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The establishment of plants in an ecosystem is limited by the availability of seeds and the availability of suitable sites for establishment. Describing plant population dynamics through the relative strength of seed and establishment limitation is an important concept in the study of natural ecosystems. To date, it is unclear whether this concept can be applied to describe populations of annual weeds in agricultural fields. Using a recruitment function, we show that limitation parameters prove valuable in describing seedling recruitment in weed populations. We conducted a seed addition experiment in three cornfields (Zea mays L.) and recorded seedling recruitment in populations of the economically important weed barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.]. Seed predation, competition with other weeds, and seed burial were prevented. We estimated the strength of seed and establishment limitation in the population with two parameters: n, which is the number of microsites, and b, which is the suitability of those sites to support a seedling. We further estimated the relative proportions of density-dependent and density-independent establishment limitation in the seedling population. Recruitment rates of E. crus-galli ranged from 31% to 36% across all evaluated seed densities and fields, which is high compared with results from other seed addition studies. Two of the three monitored populations were predominantly establishment limited at the highest evaluated seed density of 2,400 added seeds m−2. Further knowledge about the relative strength of limitations in other weed populations will provide important information on how effective different weed management strategies can be.

Barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.] is a noxious grass weed that infests rice fields and causes huge crop yield losses. In this study, we collected 12 E. crus-galli populations from rice fields of Ningxia Province in China and investigated the resistance levels to the acetolactate synthase (ALS) inhibitor penoxsulam and the acetyl-CoA carboxylase (ACCase) inhibitor cyhalofop-butyl. The results showed that eight populations exhibited resistance to penoxsulam and four populations evolved resistance to cyhalofop-butyl. Moreover, all four cyhalofop-butyl–resistant populations (NX3, NX4, NX6, and NX7) displayed multiple herbicide resistance to both penoxsulam and cyhalofop-butyl. The alternative herbicides bispyribac-sodium, metamifop, and fenoxaprop-p-ethyl cannot effectively control the multiple herbicide–resistant (MHR) plants. To characterize the molecular mechanisms of resistance, we amplified and sequenced the target site–encoding genes in resistant and susceptible populations. Partial sequences of three ALS genes and six ACCase genes were examined. A Trp-574-Leu mutation was detected in EcALS1 and EcALS3 in two high-level (65.84- and 59.30-fold) penoxsulam-resistant populations, NX2 and NX10, respectively. In addition, one copy (EcACC4) of ACCase genes encodes a truncated aberrant protein due to a frameshift mutation in E. crus-galli populations. None of the amino acid substitutions that are known to confer herbicide resistance were detected in ALS and ACCase genes of MHR populations. Our study reveals the wide spread of MHR E. crus-galli populations in Ningxia Province that exhibit resistance to several ALS and ACCase inhibitors. Non–target site based mechanisms are likely to be involved in E. crus-galli resistance to the herbicides, at least in four MHR populations.

Resistance to penoxsulam among barnyardgrass populations is prevalent in rice fields in China. Seeds of penoxsulam-resistant (AXXZ-2) and penoxsulam-susceptible (JLGY-3) barnyardgrass populations, as well as the seeds of two rice varieties, including Wuyungeng32 (WY) and Liangyou669 (LY), were planted in plastic pots and then treated with a rate titration of acetochlor, anilofos, butachlor, clomazone, oxadiazon, pendimethalin, pretilachlor, pyraclonil, or thiobencarb. The two barnyardgrass populations exhibited similar susceptibility to acetochlor, anilofos, butachlor, oxadiazon, pretilachlor, or pyraclonil. However, the susceptibility differed between the barnyardgrass populations in response to clomazone, pendimethalin, and thiobencarb. For AXXZ-2, herbicide rates that caused 50% reduction in shoot biomass from the nontreated control (GR50) were 179, >800, and 1,798 g ha−1 for clomazone, pendimethalin, and thiobencarb, respectively; whereas JLGY-3 GR50 values were 61, 166, and 552 g ha−1, respectively. Both rice varieties demonstrated excellent tolerance to acetochlor, butachlor, oxadiazon, pretilachlor, and thiobencarb. However, substantial rice damage was observed when anilofos and clomazone were used. Anilofos at 352 g ha−1 and clomazone at 448 g ha−1 reduced rice shoot biomass by 41% and 50% from the nontreated, respectively. Averaged across herbicide rates, clomazone use resulted in a reduction in rice shoot biomass from that of the nontreated control by 52% and 34% for WY and LY, respectively; and pendimethalin use resulted in a reduction in rice shoot biomass from the nontreated control by 25% and 9% for WY and LY, respectively.

Barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.] is a problematic weed in rice (Oryza sativa L.) fields. Overapplication of herbicides causes environmental pollution and the emergence of resistant weeds, and integrated weed management methods can reduce dependence on herbicides. The growth of E. crus-galli and rice seedlings was shown to be significantly inhibited by high concentrations of fulvic acid (FA, C14H12O8) under flooding conditions (HF, 0.80 g L−1) (P < 0.05). In contrast, seedling growth was promoted by the application of very low concentrations of FA (LF, 0.02 g L−1). The activities of glutathione S-transferase (GST) and antioxidant enzymes, including total superoxide dismutase (T-SOD), peroxidase (POD), and catalase (CAT), in E. crus-galli seedlings were enhanced by the LF treatment; while POD activity decreased and GST, T-SOD, and CAT activity was not significantly altered by the HF treatment. The metabolomic and transcriptomic analyses showed that FA regulated E. crus-galli seedling growth by affecting the synthesis of indole derivatives and flavonoid compounds. Compared with the blank control (CK, 0 g L−1), the levels of four indole derivatives were upregulated under the HF treatment, and the indole derivatives were slightly downregulated under the LF treatment. The flavonoids, including naringenin, naringenin chalcone, eriodictyol, kaempferol, and epigallocatechin, were downregulated under HF treatment, and the growth of E. crus-galli was reduced. In contrast, the metabolism and transcription of flavonoids were not significantly altered by the LF treatment. The addition of 0.80 g L−1 FA obviously inhibited the growth of newly sprouted E. crus-galli, whereas rice growth was significantly promoted 8 d after rice planting (P < 0.05). The application of FA, therefore, might be a potential integrated weed management method to control the damage caused by E. crus-galli in paddy fields.

Barnyardgrass [Echinochloa crus-galli(L.) P. Beauv] is the foremost weed in rice (Oryza sativaL.) systems, and its control is crucial to successful rice production. Quinclorac, a synthetic auxin herbicide, has been used effectively to manageE. crus-galli. However, occurrences of quinclorac-resistant genotypes are frequently reported, and its resistance evolution has led to questions about the continued utility of quinclorac for grass control. Identification of the resistance mechanism(s) of resistant genotypes will facilitate development of integrated weed management strategies that sustain quinclorac use for management ofE. crus-galli. We evaluated the responses to quinclorac of two contrasting genotypes: E7 (resistant, R) and LM04 (susceptible, S). Quinclorac induced ethylene and cyanide biosynthesis in the S-genotype. Both genotypes responded similarly to an increasing application of exogenous 1-carboxylic acid aminocyclopropane (ACC) and potassium cyanide, and their growth was inhibited at higher doses. The key mechanism for cyanide (HCN) detoxification in plants, β-cyanoalanine synthase (β-CAS) activity, was evaluated in both genotypes, and no significant difference was observed in the basal activity. However, quinclorac significantly induced β-CAS–like activity in the S-genotype, which is consistent with the increased synthesis of ethylene and cyanide. This work suggests that the resistance to quinclorac of the E7 R-genotype is likely related to an alteration in the auxin signal transduction pathway, causing a lower stimulation of ACC synthase and, therefore, limited synthesis of ethylene and HCN after quinclorac treatment.

In wetlands, dormancy may be a key functional trait enabling seeds to avoid underwater germination, which could be lethal for seedling establishment. Our objectives were to find out (i) if shallow dormant (i.e. conditionally dormant) Echinochloa crus-galli seeds from an anaerobic germination resistant accession can break dormancy under hypoxic submergence and (ii) if underwater germination can be restored in scarified, non-dormant seeds. Shallow dormant E. crus-galli seeds perceived diurnally alternating temperatures (AT) and red light (R) pulses (i.e. dormancy-breaking cues) under hypoxic submergence; however, an inhibitory far-red light pulse given at the end of the 4-d inundation period demonstrated that most of the seeds (85%) were unable to break dormancy. Scarified E. crus-galli seeds, which did not express dormancy under drained conditions, were unable to germinate under hypoxic submergence, despite being exposed to dormancy-breaking cues (AT + R). Lastly, the temporal window for germination sensitivity to the inhibitory action of hypoxia, once dormancy-breaking signals have been applied, is progressively lost and bounded to approximately 18 h for half of the seed lot. These results highlight the importance of dormancy as a trait enabling E. crus-galli seeds to avoid underwater germination, a risky scenario for seedling emergence and establishment in this facultative hydrophyte.

Barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.] is one of the most troublesome and competitive weed species in rice (Oryza sativa L.) grown under conventional flood (FLD) irrigation and is expected to be similarly damaging under alternate-wetting-and-drying (AWD) irrigation. Several indica rice genotypes have been shown to suppress E. crus-galli under FLD management, but very little is known about the nature and potential of weed suppression using AWD irrigation. In this 3-yr field study, we evaluated seven diverse rice genotypes for their weed suppression and grain yield potential under FLD and AWD irrigation. The E. crus-galli dry biomass at midseason was 11% less under AWD compared with FLD. Overall, plots of an indica variety and a hybrid resulted in less E. crus-galli dry biomass under AWD compared with FLD. Grain yield in weed-free AWD plots averaged 12% less than in weed-free FLD plots. Grain yield of the tropical japonica (TRJ) type, ‘Bengal’, was 32% lower under AWD than FLD, whereas grain yields of the two indica genotypes, PI 312777 and ‘Rondo’, and a hybrid were similar in both irrigation systems. Grain yield reduction in E. crus-galli–infested AWD plots averaged greater than 90%. Thus, E. crus-galli greatly reduced the grain yield of all rice genotypes tested in both FLD and AWD systems in this study. Grain yield of the indica and hybrid genotypes was relatively less affected by the AWD treatments compared with the TRJ genotypes, particularly with Bengal, suggesting that the weed-suppressive genotypes, PI 312777 and Rondo, would be better suited to AWD irrigation systems and for inclusion in weed-suppression rice-breeding programs in the southern United States.

Barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.] is acknowledged to be the most troublesome weed in rice fields in Anhui and Jiangsu provinces of China. It cannot be effectively controlled using certain acetolactate synthase (ALS)-inhibiting herbicides, including penoxsulam. Echinochloa crus-galli samples with suspected resistance to penoxsulam were collected to identify the target site–based mechanism underlying this resistance. Populations AXXZ-2 and JNRG-2 showed 33- and 7.3-fold resistance to penoxsulam, respectively, compared with the susceptible JLGY-3 population. Cross-resistance to other ALS inhibitors was reported in AXXZ-2 but not in JNRG-2, and occasionally showed higher sensitivity than JLGY-3. In vitro ALS activity assays revealed that penoxsulam concentrations required to inhibit 50% of ALS activity were 11 and 5.2 times greater in AXXZ-2 and JNRG-2, respectively, than in JLGY-3. DNA and predicted amino acid sequence analyses of ALS revealed Ala-205-Val and Ala-122-Gly substitutions in AXXZ-2 and JNRG-2, respectively. Our results indicate that these substitutions in ALS are at least partially responsible for resistance to penoxsulam.

Field and greenhouse studies were conducted to evaluate the antagonism potential of glufosinate applied sequentially or mixed with graminicides on barnyardgrass control. Applications of glufosinate alone provided variable control throughout the growing season in both field and greenhouse experiments. In the field, barnyardgrass control was not adversely affected by glufosinate- and clethodim-mix applications or sequential applications of glufosinate before or after clethodim. Soybean yield was not affected by application timing or clethodim rate, with yield ranging from 1,748 to 2,733 kg ha−1. In the greenhouse, glufosinate applied 1 and 3 d before graminicides generally reduced barnyardgrass control compared with the graminicides applied alone. The response with quizalofop-P was not as dramatic as with the other graminicides. Although significant visual barnyardgrass control differences were detected due to application timing of glufosinate, barnyardgrass biomass with fluazifop-P and quizalofop-P did not differ between the application timings of glufosinate. However, glufosinate applied 1 and 3 d before clethodim had significantly greater biomass compared with glufosinate applied 1 and 3 d after clethodim. The differences in environmental conditions and growth stages at the time of application may have contributed to barnyardgrass control response differences between the field and greenhouse experiments. Although barnyardgrass control in the field was not affected by glufosinate application timing, data from the greenhouse indicate potential exists for reduced control if glufosinate is applied 1 or 3 d before graminicides.

Harvest weed seed control is an alternative non-chemical approach to weed management that targets escaped weed seeds at the time of crop harvest. Relatively little is known on how these methods will work on species in the US. Two of the most prominent weeds in soybean production in the midsouthern US are Palmer amaranth and barnyardgrass. Typically, when crop harvesting occurs the weed seed has already either shattered or is taken into the combine and may be redistributed in the soil seedbank. This causes further weed seed spread and may contribute to the addition of resistant seeds in the seedbank. There is little research on how much seed is retained on different weed species at or beyond harvest time. Thus, the objective of this study was to determine the percentage of total Palmer amaranth and barnyardgrass seed production that was retained on the plant during delayed soybean harvest. Retained seed over time was similar between 2015 and 2016, but was significantly different between years for only Palmer amaranth. Seed retention did not differ between years for either weed species. Palmer amaranth and barnyardgrass retained 98 and 41% of their seed at soybean maturity and 95 and 32% of their seed one month after soybean maturity, respectively. Thus, this research indicates that if there are escaped Palmer amaranth plants and soybean is harvested in a timely manner, most seed will enter the combine and offer potential for capture or destruction of these seeds using harvest weed seed control tactics. While there would be some benefit to using HWSC for barnyardgrass, the utility of this practice on mitigating herbicide resistance would be less pronounced than that of Palmer amaranth because of the reduced seed retention or early seed shatter.

Monoterpenes, the major constituents of essential oils, are known for their diverse biological activities. This study was conducted to assess the herbicidal effect of six monoterpenes viz. (R)-carvone, 1,8-cineole, cuminaldehyde, (S)-fenchone, geraniol, (S)-limonene, and (R)-linalool on barnyardgrass under laboratory and glasshouse conditions with a view to explore the possibility of their utilization for future weed management. The effect of monoterpenes on chlorophyll contents and total phenolic compounds was also evaluated. The inhibitory effects of monoterpenes on seed germination and seedling growth were tested at concentrations of 1, 2, 4, 6, and 8 mM. The results showed that geraniol and (R)-carvone caused greatest reduction of seed germination with complete inhibition at the concentrations > 2 mM. Similarly, these two compounds were the most potent inhibiters for root and shoot growth. In general, monoterpenes were less effective against seed germination than seedling growth. Furthermore, the inhibition of root growth by all compounds was greater than that of shoot growth. In foliar application treatments under glasshouse conditions, the monoterpenes reduced the fresh and dry weights, and shoot length of two-leaf stage barnyardgrass at concentrations of 1 and 2%. In addition, the tested monoterpenes caused phytotoxicity symptoms, mainly chlorosis and necrosis, followed by weed death. Complete weed control was observed in the treatments with 1 and 2% of geraniol, and 2% of cuminaldehyde. Further, a reduction of chlorophyll contents and total phenolic compounds of barnyardgrass leaves was noticed, indicating that the monoterpenes cause adverse effect on photosynthesis and weed metabolism. Based on the results of this study, it can be concluded that the monoterpenes, particularly geraniol, (R)-carvone, and cuminaldehyde, can be used as potential natural herbicides.

SUMMARYThe lack of control of barnyardgrass in flooded rice cultivated with imidazolinone-resistant rice cultivars is challenging the utilization of this system, which is continuously expanding for new rice areas worldwide. The objectives of the present study were to evaluate the frequency, distribution and mechanisms of imidazolinone resistance in barnyardgrass to establish the best practices to control and prevent this problem. The distribution of resistance was evaluated in 624 populations collected in Southern Brazil. The frequency of imidazolinone-herbicide resistance was 0·81, broadly distributed in all sampled regions. Resistance to quinclorac was also found in 0·19 of the populations, but all of the evaluated populations were susceptible to cyhalofop-butyl. Further studies were conducted in six populations. The enhanced metabolism was assessed with the metabolic inhibitors that reversed the resistance to quinclorac from 0·54 to 1·00 in two populations and the resistance to imazethapyr from 0·15 to 0·41 in three populations. The acetolactase synthase (ALS) enzyme activity also indicated the occurrence of altered target site resistance in two populations caused by the ALS gene mutations Trp574Leu and Ser653Asn, which is a novel finding in this species. The herbicide resistance in barnyardgrass in Southern Brazil presented a complex basis of resistance because it is associated with resistance to multiple herbicides due to multiple mechanisms and with multiple mutations of the ALS gene. This indicates that it is necessary to adopt specific measures to prevent and control the evolution of multiple herbicide resistance in this species.

The goal of this research was to develop herbicide programs for controlling acetolactate synthase (ALS)–, propanil-, quinclorac-, and clomazone-resistant barnyardgrass. Two applications of imazethapyr alone at 70 g ai ha−1failed to control the ALS-resistant biotype more than 36%; however, when imazethapyr at 70 g ha−1was applied early POST (EPOST) followed by imazethapyr at 70 g ha−1plus fenoxaprop at 120 g ai ha−1immediately prior to flooding (PREFLD), barnyardgrass control improved to 78% at 10 wk after planting. When imazethapyr was applied twice following PRE or delayed PRE applications of clomazone at 336 g ai ha−1, quinclorac at 560 g ai ha−1, pendimethalin at 1,120 g ai ha−1, or thiobencarb at 4,480 g ai ha−1control was 92 to 100%. A single-pass program consisting of a delayed PRE application of clomazone at 336 g ha−1plus quinclorac at 560 g ha−1plus pendimethalin at 1,120 g ha−1plus thiobencarb at 4,480 g ha−1controlled all herbicide-resistant barnyardgrass biotypes at the same level as a standard multiple application program.

Herbicide-resistant barnyardgrass has become widespread in the rice production systems of the midsouthern United States, leaving few effective herbicide options for controlling this weed. The acetolactate synthase (ALS)- and acetyl-CoA carboxylase (ACCase)-inhibiting herbicides remain largely effective in Clearfield®rice production, but strategies need to be developed to protect the long-term utility of these options. A two-trait model was developed to understand simultaneous evolution of resistance in barnyardgrass to the ALS- and ACCase-inhibiting herbicides in Clearfield rice. The model was used to predict resistance under a number of common weed management scenarios across 1,000 hypothetical rice fields in the Mississippi Delta region and answer some key management questions. Under an ALS inhibitor–only program consisting of three annual applications of imidazolinone herbicides (imazethapyr or imazamox) in continuous Clearfield rice, resistance was predicted within 4 yr with 80% risk by year 30. Weed management programs that consisted of ALS- and ACCase-inhibiting herbicides such as fenoxaprop and cyhalofop greatly reduced the risk of ALS-inhibiting herbicide resistance (12% risk by year 30), but there was a considerable risk for ACCase resistance (evolving by year 14 with 13% risk by year 30) and multiple resistance (evolving by year 16 with 11% risk by year 30) to both of these mechanisms of action. A unique insight was that failure to stop using a herbicide soon after resistance evolution can accelerate resistance to the subsequent herbicide option. Further, a strong emphasis on minimizing seedbank size is vital for any successful weed management strategy. Results also demonstrated that diversifying management options is not just adequate, but diversity combined with timely herbicide applications aimed at achieving high efficacy levels possible is imperative.

Mechanisms of herbicide resistance were studied in a quizalofop–ethyl-resistant barnyardgrass biotype. Acetyl-coenzyme A carboxylase (ACCase) sensitivity to quizalofop-p-ethyl was measured by high-performance liquid chromatography and the trend in ACCase gene expression over time was determined using real-time polymerase chain reaction. The results showed that an insensitive ACCase was present in Geqiushan resistant plants (R), with a resistance index of 106. The basal ACCase activities in Geqiushan R and Geqiushan susceptible plants (S) were similar, at 1.20 and 1.17 ng malonyl-CoA min−1µg−1extract protein, respectively. Basal ACCase gene expression in Geqiushan R was similar to that in Geqiushan S. The relative expression of ACCase gene decreased after spraying quizalofop–ethyl at 60 g ai ha−1in Geqiushan S, whereas it was almost not changed in Geqiushan R. From these results we concluded that plastid ACCase sensitivity change might be responsible for the resistance and gene overexpression does not play a role in this resistance.

Glyphosate-resistant (GR) weeds have been a prime challenge to the sustainability of GR cotton-based production systems of the midsouthern United States. Barnyardgrass is known to be a high-risk species for evolving herbicide resistance, and a simulation model was developed for understanding the likelihood of glyphosate resistance evolution in this species in cotton-based systems. Under a worst-case scenario of five glyphosate applications in monoculture GR cotton, the model predicts resistance evolution in about 9 yr of continuous glyphosate use, with about 47% risk by year 15. A unique insight from this model is that management in response to GR Palmer amaranth in this system (a reactive response) provided a proactive means to greatly reduce the risks of glyphosate resistance evolution in barnyardgrass. Subsequent model analysis revealed that the risk of resistance is high in fields characterized by high barnyardgrass seedbank levels, seedling emergence, and seed production per square meter, whereas the risk is low in fields with high levels of postdispersal seed loss and annual seedbank loss. The initial frequency of resistance alleles was a high determinant of resistance evolution (e.g., 47% risk at year 15 at an initial frequency of 5e−8vs. 4% risk at 5e−10). Monte Carlo simulations were performed to understand the influence of various glyphosate use patterns and production practices in reducing the rate and risk of glyphosate resistance evolution in barnyardgrass. Early planting and interrow cultivation are useful tools. Crop rotation is effective, but the diversity of weed management options practiced in the rotational crop is more important. Diversifying weed management options is the key, yet application timing and the choice of management option is critical. Model analyses illustrate the relative effectiveness of a number of diversified glyphosate use strategies in preventing resistance evolution and preserving the long-term utility of glyphosate in midsouthern U.S. cotton-based production systems.

In Asia, a significant area under rice is affected by salinity. Salt stress can affect growth of crops as well as weeds. A study was conducted in a greenhouse to determine the effect of salinity (electrical conductivity [EC] of 1, 6, 12, 18, and 24 dS m−1) on growth of barnyardgrass, horse purslane, junglerice, and rice. Growth variables were analyzed using regression analysis. The tested levels of EC influenced leaf production of barnyardgrass and junglerice but not that of horse purslane. As compared with the control treatment (EC of 1 dS m−1), shoot biomass of barnyardgrass decreased by only 24% at 12 dS m−1, whereas rice biomass declined by 59% at this level of EC. At EC of 24 dS m−1, barnyardgrass still produced 4% of the biomass of the control treatment, whereas rice did not survive at this level of EC. Junglerice shoot biomass decreased by 73% at 18 dS m−1EC compared with the control treatment, whereas rice shoot biomass declined by more than 86% at 18 dS m−1EC. An EC of 10 dS m−1was required to inhibit 50% shoot biomass of rice, whereas the EC to inhibit 50% shoot biomass of barnyardgrass and junglerice was 15 and 13 dS m−1, respectively. Shoot biomass of horse purslane was not influenced by the tested levels of EC. At the highest EC (24 dS m−1), at which rice did not survive, horse purslane shoot biomass was similar to that of the control treatment. In all weed species, data for root biomass showed trends similar to those of shoot biomass. The results of this study suggest that weeds were more tolerant to salt than rice, and horse purslane was the most tolerant species among the weeds.

The increasing use of synthetic chemicals for pest control in rice has become an overwhelming economical border, and more important, it could pose a serious threat of the environment. In addition, the extensive use of synthetic herbicides has been the cause for the evolution of herbicide-resistant barnyardgrass worldwide. This weed species is the most competitive weed in rice after the red rice. Thus, this study was conducted to examine the combination effects of aqueous sunflower leaf extracts with lower rate of pretilachlor on barnyardgrass emergence and growth in Marang (sandy loam) and Seberang (silt loamy) soil series under glasshouse conditions. Interestingly, the ED95values (rate that causes 95% inhibition) of pretilachlor for emergence and shoot fresh weight (SFW) of barnyardgrass were reduced by 79 and 82%, respectively, when being mixed with sunflower leaf extracts in Marang series. In contrast, the addition of sunflower leaf extracts increased ED95value of pretilachlor in Seberang series. Rice seedlings at 4 and 8 d after sowing (DAS) were found to be tolerant to this mixture treatment. However, root growth of rice seedlings were inhibited at 0 and 2 DAS. These results suggest that sunflower leaf extracts have potential to reduce rate of pretilachlor for inhibiting emergence and growth of barnyardgrass without injuring rice seedlings in rice fields depending on soil types and growth stage of rice.

Barnyardgrass is one of the most problematic weeds in Arkansas, and with the documentation of herbicide-resistant biotypes, there is a need to gain a detailed understanding of its ecology. In particular, knowledge on barnyardgrass seedbank size and emergence pattern is vital. An extensive seedbank survey was carried out in 2008 in 12 counties in eastern Arkansas to determine barnyardgrass seedbank size across the region. There was a great variability in seedbank size with a maximum of 215,000 seeds m−2. Among the fields surveyed, barnyardgrass seedbank was found only in 7% of the cotton fields, while it was 22 and 20%, respectively, for rice and soybean. To examine the emergence pattern of barnyardgrass, experiments were conducted in Rohwer (two sites), Stuttgart (one site), and Fayetteville (one site), Arkansas in 2008 and 2009. In each site, barnyardgrass emergence was quantified from naturally occurring seedbanks. Barnyardgrass exhibited an extended period of emergence with days to 100% emergence ranging from 99 to 165 across sites and years. Nevertheless, effective management may be achieved by targeting the peak emergence periods, which range from mid-April to mid-June in Arkansas. The four-parameter Weibull model provided a better fit to the cumulative emergence data. However, the thermal time (growing degree days, GDDs) or hydrothermal time (HTT) models did not predict barnyardgrass emergence any better than calendar days, perhaps because of the inherent variations associated with natural seedbanks. This study establishes seedbank size and general emergence pattern for barnyardgrass in Arkansas. Additionally, these results will be useful for parameterizing herbicide-resistance simulation models for barnyardgrass.

Rice flatsedge and barnyardgrass are widespread and competitive weeds in direct-seeded rice. Developing integrated weed management strategies that elevate suppression of weeds by rice through crop density, nutrition, and cultivar choice requires better understanding of the extent to which rice interferes with the growth of these weeds and how these species respond to resource limitation with crop interference. Rice interference reduced the height of barnyardgrass but did not affect height of rice flatsedge. These weed species were able to grow taller than rice and thus avoided being shaded. Increased specific stem length under crop interference may demonstrate a strategy of stem elongation to allow the top portion of the weeds to be kept out of shade. Rice interference reduced inflorescence and shoot biomass of both weed species. Barnyardgrass showed the ability to reduce the effects of rice interference by increasing leaf weight ratio. The present study shows that crop interference alone may reduce weed growth but may not give complete control of these weed species. This highlights the need for the integration of management practices to achieve control of these weed species.

Experiments were conducted to determine the efficacy, absorption, and translocation of nicosulfuron, rimsulfuron, and nicosulfuron + rimsulfuron on barnyardgrass, green foxtail, longspine sandbur, and large crabgrass. In the greenhouse, nicosulfuron, rimsulfuron, and nicosulfuron + rimsulfuron were applied at 0.0625, 0.125, 0.25, 0.5, 0.75, 1, and 2 times their label rates of 35, 13, and 26 + 13 g ai ha−1, respectively, on 5- to 10-cm plants. Three weeks after treatment (WAT), barnyardgrass was the most susceptible species to all three herbicides, and large crabgrass was the least susceptible. The nicosulfuron, rimsulfuron, or nicosulfuron + rimsulfuron rates causing 50% visible injury (GR50) for barnyardgrass were 10.9, 4.8, and 6 + 3 g ai ha−1, respectively. Similarly, the GR50for large crabgrass were 25.6, 9.9, and 14.3 + 7.2 g ai ha−1, respectively, 3 WAT. Absorption of nicosulfuron, rimsulfuron, and nicosulfuron + rimsulfuron was greater in barnyardgrass than in large crabgrass. Absorption of nicosulfuron + rimsulfuron in barnyardgrass and large crabgrass was 74% and 57%, respectively, 7 d after treatment (DAT). In addition, translocation of nicosulfuron, rimsulfuron, and nicosulfuron + rimsulfuron out of the treated leaf was 14, 12, and 14% higher, respectively, in barnyardgrass than in large crabgrass. The differential response of these weed species to nicosulfuron, rimsulfuron, and nicosulfuron + rimsulfuron might be due to differences in herbicide absorption and translocation.

Intensive selection pressure from repeated use of propanil and quinclorac led to the evolution of herbicide-resistant barnyardgrass biotypes. Twenty-two composite field samples were tested for level of resistance in 2002 and 2003, and field studies were conducted at the Rice Research and Extension Center, Stuttgart, AR, in 2002 and 2003 to evaluate alternative rice herbicides to control propanil-resistant (PR) and quinclorac-resistant (QR) barnyardgrass. Of the 22 composite samples, four were PR (30 to 40% control); four had a mixed population of PR, QR, and susceptible (S) barnyardgrass; and two had multiple resistance to propanil and quinclorac (P/QR), with control from propanil of 15 to 30% and control from quinclorac of 5 to 10%. ‘Wells’ rice was used where conventional herbicide programs were evaluated, and Clearfield rice ‘CL-161’ (imidazolinone-resistant) was used for herbicide programs involving imazethapyr. All PR and QR barnyardgrass were controlled > 90% by alternative herbicides, including all preemergence (PRE) and delayed preemergence (DPRE) treatments. By 56 d after emergence (DAE), cyhalofop or fenoxaprop applied to two- to three-leaf barnyardgrass (early postemergence [EPOST]), followed by (fb) a preflood application, controlled barnyardgrass > 93%. Pendimethalin controlled PR barnyardgrass 21 DAE, but not all season long. In contrast, imazethapyr in Clearfield rice controlled all grass weeds 100% all season long. Midpostemergence (MPOST) bispyribac application at the four- to five-leaf stage also provided season-long control of all barnyardgrass biotypes (> 88%, 56 DAE). Rice yields ranged from 5,300 to 5,700 kg ha−1in conventional weed-control treatments and from 2,800 to 5,000 kg ha−1in imazethapyr-treated plots. Nontreated plots yielded 1,500 kg ha−1.

Assessing belowground plant competition is complex because it is very difficult to separate weed and crop roots from each other by physical methods. Alternative techniques for separating crop and weed roots from each other are needed. This article introduces a stable isotope method that can quantify the amounts of roots of rice and barnyardgrass intermixed in flooded field soils. It relies on the biological principle that rice, a C3 (photosynthetic pathway) species, discriminates more effectively than barnyardgrass, a C4 species, against a relatively rare isotopic form (13C) of CO2. This results in different 13C: 12C isotope ratios (expressed as δ13C) in root tissues of the two species. δ13C values for monoculture barnyardgrass and rice grown in a standard flood-irrigated system were highly stable over 4 crop-years, averaging −13.12 ± 0.80 (SD) and −28.5 ± 0.11 (SD)‰, respectively, based on analysis by an isotope ratio mass spectrometer. Standard concentration curves relating measured δ13C values to set proportions of rice:barnyardgrass root biomass were described by linear regressions, typically with r2 values of 0.96 or greater. Quantities of intermixed rice and barnyardgrass roots sampled 0 to 5 cm deep from soil between rice rows were estimated by extrapolation from standard curves based on δ13C values. About 50% more barnyardgrass root tissue was detected in plots of Lemont long-grain rice than in weed-suppressive PI 312777 indica rice, demonstrating the feasibility of using this stable carbon isotope method in flooded rice systems.

A fungal pathogen, Helminthosporium gramineum Rabehn f.sp. echinochloae (HGE), has been developed as a mycoherbicidal agent for the control of barnyardgrass in China. Under greenhouse conditions, the effect of the pathogen on disease incidence, mortality, and dry weight reduction of barnyardgrass was tested to determine the potential of this mycoherbicide. Field experiments during 2007 and 2008 showed that the conidia of HGE displayed excellent activity on barnyardgrass and good activity on a few other weed species. The HGE treatment increased the rice yield when compared with the untreated control and had no negative impact on the rice plant. In addition, the progression of HGE infection and the alteration of cellular ultrastructure in infected barnyardgrass were observed by using scanning and transmission electron microscopes. After infection, cell membranes of barnyardgrass leaves were found dramatically changed, and the ultrastructure of cells was severely deformed. This study clearly shows the scope of HGE as a potential mycoherbicide for control of barnyardgrass in agricultural cropping systems and has laid the groundwork for further studies on the mode of infection and the nosogenesis of HGE.

Rice was planted at 78 kg/ha with different densities of creeping rivergrass. Creeping rivergrass at stand densities (SD) of 10,000 to 70,000 stolon segments/ha produced 5.4 to 6 stolons per introduced segment; however, 130,000 through 520,000 SD produced 1.4 to 2.1 stolons per segment. Stolon production was 60,000 plants/ha and increased to 760,000 stolons/ha at planting densities of 10,000 to 520,000/ha, respectively. The 260,000 and 520,000 SD produced similar stolon lengths of 217 and 318 km/ha; however, stolon length was less at lower SD. Total node production was 290,000 nodes per ha with an average of 29 nodes per segment in the 10,000 SD and 5.4 to 9.8 nodes per segment with 70,000 or greater SD. Total biomass increased as SD increased. Creeping rivergrass shoot emergence from soil was 31, 63, 44, and 25% for segments planted at 0, 1.3, 2.5, and 5 cm deep, respectively. In a greenhouse study, glyphosate at 1,260 g ai/ha controlled creeping rivergrass 91% and biomass production was 19% of the nontr...

An experiment was conducted to determine the population dynamics of barnyardgrass under a range of weed management treatments in glyphosate-resistant (GR) cotton in Australia. These treatments consisted of glyphosate herbicide only (Glyphosate only), glyphosate herbicide plus a combination of conventional or integrated weed management practices (Glyphosate + IWM), glyphosate herbicide plus a reduced residual herbicide program (Glyphosate + Res.), glyphosate herbicide plus a grass herbicide (Glyphosate + Grass), and a combination of conventional weed management practices (IWM only). The experiment investigated the effects of weed management on the weed seed bank, weed emergence patterns, and weed populations. After three years, all treatments resulted in commercially acceptable control of barnyardgrass. However, treatments containing soil-applied residual herbicides proved more effective over the period of the experiment. Seed bank reductions were in the order of 100-fold over the period of the experiment ...

As atmospheric carbon dioxide concentration ([CO2]) increases, it is anticipated that the competitive ability of C3 crops could be enhanced relative to C4 weeds in agricultural systems. However, given the different nitrogen use efficiencies of C3 and C4 plants, it is unclear whether any effect of increasing [CO2] on C3/C4 competition is nitrogen dependent. To determine the interaction of [CO2] and N availability on species growth and competitive outcomes, the growth of rice (C3 photosynthetic pathway) was examined in both monoculture and in competition with a common weed, barnyardgrass (C4 photosynthetic pathway) at two levels of N supply (0.357 and 1.071 mmol N L−1) and two levels of [CO2] (ambient and ambient + 200 µmol mol−1) under field conditions in eastern China. In monoculture, the biomass response of rice to elevated [CO2] depended on N supply, whereas the response of barnyardgrass to elevated [CO2] was less dependent on nitrogen. Consequently, when grown in mixture, the proportion of ric...

Bispyribac is registered for postemergence control of broadleaf, sedge, and grass weeds in rice. Bispyribac inhibits the acetolactate synthase enzyme in sensitive plants. Herbicides in this class of chemistry require a spray adjuvant to achieve optimal efficacy, often achieve different levels of weed control according to the spray adjuvant used, and typically have rainfast periods of at least 6 to 8 h. Efficacy and rainfastness of bispyribac can be affected by spray adjuvant and the addition of urea ammonium nitrate (UAN). Greenhouse experiments were conducted to investigate the effect of spray adjuvant type, addition of UAN, and soil moisture on bispyribac efficacy on barnyardgrass. Control of barnyardgrass was improved when UAN was added to bispyribac at 0.4 or 0.8 g ha−1plus an organosilicone-based nonionic surfactant (OSL/NIS) or methylated seed oil/organosilicone (MSO/OSL) spray adjuvant. The type of adjuvant added to the spray solution affected bispyribac efficacy on barnyardgrass. The addition of UAN decreased the rainfast period from 8 h (registered rainfast period) to 1 or 4 h (99 to 100% control) when either the OSL/NIS or MSO/OSL adjuvant was applied with bispyribac, respectively. Applying UAN and OSL/NIS or MSO/OSL adjuvant with bispyribac enhanced efficacy and reduced the time period required between bispyribac application and washoff during a rainfall event. Increasing soil moisture conditions resulted in greater efficacy from bispyribac when applied with and without UAN.

Inconsistent control of barnyardgrass with bispyribac may be alleviated through adjuvant technology. Experiments were conducted to determine the effect of adjuvant and urea ammonium nitrate (UAN) on absorption and translocation of bispyribac in barnyardgrass. Additional experiments were conducted to determine when maximum absorption and translocation occurred with the use of a methylated seed oil/organosilicone adjuvant (MSO/OSL) plus UAN (0.37 L ha−1and 2% v/v). In the initial experiment,14C-bispyribac–treated leaves, nontreated leaves, and roots were collected 6 and 24 h after application. Absorption was greatest with tank-mixed MSO/OSL (0.37 L ha−1) plus UAN (2% v/v) and the proprietary blend of MSO/OSL/UAN (2% v/v) at 80 and 74% of applied14C-bispyribac, respectively. Translocation to nontreated leaves and roots was also highest with these treatments. Increased translocation appeared to be due to greater herbicide absorption, not an increase in translocation rate. The addition of 32% UAN to MSO/OSL and nonionic organosilicone (OSL/NIS) adjuvant systems resulted in a four to fivefold increase in absorption compared with treatments without UAN. Recovery of14C-bispyribac in additional experiments generally decreased as time after application increased; however, recovery was 86% or greater for all time intervals. By 12 h after application, 68% of applied14C-bispyribac was absorbed. At this time,14C-bispyribac was partitioned within the plant in the following manner: 48% in the treated area, 10% in leaf tissue from treated area to tip of the treated leaf, 1.9% in leaf tissue from treated area to collar region of the treated leaf, 1.6% in remaining leaves from collar of treated leaf upward, 5.3% in remaining leaves from collar of treated leaf downward to soil line, and 0.7% in the roots. These data indicate that maximum absorption was achieved within 12 h with a tank mix of MSO/OSL and UAN or the MSO/OSL/UAN blend.

Studies were initiated to evaluate absorption, translocation, and distribution of 14C-quinclorac in propanil- and quinclorac-resistant (R-BYG) and -susceptible (S-BYG) barnyardgrass. No differential absorption of 14C-quinclorac was observed between R-BYG and S-BYG, but more 14C remained in the treated leaf of S-BYG (57% of total absorbed) compared with the R-BYG leaf (34% of total absorbed) at 72 h after treatment (HAT). After 12 HAT, 20 and 15% of the amount absorbed had been translocated basipetally by R-BYG and S-BYG, respectively. At 72 HAT, 27 and 17% of the total absorbed 14C had been translocated acropetally by R-BYG and S-BYG, respectively. The levels of 14C above the treated leaf continued to increase throughout the duration of the experiment in R-BYG while levels of 14C above the treated leaf in S-BYG remained relatively constant. Seven percent more of the total absorbed 14C was exuded from roots of R-BYG than S-BYG at 72 HAT. Although differential translocation was observed between R-BYG and S-...

Arkansas has been the leading state in rice production in the United States for many years. Barnyardgrass is the dominant weed in Arkansas rice. Propanil was the first highly effective herbicide for weed control in rice and has been used in Arkansas since 1959 as the primary herbicide for rice weed control. By 1989, its continual use led to the development of propanil-resistant barnyardgrass, which had spread to 16 of the 38 rice-producing counties in Arkansas by 1992. Arkansas rice growers are dependent on herbicides for the control of weeds in this drill-seeded crop. The residual herbicides thiobencarb, molinate, and pendimethalin mixed with propanil applied early postemergence improved control of propanilresistant barnyardgrass. But it was quinclorac, introduced in 1992, that became the real replacement treatment for propanil-resistant barnyardgrass. Then in 1999, a barnyardgrass biotype with resistance to both quinclorac and propanil was confirmed in Craighead County, Arkansas. Additionally, problems with quinclorac drift to other crops, especially tomato, led to restrictions on application of quinclorac in Arkansas by 1994. Fortunately, alternative herbicides for barnyardgrass control were developed, and clomazone was introduced in 2000. Clomazone is currently the standard herbicide for annual grasses in rice, including barnyardgrass. Herbicides recently developed for rice allow a broad range of options for a resistance management program, based on rotational and sequential herbicide applications. These include fenoxaprop and cyhalofop (both acetyl-CoA carboxylase [ACCase] inhibitors), bispyribac and penoxsulam (acetolactate synthase [ALS] inhibitors), and imazethapyr and imazamox (also ALS inhibitors for imidazolinone-resistant rice). From a global standpoint, there is considerable evidence that barnyardgrass has the propensity to develop resistance to most of these herbicide groups. Therefore, efforts to manage and monitor for herbicide resistance in this species need to be diligently continued. Research on nonchemical options is in progress utilizing weed-suppressive rice breeding lines to control barnyardgrass. Nomenclature: 2,6-bis[(4,6-dimethoxy-2-pyrimidinyl)oxy]benzoic acid, proposed common name bispyribac-sodium; clomazone; cyhalofop; fenoxaprop; imazamox; imazethapyr; molinate; pendimethalin; {2-(2,2-difluoroethoxy)-6(trifluoromethyl-N-(5,8-dimethoxy[1,2,4]triazolo[1,5-c]pyrimidin-2-yl))benzenesulfonamide}, proposed common name penoxsulam; propanil; quinclorac; thiobencarb; barnyardgrass, Echinochloa crus-galli (L.) Beauv. ECHCG; rice, Oryza sativa L.; tomato, Solanum lycopersicum L.

The response of new, high-yielding rice cultivars to varying barnyardgrass control levels and rice plant densities is not well understood. Studies were conducted in 2002 through 2004 at the Rice Research and Extension Center near Stuttgart, AR, to evaluate three new rice cultivars at plant densities ranging from 79 to 392 plants/m2grown in competition with barnyardgrass. Representatives from each of the three classes of long-grain rice produced in the United States were selected. ‘Wells’ represented conventional, long-grain rice, ‘CL161’ represented semidwarf, imidazolinone-tolerant, long-grain rice, and ‘XL8’ represented hybrid, long-grain rice. Rice density and barnyardgrass control affected rice total aboveground biomass production, panicle weight, and harvest index. As rice density decreased or barnyardgrass control increased, total aboveground biomass production and harvest index increased. Rice density had no effect on panicle density or yield. Panicle density increased 14 panicles/m2for every 10% increase in barnyardgrass control. Cultivar yields were affected similarly by barnyardgrass control, and increased 750 kg/ha for each 10% increase in barnyardgrass control. XL8 and Wells produced the highest average yields, with CL161 producing the lowest over the three-year experiment. XL8 yield components fluctuated more than CL161 or Wells over the range of rice densities and barnyardgrass control levels, indicating that XL8 is highly sensitive to intra- and interspecific competition at high plant populations.

Laboratory studies were conducted to examine the leaf surface, epicuticular wax content, and spread area of primisulfuron spray droplet with and without surfactant on leaf surface of barnyardgrass and green foxtail. Adaxial and abaxial leaf surfaces were examined using scanning electron microscopy and leaf wax was extracted and quantified. The spread of 1-μl droplets of distilled water, primisulfuron solution (without surfactant), primisulfuron solution with a nonionic low foam wetter/spreader adjuvant (0.25% v/v), and with an organosilicone wetting agent (0.1% v/v) was determined on the adaxial leaf surfaces of each of the weed species. Stomata and trichomes were present on adaxial and abaxial leaf surfaces in both species. Green foxtail had more stomata per unit area on the adaxial as compared to the abaxial leaf surface. Barnyardgrass had more stomata on the abaxial than on the adaxial leaf surface. There was no significant variation in the number of trichomes per unit leaf area of green foxtail, and the number of prickles per unit area of leaf was significantly higher in adaxial than the abaxial leaf surface, in both young and old leaves. In barnyardgrass, there were more trichomes on abaxial than adaxial leaf surface. The mean value of the wax content per unit of leaf area in barnyardgrass and green foxtail was 35.9 μg cm−2and 19.1 μg cm−2, respectively. On both species primisulfuron with a nonionic surfactant had more spread area than that without a surfactant, and the spread was even greater with organosilicone wetting agent. The spread area of primisulfuron droplet was higher on the leaf surface of barnyardgrass than on green foxtail when surfactant was added.

Three field experiments were conducted to evaluate efficacy of BAS 625, cyhalofop, and fenoxaprop plus isoxadifen when applied alone or with acifluorfen plus bentazon, acifluorfen, bentazon, triclopyr, bensulfuron, halosulfuron, carfentrazone, and propanil. Broadleaf signalgrass control with fenoxaprop plus isoxadifen was reduced by triclopyr and halosufuron. None of these herbicides reduced Amazon sprangletop control by fenoxaprop plus isoxadifen. Barnyardgrass control with fenoxaprop plus isoxadifen was reduced by halosulfuron and propanil. Barnyardgrass and Amazon sprangletop control by cyhalofop was reduced by bentazon plus acifluorfen, bentazon, acifluorfen, triclopyr, bensulfuron, or halosulfuron. Broadleaf signalgrass control with cyhalofop was reduced by triclopyr and halosulfuron. Amazon sprangletop control by cyhalofop was reduced by all herbicides. Barnyardgrass control with BAS 625 was reduced when applied with propanil, bentazon, or bentazon plus acifluorfen. Broadleaf signalgrass control with BAS 625 was reduced when applied with bentazon plus acifluorfen. Amazon sprangletop control with BAS 625 was reduced when applied with bentazon, acifluorfen, halosulfuron, or propanil.

Field studies were conducted over 3 yr at two locations to evaluate the effects of glyphosate rate and application timing on barnyardgrass control, seed production, seed viability, and seedbank density the year after herbicide application in glyphosate-resistant corn. Glyphosate was applied at 0, 112, 225, 450, 675, or 900 g ai/ha when barnyardgrass was at the two-, four-, or six-leaf stage of growth. Visual estimates of percent control increased whereas density, dry weight, seed production, and seedbank density the year after treatment decreased as the rate of glyphosate was increased from 0 to 450 g/ha. Increasing the rate of glyphosate from 450 to 900 g/ha (registered rate) had no further effect on any measured parameter. Seed viability was not affected by glyphosate rate nor application timing. Corn yield declined only at a glyphosate rate of 225 g/ha and below. Barnyardgrass control improved as application was delayed to the six-leaf stage because this weed had an extended period of emergence. There ...

Field experiments in 2001 and 2002 at Stuttgart, AR, demonstrated cyhalofop applied POST controlled propanil-susceptible barnyardgrass, propanil-resistant barnyardgrass, and broadleaf signalgrass 67, 65, and 71%, respectively 28 days after treatment (DAT). Halosulfuron applied 1 d before cyhalofop or applied in a tank-mixture reduced broadleaf signalgrass and propanil-resistant or propanil-susceptible barnyardgrass by 35 to 59 percentage points. Triclopyr applied 5, 3, or 1 d before, or mixed with, cyhalofop reduced control of propanil-resistant and propanil-susceptible barnyardgrass, 16 to 41 percentage points, respectively. Triclopyr applied 1 d before cyhalofop or combined with cyhalofop reduced broadleaf signalgrass control 38 and 18 percentage points, respectively compared with cyhalofop alone. Propanil reduced cyhalofop grass control only when mixed with cyhalofop. Application timings, averaged across halosulfuron, propanil, and triclopyr, made 1 d before cyhalofop or tank-mixed with cyhalofop reduc...

The response of rice and barnyardgrass to clomazone rates (0.22, 0.45, 0.67, 0.9, and 1.12 kg ai/ha) and application timings of preplant incorporated, preemergence (PRE), and delayed PRE (DPRE) were evaluated from 1997 to 1999. Rice bleaching at 14 d after planting (DAP) increased from 5 to 35% as the clomazone rate increased. At 14 and 49 DAP, barnyardgrass control decreased with clomazone at 0.22 kg/ha compared with control at higher rates. Clomazone at 0.22 kg/ha resulted in lower grain yield. Clomazone PRE caused less rice bleaching and lower grain yield compared with DPRE timings. These results indicate that initial bleaching by high clomazone rates may not translate into yield loss. Higher clomazone rates may increase rice yield by improving weed control. Nomenclature: Clomazone; barnyardgrass, Echinochloa crus-galli (L.) Beauv. #3 ECHCG; rice, Oryza sativa L., ‘Lemont’, ‘Cypress’. Additional index words: Application timing, herbicide rate response. Abbreviations: DAP, days after planting; DPRE, del...

A study was conducted in 2000 and 2001 to evaluate interaction of fenoxaprop with other herbicides for barnyardgrass control in rice. Changes in herbicide interaction over time were also evaluated, and herbicide combinations were ranked on the basis of compatibility. Fenoxaprop at 0.075 kg/ha plus bentazon or propanil plus molinate resulted in an additive response for barnyardgrass control at 10, 20, and 30 d after treatment (DAT); however, when the rate of fenoxaprop increased to 0.089 kg/ha, an antagonistic effect was found. Carfentrazone and halosulfuron consistently antagonized the activity of fenoxaprop at both rates on barnyardgrass. Bensulfuron at 10 and 20 DAT and triclopyr at 20 DAT were antagonistic to fenoxaprop. An increase in interaction over time was detected when fenoxaprop at 0.089 kg/ha was applied in mixture with carfentrazone at 0.04 kg/ha or halosulfuron at 0.05 kg/ha. These results indicate that propanil plus molinate and bentazon are more compatible with fenoxaprop at 0.075 kg/ha for barnyardgrass control, whereas bensulfuron, carfentrazone, halosulfuron, and triclopyr can antagonize fenoxaprop activity on barnyardgrass.

Barnyardgrass (BYG) has been the most frequently reported troublesome weed in rice because it is an aggressive invader, is difficult to control, and reduces yields significantly. A replacement series study was conducted to determine how a naturally suppressive cultivar (T65*2/TN 1; ‘PI 312777’), a nonsuppressive cultivar (‘Lemont’), and an F3 cross between the two (‘PI 312777 × Lemont’) would interfere with BYG in the southern United States. The rice cultivars did not differentially affect BYG height. The PI 312777 produced more tillers and greater shoot dry weight but was only moderately competitive (relative yield [RY]) or aggressive (relative crowding coefficient) against BYG. Competitiveness at 2:2 rice and BYG mixture proportion and replacement series illustrations on RYs for the number of tillers and shoot dry weights for the three rice cultivars indicated that PI 312777 suppressed BYG growth relatively better than the other two cultivars. Plant-for-plant, PI 312777 was more competitive tha...

Thirteen hard red winter wheat cultivars were evaluated for their ability to suppress summer annual weeds in grain production systems near North Platte, NE, from 1993 through 1997. ‘Turkey’, a 125-yr-old landrace selection, suppressed both broadleaf and grass weeds more than other cultivars. Some relatively new cultivars, such as ‘Arapahoe’, ‘Jules’, ‘Pronghorn’, and ‘Vista’ suppressed summer annual grasses almost as well as Turkey. Total weed density was negatively correlated with number of winter wheat stems/m2, mature winter wheat height, and lodging. Weed density after wheat harvest was positively correlated with delay in winter wheat seeding date and was negatively correlated with precipitation 0 to 30 d after winter wheat seeding, during tillering, tillering to boot stage, and heading to maturity stage. Mean air temperature 0 to 30 d after wheat seeding was positively correlated with weed density. In the spring, weed density was positively correlated with temperatures during the tillering stage, tillering to boot stage, and heading to maturity stage. Stinkgrass and witchgrass densities were positively correlated with severity of wheat leaf rust. The highest grain-producing cultivars included three medium height cultivars ‘Alliance’, Arapahoe, and ‘Niobrara’. Alliance wheat produced 53% more grain than Turkey, and the other two produced 43% more grain.

Competition between wet-seeded rice and barnyardgrass under two distinct environments was analyzed using a two-parameter response–surface model at the International Rice Research Institute in the Philippines. The findings showed that this model could predict crop yield loss due to weed competition over a wide range of crop and weed densities. The low-tillering, new plant–type cultivar was a weaker competitor and had a higher yield loss than high-tillering cultivar ‘IR72’ and a hybrid. Increasing the crop density could reduce yield loss due to weed competition. This effect was greater for the new plant type than for IR72 and the hybrid when barnyardgrass density was low. In contrast, this effect was less for the new plant type than for IR72 and the hybrid when the weed density was high. Competitiveness of the three rice cultivars was also affected by season. Crop yield loss was higher in the wet season than in the dry season. Nomenclature: Barnyardgrass, Echinochloa crus-galli (L.) Beauv. ECHCG; r...

Field studies were conducted to compare the barnyardgrass suppression by four U.S. (‘Starbonnet’, ‘Kaybonnet’, ‘Lemont’, and ‘Cypress’) and three highly competitive, high-yielding Asian cultivars (‘PI 312777′, ‘Guichao’, and ‘Teqing’). The economic consequence of applying less than the recommended propanil rates to these cultivars was also evaluated. Grain yields increased, and barnyardgrass biomass decreased with increasing propanil rates. With or without propanil, the Asian rice cultivars consistently suppressed barnyardgrass more and consequently produced higher grain yields than did U.S. cultivars. The economic benefit derived from propanil application was less for Asian than for U.S. cultivars. Asian cultivars produced higher rough rice yields, resulting in higher net returns (not adjusted for milling) than did the commercial cultivars, but this advantage was usually reduced when adjusting for their lower milling yields. These results suggest that growing weed-suppressive Asian rice cultivar...

A 3-yr study was conducted to establish if the presence of corn had an effect on the emergence patterns and total weed seedling density under growing conditions in southwestern Quebec. Weed seedling emergence was monitored in permanent quadrats throughout the growing season in the presence and absence of growing corn. Common lambsquarters and barnyardgrass were prevalent in most site-years. The presence of corn did not affect the patterns of common lambsquarters and barnyardgrass emergence nor their total weed seedling density except in 1994. Corn canopy was probably not sufficiently developed to affect light levels or soil temperature needed for weed germination and, consequently, seedling emergence. In 1994, in the absence of corn, some soil crusting was observed on a fine-textured soil, and the total number of seedlings was reduced. The results of these weed emergence studies in corn can be extended to other crops growing with wide row spacing and relatively slow canopy closure similar to those of grai...

The cause of differential susceptibility of barnyardgrass, hemp sesbania, pitted morningglory, and prickly sida to glyphosate was examined by measuring the absorption of 14C-glyphosate, quantifying the amount of epicuticular wax, and observing the wettability of leaf surfaces. In greenhouse experiments, the biomass of barnyardgrass and prickly sida was reduced by 95% by Roundup Ultra®. Hemp sesbania and pitted morningglory showed more tolerance, with 66 and 51% average biomass reduction, respectively. Absorption of 14C-glyphosate in a controlled environment did not follow the trend in species susceptibility with barnyardgrass, 30%; prickly sida, 18%; hemp sesbania, 52%; and pitted morningglory, 6%; absorption. The high tolerance of pitted morningglory to glyphosate can be attributed mostly to limited absorption, but the tolerance of hemp sesbania is due to other mechanisms. The addition of nonionic surfactant (NIS) to a low rate of Roundup Ultra® reduced absorption of 14C-glyphosate by barnyardgrass and h...

Field experiments were conducted during 1998, 1999, and 2000 in Santa Fe, Argentina, to evaluate chlorimuron, clethodim, flumetsulam, 2,4-DB, glyphosate, and haloxyfop for control of broadleaf and grass weeds in established alfalfa. Herbicides were applied to alfalfa, 6 to 10 cm in height, after first or second cuttings. Clethodim and haloxyfop were highly effective against bermudagrass, barnyardgrass, and knotroot foxtail, and alfalfa yields were increased. 2,4-DB was effective against Scotch thistle, swinecress, turnipweed, and birdsrape mustard, but no effects were observed on alfalfa yields. Chlorimuron, flumetsulam, and glyphosate were effective against burning nettle, common chickweed, common lambsquarters, spiny cocklebur, and Persian speedwell, but they injured alfalfa and decreased yields. Nomenclature: Chlorimuron; clethodim; 2,4-DB; flumetsulam; glyphosate; haloxyfop; alfalfa, Medicago sativa (L.); barnyardgrass, Echinochloa crus-galli (L.) Beauv. #3 ECHGA; bermudagrass, Cynodon dacty...

Greenhouse studies were conducted to evaluate weed control from various formulations of glyphosate alone and in combination with postemergence herbicides. Tank mixtures did not increase barnyardgrass control 2 wk after treatment (WAT) when compared with glyphosate alone; however, tank mixtures did reduce barnyardgrass fresh weight 4 WAT when compared with glyphosate alone in several instances. Antagonism was observed when chlorimuron was combined with all formulations of glyphosate 4 WAT, but control was not reduced when compared with glyphosate alone. Selective herbicides added to glyphosate had an additive or antagonistic effect on prickly sida fresh-weight reductions. Antagonism of pitted morningglory fresh-weight reductions occurred when glyphosate was combined with all herbicides except acifluorfen, which had an additive effect. Fomesafen or lactofen effectively controlled hemp sesbania 2 WAT without the addition of glyphosate. Acifluorfen and chlorimuron combined with glyphosate Cheminova,...

A greenhouse study was conducted to evaluate the efficacy of imazethapyr at 35 and 53 g ai/ha applied preplant incorporated (PPI) or postemergence (POST) under various soil moisture regimes (13, 19, 25, and 50%) on barnyardgrass and red rice. Response of barnyardgrass and red rice to imazethapyr PPI was affected by soil moisture. With imazethapyr PPI, control of barnyardgrass was reduced at 50% soil moisture compared with other soil moisture regimes, and height of barnyardgrass increased as soil moisture increased from 19 to 50% 2 weeks after treatment (WAT). Barnyardgrass control declined and plant dry weight increased with the increase of soil moisture from 19 to 50% at 3 WAT. Imazethapyr PPI activity on red rice was reduced at 50% compared with other soil moisture regimes, as reflected by decreased control ratings as well as increased plant height and dry weight. Imazethapyr activity on barnyardgrass and red rice was increased at 50% soil moisture when applied POST compared with PPI. Imazetha...

A study was conducted in 1999 to evaluate weed control and imidazolinone-tolerant (IMI-tolerant) rice response to 140 g ai/ha imazethapyr at different application timings and to flood depths of 5, 10, and 20 cm at the Rice Research Station near Crowley, LA, and at the Northeast Research Station near St. Joseph, LA. Barnyardgrass control, rice injury, and rice yield were not influenced by flood depth; therefore, data were averaged over flood depths. Season-long barnyardgrass control was greater than 80% at both locations with imazethapyr applied preplant incorporated (PPI), early postemergence (EPOST), and mid-postemergence (MPOST). IMI-tolerant rice injury at 7 d after late postemergence (LPOST) treatment (DAT) was less than 5% for all imazethapyr treatments at Crowley and 1 to 8% for PPI, preemergence (PRE), delayed preemergence (DPRE), and EPOST timings at St. Joseph. However, IMI-tolerant rice injury was 5% or less at 42 DAT for all treatments at Crowley and St. Joseph. IMI-tolerant rice yiel...

A study was established to evaluate weed control and crop response of imidazolinone-tolerant (IMI-tolerant) rice in water-seeded culture with imazethapyr at 70, 105, and 140 g ai/ha at four different soil application timings with or without 70 g/ha imazethapyr postemergence (POST). Application timings included preplant incorporated (PPI), surface application prior to seeding (SURFACE), following seeding (SEED), and at pegging (PEG). The PEG treatments were applied when green leaf tissue had emerged from the seed and the root had begun to extend downward into the soil. Response of barnyardgrass, red rice, and rice injury was not affected by the addition of an imazethapyr POST application. Barnyardgrass control was above 90% at 28 d after treatment (DAT) for both years. At 42 DAT, barnyardgrass control decreased to 88% in 1998, compared with 94% control in 1999. Averaged over years, Indian jointvetch control ranged from 44 to 74% at 28 DAT and 41 to 71% at 42 DAT. Indian jointvetch control was inc...

Glyphosate is often mixed with residual herbicides to control emerged weeds in no-till crop production systems. Field studies were conducted in Ohio from 1992 to 1994 to evaluate the weed control provided by residual herbicides and reduced rates of glyphosate in full-season, no-till soybean. Herbicide treatments were applied at two timings to examine the effect of weed size. At 4 wk after treatment, greater than 85% Pennsylvania smartweed control was obtained with metribuzin plus chlorimuron and linuron plus chlorimuron applied with 280 g ai/ha glyphosate and with imazethapyr and imazaquin applied with 560 g/ha glyphosate. All residual herbicides provided at least 85% common lambsquarters control when applied with 560 g/ha glyphosate. In 1992, the early application of residual herbicides provided this level of common lambsquarters control without glyphosate due to the small weed size at the time of application. Residual herbicides applied with 280 g/ha glyphosate controlled giant foxtail 85% or ...