Yellow nutsedge is one of the most problematic weedy sedges in rice–soybean systems of the Mississippi Delta region. An acetolactate synthase (ALS)-inhibiting, herbicide-resistant (Res) yellow nutsedge biotype was recently documented in eastern Arkansas, which showed intermediary growth habit between yellow nutsedge and purple nutsedge and also exhibited differential photoperiodic sensitivity to flowering. The objectives of this study were to: (a) determine variation in reproductive characteristics of the Res biotype and three susceptible (Sus) yellow nutsedge biotypes, (b) understand the influence of photoperiod on growth and reproduction, (c) understand the potential role of seeds in population establishment, and (d) elucidate the phylogenetic relationships between the Res yellow nutsedge biotype and purple nutsedge. Tuber production per plant and tuber weight of the Res biotype were less than that of the Sus biotypes. Differences in quantitative traits, such as shoot and tuber production existed between the Res and Sus biotypes for photoperiods ranging from 12 to 16 h. Generally, photoperiods greater than 12 h increased shoot development in all yellow nutsedge biotypes, with differential responses among the biotypes. Number of tubers reached the maximum for the Res biotype at a 14-h photoperiod. Over a 90-d period, inflorescence formation was only observed in the Res biotype with maximum flowering and seed production in the 14-h photoperiod. Subsequent tests revealed up to 18% seed germination, suggesting that seed could also play a role (in addition to tubers) in the persistence and spread of the Res yellow nutsedge. Phylogenetic analysis based on ribosomal DNA internal transcribed spacer (ITS) regions and mitochondrial nad4 gene intergenic spacer sequences indicated that the Res biotype was more closely associated with Sus yellow nutsedge biotypes. Nevertheless, 100% similarity for the nad4 gene sequences between the Res yellow nutsedge biotype and a reference purple nutsedge suggests that the Res biotype is likely a result of hybridization between yellow and purple nutsedges, which perhaps explains the intermediary growth characteristics observed in the Res biotype.

Due to the looming water and labor crisis, farmers are adopting dry-seeded rice establishment, in which Chinese sprangletop is becoming a major weed. Concerns about the excessive use of herbicides in controlling Chinese sprangletop highlight the need for cultural weed management strategies. Such strategies require an adequate understanding of Chinese sprangletop response to rice plant density, nutrition, and water regime. Therefore, a greenhouse study was conducted to determine the effect of water regime (aerobic and saturated), nitrogen (N) fertilization (50 and 150 kg ha−1), and rice density (0, 160, and 640 plants m−2) on the growth and reproduction of Chinese sprangletop. Chinese sprangletop plants were taller than rice in aerobic conditions than in saturated conditions. All growth parameters (shoot, root, and inflorescence biomass and leaf area, leaf weight, and inflorescence weight ratio) of Chinese sprangletop were higher in aerobic conditions than in saturated conditions when grown without rice. However, no difference was observed for these parameters between water regimes when Chinese sprangletop was grown with rice. Chinese sprangletop growth and seed production was not affected, but rice growth was affected by N rates. Irrespective of N rate and water regime, Chinese sprangletop height (34 to 59%), tiller number (87 to 92%), leaf number (83 to 89%), shoot biomass (93 to 99%), and inflorescence biomass (95 to 99%) decreased as rice density increased from 0 to 640 plants m−2. The ability of Chinese sprangletop to grow taller and produce more plant biomass (107%) and inflorescence biomass (183%) under aerobic than saturated conditions suggests the need for integrated weed management strategies for controlling weeds under water-limited environments. Such strategies might include the use of weed-competitive and drought-tolerant rice cultivars, high seeding rates, and optimum rate of fertilizer application.

Johnsongrass is a common weed of corn in Chile, which is most often controlled by nicosulfuron, an acetohydroxyacid synthase (AHAS)-inhibiting herbicide. Recurrent nicosulfuron use has resulted in selection for resistant johnsongrass biotypes. We conducted studies to determine nicosulfuron resistance levels in two johnsongrass biotypes from Chile and to investigate if this resistance was target-site mediated. Whole-plant resistance to nicosulfuron was 33 and 46 times higher in resistant (R) than in susceptible (S) plants grown from seed and rhizomes, respectively. The nicosulfuron concentrations for 50% inhibition of AHAS enzyme activity in vitro were more than 11 times higher in R than in S plants. Sequencing analysis of the AHAS coding sequence revealed a Trp-574-Leu substitution in both R biotypes. This study shows that resistance to nicosulfuron in the two R biotypes is conferred by an altered target site. We also report the first consensus sequence of the johnsongrass AHAS gene corresponding to the known mutation sites conferring resistance to AHAS-inhibiting herbicides.

Glyphosate-resistant giant ragweed has been confirmed in several Midwestern states. In some cases, weed resistance to glyphosate has been shown to carry a fitness penalty. Previous research has found that a glyphosate-resistant giant ragweed biotype from Indiana with a rapid necrosis response to glyphosate displayed early, rapid growth in the absence of glyphosate, flowered earlier, but produced 25% less seed than a sensitive biotype, suggesting that there may be a fitness penalty associated with the rapid necrosis resistance trait. In Wisconsin, we have recently identified a giant ragweed accession with a 6.5-fold level of resistance to glyphosate that does not demonstrate the rapid necrosis response. Our objective was to determine the noncompetitive growth and fecundity of the resistant accession in the absence of glyphosate, relative to a sensitive accession from a nearby field border population. In greenhouse experiments, plant height, leaf area, and dry shoot biomass were similar between the resistant and sensitive accessions during vegetative growth to the onset of flowering. The instantaneous relative growth rate, instantaneous net assimilation rate, and instantaneous leaf area ratio also did not differ between accessions. However, fecundity of resistant plants (812 seeds plant−1) was greater (P = 0.008) than sensitive plants (425 seeds plant−1). The percentage of intact viable seeds, intact nonviable seeds, and empty involucres did not differ between resistant and sensitive accessions. These results indicate that resistance of this accession of giant ragweed to glyphosate has not affected its growth and development relative to a sensitive accession. The greater fecundity and similar viability of resistant plants relative to sensitive plants suggests that in the absence of selection by glyphosate, the frequency of the resistance trait for glyphosate may increase in the giant ragweed field population over time.

Herbicide degradation in soil is a major research issue, as evidenced by the number of refereed articles on the subject (Figure 1). The approximate number of total citations per year has increased from about 70 in the mid-1990s to approximately 170 per year in the last few years (Figure 1a). From a weed science perspective, publications in the journals Weed Science and Weed Technology have tended to decline from an average of eight per year in the 1995 to 2005 interval to about three per year in the last 10 yr (Figure 1b). This discrepancy of total citations vs. Weed Science Society of America journals may be due to funding availability and other more immediate research needs in weed science. Other reasons might be a lack of reader interest (indicating low potential impact in this specific topic area) or the perception that Weed Science and Weed Technology are light venues for such papers and therefore not the first choice for publication. The authors believe this research topic to be important and relevant to the discipline of weed science, even more so as herbicide use patterns become more complicated because of glyphosate-resistant (GR) weeds.

Japanese brome is a winter annual weed commonly found in wheat fields in China. Laboratory and greenhouse experiments were carried out to determine the effect of temperature, light, pH, osmotic stress, salt stress, and burial depth on the germination and emergence of Japanese brome. Germination was greater than 98% under a wide temperature range of 5 to 30 C and onset of germination was shortened as temperature increased. Light was not required for germination to occur and pH values from 5 to 10 had insignificant effect on germination. Germination was reduced by osmotic stress or salt stress and no germination occurred at −1.3 MPa or 360 mM, suggesting that Japanese brome seed was quite tolerant to osmotic potential and salinity. Seedling emergence was greatest (98%) when seeds were placed on the soil surface but decreased with increasing of burial depth. Only 7% of seedlings emerged at a depth of 5 cm. The results of this study have contributed to our understanding of the germination and emergence of Japanese brome and should enhance our ability to develop better control strategies in wheat farming systems of the Huang-Huai-Hai Plain of China.

Field experiments were conducted near Boone, IA, to quantify postdispersal seed predation of common lambsquarters and common waterhemp in corn (2003) and soybean (2004) managed with conventional, reduced, and zero-tillage systems. Seed predation in each tillage regime was quantified using selective exclusion treatments during July through September 2003 and June through October 2004. In addition, the activity density of ground-dwelling invertebrates was estimated with pitfall traps. Choice and no-choice feeding trials were conducted in the laboratory using the most abundant weed seed predators found in the field to determine seed preferences of the potential predator organisms. The greatest seed loss occurred during July and August. In 2003, seed predation was lower in zero tillage than in conventional and reduced tillages, but no differences in seed predation between tillage regimes were observed in 2004. Maximum seed predation for common lambsquarters was 53% in 2003 and 64% in 2004. Common waterhemp seed predation reached 80% in 2003 and 85% in 2004. The majority of seed predation was by invertebrate organisms. The most common invertebrate species captured with pitfall traps were field crickets (Gryllus pennsylvanicus De Geer [Gryllidae, Orthoptera]) and ground beetles (Harpalus pensylvanicus Burmeister [Coleoptera, Carabidae]). In 2003, field crickets were relatively more abundant in conventional and reduced tillage than in zero-tillage plots. In 2004, field crickets were more abundant in the reduced tillage than in the other two tillage regimes. No differences were detected for ground beetles among tillage regimes (P = 0.57). Choice and no-choice feeding experiments confirmed the preferences of field crickets and ground beetles for common lambsquarters and common waterhemp seeds over the larger seeds of giant foxtail and velvetleaf. Under field conditions, the activity density of field crickets was a significant predictor of common lambsquarters (r2 = 0.47) and common waterhemp (r2 = 0.53) seed predation. Positive relationships were also detected between the activity density of ground beetles and common lambsquarters (r2 = 0.30) and common waterhemp (r2 = 0.30) seed predation. This research demonstrated that weed seed predation is an important component affecting weed seedbanks and that crop management practices that favor the activity of predators such as field crickets or ground beetles could influence weed populations. Also, the results suggested that tillage is more important in determining the number of weed seeds available on the soil surface to predators than directly affecting predator activity density.

Crop yields can be similar in organic and conventional systems even when weed biomass is greater in organic systems. Greater weed tolerance in organic systems may be due to differences in management-driven soil fertility properties. The goal of this experiment was to determine whether soil collected from a long-term organic cropping system with a diverse crop rotation and organic fertility inputs would support higher soil nitrogen (N) resource partitioning, as indicated by overyielding of corn–weed mixtures, than a cropping system with a less diverse crop rotation and inorganic N inputs. A replacement series greenhouse experiment was conducted using corn : smooth pigweed and corn : giant foxtail proportions of 0 : 1, 0.25 : 0.75, 0.5 : 0.5, 0.75 : 0.25, and 1 : 0 and harvested at 29, 40, or 48 d after experiment initiation (DAI). The monoculture density of corn was 4 plants pot−1 and the monoculture density of each weed species was 36 plants pot−1. Corn was consistently more competitive than both weed species at 40 and 48 DAI when soil inorganic N was limiting to growth. Corn–smooth pigweed mixtures had greater shoot biomass and shoot N content than expected based on the shoot biomass and shoot N content of monocultures (i.e., overyielding) at the onset of soil inorganic N limitation, providing some evidence for N resource partitioning. However, soil management effects on overyielding were infrequent and inconsistent among harvest dates and corn–weed mixtures, leading us to conclude that management-driven soil fertility properties did not affect corn–weed N resource partitioning during the early stages of corn growth.

Cover crops represent a potentially important biological filter during weed community assembly in agroecosystems. This filtering could be considered directional if different cover-crop species result in weed communities with predictably different species composition. We examined the following four questions related to the potential filtering effects of cover crops in a field experiment involving five cover crops grown in monoculture and mixture: (1) Do cover crops differ in their effect on weed community composition? (2) Is competition more intense between cover crops and weeds that are in the same family or functional group? (3) Is competition more intense across weed functional types in a cover-crop mixture compared with cover crops grown in monocultures? (4) Within a cover-crop mixture, is a higher seeding rate associated with more effective biotic filtering of the weed community? We found some evidence that cover crops differentially filtered weed communities and that at least some of these filtering effects were due to differential biomass production across cover-crop species. Monocultures of buckwheat and sorghum–sudangrass reduced the number of weed species relative to the no-cover-crop control by an average of 36 and 59% (buckwheat) and 25 and 40% (sorghum–sudangrass) in 2011 and 2012, respectively. We found little evidence that competition intensity was dependent upon the family or functional classification of the cover crop or weeds, or that cover-crop mixtures were stronger assembly filters than the most effective monocultures. Although our results do not suggest that annual cover crops exert strong directional filtering during weed community assembly, our methodological framework for detecting such effects could be applied to similar future studies that incorporate a greater number of cover-crop species and are conducted under a greater range of cover-cropping conditions.

2,4-D, discovered independently in the United States and Europe in the mid-1940s, was one of the first synthetic herbicides to be used selectively for weed control (Cobb and Reade 2010). Since then, several herbicides belonging to different chemical classes and possessing diverse mechanisms of action have been synthesized and marketed globally. Herbicides have vastly contributed to increasing world food, fiber, fuel, and feed production in an efficient, economic, and environmentally sustainable manner. Before receiving regulatory approval, all herbicides (pesticides) undergo rigorous testing for their toxicological, residual, physicochemical, and biological properties. Additionally, herbicides are suitably formulated to reach their target site and maximize their efficacy on target weeds while being safe on crops.

Prevention programs are often assumed to be the most cost-effective method for managing invasive plants. However, there is very little information available about economic and biological factors that determine the forage benefits resulting from prevention programs. We developed an easy to use economic model to assess potential savings in livestock forage that might result from implementing prevention programs. The model can be used to determine potential loss in forage production caused by invasive plants and to estimate potential income savings by preventing invasive plant infestations. The model compares a prediction of populations with and without a prevention program using a logistic growth function. Animal unit month (AUM) price and interest rates are the primary economic input variables. The primary biological input variables are amount of invasive plant utilization, size of the initial infestation, and the spread rate with and without prevention. Our model suggests that as the AUM price increases and/or the interest rate decreases, the total savings increases for each AUM that was protected through a prevention program. The model also shows savings per AUM increases as the size of the initial infestation decreases, suggesting that prevention should focus on eliminating seed sources and seed production early in the program. Using our model inputs, the savings per AUM was about $9.20 for each percent reduction in spread rate over 100 yr.

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?

Weedy red rice is a major weed pest of rice in the southern United States. Outcrossing between red rice and commercial tropical japonica rice cultivars has resulted in new weed biotypes that further hinder the effectiveness of weed management. In recent years, indica rice has been used increasingly as a germplasm source for breeding and for reduced-input systems in the United States, but little is known about its outcrossing potential with U.S. weedy red rice biotypes. In a 2-yr study, simple sequence repeat marker analysis was used to show that blackhull (BH) red rice (PI 653424) outcrossing to four, late-maturing indica cultivars averaged 0.0086% and ranged from 0.002% for ‘TeQing’ to 0.0173% for ‘4484’ (PI 615022). Rates of outcrossing to a tropical japonica cultivar standard, ‘Kaybonnet’ (0.032%), were substantially greater than for the indica cultivars. These differences in outcrossing were due largely to synchronization of flowering times between rice and red rice, with Kaybonnet and TeQing exhibiting the greatest and least synchronization, respectively. Outcrossing rates also may have been affected by rice–red rice flower density differences within the rice plots. Outcrossing from cultivated rice to the red rice (as pollen recipient), which was taller than all rice cultivars, was undetectable in these studies, and environmental conditions (e.g., temperature, humidity, solar radiation, and rainfall) were not strongly correlated with the outcrossing rates observed. Grain yields of the original BH red rice line were greatest in the Kaybonnet plots, demonstrating that the indica cultivars were superior competitors against this weed. Collectively, these results suggest that red rice biotypes that flower synchronously with rice cultivars are a potential source of pollen for outcrossing and gene flow in rice fields in the southern United States.

Insufficient weed control is a major constraint to adoption of reduced-tillage practices for organic grain production. Tillage, cover crop management, and crop planting date are factors that influence emergence periodicity and growth potential of important weed species in these systems. We assessed two hairy vetch cover crop management practices, disk-kill and roll-kill, across a range of corn planting dates from early May to late June in three experiments in Beltsville, MD. Patterns of seed dormancy, emergence, and early weed growth were determined for overseeded populations of common ragweed, giant foxtail, and smooth pigweed, three important species in the Mid-Atlantic states that represent early to late emergence. Common ragweed emergence was lowest and dormancy was highest of the three species across all planting dates. Giant foxtail emergence was higher than the other species in roll-killed hairy vetch and included a significant number of seeds that germinated before rolling operations in late June. Smooth pigweed had the highest emergence and lowest dormancy in disk-killed hairy vetch in June. Individual giant foxtail plant weight was higher in roll-killed than disk-killed hairy vetch in 2 of 3 yr, whereas that of smooth pigweed plants was higher in disk-killed than roll-killed vetch in 2 of 3 yr. Giant foxtail was the dominant species in roll-killed hairy vetch (averaged 79% of total weed biomass at corn silking), probably because of early germination and establishment before rolling operations. Smooth pigweed was the dominant species in disk-killed hairy vetch at June planting dates (averaged 77% of total weed biomass), probably because of high growth rates under warm conditions in tilled soil. This research demonstrated that cover crop management practices and the timing of planting operations can shift the dominant species of weed communities in organic farming systems and must be considered in long-term weed management planning.

Molecular assays are often implemented by weed scientists for detection of herbicide-resistant individuals; however, the utility of these assays can be limited if multiple mechanisms of evolved resistance exist. Waterhemp resistant to protoporphyrinogen oxidase (PPO)– inhibiting herbicides is conferred by a target-site mutation in PPX2L (a gene coding for PPO), resulting in the loss of a glycine at position 210 (ΔG210). This ΔG210 mutation of PPX2L is the only known mechanism responsible for PPO-inhibitor resistance (PPO-R) in waterhemp from five states (Illinois, Indiana, Iowa, Kansas, and Missouri); however, a limited number of populations have been tested, especially in Illinois. To verify the ubiquity of the ΔG210 in PPO-R waterhemp populations in Illinois, a previously published allele-specific PCR (asPCR) was used for the detection of the ΔG210 mutation to associate this mutation with phenotypic resistance in 94 Illinois waterhemp populations. The ΔG210 mutation was detected in all populations displaying phenotypic resistance to lactofen (220 g ai ha−1), indicating the deletion is likely the only mechanism of resistance. With evidence that the ΔG210 mutation dominates PPO-R waterhemp biotypes, molecular detection techniques have considerable utility. Unfortunately, the previously published asPCR is time consuming, very sensitive to PCR conditions, and requires additional steps to eliminate the possibility of false negatives. To overcome these limitations, a streamlined molecular method using the TaqMan® technique was developed, utilizing allele-specific, fluorescent probes for high-throughput, robust discrimination of each allele (resistant and susceptible) at the 210th amino acid position of PPX2L.

There are various reasons for using statistics, but perhaps the most important is that the biological sciences are empirical sciences. There is always an element of variability that can only be dealt with by applying statistics. Essentially, statistics is a way to summarize the variability of data so that we can confidently say whether there is a difference among treatments or among regression parameters and tell others about the variability of the results. To that end, we must use the most appropriate statistics to get a “correct” picture of the experimental variability, and the best way of doing that is to report the size of the parameters or the means and their associated standard errors or confidence intervals. Simply declaring that the yields were 1 or 2 ton ha−1 does not mean anything without associated standard errors for those yields. Another driving force is that no journal will accept publications without the data having been subjected to some kind of statistical analysis.

Field studies were conducted to determine the effect of emergence timing on the fitness of the next generation as represented by seed mass, maturation, and afterripening of common waterhemp cohorts. Five natural cohorts were documented both in 2009 and 2010. Different maternal environments resulting from varied cohort emergence timings did not influence seed maturation time and seed mass, but had an inconsistent effect on seed afterripening. Here are our major findings. (1) Waterhemp cohorts needed similar amounts of time to generate viable seeds (20 to 27 d after flower initiation) and the seeds produced were of similar size (2.0 to 2.35 g), and (2) waterhemp has strong primary dormancy that may be released within 4 mo during the afterripening process, depending on the dormancy level. Seeds produced by later cohorts were more sensitive to the afterripening period, suggesting more flexibility in life strategy. Seeds from the 2009 cohorts had similar afterripening patterns; newly harvested seeds had strong primary dormancy (<10% germination), which was gradually released during dry storage and reached the maximum germination (>80%) rate 4 mo after harvest (MAH). However, germination then dropped to 40% 6 and 8 MAH, suggesting the induction of secondary seed dormancy. Strong primary dormancy at harvest for 2010 seeds was sustained in dry afterripening, perhaps because of higher dormancy level, which was the result of less-favorable parental environments brought by 10 to 30 times higher population densities and 2.5 to 5 times higher accumulative precipitation than in 2009 (see Wu and Owen 2014). We also tested the soil seed-bank seed population densities for each waterhemp cohort and found that early cohorts greatly influenced the seed population densities at the soil surface level and the turnover rate of the soil seed bank. Results from this research will provide insights into better management of waterhemp, targeting a better understanding of the seed bank.

This study was conducted in the laboratory and screenhouse to determine the effects of temperature, light, osmotic stress, salt stress, burial depth, use of crop residues as mulch, depth of flooding, and use of POST herbicides on the emergence, survival, and growth of doveweed. In the light/dark regime, germination was higher at alternating day/night temperatures of 35/25 C (95%) than at 30/20 C (72%), and no germination occurred at 25/15 C. Light strongly influenced germination (95%) and dark completely inhibited germination. No germination occurred at an osmotic potential of −0.8 MPa and a salt concentration of 150 mM and above. The highest germination (91%) was observed from the seeds sown on the soil surface and emergence decreased by 78, 86, and 92% when burial depths were increased to 0.5, 1, and 2 cm, respectively. No seedlings emerged from seeds buried at depths of more than 2 cm. The use of rice residues as mulch significantly reduced the emergence and growth of doveweed seedlings. The amount of residue required to suppress 50% of the maximum biomass was 2.5 t ha−1. Flooding had a more pronounced effect on seedling biomass than seedling emergence. Biomass was reduced by 78, 92, and 96% when flooding depths increased from 0 to 2, 4, and 6 cm, respectively, for the seeds placed on the soil surface, whereas for the seeds buried at 0.5 cm, these values were 78, 100, and 100%. Bentazon (100 g ha−1) and bispyribac-sodium (30 g ha−1) provided 100% control of doveweed when applied at the three-leaf stage. Doveweed control was less than 31% with glyphosate rates up to 2,000 g ha−1. The application of 2,4-D (500 g ha−1) provided 100% control of doveweed even when applied at the seven-leaf stage. The information from this study could help in developing more sustainable and effective integrated weed management strategies for the control of this weed and weeds with similar response in dry-seeded rice systems.

Palmer amaranth, a dioecious summer annual forb, originating in Sonoran desert washes, compromises crop yields in much of the southern United States and its range is expanding northward. Appropriate tactics for managing this weed proactively in the Upper Midwest will depend on characterizing its damage niche, the geographic range in which it can reduce crop yields. We implemented a common garden study in 2011 and 2012, planting eight accessions of Palmer amaranth from the southern and midwestern United States, into soybean crops in southern, central, and northern Illinois, at a population density of 8 plants m−2 with a biocontainment protocol. Once Palmer amaranth plants initiated flowering, they were removed and burned. Weed survival, flowering, and weed biomass were measured, in addition to soybean yield and weather data. Analyses indicated that Palmer amaranth's damage niche in Illinois soybean was independent of weed genotype or maternal environment. Despite competing only briefly, Palmer amaranth reduced soybean yields in all site–years, indicating its damage niche in Illinois, and much of the Midwest, is limited primarily by seed immigration rate. These results highlight the urgent need for weed managers to learn Palmer amaranth identification, prevent seed introduction, and maintain a policy of zero seed return.

Meadowfoam seed meal (MSM), a by-product after oil extraction, has potential uses for crop growth enhancement or weed control. The herbicidal effect of MSM is the result of a secondary metabolite, glucosinolate glucolimnanthin (GLN). Field evaluations were conducted using concentrations of 3, 5, and 7% by weight and two forms (nonactivated and activated) of MSM applied as soil amendments. No injury was observed on lettuce transplanted 7 d after MSM incorporation in 2011. Activated MSM at 7% reduced weed emergence up to 71%. Lettuce leaf N content was at least 8.5-fold greater in MSM treatments compared to the untreated control. Greater soil nitrate levels correlated with greater weed biomass in MSM-amended plots. Isothiocyanate, a potent herbicidal compound, was detected in soil incorporated with 7% activated MSM. In 2012, 2.86 g m−2 of activated MSM, applied as a split or single dose, was evaluated for weed control efficacy and crop injury response. The split MSM application provided weed control similar to that from the single MSM application. The split and single MSM applications inhibited spiny sowthistle emergence more than 95% compared to the untreated control. A single application of activated MSM as a PRE soil amendment suppressed weeds and increased lettuce yield.