of 7 to 22%. In this region water is weakly associated with macromolecular surfaces and begins to have solvent properties. Because afterripening occurs mainly in the second region of water binding it is likely that individual enzymatic and nonenzymatic reactions, rather than metabolic processes, mediate the conversion of wild oat seed to the nondormant state.

Field experiments were established near Portageville, MO, to determine the effects of giant ragweed interference in soybeans. Threshold densities of giant ragweed that reduced soybean yield were less than two plants 9 m−1 of soybean row. This density reduced soybean seed yield 46 and 50% in 1988 and 1989, respectively, after full-season interference. The critical duration of giant ragweed interference in soybeans was between 4 and 6 weeks after emergence (WAE) in 1988 and between 2 and 4 WAE in 1989. Full-season giant ragweed interference at densities of 220 000 and 360 000 plants ha−1 in 1988 and 1989, respectively, resulted in almost complete soybean yield loss. Eight to 10 weeks after emergence of giant ragweed-free conditions were required to prevent soybean yield reductions.

Effective weed seedbank management requires mechanistic understanding of ecological determinants of seed persistence in the soil seedbank. Chemical and physical defense of common lambsquarters, field pennycress, giant foxtail, kochia, velvetleaf, and yellow foxtail seeds were quantified in relation to short- and long-term seedbank persistence. Seed content of ortho-dihydroxyphenols (o-DHP), a class of putative seed defense compounds, varied more than threefold between the least protected species (common lambsquarters, 9.2 µg g seed−1) and the most protected species (kochia, 34.1 µg g seed−1). Seed o-DHP was inversely related (r = −0.77, P < 0.001) to seed half-life in the soil and to short-term seed persistence in burial assays (r = −0.82, P < 0.05). The relative importance of chemical seed protection in comparison to physical seed protection, as represented by the ratio of seed o-DHP concentration to seed coat thickness, decreased linearly with increasing short-term seed persistence (r = −0.96, P < 0.01) and nonlinearly with increasing long-term seed persistence in the soil seedbank (y = 0.16 + 0.21/(0.0432 + x), R2 = 0.99, P < 0.001). Mechanical damage to the seed coat, via piercing, slicing, or grinding treatments, increased short-term mortality during burial for all six species. Mortality of pierced seeds was negatively associated (r = −0.35, P < 0.05) with seed phenol concentration and positively associated with seed half-life (r = 0.42, P < 0.01) and seed coat thickness (r = 0.36, P < 0.05). Seed phenolics, as a class, supported the results for o-DHPs. Overall, these findings suggest a potential weakness, with respect to seedbank management, in the way weed seed defenses are constructed. Weed species with transient seedbanks appear to invest more in chemical defense than those species with highly persistent seedbanks. As a result, seeds in the latter category are relatively more dependent upon physical seed protection for persistence in the soil seedbank, and more vulnerable to management tactics that reduce the physical integrity of the weed seed coat.

Cover crop residues are not widely used for weed control because, as a stand-alone tactic, they do not effectively suppress all weeds and their duration of weed control is too short. Field experiments were conducted in 1995 and 1996, under both irrigated and rainfed conditions, to quantify Amaranthus spp., Setaria spp., and soybean emergence and growth in residues of fall-planted, spring-killed barley, rye, triticale, wheat, and hairy vetch. For both weed species, seedling emergence was reduced 3 wk after soybean planting by rye and wheat residues (≥ 2, 170 kg ha−1) in 1996. In 1996, Amaranthus spp. canopy volume was reduced 38 to 71% by residues 3 wk after planting. Likewise, Setaria spp. canopy biomass was reduced 37 to 97% in residues 5 wk after planting over both years. The response comparison index was used to identify frequency by which weed growth was placed at a disadvantage relative to soybean growth. Amaranthus spp. and Setaria spp. growth suppressions 3 to 5 wk after planting indicate potential times for intervention with other integrated weed management tactics such as reduced postemergence herbicide rates and interrow cultivation.

Resistance to herbicides and the lack of viable control options have led to an interest in increasing the role of crop competition as a weed management tool in water-seeded rice production. Weed-suppressive rice cultivars have been suggested as a tool that could improve weed control and reduce the reliance of growers on herbicides. Field studies were conducted at Biggs, CA, in 1999 and 2000 with six to eight semidwarf rice cultivars to identify water-seeded rice traits related to the suppression of watergrass growth. Cultivars S-201 and M-302 were the most suppressive in both years. The dry weight (DW) of watergrass grown with the most suppressive cultivar was only 16% in 1999 and 57% in 2000 of the DW of watergrass grown with the least suppressive cultivar. Rice leaf area and root DW in weed-free plots were linearly related to watergrass DW in both years. Weed-suppressive traits were not inversely correlated with rice yields in monoculture; competitive cultivars also had high yields. This study suggests that an indirect selection program, based on traits that can be identified early in the season under weed-free conditions, has great potential for developing more competitive cultivars for water-seeded rice.

Postdispersal weed seed predation by animals during the summer fallow period may lead to a reduction in the number of weeds that grow in the following winter cropping season. In this study, we investigated the patterns of weed seed removal, the influence of crop residue cover on seed removal, the types of granivores present and their seed preferences in a 16-ha postharvest cropping field in Western Australia during the summer months over 2 yr. Seed removal from caches was extremely variable (from 0 to 100%). Removal rates were generally highest along the edges of the field near bordering vegetation and lowest in the center of the field and within the bordering vegetation. However, there were many deviations from this general pattern. There was no change in rates of predation with different levels of residue cover. Ants or other small invertebrates were found to remove the most seeds. However, seed removal by other animals, such as rodents, was also evident. Annual ryegrass seeds were preferred over wild oat seeds, followed by wild radish pod segments. Seed harvesting was lowest in late January, peaked in February, and decreased in March. Results from this study suggest seed harvesters could reduce the number of surface seeds in the field, reducing the weed seed bank. Management options that increase the activity of the seed harvesters may lead to less variability in seed predation and could, therefore, be incorporated into an integrated weed management program.

Cotton (Gossypium hirsutum L. ‘Stoneville 213’) was grown with densities of sicklepod (Cassia obtusifolia L.) or redroot pigweed (Amaranthus retroflexus L.) ranging from 0 to 32 weeds/15 m of row. Regression of seed cotton yields on weed density revealed a linear decrease in yield with increasing weed densities. In the 3 yr these studies were conducted, losses in hand harvested yields of seed cotton ranged from 34 to 43 kg/ha for each sickledpod plant/15 m of row and 21 to 38 kg/ha for each redroot pigweed plant per 15 m of row. Under comparable weed densities, yields of seed cotton differed only slightly when hand harvested or mechanically harvested. Mechanical harvesting efficiencies of cotton were reduced only at higher densities of weeds. The percentage of trash in cotton generally increased with increasing density of weeds. Neither sicklepod nor redroot pigweed affected cotton grade or micronaire.