American sloughgrass [Beckmannia syzigachne (Steud.) Fernald] is one of the most predominant and troublesome weeds in wheat (Triticum aestivum L.) fields rotated with rice (Oryza sativa L.) in China. Mesosulfuron-methyl is one of the main herbicides used to selectively control B. syzigachne in winter wheat fields in China. After many years of application, mesosulfuron-methyl failed to control B. syzigachne in Yutai County. The objectives of this study were to determine the resistance level to mesosulfuron-methyl and other acetolactate synthase (ALS) inhibitors in the B. syzigachne population collected from Yutai County (R) and identify the mechanism of resistance. The results indicated that the R population was 4.1-fold resistant to mesosulfuron-methyl and was cross-resistant to pyroxsulam (600-fold), imazethapyr (4.1-fold), flucarbazone (12-fold), and bispyribac-sodium (12-fold). In vitro assays revealed that ALS in the R population was as sensitive as that in a susceptible (S) population. Gene sequence analysis identified no known resistant mutations in the ALS gene of the R population. Furthermore, real-time quantitative reverse transcriptase PCR experiments indicated that the expression level of the ALS gene in the R population was not different from that of the S population. However, the cytochrome P450 inhibitor malathion reversed the R population's resistance to mesosulfuron-methyl. The result of ultraperformance liquid chromatography–tandem mass spectrometry (UPLC-MS-MS) spectral analysis indicated that the metabolic rates of mesosulfuron-methyl in the R population were significantly faster than in the S population. Therefore, non-target resistance to mesosulfuron-methyl has been demonstrated in the R population. The resistance was very likely caused by enhanced herbicide metabolism.

Glyphosate-resistant junglerice [Echinochloa colona (L.) Link] is a problematic weed in mungbean [Vigna radiata (L.) R. Wilczek] crops in Australia. Due to limited herbicide options in mungbean, there is an increased interest in developing integrated management strategies for the sustainable control of E. colona. Pot experiments were conducted in a screenhouse in 2017 and 2018 by growing E. colona plants (glyphosate-resistant [GR] and glyphosate-susceptible [GS] biotypes) alone (1 plant pot−1) and in competition with 4 and 8 mungbean plants pot−1. Both biotypes were developed from a single population using the clone method. The growth and seed production of both GR and GS biotypes were similar in response to mungbean competition. Averaged over biotypes, there was a reduction in the growth and seed production of E. colona as crop plants increased. Compared with the weed plants grown alone, crop interference reduced E. colona height by 17% to 19%, tiller numbers by 69% to 82%, total shoot biomass by 85% to 91%, and inflorescence numbers by 74% to 91%. When E. colona was grown with 8 mungbean plants pot−1, leaf weight ratio increased by 42% compared with plants grown alone. Compared with weed plants grown alone, mungbean interference (4 and 8 plants pot−1) reduced weed seed production by 85% to 95%. These reductions were similar for both biotypes (GR and GS), suggesting that there was no fitness penalty associated with resistance. The results of this study suggest that mungbean interference can reduce E. colona growth and seed production, but it should not be considered as a stand-alone strategy to manage E. colona and similar species in mungbean. These results also highlight the need for integrating crop competition with other management strategies to achieve complete and sustainable management of this weed.

In a rice (Oryza sativa L.)–wheat (Triticum aestivum L.) rotation system, a study was conducted to determine the effects of different fertilization regimens (no fertilization, replacement of a portion of chemical fertilizer with composted pig manure, chemical fertilizer only, and straw return combined with chemical fertilizer) on the weed communities and wheat yields after 4 and 5 yr. The impact of the long-term recurrent fertilization regimen initiated in 2010 on the composition and diversity of weed communities and the impact of the components and total amount of fertilizer on wheat yields were assessed in 2014 and 2015. Totals of 19 and 16 weed species were identified in experimental wheat fields in 2014 and 2015, respectively, but the occurrence of weed species varied according to the fertilization regimen. American sloughgrass [Beckmannia syzigachne (Steud.) Fernald], water starwort [Myosoton aquaticum (L.) Moench], and lyrate hemistepta (Hemistepta lyrata Bunge.) were adapted to all fertilization treatments and were the dominant weed species in the experimental wheat fields. The greatest number of weed species were observed under the no-fertilization treatment, in which 40% of the weed community was composed of broadleaf weeds and the lowest wheat yields were obtained. With fertilizer application, the number of weed species was reduced, the height of weeds increased significantly, the density of broadleaf weeds was significantly reduced, the biodiversity indices of weed communities decreased significantly, and higher wheat yields were obtained. Only the chemical fertilizer plus composted pig manure treatment and the chemical fertilizer–only treatment increased the density of grassy weeds and the total weed community density. The treatment with chemical fertilizer only also resulted in the highest density of B. syzigachne. Rice straw return combined with chemical fertilizer yielded the lowest total weed density, which suggests that it inhibited occurrence of weeds. The different fertilizer regimens not only affected the weed species composition, distribution, and diversity, but also the weed density. Our study provides new information from a rice–wheat rotation system on the relationship between soil amendments and agricultural weed infestation.

Kochia [Bassia scoparia (L.) A. J. Scott] is a problematic weed species across the Great Plains, as it is spreading fast and has developed herbicide-resistant biotypes. It is imperative to understand key life-history stages that promote population expansion of B. scoparia and control strategies that would provide effective control of these key stages, thereby reducing population growth. Diversifying weed control strategies has been widely recommended for the management of herbicide-resistant weeds. Therefore, the objectives of this study were to develop a simulation model to assess the population dynamics of B. scoparia and to evaluate the effectiveness of diverse weed control strategies on long-term growth rates of B. scoparia populations. The model assumed the existence of a glyphosate-resistant (GR) biotype in the B. scoparia population, but at a very low proportion in a crop rotation that included glyphosate-tolerant corn (Zea mays L.) and soybean [Glycine max (L.) Merr.]. The parameter estimates used in the model were obtained from various ecological and management studies on B. scoparia. Model simulations indicated that seedling recruitment and survival to seed production were more important than seedbank persistence for B. scoparia population growth rate. Results showed that a diversified management program, including glyphosate, could provide excellent control of B. scoparia populations and potentially eliminate already evolved GR B. scoparia biotypes within a given location. The most successful scenario was a diverse control strategy that included one or two preplant tillage operations followed by preplant or PRE application of herbicides with residual activities and POST application of glyphosate; this strategy reduced seedling recruitment, survival, and seed production during the growing season, with tremendous negative impacts on long-term population growth and resistance risk in B. scoparia.

Field studies were conducted in 2016 and 2017 in Clinton, NC, to determine the interspecific and intraspecific interference of Palmer amaranth (Amaranthus palmeri S. Watson) or large crabgrass [Digitaria sanguinalis (L.) Scop.] in ‘Covington’ sweetpotato [Ipomoea batatas (L.) Lam.]. Amaranthus palmeri and D. sanguinalis were established 1 d after sweetpotato transplanting and maintained season-long at 0, 1, 2, 4, 8 and 0, 1, 2, 4, 16 plants m−1 of row in the presence and absence of sweetpotato, respectively. Predicted yield loss for sweetpotato was 35% to 76% for D. sanguinalis at 1 to 16 plants m−1 of row and 50% to 79% for A. palmeri at 1 to 8 plants m−1 of row. Weed dry biomass per meter of row increased linearly with increasing weed density. Individual dry biomass of A. palmeri and D. sanguinalis was not affected by weed density when grown in the presence of sweetpotato. When grown without sweetpotato, individual weed dry biomass decreased 71% and 62% from 1 to 4 plants m−1 row for A. palmeri and D. sanguinalis, respectively. Individual weed dry biomass was not affected above 4 plants m−1 row to the highest densities of 8 and 16 plants m−1 row for A. palmeri and D. sanguinalis, respectively.

Bearded sprangletop [Diplachne fusca (L.) P. Beauv. ex Roem. & Schult. ssp. fascicularis (Lam.) P. M. Peterson & N. Snow] is a noxious annual grass weed of paddy fields, distributed in coastal regions of the Jiangsu and Hebei provinces in China. Cyhalofop-butyl has been widely used to control grass weeds since 2006 in China. Overreliance on cyhalofop-butyl has led to the evolution of resistant weeds. In this study, the resistance level and cyhalofop-butyl resistance mechanisms were investigated in the putative resistant (JSHH) population. The dose–response experiments showed that the JSHH D. fusca population had evolved 8.9-fold resistance to cyhalofop-butyl. Acetyl-CoA carboxylase (ACCase) sequencing revealed a point mutation (GGC to GCC) at amino acid position 2096, resulting in a Gly-2096-Ala substitution in the resistant population. To our knowledge, this is the first case of cyhalofop-butyl resistance in D. fusca and the first report of a target-site mutation conferring resistance to ACCase-inhibiting herbicides in D. fusca. In addition, the resistant D. fusca population (JSHH) with the Gly-2096-Ala mutation was cross-resistant to the aryloxyphenoxypropionate herbicide metamifop, the cyclohexanedione herbicide sethoxydim, and the phenylpyrazolin herbicide pinoxaden.

Susceptibility of a system to colonization by a weed is in part a function of environmental resource availability. Doveweed [Murdannia nudiflora (L.) Brenan] can establish in a variety of environments; however, it is found mostly in wet or low-lying areas with reduced interspecies competition. Four studies evaluated the effect of mowing height, interspecies competition, and nitrogen, light, and soil moisture availability on M. nudiflora establishment and growth. A field study evaluated the effect of mowing height on M. nudiflora establishment. In comparison with unmowed plots, mowing at 2 and 4 cm reduced spread 46% and 30%, respectively, at 9 wk after planting. Effect of mowing height and nitrogen fertilization on ‘Tifway’ bermudagrass (Cynodon dactylon Burtt-Davy×C. transvaalensis L. Pers.) and M. nudiflora interspecies competition was evaluated in a greenhouse trial. Murdannia nudiflora coverage was 62% greater in flats maintained at 2.6 cm than flats maintained at 1.3 cm. Supplemental application of 49 kg N ha−1 mo−1 increased M. nudiflora coverage 75% in comparison with 24.5 kg N ha−1 mo−1. A difference in M. nudiflora coverage could not be detected between flats receiving 0 and 24.5 kg N ha−1 mo−1, suggesting moderate nitrogen fertilization does not encourage M. nudiflora colonization. Effect of light availability on M. nudiflora growth and development was evaluated in a greenhouse study. Growth in a 30%, 50%, or 70% reduced light environment (RLE) did not affect shoot growth on a dry weight basis in comparison with plants grown under full irradiance; however, internode length was 28% longer in a 30% RLE and 39% longer in a 50% and 70% RLE. Effect of soil moisture on M. nudiflora growth and development was evaluated in a greenhouse study. Plants maintained at 50%, 75%, and 100% field capacity (FC) increased biomass>200% compared with plants maintained at 12.5% or 25% FC.

Turnipweed [Rapistrum rugosum (L.) All.] and Mexican pricklepoppy (Argemone mexicana L.) are increasingly prevalent in the northern cropping regions of Australia. The effect of different densities of these two weeds was examined for their potential to cause yield loss in wheat (Triticum aestivum L.) through field studies in 2016 and 2017. There was 72% to 78% yield reduction in wheat due to competition from R. rugosum. Based on the exponential decay model, 18.2 and 24.3 plants m−2 caused a yield reduction of 50% in 2016 and 2017, respectively. Rapistrum rugosum produced a maximum of 32,042 and 29,761 seeds m−2 in 2016 and 2017, respectively. There was 100% weed seed retention at crop harvest. Competition from A. mexicana resulted in a yield loss of 17% and 22% in 2016 and 2017, respectively; however, plants failed to set seeds due to intense competition from wheat. Among the yield components, panicles per square meter and grains per panicle were affected by weed competition. The studies indicate a superior competitiveness of R. rugosum in wheat and a suppressive effect of wheat on A. mexicana. The results indicate that a wheat crop can be included in crop rotation programs where crop fields are infested with A. mexicana. High seed retention in R. rugosum indicates the possibility to manage this weed through seed catching and harvest weed seed destruction.

Row-middle weed control in Florida vegetable production is challenging and often necessitates several PRE and POST herbicide applications. Coordinating POST spray applications with emergence timings should increase herbicide efficacy by targeting susceptible growth stages. Most published emergence models were developed in temperate climates, and adapting them to subtropical climates can be complex and requires reductionist insights into seed ecology, particularly germination and dormancy. The study objective was to examine the influence of temperature and osmotic potential on seed germination of carpetweed (Mollugo verticillata L.), Carolina geranium (Geranium carolinianum L.), eclipta [Eclipta prostrata (L.) L.], and goosegrass [Eleusine indica (L.) Gaertn.]. Mollugo verticillata seed germination was positively photoblastic, with increased germination at high temperatures (≥35 C), more so with high fluctuating temperatures (35/20 and 35/25 C), and occurred at osmotic potentials as low as −0.5 MPa. Geranium carolinianum seed germinated between 10 and 20 C in light or darkness and at osmotic potentials as low as −0.4 MPa. Eclipta prostrata seed germination was entirely positively photoblastic, occurring optimally between 15 and 25 C and at osmotic potentials as low as −1 MPa. Eleusine indica seed germination demonstrated some degree of positive photoblasticity, with greater germination in the light, peak germination at 35 C, and germination occurring at osmotic potentials as low as −0.5 MPa. Described germination ecology for selected species will provide insights for building ecology-based growing degree-day accounting restrictions for empirically derived emergence models.

Weed management is a major challenge in organic crop production, and organic farms generally harbor larger weed populations and more diverse communities compared with conventional farms. However, little research has been conducted on the effects of different organic management practices on weed communities and crop yields. In 2014 and 2015, we measured weed community structure and soybean [Glycine max (L.) Merr.] yield in a long-term experiment that compared four organic cropping systems that differed in nutrient inputs, tillage, and weed management intensity: (1) high fertility (HF), (2) low fertility (LF), (3) enhanced weed management (EWM), and (4) reduced tillage (RT). In addition, we created weed-free subplots within each system to assess the impact of weeds on soybean yield. Weed density was greater in the LF and RT systems compared with the EWM system, but weed biomass did not differ among systems. Weed species richness was greater in the RT system compared with the EWM system, and weed community composition differed between RT and other systems. Our results show that differences in weed community structure were primarily related to differences in tillage intensity, rather than nutrient inputs. Soybean yield was lower in the EWM system compared with the HF and RT systems. When averaged across all four cropping systems and both years, soybean yield in weed-free subplots was 10% greater than soybean yield in the ambient weed subplots that received standard management practices for the systems in which they were located. Although weed competition limited soybean yield across all systems, the EWM system, which had the lowest weed density, also had the lowest soybean yield. Future research should aim to overcome such trade-offs between weed control and yield potential, while conserving weed species richness and the ecosystem services associated with increased weed diversity.

Sesbania [Sesbania cannabina (Retz.) Pers.] is a problematic emerging weed species in Australian cotton-farming systems. However, globally, no information is available regarding its seed germination biology, and better understanding will help in devising superior management strategies to prevent further infestations. Laboratory and glasshouse studies were conducted to evaluate the impact of various environmental factors such as light, temperature, salt, osmotic and pH stress, and burial depth on germination and emergence of two Australian biotypes of S. cannabina. Freshly harvested seeds of both biotypes possessed physical dormancy. A boiling-water scarification treatment (100±2 C) of 5-min duration was the optimum treatment to overcome this dormancy. Once dormancy was broken, the Dalby biotype exhibited a greater germination (93%) compared with the St George biotype (87%). The nondormant seeds of both biotypes showed a neutral photoblastic response to light and dark conditions, with germination marginally improved (6%) under illumination. Maximum germination of both biotypes occurred under an alternating temperature regime of 30/20 and 35/25 C and under constant temperatures of 32 or 35 C, with no germination at 8 or 11 C. Seed germination of both biotypes decreased linearly from 87% to 14% with an increase in moisture stress from 0.0 to −0.8 MPa, with no germination possible at −1.0 MPa. There was a gradual decline in germination for both biotypes when imbibed in a range of salt solutions of 25 to 250 mM, with a 50% reduction in germination occurring at 150 mM. Both biotypes germinated well under a wide range of pH values (4.0 to 10.0), with maximum germination (94%) at pH 9.0. The greatest emergence rate of the Dalby (87%) and St George (78%) biotypes was recorded at a burial depth of 1.0 cm, with no emergence at 16.0 cm. Deep tillage seems to be the best management strategy to stop S. cannabina’s emergence and further infestation of cotton (Gossypium hirsutum L.) fields. The findings of this study will be helpful to cotton agronomists in devising effective, sustainable, and efficient integrated weed management strategies for the control of S. cannabina in cotton cropping lands.

Proactive integrated weed management (IWM) is critically needed in no-till production to reduce the intensity of selection pressure for herbicide-resistant weeds. Reducing the density of emerged weed populations and the number of larger individuals within the population at the time of herbicide application are two practical management objectives when integrating cover crops as a complementary tactic in herbicide-based production systems. We examined the following demographic questions related to the effects of alternative cover-cropping tactics following small grain harvest on preplant, burndown management of horseweed (Erigeron canadensis L.) in no-till commodity-grain production: (1) Do cover crops differentially affect E. canadensis density and size inequality at the time of herbicide exposure? (2) Which cover crop response traits are drivers of E. canadensis suppression at time of herbicide exposure? Interannual variation in growing conditions (study year) and intra-annual variation in soil fertility (low vs. high nitrogen) were the primary drivers of cover crop response traits and significantly affected E. canadensis density at the time of herbicide exposure. In comparison to the fallow control, cover crop treatments reduced E. canadensis density 52% to 86% at the time of a preplant, burndown application. Cereal rye (Secale cereale L.) alone or in combination with forage radish (Raphanus sativus L.) provided the most consistent E. canadensis suppression. Fall and spring cover crop biomass production was negatively correlated with E. canadensis density at the preplant burndown application timing. Our results also show that winter-hardy cover crops reduce the size inequality of E. canadensis populations at the time of herbicide exposure by reducing the number of large individuals within the population. Finally, we advocate for advancement in our understanding of complementarity between cover crop– and herbicide-based management tactics in no-till systems to facilitate development of proactive, herbicide-resistant management strategies.

Wild radish (Raphanus raphanistrum L.) is a problematic and economically damaging dicotyledonous weed infesting crops in many regions of the world. Resistance to the auxinic herbicides 2,4-D and dicamba is widespread in Western Australian R. raphanistrum populations, with the resistance mechanism appearing to involve alterations in the physiological response to synthetic auxins and in plant defense. This study aimed to determine whether these alterations cause inhibition in plant growth or reproduction that could potentially be exploited to manage 2,4-D–resistant populations in cropping areas. Therefore, the morphology and seed production of resistant and susceptible populations were compared in an outdoor pot study, with plants grown in the presence and absence of competition by wheat (Triticum aestivum L.). The susceptible and resistant R. raphanistrum populations were equally suppressed by wheat competition, with plant growth and seed production being decreased by approximately 50%. Although resistant populations produced less vegetative biomass than susceptible populations, there was no negative association between resistance and seed production. Therefore, it is unlikely that any nonherbicidal management practices will be more efficacious on 2,4-D–resistant than 2,4-D–susceptible R. raphanistrum populations.

Field experiments determined the critical period for weed control (CPWC) in grafted and nongrafted watermelon [Citrullus lanatus (Thumb.) Matsum. & Nakai] grown in plasticulture. Transplant types included ‘Exclamation’ seedless watermelon as the nongrafted control as well as Exclamation grafted onto two interspecific hybrid squash (ISH) rootstocks, ‘Carnivor’ and ‘Kazako’. To simulate weed emergence throughout the season, establishment treatments (EST) consisted of two seedlings each of common purslane (Portulaca oleracea L.), large crabgrass [Digitaria sanguinalis (L.) Scop.], and yellow nutsedge (Cyperus esculentus L.) transplanted in a 15 by 15 cm square centered on watermelon plants at 0, 2, 3, 4, and 6 wk after watermelon transplanting (WATr) and remained until the final watermelon harvest at 11 WATr. To simulate weed control at different times in the season, removal treatments (REM) consisted of two seedlings of the same weed species transplanted in a 15 by 15 cm square centered on watermelon plants on the same day of watermelon transplanting and allowed to remain until 2, 3, 4, 6, and 11 WATr, at which time they were removed. Season-long weedy and weed-free controls were included for both EST and REM studies in both years. For all transplant types, aboveground biomass of weeds decreased as weed establishment was delayed and increased as weed removal was delayed. The predicted CPWC for nongrafted Exclamation and Carnivor required only a single weed removal between 2.3 and 2.5 WATr and 1.9 and 2.6 WATr, respectively, while predicted CPWC for Kazako rootstock occurred from 0.3 to 2.6 WATr. Our study results suggest that weed control for this mixed population of weeds would be similar between nongrafted Exclamation and Exclamation grafted onto Carnivor. But the observed CPWC of Exclamation grafted onto Kazako suggests that CPWC may vary with specific rootstock–scion combinations.

Weed management in container crops is primarily accomplished through frequent PRE herbicide applications and supplemental hand weeding. However, many ornamental species are sensitive to herbicides, and a significant number of tropical plants, ornamental grasses, and foliage crops have not been screened for herbicide tolerance. As nursery crops are produced in inert substrates that are largely composed of bark or peat, strategic fertilizer placement has the potential to significantly reduce weed growth in container-grown ornamentals. Growth and reproduction of three common container nursery weed species, eclipta [Eclipta prostrata (L.) L.], large crabgrass [Digitaria sanguinalis (L.) Scop.], and spotted spurge (Euphorbia maculata L.), were evaluated following fertilization via alternative methods, including subdressing or dibbling in comparison with industry standard practices of topdressing or incorporating a controlled-release fertilizer (17-5-11 [8 to 9 mo.]) to each 3.8-L container at 36.5 g per container. Fertilizer placement had little to no effect on germination of Eclipta prostrata or D. sanguinalis, but incorporation increased E. maculata germination by 77% to 183% compared with other placements or a nonfertilized control. Subdressing reduced seed production by 94%, 63%, and 92% for Eclipta prostrata, D. sanguinalis, and E. maculata, respectively, compared with the average number of seeds produced in the conventional placement methods (average of incorporation and topdressing). Dibbling fertilizer resulted in similar decreases in the case of D. sanguinalis and E. maculata, while Eclipta prostrata produced no seeds when fertilizer was dibbled. Similar to reductions observed in reproduction, subdressing fertilizer resulted in biomass decreases of 90%, 81%, and 85% compared with the average biomass of the incorporation and topdressed placements. Results suggest alternative fertilizer placements could be implemented as part of an integrated weed management program in container production to reduce weed growth.

Asia Minor bluegrass (Polypogon fugax Nees ex Steud.) is a problem grass weed of winter crops in China, where a population has become resistant to aryloxyphenoxypropionate (APP) herbicides. The mechanism of resistance is due to an Ile-2041-Asn mutation of the ACCase gene. Screen house experiments were conducted to study the growth, fecundity characteristics, and competitive ability of this aryloxyphenoxypropionate-resistant (APP-R) biotype compared with a susceptible (APP-S) biotype. When grown under noncompetitive conditions, the APP-R P. fugax developed more rapidly than the APP-S plants, with earlier tiller and panicle emergence and seed shedding; the APP-R P. fugax set seeds nearly 12 d earlier than the APP-S biotype. APP-R and APP-S biotypes had similar aboveground dry weight before the flowering stage. Fecundity of the APP-R biotype was similar to the APP-S biotype (8.57 g seeds plant−1 and 0.17 g seeds panicle−1 versus 8.22 g seeds plant−1 and 0.13 g seeds panicle−1, respectively). Ultimately, the relatively slower-developing APP-S P. fugax had 50% more shoot dry weight than the APP-R plants. Relative competitiveness among the APP-R and APP-S P. fugax biotypes was investigated through replacement series experiments. No difference in competitive ability was measured between APP-R and APP-S biotypes on the basis of shoot dry weight before the tillering stage. These results indicate that there is no apparent fitness penalty for the APP-R P. fugax. The shorter growth cycle of APP-R with no apparent fitness penalty suggests that growers will need begin weed control earlier and possibly include vegetative crops with an even shorter growth cycle in their rotations.

Watermelon [Citrullus lanatus (Thunb.) Matsum & Nakai] grafting is commonly used for management of diseases caused by soilborne pathogens; however, little research exists describing the effect of grafting on the weed-competitive ability of watermelon. Field experiments determined the response in yield, fruit number, and fruit quality of grafted and nongrafted watermelon exposed to increasing densities of Palmer amaranth (Amaranthus palmeri S. Watson). Grafting treatments included ‘Exclamation’ triploid (seedless) watermelon grafted on two interspecific hybrid squash rootstocks ‘Carnivor’ and ‘Kazako’, with nongrafted Exclamation as the control. Weed treatments included A. palmeri at densities of 1, 2, 3, and 4 A. palmeri plants per watermelon planting hole (0.76-m row) and a weed-free control. Increasing A. palmeri densities caused significant reductions (P <0.05) in marketable watermelon yield and marketable fruit number. Watermelon yield reduction was described by a rectangular hyperbola model, and 4 A. palmeri plants planting hole−1 reduced marketable yield 41%, 38%, and 65% for Exclamation, Carnivor, and Kazako, respectively. Neither grafting treatment nor A. palmeri density had a biologically meaningful effect on soluble solids content or on the incidence of hollow heart in watermelon fruit. Amaranthus palmeri seed and biomass production was similar across weed population densities, but seed number per female A. palmeri decreased according to a two-parameter exponential decay equation. Thus, increasing weed population densities resulted in increased intraspecific competition among A. palmeri plants. While grafting may offer benefits for disease resistance, no benefits regarding weed-competitive ability were observed, and a consistent yield penalty was associated with grafting, even in weed-free treatments.

Prairie groundcherry [Physalis hederifolia (A. Gray) var. fendleri (A. Gray) Cronquist] is an invasive perennial weed with the potential to become a significant summer weed across 409 million hectares in Australia. Current management practices do not provide effective control of established populations. A better understanding of the seed biology is needed to effectively manage this weed. A series of field and laboratory studies were conducted to determine plant fecundity, soil seedbank longevity, and the factors that affect seed germination. Physalis hederifolia has the capacity to produce 66 to 86 berries plant−1, 51 to 74 seeds berry−1, and approximately 4,500 seeds plant−1, with the seeds potentially able to persist in the soil seedbank for 20 yr if buried in an intact dry berry pod. The bare-seed component of the soil seedbank can be virtually exhausted within 3 yr if cultivation is minimized to avoid burial of seed. Optimal temperature for germination is diurnal fluctuations of 15 C within the temperature range of 10 and 30 C. Increasing osmotic stress levels reduced the germination under all temperature regimes, with less than 6% germination occurring at −0.96 MPa. Physalis hederifolia seed germination was not significantly affected by substrate pH 4 to 10 or salt levels less than 160 mM, while the germination was significantly reduced at NaCl concentrations above 160 mM. These results suggest that P. hederifolia can adapt to a range of substrate conditions. Stopping seed set, avoiding grazing plants with viable seeds, and minimizing seed burial in the soil are some effective strategies to control this weed.

Nitrogen (N) inputs have been found to exert strong influence on leaf stoichiometry in natural ecosystems, but there are few studies investigating the effects of N in agroecosystems. Using a 5-yr fertilization experiment in rice fields, we examined the effects of N inputs on leaf stoichiometry of one crop, rice (Oryza sativa L.), and its four common weeds, barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.], Monochoria korsakowii Regel and Mack, alligatorweed [Alternanthera philoxeroides (Mart.) Griseb.], and Japanese mazus [Mazus pumilus (Burm. f.) Steenis], and further evaluated whether and how straw return mediates these effects. We found that rice and weed leaf nitrogen:phosphorus:potassium (N:P:K) stoichiometry exhibited divergent responses to N fertilizer. Weed leaf N:P:K stoichiometry was not sensitive to low (120 kg N ha−1) and regular (240 kg N ha−1) N inputs, but rice plants were, with significantly increased leaf N concentration and N:P and N:K ratios. The opposite trend was found for high N inputs (360 kg N ha−1). Rice leaf N concentration [N] did not increase further, and N:P ratios even decreased, whereas E. crus-galli and M. korsakowii had significantly increased [N] and N-related stoichiometry. We also found that the positive effects of regular N inputs on rice leaf N:P and N:K ratios were significantly dampened by straw return, but the positive effects on N:P ratios in M. pumilus leaves were enhanced by straw return. Compared with weeds, rice leaves contained low elemental concentrations across fertilization levels at grain-filling stages. These results indicate that rice has a lower N requirement than weeds at grain-filling stages, and the N supply should be managed at a relative low level to reduce the nutrient acquisition and competitive abilities of weeds. From a stoichiometric perspective, this study highlights the importance of N management in combination with straw return in controlling weeds and increasing the nutrient-use efficiency of crops.

Weedy rice (Oryza spp.) is considered one of the main weeds in cultivated rice (Oryza sativa L.) around the world, having a great impact on both yield and quality of crop rice. Recent studies have characterized the range of morphological and genetic diversity in weedy rice from different locations and have revealed that there is often great morphological diversity within growing regions. No systematic attempt to characterize phenotypic diversity of weedy rice in Colombia, where this group of weeds greatly affects rice production, has yet been carried out. This study seeks (1) to establish the range of variation in various morphological characters for weedy rice collected in the five production zones of Colombia and to compare these with commercial varieties and landraces sown in the country, (2) to determine the association between weedy rice morphotypes and rice production areas in the country, and (3) to assess whether any association exists between morphology and recently discovered genetic groupings for weedy rice in Colombia. Based on a sampling of rice production areas in Colombia and evaluation of 27 phenotypic traits, a two-step cluster analysis identified four morphological groups for weedy rice in Colombia. These groupings had some limited association with geography and the genetic ancestries of weedy rice. Clustering showed that awn and apiculus color and awn length and presence are the most important predictors in defining morphological groupings. Understanding and classifying the morphological diversity may be helpful in understanding weedy rice origins, evolution, and potential management practices.