Wild radish is a prevalent annual weed throughout the cropping regions of southern Australia. Field experiments were conducted at Wagga Wagga, New South Wales, in 1998 and 1999 to determine the effect of various densities and emergence times of wild radish on yield and quality of canola and on wild radish seed production. As few as 4 wild radish m−2 emerging with canola reduced canola yield 9 to 11%, whereas 64 wild radish m−2 reduced canola yield 77 to 91%. Wild radish interference in canola was greatly affected by its time of emergence relative to canola. At 64 wild radish m−2, canola yield was reduced 77, 54, 33, and 19% in 1998 and 91, 65, 56, and 19% in 1999 when wild radish emerged 0, 2, 4, and 7 wk after canola, respectively. Wild radish that emerged 10 wk after canola did not reduce canola yield. Maximum wild radish seed production ranged from 24,183 to 32,167 seed m−2 when they emerged with canola at high densities. Wild radish that emerged later than canola produced much less seed, but some seed production still occurred in one of the 2 yr when it emerged as late as 10 wk after canola. Wild radish did not directly reduce canola quality in either year, but if wild radish seed were not separated from canola seed, the amount of erucic acid and glucosinolates was increased above marketable levels in some cases. The results of this study will be used to advise growers on wild radish control in canola and will aid the development of a multiyear management strategy for this troublesome weed in annual cropping systems.

Six vegetable crops were greenhouse-grown to maturity in three Maryland soils treated with up to 500 ppm of arsenic (As) added as sodium arsenate. Phytotoxicity of arsenic residues was highest on Lakeland loamy sand and lowest on Hagerstown silty clay loam. Correlations between available arsenic and plant dry weight indicated that crop sensitivity proceeded as follows: green beans (Phaseolus vulgaris L. ‘stringless greenpod’) > lima beans (Phaseolus linensis L. ‘Fordhook 242’) spinach (Spinacia oleracea L. ‘Longstanding Bloomsdale Savoy’) > radish (Raphanus sativus L. ‘Champion’) > tomato (Lycopersicon esculentum Mill. ‘F-7’) > cabbage (Brassica oleracea L. var. capitata L. ‘Jersey Wakefield’). Residues in the total dry plant at the available arsenic level at which growth was reduced 50% (GR50) were highest with radish (43.8 ppm) and spinach (10.0 ppm). Residues in the dry-edible portion increased to 76.0 ppm arsenic for unpeeled, washed radish at the GR50 point. The soil at the GR50 level for radish contained about 19 ppm of available arsenic from a 50 ppm arsenate treatment to Lakeland loamy sand and a 100 ppm treatment to Hagerstown silty clay loam and to Christiana clay loam.

Site properties and weed species abundance are known to vary spatially across fields. The extent to which they covary is not well understood. The objective of this research was to assess how canonical correlation analysis could be used to identify associations among site properties and weed species abundance within an agricultural field. A farmer-managed field rotated between Zea mays and Glycine max in Boone County, IA, was grid-sampled for site properties in 1992 and for weed species abundance between 1994 and 1997. Twelve site properties were considered in relation to five weed species that were identified and counted after all weed control operations were completed. Site properties such as total nitrogen, Bray-1 P, percent organic carbon, and texture were spatially variable. Weed species abundance was also spatially variable such that most weeds were found in patches and much of the field was weed-free. Canonical correlation analysis identified one to four significant correlations between linear combinations of site properties and weed species abundance. The first and second pairs of linear combinations explained the majority of variation in the data and were used to identify associations among site properties and weed species abundance. In years with Z. mays, the first pair of linear combinations described an association between herbicide activity and weed presence, and the second described topography and soil texture associations with weed presence. In years with G. max, the single observed association described a link between soil texture and presence of Setaria species and Polygonum coccineum. Several consistent associations were identified across years, indicating that site properties can influence weed abundance. However, annual variation in the associations may be attributed to differences in agronomic and weed management practices for each crop, as well as temporal weather variation influencing weed abundance from year to year. This multivariate technique is an important tool to identify associations between site properties and weed abundance that could help explain observed patchy patterns of weed abundance. These associations are an important first step in the generation of hypotheses to be tested at the whole field scale.

A population of common ragweed from Delaware was not controlled in the field by herbicides that inhibit acetolactate synthase (ALS) or protoporphyrinogen oxidase (PPO). Research was conducted to ascertain if this population was resistant to these herbicidal modes of action and, if so, to determine the resistance mechanism(s). Resistance was confirmed by dose-response studies on greenhouse-grown plants with multiple ALS- and PPO-inhibiting herbicides. DNA sequence data revealed that resistance to ALS-inhibiting herbicides was due to the previously reported W574L ALS mutation. To assist in determining the mechanism of resistance to PPO-inhibiting herbicides, an F2 population was derived from a cross between the resistant biotype (Del-R) and a sensitive biotype (DV1-S). This population segregated in the ratio of three resistant : one sensitive when treated with fomesafen, indicating that resistance to PPO-inhibiting herbicides was conferred by a single, (incompletely) dominant, nuclear gene. Sequences of the target-site genes, PPX1 and PPX2, for PPO-inhibiting herbicides were obtained through the screening of a common ragweed cDNA library and subsequent cDNA extension (5′-RACE). Molecular marker analysis with the F2 population revealed that the PPX2 gene cosegregated with resistance to PPO-inhibiting herbicides. A mutation substituting an arginine codon for a leucine codon at a conserved location (R98L) of the PPX2 gene was suspected of being responsible for resistance. By using a transgenic Escherichia coli system, it was demonstrated that the R98L mutation was sufficient to confer resistance to PPO-inhibiting herbicides. The level of resistance to acifluorfen conferred by the R98L mutation in the E. coli system was about 31-fold, similar to the level of resistance seen in the whole-plant dose-response study. Last, a DNA-based assay was developed to identify the presence or absence of the common ragweed PPX2 R98L mutation. The R98L PPX2 mutation is the second mechanism identified for evolved resistance to PPO-inhibiting herbicides.

Combine harvesters have the potential to disperse weed seeds great distances. Reducing this dispersal may be important in an integrated weed management system. The objectives of this study were to determine the distance that wild oat seeds are dispersed by a combine harvester and the effect of chaff collection on combine harvester seed dispersal. This was measured by sampling wild oat seeds at varying distances behind a combine equipped with a removable chaff collection system after it passed through a wild oat patch. Chaff collection consistently reduced the amount and distance that wild oat seeds were dispersed. This occurred because more than 74% of the total wild oat seed that were ejected from the combine were in the chaff. Because most of the chaff falls in a row directly behind the combine, chaff collection only affected dispersal in this area. In 1996, chaff collection reduced wild oat seed dispersal past the wild oat patch to less than 10 seeds m−2 at 45 m, whereas without chaff collection, there was greater than 10 seeds m−2 up to 145 m. At distances beyond 145 m, chaff collection had no significant effect on seed dispersal. Chaff collection may be an important tool in an integrated weed management program because it may slow weed invasions and reduce the expansion of weed patches.

The concept of herbicide partition into adsorbed and solution phases in the soil was utilized to predict the effect of soil water content on the concentration of herbicide in solution. Calculated predictions based on measured partition coefficients and water contents were tested experimentally by equilibrating 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine) with soil from three horizons of a latosolic soil at four water contents (45 to 60%). The effects of adsorption and water content were predicted with reasonable accuracy when only that water held at tensions less than 15 bars was considered available for equilibration with the applied herbicide. Only on low adsorption soils will water content variations alter significantly herbicide concentration in the soil solution. The principal effect of soil water content on herbicide phytotoxicity probably is associated with herbicide transport, which is more sensitive to changes in water content than is the concentration of herbicide in soil solution.

A suspected glyphosate-resistant (R) junglerice population was collected from a glyphosate-R corn field near Durham in northern California where glyphosate had been applied at least twice a year for over 6 yr. Based on the amount of glyphosate required to reduce growth by 50% (ED50), the R population was 6.6 times more R than the susceptible (S) standard population. Based on the glyphosate concentration that inhibits EPSPS by 50% based on shikimate accumulation (I50) in leaf discs, R plants were four times more R than S plants. By 3 d after treatment with 0.42 kg ae ha−1 glyphosate, the S population had accumulated approximately five times more shikimate than the R population. No differences in [14C]-glyphosate uptake and translocation were detected between R and S plants. However, partial sequencing of the EPSPS gene revealed a mutation in R plants causing a proline to serine change at EPSPS position 106 (P106S). Our results reveal the first case of a P106S target site mutation associated with glyphosate resistance in junglerice.

Control of early-emerging weeds is essential to protect the yield potential of maize. An understanding of the physiological changes that occur as a result of weed interference is required to address variability in yield loss across sites and years. Field trials were conducted at the University of Guelph (UG), the Ohio State University (OSU), and Colorado State University (CSU) during 2009 and 2010. There were six treatments (season-long weedy and weed-free, and weed control at the 1st-, 3rd-, 5th-, and 10th-leaf-tip stages of maize development) and 20 individual plants per plot were harvested at maturity. We hypothesized that, as weed control was delayed, weed interference in the early stages of maize development would increase plant-to-plant variability in plant dry-matter accumulation, which would result in a reduction of grain yield at maturity. The onset of the critical period for weed control (CPWC) occurred on average between the third and fifth leaf tip stages of development (i.e., V1 to V3, respectively). Rate of yield loss following the onset of the CPWC ranged from 0.05 MG ha−1 d−1 at UG 2009 to 0.22 MG ha−1 d−1 at CSU 2010 (i.e., 0.5 and 1.6% d−1, respectively). On average, reductions in kernel number per plant accounted for approximately 65% of the decline in grain yield as weed control was delayed. Biomass partitioning to the grain was stable through early weed removal treatments, increased and peaked at the 10th-leaf-tip time of control, and decreased in the season-long weedy treatment. Plant-to-plant variability in dry matter at maturity and incidence of bareness increased as weed control was delayed. As weed control was delayed, the contribution of plant-to-plant variability at maturity to the overall yield loss was small, relative to the decline of mean plant dry matter.

A survey of soybean fields containing waterhemp was conducted just prior to harvest in 2012 to determine the scope and extent of herbicide resistance and multiple herbicide resistances among a sample of Missouri waterhemp populations. Resistance was confirmed to glyphosate and to acetolactate synthase (ALS), protoporphyrinogen oxidase (PPO), photosystem II (PSII), and 4-hydroxyphenylpyruvate dioxygenase (HPPD) inhibitors, but not to 2,4-D. Of the 187 populations tested, 186 exhibited resistance to chlorimuron. The proportions of populations with atrazine or glyphosate resistance were similar, with 30 and 29% of the populations surviving the 3× rates. Lactofen resistance was observed in 5% of the populations, whereas mesotrione resistance was only found in 1.6% of the populations. All populations tested were susceptible to 2,4-D at the 3× rate. At least 52% of the waterhemp populations tested exhibited resistance to herbicides from two mechanism of action. Resistance to atrazine plus chlorimuron as well as glyphosate plus chlorimuron was present in 29% of the populations. Three-way resistance, primarily comprised of resistance to atrazine plus chlorimuron plus glyphosate, was present in 11% of the populations. Resistance to herbicides from four mechanisms of action was found in 2% of the populations, and one population exhibited resistance to herbicides from five mechanisms of action. DNA analysis of a subsample of plants revealed that previously documented mechanisms of resistance in waterhemp, including the ΔG210 deletion conferring PPO-inhibitor resistance, the Trp574Leu amino acid substitution conferring ALS-inhibitor resistance, and elevated 5-enolypyruvyl-shikimate-3-phosphate synthase copy number and the Pro106Ser amino acid substitution resulting in glyphosate resistance, explained survival in many, but not all, instances. Atrazine resistance was not explained by the Ser264Gly D1 protein substitution. Overall, results from these experiments indicate that Missouri soybean fields contain waterhemp populations with resistance to glyphosate, ALS-, PPO-, PSII-, and HPPD-inhibiting herbicides, which are some of the most common mechanisms of action currently utilized for the control of this species in corn and soybean production systems. Additionally, these results indicate that slightly more than half of the populations tested exhibit resistance to more than one herbicide mechanisms of action. Managing the current resistance levels in existing populations is of utmost importance. The use of multiple, effective herbicide modes of action, both preemergence and postemergence, and the integration of optimum cultural and mechanical control practices will be vital in the management of Missouri waterhemp populations in the future.

Much of agriculture-related research today involves weed resistance to herbicides. Resistance evolution is perhaps the strongest driver for the quest for new herbicide targets, novel weed intervention technologies, and the promotion of best management practices for sustainable crop production (Burgos et al., 2006; Norsworthy et al. 2012; Vencill et al. 2012). To date, 222 weedy species collectively have evolved resistance to 150 herbicides representing 21 sites of action (Heap 2014). For decades, scientists have developed numerous protocols for resistance confirmation using seeds, different plant parts, or whole plants. These have been reviewed by Beckie et al. (2000) and Burgos et al. (2013). We draw from these and other sources to present general guidelines for resistance confirmation that students and new researchers can use in planning their experiments. The most immediate questions that stakeholders seek to answer with resistance bioassays include:

1. 1. Is the population resistant?

2. 2. What is the level of resistance?

3. 3. What alternative herbicides can be used?

Competitiveness of three high-yielding, lodging-resistant rice (Oryza sativa L.) cultivars with barnyardgrass [Echinochloa crus-galli (L.) Beauv.] was compared during 3 years at Stuttgart, Arkansas. ‘Starbonnet,’ ‘Nova 66,’ and ‘Bluebelle,’ which matured in 145, 135, and 125 days, respectively, competed with barnyardgrass for periods ranging from 10 days after crop emergence to all season. ‘Starbonnet’ was the most competitive and ‘Bluebelle’ was the least. Barnyardgrass competition for 10 or 20 days did not reduce grain yields of any cultivar. Weed competition for 40 days or longer reduced yields of ‘Nova 66’ and ‘Bluebelle,’ and competition for 60 days or longer lowered yields of ‘Starbonnet.’ As the period for maturity increased, each cultivar competed more effectively with barnyardgrass.

Morphological and physiological variation among annual bluegrass populations from the green, fairway, and rough were measured to determine the role annual bluegrass infestations outside the green play in maintaining the population in the green. Annual bluegrass populations from the fairway and the rough were similar to each other but differed from that of the green in habit, dry mass production, flowering, seed size, and germination. Seeds from each population established best in the type of turf from which they originated. It is concluded that, since few genotypes were common to the green and its surroundings, the populations in the fairway and rough played little or no role in maintaining the population of the green. The same is likely to be true wherever the management of greens and their surroundings differs sufficiently for marked population differentiation to occur. This finding does not preclude the possibility that some genotypes adapted to the green persist in the surroundings, and that these genotypes serve as sources of seeds for the colonization or reinfestation of annual bluegrass-free greens.

Ford soybean yields and weed yields were studied under combinations of tillage treatments, 3-amino-2,5-dichlorobenzoic acid (amiben) applications, hand weedings, and row spacings on irrigated and non-irrigated locations in Nebraska during 1961 and 1962. Soybean yields increased and weed yields decreased with tillage, amiben applications, hand weedings, and narrow-row spacing of soybeans. Combinations of these weed control treatments gave more dependable results than any one alone. Each 86 lb/A of weeds present produced an average soybean yield reduction of 1 bu/A. Advantages of growing soybeans with amiben applications in 10-inch rows as compared to 40-inch rows are: (a) an average soybean yield increase of 39%, (b) elimination or reduction of tillage (c) a 50 to 75% reduction in amiben application rate, and (d) little or no soybean injury from the lower rates of amiben required in the 10-inch rows.

Field experiments were conducted to determine the influence of time of emergence and density of single and multispecies populations of pigweed and barnyardgrass on soybean yield and competitive abilities of pigweed and barnyardgrass. Pigweed and barnyardgrass were established at selected densities within 12.5 cm on either side of the soybean row. Pigweed and barnyardgrass seeds were sown concurrently with soybean and at the cotyledon stage of soybean growth. Time and density of pigweed and barnyardgrass seedling emergence relative to soybean influenced the magnitude of soybean yield loss. Maximum soybean yield loss ranged from 32 to 99%, depending upon time of emergence relative to soybean. Pigweed was more competitive than barnyardgrass across all locations, years, and time of weed emergence. When pigweed was assigned a competitive index of 1 on a scale from 0 to 1, the competitive ability of barnyardgrass ranged from 0.075 to 0.40 of pigweed, depending upon location and time of emergence. This is the first multiple weed species study to include time of weed emergence relative to the crop. Competitive index values for multiple weed species must be calculated from field experiments in which weeds are grown with the crop under differing environmental conditions.

The C4 weed johnsongrass [Sorghum halepense (L.) Pers. ♯3 SORHA] and the C3 crop soybean [Glycine max (L.) Merr. ‘Ransom’] were grown separately and in inter- and intraspecific competition at 350 and 675 ppm CO2 in controlled environment chambers with 29/23 C day/night temperatures and 950 μE·m-2 · s-1 PPFD (photosynthetic photon flux density). In the absence of competition, the higher CO2 concentration stimulated dry matter accumulation, leaf area expansion, net assimilation rate, and leaf area duration of soybean more than that of johnsongrass. The plant relative yield (PRY) of soybean in competition with johnsongrass increased, and the PRY of johnsongrass in competition with soybean decreased, as the CO2 concentration was increased from 350 to 675 ppm. Thus, the competitiveness of the C3 crop with the C4 weed increased with increasing CO2 concentration. Relative yield totals were not significantly different from 1.0, indicating that the two species were competing for the same resources. With the increases in global atmospheric CO2 concentration predicted for the next 50 to 100 yr, the competitiveness of C3 crops with C4 weeds could be increased.

The evolution of glyphosate resistance in weedy species places an environmentally benign herbicide in peril. The first report of a dicot plant with evolved glyphosate resistance was horseweed, which occurred in 2001. Since then, several species have evolved glyphosate resistance and genomic information about nontarget resistance mechanisms in any of them ranges from none to little. Here, we report a study combining iGentifier transcriptome analysis, cDNA sequencing, and a heterologous microarray analysis to explore potential molecular and transcriptomic mechanisms of nontarget glyphosate resistance of horseweed. The results indicate that similar molecular mechanisms might exist for nontarget herbicide resistance across multiple resistant plants from different locations, even though resistance among these resistant plants likely evolved independently and available evidence suggests resistance has evolved at least four separate times. In addition, both the microarray and sequence analyses identified non–target-site resistance candidate genes for follow-on functional genomics analysis.

Combinations of row spacings, tillage, and herbicides were evaluated on Clark soybeans grown on Mexico silt loam. Weeds were not always completely controlled with 3-amino-2,5-dichlorobenzoic acid (amiben) or sodium pentachlorophenate (PCP). When herbicides were used, soybeans in 20- and 24-inch rows usually needed no more than one cultivation, while those in 32- and 40-inch rows usually needed at least one and sometimes two cultivations for good weed control and high soybean yields. Soybeans in narrow rows always equalled and sometimes produced higher yields than those in wide rows. The soybean canopy covered the ground more rapidly in narrow rows than in wide rows so that when herbicides suppressed early weed growth, less weeds were produced in narrow rows than in wide rows. In some situations, cultivations increased soybean yields when few or no weeds were present.

The present paper is supplementary to one previously published which received more comment than had been expected. In the meantime Salisbury's book appeared in which reference was made to the paucity of data on the subject and many records from the previous paper were quoted. Salisbury made statistical and ecological studies of various species in Great Britain.

Pot experiments were conducted in the field at Stuttgart, AR, during 1982 and 1983 to evaluate growth and morphological differences between strawhull and blackhull red rice (Oryza sativa L. ♯ ORYSA) biotypes collected from Arkansas, Louisiana, and Texas. All red rice biotypes were compared with rice (Oryza sativa L.) cultivars ‘Lebonnet’ and ‘Nortai’. Growth and morphological differences were greatest between cultivars and red rice biotypes, less between blackhull and strawhull types, and least among collections within blackhull or strawhull. Cultivars emerged slower, were shorter, tillered less, produced less straw and fewer panicles/plant, had a lower leaf area index, and had less grain shattering than most of the red rice biotypes. Blackhull red rice biotypes tillered 27% more, produced 18% more straw, and had later maturity than strawhull. Blackhull red rice from Arkansas emerged earlier, tillered 6 to 38% more, and produced 8 to 38% more panicles per plant than other red rice biotypes, whereas blackhull red rice from Texas was 11 to 26% taller at maturity than other biotypes.

A greenhouse experiment was conducted to evaluate the herbicidal activity of five aliphatic (ethyl, propyl, butyl, allyl, and 3-methylthiopropyl) and three aromatic (phenyl, benzyl, and 2-phenylethyl) isothiocyanates (ITCs) on Palmer amaranth, pitted morningglory, and yellow nutsedge. All ITCs were applied to soil at 0, 10, 100, 1,000, and 10,000 nmol g−1 of soil and incorporated. All ITCs had a deleterious effect on Palmer amaranth and pitted morningglory emergence. LC50 values for Palmer amaranth emergence inhibition from aliphatic and aromatic ITCs ranged from a low of 32 nmol g−1 of soil for phenyl ITC to a high of 941 nmol g−1 of soil for propyl ITC. Pitted morningglory was slightly more tolerant than Palmer amaranth to each of the ITCs, with LC50 values for emergence ranging from 347 to 2,855 nmol g−1 of soil for 3-methylthiopropyl and butyl ITC, respectively. Yellow nutsedge was the most tolerant of the three species, with LC50 values for ethyl, butyl, benzyl, and 2-phenylethyl being greater than the highest evaluated concentration of 10,000 nmol g−1 of soil. Phenyl and 3-methylthiopropyl at 10,000 nmol g−1 of soil were the most effective ITCs against yellow nutsedge, reducing emergence by 92%. Effectiveness of the ITCs varied across structure and species, but 3-methylthiopropyl and phenyl ITC were generally the most efficacious for the three species evaluated.