Weed Survival Research Articles

Relative uptake of organic and inorganic nitrogen by common weed species

AbstractThe extent to which weed species vary in their ability to acquire and use different forms of nitrogen (N) (inorganic and organic) has not been investigated but could have important implications for weed survival and weed–crop competition in agroecosystems. We conducted a controlled environment experiment using stable isotopes to determine the uptake and partitioning of organic and inorganic N (amino acids, ammonium, and nitrate) by seven common weed and non-weed species. All species took up inorganic and organic N, including as intact amino acids. Concentrations of 15N derived from both ammonium and amino acids in shoot tissues were higher in large crabgrass [Digitaria sanguinalis (L.) Scop.] and barnyardgrass [Echinochloa crus-galli (L.) P. Beauv] than in common lambsquarters (Chenopodium album L.), redroot pigweed (Amaranthus retroflexus L.), and sorghum-sudangrass [Sorghum bicolor (L.) Moench × Sorghum bicolor (L.) ssp. drummondii (Nees ex Steud.) de Wet & Harlan]. In contrast, the concentration of 15N derived from nitrate was higher in wild mustard (Sinapis arvensis L.) shoots than in wild oat (Avena fatua L.) shoots. Root concentration of 15N derived from ammonium was lower in sorghum-sudangrass compared with other species, except for A. retroflexus and A. fatua, while root concentration of 15N derived from nitrate was lower in A. retroflexus compared with other species, except for C. album and S. arvensis. Discriminant analysis classified species based on their uptake and partitioning of all three labeled N forms. These results suggest that common agricultural weeds can access and use organic N and differentially take up inorganic N forms. Additional research is needed to determine whether species-specific differences in organic and inorganic N uptake influence the intensity of competition for soil N.

Integration of Halosulfuron and Anaerobic Soil Disinfestation for Weed Control in Tomato

Anaerobic soil disinfestation (ASD) is a preplant pest management technique that involves amending the soil with a labile carbon source, irrigating the soil to stimulate decomposition, and then covering the soil with polyethylene film (polyfilm) to limit gas exchange. During the ASD process, soil microorganisms shift from aerobic to anaerobic metabolism and release phytotoxic byproducts such as organic acids and gases. Although it has been shown that these phytotoxic by-products have a negative impact on weed survival, questions remain about whether commercial-level weed control can be achieved using ASD alone or in combination with other chemicals. Greenhouse and field studies were conducted to evaluate ASD with mustard (Brassica sp.) meal, molasses, and herbicide applications for yellow nutsedge (Cyperus esculentus) control in tomato (Solanum lycopersicum). The treatments in these studies included factorial of two carbon sources [mustard meal + molasses (MMM) or no carbon amendment], three herbicide treatments [halosulfuron applied preemergence (PRE), halosulfuron applied postemergence (POST), and no herbicide] and two polyfilm treatments (polyfilm cover or polyfilm uncover). In field trials two polyfilm cover treatments were punctured and nonpunctured. Soil treatments included molasses at 14,000 L·ha−1 and mustard meal at 2100 kg·ha−1. Halosulfuron was applied at a rate of 1 oz/acre for PRE or POST applications. Greater anaerobic conditions were achieved in polyfilm cover treatments amended with MMM. In greenhouse and field trials, the most effective treatments for reducing yellow nutsedge populations were ASD with MMM or combined with halosulfuron application (PRE- or POST-ASD), which delivered significantly higher weed control than all other treatments tested or controls. In field trials, ASD with MMM caused plant growth stunting 14 d after transplantation (DAT); however, plants recovered, and stunting or injury was often not observed at 42 DAT. These studies demonstrated that ASD using MMM can be an effective strategy for reducing yellow nutsedge populations; however, the more research is needed to ensure crop safety while using ASD technology.

Cyperus iria Weed Growth, Survival, and Fecundity in Response to Varying Weed Emergence Times and Densities in Dry-Seeded Rice Systems

Cyperus iria is amongst the most threatening weeds of rice in Sri Lanka, India, Pakistan, and the Philippines. Broad knowledge about the ecology and fecundity of C. iria is important for its effective management. Field studies were conducted over two seasons (wet (WS) and dry seasons (DS)) to investigate the influence of C. iria populations (40 and 80 plants m−2) on its growth, survival, and fertility, with four emergence times, 3, 15, 30, and 45 d after rice emergence (DARE), during 2013. We postulated that (a) higher plant density of this weed would result in more biomass production and viable seeds per unit area, (b) interference of rice would reduce the biomass production and fecundity of this weed, and (c) delaying weed emergence would lead to downsizing of its percent survival and seed bank enrichment in soil. The results indicated that rice interference decreased C. iria growth and seed production as compared with those of plants established without rice interference. A linear decrease in the percent survival of C. iria without rice and a sigmoid decrease with rice were observed during both seasons. Plant height of C. iria was moderately affected up to 30 DARE, and a significant reduction was observed at 45 DARE. There was a linear relationship between C. iria shoot dry weight and seed number plant −1, across-weed density, seeding rate of rice, and emergence time. C. iria seed production, 1000-seed weight, and seed yield were greater when seedlings emerged simultaneously with the rice crop (3 DARE) than when they emerged late. Under rice weed interference growth, the production of viable seeds was completely stopped at 45 DARE. The delay in the emergence of C. iria up to 45 DARE was unable to produce seed in both seasons. The results of the current studies advocate that the emergence, weed biomass, and seed production of C. iria can be checked by adopting suitable cultural weed management practices, which can impede the emergence of weed relative to rice. These practices, enabling the respective crops to be more competitive, will foster integrated weed management approaches, thus offering a key role in seed detection to the soil seed reservoir or pool by notorious weeds in the field.

Effect of soil moisture regimes on the growth and fecundity of slender amaranth (Amaranthus viridis) and redroot pigweed (Amaranthus retroflexus)

AbstractSlender amaranth (Amaranthus viridisL.) and redroot pigweed (Amaranthus retroflexusL.) are increasingly problematic weeds of summer crops in Australia. Water is considered the most limiting factor in an agroecosystem, and water stress adversely impacts the growth and reproduction of plant species. The primary objective of this study was to determine the growth and fecundity of two Australian biotypes (Goondiwindi and Gatton) ofA. viridisandA. retroflexusunder water-stress conditions. Four water-stress treatments (100%, 75%, 50%, and 25% field capacity [FC]) at a 4-d irrigation interval were chosen. No difference was observed for growth and seed production between the two biotypes of both species when grown under varying soil moisture regimes. At 100% FC,A. viridisproduced 44 g plant−1aboveground biomass and 1,740 seeds plant−1. The maximum growth (46 g plant−1) and seed production (3,070 seeds plant−1) ofA. retroflexuswere observed at 100% FC. The growth and seed production of both species were reduced with increased water-stress levels. Both weeds responded to water stress by decreasing the shoot:root biomass ratio. However,A. viridis(290 seeds plant−1) andA. retroflexus(370 seeds plant−1) were able to produce a significant number of seeds per plant even at 25% FC. Results suggest that both weeds will produce seeds under water-limiting conditions. Therefore, management strategies are required to minimize the growth and survival of weeds in water-deficit conditions.

Integrating emergence and phenology models to determine windows of action for weed control: A case study using Senna obtusifolia

• Weed phenology can be modeled as a function of seedling emergence timing. • Phenology models can be used to determine windows of action for weed control. • Windows of action can complement the critical period of weed control concept. The success of integrated weed management strategies is contingent on the accuracy of control actions in both time and space. While emphasis has been given to spatial accuracy, timing accuracy has been largely neglected. Weed control timing must consider not only the total duration of weed interference with the crop, as done by the traditional critical period of weed control (CPWC) based on yield protection only, but also weed growth, size, and susceptible phenological stages. In this study, we expand upon the idea of using weed emergence models for timing weed control by integrating them with phenology probability models for key weed growth stages to optimize timing of control actions, here referred as Critical Control Windows (CCW). Combining the CCW with thresholds for yield loss due to weed interference and thresholds for weed survival risk makes it possible determining the frequency with which control actions should be implemented to maintain crop yield and weed populations at desired levels. Using Senna obtusifolia as a study case, vegetative and reproductive phenological stages were modeled as a function of seedling emergence for different cohorts. Chronological and thermal-time models provided robust predictions of S. obtusifolia phenology. CCW did not always coincided with CPWC for several crops when considering 10-cm tall plants as the threshold for control. In general, for summer row crops, CCW required 2 postemergence control actions and sometimes 1 action outside the CPWC. The results of the present research illustrate how predictive models can be used to develop CCW that will complement the traditional CPWC. These two concepts when used complementary can increase not only timing accuracy, but also efficiency of weed control.

Effect of crop canopy and herbicide application on kochia (Bassia scoparia) density and seed production

AbstractUnderstanding the effects of crop management practices on weed survival and seed production is imperative in improving long-term weed management strategies, especially for herbicide-resistant weed populations. Kochia [Bassia scoparia (L.) A.J. Scott] is an economically important weed in western North American cropping systems for many reasons, including prolific seed production and evolved resistance to numerous herbicide sites of action. Field studies were conducted in 2014 in a total of four field sites in Wyoming, Montana, and Nebraska to quantify the impact of different crop canopies and herbicide applications on B. scoparia density and seed production. Crops used in this study were spring wheat (Triticum aestivum L.), dry bean (Phaseolus vulgaris L.), sugar beet (Beta vulgaris L.), and corn (Zea mays L.). Herbicide treatments included either acetolactate synthase (ALS) inhibitors effective on non-resistant B. scoparia or a non–ALS inhibiting herbicide effective for both ALS-resistant and ALS-susceptible B. scoparia. Bassia scoparia density midseason was affected more by herbicide choice than by crop canopy, whereas B. scoparia seed production per plant was affected more by crop canopy compared with herbicide treatment. Our results suggest that crop canopy and herbicide treatments were both influential on B. scoparia seed production per unit area, which is likely a key indicator of long-term management success for this annual weed species. The lowest germinable seed production per unit area was observed in spring wheat treated with non–ALS inhibiting herbicides, and the greatest germinable seed production was observed in sugar beet treated with ALS-inhibiting herbicides. The combined effects of crop canopy and herbicide treatment can minimize B. scoparia establishment and seed production.

Influence of soil moisture regimes on growth, photosynthetic capacity, leaf biochemistry and reproductive capabilities of the invasive agronomic weed; Lactuca serriola.

Global temperatures are predicted to increase by 1.5–5.9°C during this century, and this change is likely to impact average rainfall, with predictions that water deficit will perhaps be the most severe threat to sustainable agriculture. In this respect, invasive weeds, which have traits better adapted to drought stress than crops, add to concerns regarding crop sustainability. Lactuca serriola, an aggressive agronomic weed is thought to be a successful weed because of its ability to maintain high water use efficiency under drought conditions. In this study, experiments were conducted to examine the influence of different soil moisture regimes (100%, 75%, 50% and 25% water holding capacity (WHC)) on growth, photosynthetic capacity, leaf biochemistry and reproduction of this species. Soil moisture significantly affected plant’s height, stem diameter, number of leaves and biomass. The highest plant height (115.14 cm ± 11.64), shoot diameter (9.4 mm ± 0.18), leaf area (1206.5 mm2 ± 73.29), plant fresh weight (83.1 ± 3.98) and dry weight (22.38 ± 1.24) were recorded at 75% soil moisture content. A fundamental adaptation to drought was observed as plants in the 25% WHC treatment had the highest root: shoot ratio. Soluble sugars and total phenolic content were highest in the 25% WHC treatment and significantly different to 100% WHC which was a response to soil moisture stress to ameliorate the damaging effects of reactive oxygen species produced under stress conditions. Results also indicate that L. serriola can survive and produce seeds under water stress as more than 6000 seeds were produced per plant in all WHC treatments. In this study, there was no significant difference in the seed weight, number of seeds produced and their germination ability. This can have a huge impact on agricultural systems as the species can survive both under low and high soil moisture conditions. We therefore suggest that the demonstrated ability of L. serriola to complete its life cycle and produce biomass and seeds under water stressed conditions leads to the introduction of strategies that minimize weed survival while maximizing irrigation efficiency for the crop. A clear understanding of the ecological and biological characteristics of this weed will help land managers take appropriate control measures to mitigate the effect of this species on economic crop productivity.

Tolerance of transplanted guayule seedlings to post-emergence herbicides

Guayule (Parthenium argentatum Gray) is gaining a wide attention as natural rubber crop that is successfully grow in the arid and semi-arid regions. Genetically improving of guayule germplasm for higher rubber production and maximizing agronomic practices are the prioritized research areas. Among agronomic practices is weed control, where weeds compete with guayule plants and thus could reduce its productivity. To study the effect of herbicides on transplanted guayule seedlings and weeds survival rates, four post-emergence herbicides were tested. Three guayule genotypes were planted in a repeated split plot arrangement and sprayed with the post-emergence herbicides at different concentrations and time intervals. Results showed that after second spray, herbicide type and concentration had significant effect on guayule seedlings survival rate. Paraquat rates were effectively able to control weed growth and viability but affected survival of guayule seedlings. Paraquat at 840 g a.i. ha−1 resulted in 96% of guayule seedlings survived, and 97.8% of weed populations were inhibited. More studies with different concentrations for other herbicides are required. The optimal concentration of paraquat herbicide has non-significant effect on guayule plant height compare with control treatment, suggests herbicide effects on guayule plant are most effective during seedling establishment stage. When use new herbicide application caution should be paid to control weed growth in the same time reduce the damage on guayule plants.

Efficacy of Irradiated Glyphosate Against Brachiaria plantaginea L.

Glyphosate irradiated with cobalt-60 type Gammacell 220 at different rates was sprayed on seedlings of Brachiaria plantaginea L., a common weed, grown in pots (one seedling with 3-4 leaves in each pot), at different concentrations equivalent to 1 L h-1 or 3 L h-1. Each sample containing 40 ml of the product (glyphosate) taken to a source of Cobalt-60 type Gammacell 220, under a dose rate of 0.312 kGy/hour in doses of: T0, control (water instead of glyphosate); T1, glyphosate at 1.0 L h-1; T2, glyphosate at 3.0 L h-1; T3, glyphosate irradiated at 250 Gy at 1.0 L h-1; T4, glyphosate irradiated at 500 Gy at 1.0 L h-1; T5, glyphosate irradiated at 750 Gy at 1.0 L h-1; and T6, glyphosate irradiated at 1000 Gy at 1.0 L h-1. The effects were assessed in terms of the extent of injury, where scale of 0% (no visible injury) to 100% (plant dead), number of tillers, weed survival and dry biomass produced by the plants. The most effective treatment was glyphosate irradiated at 250 Gy h-1 and applied at 1.0 L h-1. It is important to know that gamma radiation in glyphosate herbicide can be used without antagonistic interactions without cause resistance.

Weed ecology is affected by succession in differently aged gardens of Citrus sinensis and C. reticulata

The concepts of destruction and rebuilding in the succession play an important role in agro-ecology. Agricultural ecosystems are always affected by tillage, soil preparation, harvest, burning, and arboriculture. A common practice in the northern province of Mazandaran (Iran) is to replace rice (Oryza sativa L.) cultivation with Citrus farming. It can be accordingly interesting to investigate the ecology of weed community specifically the dominance and characters during the different stages of succession, using constitutive and physiological aspects. In a 2-year research work, some functional (species composition and stability) and physiological characteristics (relative growth rate, biomass, and N and P uptake) of different weeds in the initial and final stages of secondary succession in Citrus gardens were compared. Three young (Citrus sinensis L.) and three old Citrus (Citrus reticulata Blanco) gardens (with the average age of 2 and 29 years, respectively), were selected in the suburb of Qaemshahr city (Iran), the centre of Citrus production. In each garden, three main fixed plots (30 m2), with five subplots (1 m2) and destructive quadrates in each one, were prepared and used for the experiment (January 2013–August 2014). Weed composition was monthly recorded. The concept of dominance diversity was used to estimate the community consistency. Thirty-three weed species were identified among which the Poaceae and Asteraceae families were the dominant ones. In the young gardens, the number of weed species was twice more than the old ones. Poa nemoralis as a perennial and sciophytes species were plentiful in the old gardens. Just in the young gardens, the weed species were moved by pappus. There was a linear and stable regression between weed frequency and dominance in the final sampling. Higher relative growth rate as well as higher N and P uptake was resulted in the young gardens. Seed size and growth rate were correlated in the young gardens, and the smaller seeds resulted in a higher rate of weed survival. Parameters including weed dominance, functional and physiological characteristics, seed size, and the environment may determine weed ecology and survival at different stages of succession in Citrus gardens.

Screening Cover Crops for Weed Suppression in Conservation Agriculture

The use of herbicides amongst smallholder farmers is minimal because herbicides are expensive and they require specialized application equipments. Weeds are problematic in conservation agriculture where herbicides are expensive for smallholder farmers. The use of cover crops can help to suppress weed growth and development by creating an environment which is not suitable for weeds survival. Cowpea (Vigna unguiculata (L.) Walp) dolichos lablab (Lablab purpureus L.) and velvet bean (Mucuna pruriens (L.) DC) were evaluated for biomass accumulation and weed suppression under conservation agriculture system in two contrasting experimental sites: Ukulinga and Bergville in KwaZulu-Natal. Bare plot and herbicide treatments served as controls. Treatments were laid in a randomized complete block design, replicated three times. Mucuna pruriens (L.) DC had the highest biomass accumulation in both sites Bergville (0.72 t/ha) and Ukulinga (1.59 t/ha). Cowpea had the lowest biomass accumulation in Bergville (0.59 t/ha) and lablab was the lowest in Ukulinga (0.88 t/ha). Lablab was effective in suppressing weed biomass in Bergville (P < 0.05). Cowpea performed best in suppressing weed biomass in Ukulinga (P < 0.05). The results suggest that cowpea and lablab can be effective for weed suppression and therefore can be recommended for use in conservation agricultural systems.

Efficacy of heat for weed control varies with heat source, tractor speed, weed species and size

ABSTRACTThe efficacy of a direct-fired, tractor-mounted steam weeder was compared with that of a flame weeder. At a tractor speed of 3 km h−1, steam killed 100% of 10-day-old mustard plants, sown as a surrogate weed, and flame killed 92%. Plant survival increased with increasing tractor speed, but was always lower for the steam weeder. Mean survival over all speeds was 40% for steam and 74% for flame. By contrast, for 15 agricultural weed species, at a tractor speed of 3 km h−1 weed survival for both weeders was similar. At 3 km h−1 both methods killed Stellaria media and Veronica agrestis at all growth stages assessed, but neither killed Lolium perenne. Plants of all the other species were killed when the treatments were applied soon after emergence (cotyledon stage), but as the plants increased in size they became more resistant to heat, either because the terminal or axillary meristems were protected by soil, or because meristems were protected by thickened stems and leaves as the plants aged.

Palmer Amaranth (Amaranthus palmeri) Damage Niche in Illinois Soybean Is Seed Limited

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

Experimental Methods for Estimation of Plant Fitness Costs Associated with Herbicide-Resistance Genes

Since the beginning of agriculture, crops have been exposed to recurrent invasion by weeds that can impose severe reductions in crop quality and yield. There have been continuing efforts to reduce the impacts of weeds on production. More than 40 yr ago, overreliance on herbicide technology to reduce weed infestations resulted in the selection of adaptive traits that enabled weed survival and reproduction under herbicide treatments (Délye et al. 2007; Powles and Yu 2010; Vila-Aiub et al. 2008). As a result, herbicide resistance in > 200 weed species has evolved worldwide (Heap 2013; Powles 2008).

WEEDS RESPONSE TO THE VARIOUS DOSES OF NEW GENERATION HERBICIDE 'VERDICT' IN A CONTROLLED ENVIRONMENT

Trial was carried out to survey the different rates of new generation post-emergence herbicide 'Verdict' in four levels involving: 0, 0.2, 0.3 and 0.5 kg ha-1 to suppress three weeds species; Chenopodium album, Poaceae sp. and Stelaria media. Experiment was conducted in a completely randomized design [CRD] with four treatments in four replications. Weeds growth diminished mostly for verdict 0.5 kg ha-1 and then 0.3 kg ha-1 compared to 0.2 kg ha-1 and control, results of the trial revealed that a satisfactory survival reduction of Chenopodium album, poaceae and also Stelaria media were achieved with labeled-dose of verdict as 0.5 kg ha-1 and also intermediate dose 0.3 kg ha-1. In contrast, the minimum dose 0.2 kg ha-1 had a significantly highest weeds survival about three varieties of mentioned weeds in comparison with other higher doses of herbicide.

Modelling population dynamics of Sinapis arvensis in organically grown spring wheat production systems

SummaryAlthough we know that alterations in crop density, crop spatial pattern and inclusion of more selective weed control can improve weed suppression for organic growers, it is unknown whether these result in changes to the weed seedbank that increase cropping system profitability over time. Data collected from field trials conducted in 2009 and 2010 in Maine, USA, comparing regional grower practices (Standard) with management that aimed to (i) facilitate better physical weed control through the use of wide row spacing and inter‐row cultivation (Wide) or (ii) enhance crop–weed competition through increased seeding rate and narrow inter‐row spacing (Narrow HD), were used to construct a matrix population model with an economic sub‐model. Using field measurements of grain yield and weed survival and fecundity, we investigated the lasting implications of employing alternative organic spring wheat (Triticum aestivum) production practices on Sinapis arvensis population dynamics. In most scenarios, the model indicated that regional production practices were not sufficient to prevent an increase in the weed seedbank, even with excellent weed control. The two alternative methods, on the other hand, were able to limit weed population growth when initial densities were low or cultivation efficacy was >80%. Due to higher seed costs in the Narrow HD system, net returns were still lower after 10 years of simulation in this system compared with wide rows with cultivation, despite a lower weed seedbank.

Growth Response of Itchgrass (Rottboellia cochinchinensis) to Water Stress

Greenhouse studies were conducted to evaluate the growth response of itchgrass to water stress. Itchgrass plants produced the greatest aboveground biomass and seeds at 75% of field capacity and these parameters at 50 and 100% of field capacity were similar. With further increase in water stress, seed production was sharply reduced, but itchgrass was still able to produce an average of 63 and 9 seeds plant−1at 25 and 12.5% of field capacity, respectively. Itchgrass plants responded to increasing water stress with increased leaf weight ratio; it was 2.5 times greater at 12.5% of field capacity than at 100% of field capacity. In another study, compared with daily irrigation, intervals of 9 d between irrigations reduced aboveground biomass of itchgrass by 27% and 12-d intervals reduced aboveground biomass by 67%. Compared with the daily irrigation regime, itchgrass seed production was reduced by 61% at intervals of 12 d between irrigations; however, the weed plants produced a considerable number of seeds (153 seeds plant−1) at the 12-d intervals. The ability of itchgrass to produce biomass and seeds under water stressed conditions necessitates strategies that minimize weed survival while maximizing irrigation efficiency for the crop at the same time.

Weed Biology and Ecology: - A Key to Successful Weed Management and Control

Weed biology is related to the study of weeds in relation to their geographical distribution, habitat, growth, population dynamics and communities. On the other hand, ecology is the interrelationship between or- ganisms and their environment. Weed ecology is thus concerned with growth characteristics and adaptations that enable weeds to survive changes in the environment. The environment includes climatic, edaphic and biotic factors and it determines the distribution, prevalence, competing ability, behavior and survival of the weeds. Development of an appropriate and effective weed management program is dependent on the sound knowledge of weed biology and ecology i.e. a thorough knowledge regarding propagation, dispersal, behavior, survival and persistence behavior of weeds. Man plays an important role in changing the environment by altering the crop husbandry practices and by managing weed-free mono crop or multi crop cultures. Mono crop culturesu- tilize moisture, nutrients and light available for plant growth in a way different from multi crop cultures and this influences the ecological requirements for weed growth, behavior, competition and survival. The various as- pects related to weed biology and ecology are discuss in this paper and the importance of weed biology in weed control have been reviewed in the paper. The discussion on weed biology may sometime overlap weed ecology.

Effect of Water Stress on the Growth and Development of <i>Amaranthus spinosus, Leptochloa chinensis</i>, and Rice

Drought is the most important abiotic constraint in rainfed rice systems. In these systems, Amaranthus spinosus and Leptochloa chinensis are the dominant weed species, which may reduce the available water to rice by competition and cause water stress in the crop. Two studies were conducted in a greenhouse to evaluate the growth response of A. spinosus and rice and L. chinensis and rice to water stress. The water stress treatments were 12.5%, 25%, 50%, 75%, and 100% of field capacity and the plants were grown until weed maturity (i.e., 63 days from seeding). Rice plants did not survive at 12.5% and 25% of field capacity, but both weed species survived in all the treatments. Both weed species produced a significant number of tillers/branches and leaves even at the lowest soil water content. The maximum amount of total shoot biomass produced by weed plants was 2.5 to 3 times more than that of rice plants. The soil water content to achieve 50% of the maximum aboveground biomass was 47%-50% of field capacity for rice, whereas it was 39% and 31% of field capacity for A. spinosus and L. chinensis, respectively. A. spinosus plants responded to increasing water stress with increased leaf weight ratio, whereas the leaf weight ratio of L. chinensis decreased with increases in water stress. The ability of A. spinosus and L. chinensis to produce shoot biomass under water-stressed conditions suggests that weed management strategies are needed that can minimize weed survival in water-limited environments. These strategies may include the use of weed-competitive and drought-tolerant cultivars, high seeding rates, and optimum timing of fertilizers.

Nisza ekologiczna chwastów w odmianach zbóż jarych uprawianych po rzepaku ozimym

The environmental conditions and ecological niches for weed survival and development in cereals crops are diversified. They are conditioned by biotic and abiotic factors of the environment and natural factors including quantitative rations between species of agrophytocenosis. The object of the studies carried out in 2009–2012 was to determine the weed species composition and infestation in crops of spring cereal species. Two cultivars were taken into consideration for each cereal species. The observations were done on the control (untreated) plots of a field experiment on evaluation of herbicide biological efficacy. The obtained results revealed that regardless of cereal species and cultivar, the weed species composition was similar, however the number of weeds and their biomass were different. The smaller weed biomass and their number were correlated with a higher competitiveness of cultivated cereal plants in relation to weeds. The rye cultivar Bojko showed the highest competitivenes, while the wheat cultivar Tybalt was the least competitive. The highest weed infestation was recorder for wheat in contrast to the lowest one in barley and rye crops. A proper selection of cereal species and cultivars with high competitive abilities should be taken into consideration by organic farms. Warunki życia chwastów i nisze ekologiczne w zasiewach rożnych gatunków zbóż nie są jednakowe. Zależą one od zespołu biotycznych i abiotycznych czynników środowiska i całokształtu czynników przyrodniczych, w tym od stosunków ilościowych pomiędzy różnymi gatunkami tworzącymi agrofitocenozę. W badaniach wykonanych w latach 2009–2012 określono skład florystyczny i stopień zachwaszczenia w 5 gatunkach zbóż jarych, przy czym w każdym gatunku uwzględniono 2 odmiany. Obserwacje przeprowadzono w obiektach kontrolnych doświadczeń z oceną skuteczności biologicznej herbicydów. Uzyskane wyniki wskazują, że niezależnie od gatunku i odmiany liczba występujących taksonów kształtowała się podobnie, natomiast ich liczebność i biomasa ulegała dużemu zróżnicowaniu w zależności od uprawianych gatunków i odmian zbóż jarych. Im mniejsza była liczba i biomasa chwastów, tym większa była konkurencyjność uprawianych roślin w stosunku do chwastów. Największą konkurencyjnością charakteryzowało się żyto odmiany Bojko, a najmniejszą pszenica odmiany Tybalt. Największe zachwaszczenie wyrażone zarówno ogólną liczbą chwastów i biomasą odnotowano w pszenicy a najmniejsze w jęczmieniu i w życie. Wybór do uprawy gatunków i odmian zbóż jarych o większej zdolności konkurencyjnej powinien być uwzględniany w gospodarstwach ekologicznych.