Annual weed competition in wheat crops: the effect of weed density and applied nitrogen

Experiments investigating the effect of weed density on the yjeld of a wheat crop at three levels of applied nitrogen were conducted in north-western Victoria. Australia, during 1970. There were five sites, each infested with a pure stand of one of the following annual broad-leaved weed species: Lithospermum arvense, Brassica tournifortii. Lamium amplt'xicaule, Amsinckia hispida and Fumaria parviflora. At the three-leaf stage of crop growth, the weed populations were systematically thinned with a specially developed spray boon) (which is described) to give a range of weed densities in competition) with the crop. The relalionship between dry matter production and population density for all but one weed species was curvilinear, but the degree of curvature was small and competition in the wheat crop was linear for four of the five weed species. There were large differences in the competitive ability of individual weed species and these have been described by regression equations. Applied nitrogen increased wheat yields at all sites but weed competition was not affected. The use of these grain yield-weed density relationships in predicting crop losses from weed competition is discussed.     

Effects of nitrogen supply on competition between wheat and three annual weed species

Three pot experiments have investigated the effects of nitrogen (N) supply on interspecific competition between three weed species ( Phalaris minor Retz., Chenopodium album L, and Sinapis arvensis L.) and spring wheat ( Triticum aestivum L.) cv, Alexandria. The treatments tested included monocultures of each species and a mix-ture containing them in equal proportions that were combined factorially with two levels of N supply (20, 120 kg N ha^-1). Low N supply decreased net photosynthetic rate (P_n), leaf N percentage, plant dry weight and N uptake of both wheat and weed species and gram dry weight of wheat. The effects of low N on P_n and dry weight of weeds were greater than the effects on wheat. In most cases the decrease in P_n at low N was due to non-stomatal factors. The relative competitive abilities of wheat and weeds were influenced by N supply. At high N, S.arvensis was more competitive than wheat, whereas P. minor was less competitive than wheat. C. album was more competitive than wheat at both N levels. The rank order of competitive ability of the weed species was C. album > P. minor > S. arvensis . The effects of interspecific competition on P_n were smaller than the effects of N supply and were not associated with corresponding effects on leaf N percentage and plant dry weight of both wheat and weed species and grain dry weight of wheat.     

The interaction between planting depth of four winter wheat cultivars, Alopecurus myosuroides Huds. and Bromus sterilis L. and their susceptibility to post-emergence applications of isoproturon and chlorotoluron

Seeds of four winter wheat cultivars, Slejpner, Galahad, Avalon and Penman, were sown at depths ranging from 6–75 mm in soil in pots, and isoproturon or chlorotoluron was then applied to the soil surface. For chlorotoluron-treated plants (both pre- and post-emergence) the dose required to produce a 50% effect (ED₅₀) was unaffected by depth of planting. In contrast, for isoproturon applied pre-emergence, the ED₅₀ for both Avalon and Slejpner was strongly affected by sowing depth. Although chlorotoluron was much more active in a second experiment when applied post-emergence to Slejpner wheat, the ED₅₀ for both herbicides increased with greater depth of sowing. Protection of wheat from isoproturon damage by deeper planting was enhanced if the adsorption capacity of the soil was raised from K_d 0.5 to 2.0 by incorporation of activated charcoal in the soil. Isoproturon entry into plants (as measured by the effect on rate of photosynthesis) was slower in those that had been sown deeper and were growing in more adsorptive soils, but there was no obvious relationship between these observations and isoproturon distribution in the soil profile. In nutrient culture the four wheat cultivars responded similarly to a range of doses of isoproturon. The chlorotoluron-sensitive cultivars, Slejpner and Galahad, were damaged by much lower doses of chlorotoluron than were Avalon and Penman. Bromus sterilis L. responded similarly to wheat with regard to its interaction with isoproturon and planting depth. Alopecurus myosuroides Huds., however, was less damaged by isoproturon when the zone above the seed was protected from the herbicide by growing the shoot through a plastic straw.     

密度对大豆群体碳氮代谢相关指标及产量、品质的影响

以垦农4号大豆为试验材料,在大田条件下,研究了密度对大豆群体碳氮代谢相关指标及产量品质的影响.结果表明,群体单株叶片氮素、可溶性蛋白、淀粉及蔗糖积累量均随着密度的增加而下降,低密度与高密度群体间差距显著;随着种植密度的增加,大豆子粒中蛋白质的含量有所上升,脂肪的含量有所下降;各生育时期氮素、可溶性蛋白、淀粉和蔗糖积累量...     

Climate change impacts on plant canopy architecture: implications for pest and pathogen management

Climate change influences on pests and pathogens are mainly plant-mediated. Rising carbon dioxide and temperature and altered precipitation modifies plant growth and development with concomitant changes in canopy architecture, size, density, microclimate and the quantity of susceptible tissue. The modified host physiology and canopy microclimate at elevated carbon dioxide influences production, dispersal and survival of pathogen inoculum and feeding behaviour of insect pests. Elevated temperature accelerates plant growth and developmental rates to modify canopy architecture and pest and pathogen development. Altered precipitation affects canopy architecture through either drought or flooding stress with corresponding effects on pests and pathogens. But canopy-level interactions are largely ignored in epidemiology models used to project climate change impacts. Nevertheless, models based on rules of plant morphogenesis have been used to explore pest and pathogen dynamics and their trophic interactions under elevated carbon dioxide. The prospect of modifying canopy architecture for pest and disease management has also been raised. We offer a conceptual framework incorporating canopy characteristics in the traditional disease triangle concept to advance understanding of host-pathogen-environment interactions and explore how climate change may influence these interactions. From a review of recent literature we summarize interrelationships between canopy architecture of cultivated crops, pest and pathogen biology and climate change under four areas of research: (a) relationships between canopy architecture, microclimate and host-pathogen interaction; (b) effect of climate change related variables on canopy architecture; (c) development of pests and pathogens in modified canopy under climate change; and (d) pests and pathogen management under climate change.     

猎物密度对七星瓢虫与异色瓢虫种间竞争的影响

通过单种群与混合种群饲养,研究了猎物密度对七星瓢虫Coccinella septempunctata和异色瓢虫Leis axyridis种间竞争影响,并用Lotka-Volterra种间竞争模型对两种瓢虫在猎物相对充足与不足条件下的种间竞争进行模拟,结果表明:猎物充足,两种瓢虫的种群增长呈线性增长趋势;猎物不足时,单独饲养时,两种瓢虫的种群增长趋势呈Logistic曲线,混合饲养时异色瓢虫的种群增长呈上升趋势,七星瓢虫趋于下降。在两种瓢虫的种间竞争中,异色瓢虫占相对优势,竞争的结局是二者可以形成一个稳定的平衡局面而得以共存。     

山西吕梁地区三种林分土壤养分与碳密度关系研究

通过在山西吕梁地区分别在山杨(Populus davidiana Dode)、落叶松(Larix principisrupprechtii)、油松(Pinus tabulaeformis)林内设置样地进行林分结构和土壤调查采样,测定了土壤有机碳密度和养分含量,分析了不同林分对土壤有机碳积累的差异性及其影响因素。结果表明:该研究区三种林地土壤有机碳密度山杨林(57.08kg/m2)>落叶松林(16.54kg/m2)>油松林(6.71kg/m2);油松林、落叶松林土壤有机碳密度随着土层深度递减,山杨林则在10-30cm土层土壤有机碳碳密度最低,土壤表层与深层都具有富集作用。土壤中的TN、TP和有机质的含量与土壤有机碳之间的差异变化规律基本相同,山杨林也表现出表层和深层含量较高的特点,相关分析结果说明三种林分类型土壤养分与土壤有机碳碳密度密切相关。     

大青山主要林型林分密度与竞争关系的研究

采取克拉夫特分级法d=r/的公式(r:林木胸径,R:林分平均胸径),进行Ⅰ-Ⅴ级木分类,以大青山油松人工林、落叶松人工林和白桦天然次生林为主要研究对象,对比分析不同密度下林木分级、竞争和径阶的变化情况,为大青山植树造林提供相对适宜的密度区间。结果表明:大青山油松人工林密度不宜超过3200株/hm2,落叶松在1700株/hm2时林木分化现象并不明显。白桦天然次生林因萌生起源、年龄因素的影响,密度对其分级木影响效应不显著。     

The selectivity of methabenzthiazuron between barley, perennial ryegrass and Poa trivialis

In field experiments methabenzthiazuron at 1.1 kg ai/ha applied pre-emergence gave good control of Poa trivialis L. in perennial ryegrass S23 which was undersown in spring barley (Zephyr); no significant damage to barley or perennial ryegrass resulted from 2.2 kg ai/ha. However this dose applied when barley had three to four leaves decreased the subsequent barley yield. In pot experiments in the glasshouse methabenzthiazuron was active mainly due to uptake through the soil. Much greater damage to barley and perennial ryegrass resulted when herbicide-treated soil was placed below seed level than above the seed. The difference was less marked with P. trivialis. The latter started to develop secondary roots near the soil surface earlier than either perennial ryegrass or spring barley.     

The influence of rice plant type on the effect of weed competition on Oryza sativa and Oryza glaberrima

Detailed studies were conducted on three rice cultivars – IG10, an Oryza glaberrima Steudel; Moroberekan, a traditional Oryza sativa L.; and IDSA6, an improved O. sativa L. – in conditions with and without competition from weeds. IG10 suffered less from competition with weeds and suppressed weeds better than the O. sativa cultivars. IG10 accumulated more biomass, produced more tillers, established a higher leaf area index, had higher specific leaf area and, in the earlier stages of growth, partitioned more of its biomass to leaves than the O. sativa cultivars. Without competition from weeds, grain yields did not significantly differ between the cultivars, but IG10 produced higher yields in competition with weeds. The implications of these findings are discussed in relation to the development of rice plants with good weed suppression characteristics.     

Integration of partial resistance,plant density and use of fungicide for management of white mould in common bean

In-row plant densities have not been studied for common beans with type II growth habit and contrasting reactions to white mould. Advanced breeding lines with partial resistance or susceptibility to white mould were combined with 4, 7, 10 or 13 plants m⁻¹ and with or without fungicide at a constant between-row spacing of 0.5 m in five sprinkler-irrigated field trials conducted during the autumn–winter season in Brazil. White mould pressures in the trials covered the whole range from zero to moderate/high (46–60% of white mould severity index). In all trials, means of white mould incidence, severity and yield did not vary significantly between 7 and 13 plants m⁻¹ for the partially resistant line, regardless of the fungicide levels. For the susceptible line, 13 plants m⁻¹ increased white mould incidence and severity under moderate disease, regardless of the fungicide levels, and decreased yield compared with 10 plants m⁻¹ when fungicide was applied twice under moderate/high disease pressure. For the susceptible line, 7 or 10 plants m⁻¹ maximized yield in all trials, with or without fungicide applications. The results suggest that the current recommendation of 11–13 plants m⁻¹ could be used for type II beans with partial resistance to white mould in either a conventional or organic system. For susceptible genotypes, 7–10 plants m⁻¹ seems to be the most appropriate in-row plant density. This study may improve the recommendation of in-row plant density for type II beans cultivated under white mould pressure.     

Common vole (Microtus arvalis) ecology and management: implications for risk assessment of plant protection products

Common voles (Microtus arvalis) are common small mammals in some European landscapes. They can be a major rodent pest in European agriculture and they are also a representative generic focal small herbivorous mammal species used in risk assessment for plant protection products. In this paper, common vole population dynamics, habitat and food preferences, pest potential and use of the common vole as a model small wild mammal species in the risk assessment process are reviewed. Common voles are a component of agroecosystems in many parts of Europe, inhabiting agricultural areas (secondary habitats) when the carrying capacity of primary grassland habitats is exceeded. Colonisation of secondary habitats occurs during multiannual outbreaks, when population sizes can exceed 1000 individuals ha^?1. In such cases, in‐crop common vole population control management has been practised to avoid significant crop damage. The species' status as a crop pest, high fecundity, resilience to disturbance and intermittent colonisation of crop habitats are important characteristics that should be reflected in risk assessment. Based on the information provided in the scientific literature, it seems justified to modify elements of the current risk assessment scheme for plant protection products, including the use of realistic food intake rates, reduced assessment factors or the use of alternativee focal rodent species in particular European regions. Some of these adjustments are already being applied in some EU member states. Therefore, it seems reasonable consistently to apply such pragmatic and realistic approaches in risk assessments for plant protection products across the EU. © 2013 Society of Chemical Industry     

The effect of atrazine,cyanazine and cyprazine on photosynthesis and growth of nine grasses*

A semi-open circuit system for measuring changes in net CO₂ exchange (NCE) in single leaves of intact grasses following herbicide treatment is described and evaluated. There were significant differences in levels of inhibition and subsequent recovery of NCE in maize and eight weedy panicoid grasses following limited root uptake of atrazine (2-chloro-4-ethyl-amino-6-isopropylamino-1,3,5-triazine). cyanazine [2-chloro-4-(1-cyano-1-methylethylamino)-6-ethylamino-1,3,5-triazine] and cyprazine (2-chloro-4-cyclopropylamino-6-isopropyl-amino-1,3.5-triazine). Rate of NCE recovery was positively correlated (P = 0.05) with growth of seedlings in nutrient solution containing the herbicides. Rates of NCE recovery >0.9 mg CO2 per dm2 per h/h reflected rapid rates of herbicide detoxification in the leaves and a significant tolerance to preplant incorporated and postemergence applications of atra-zine, cyanazine and cyprazine. In contrast, some species, e.g. large crabgrass [Digitaria sanguinalis (L.) Scop.] and proso millet (Panicum miliaceum L.) treated with cyanazine demonstrated considerable tolerance to these treatments in spite of low NCE recovery rates indicating that factors other than foliar detoxification may play an important role in the tolerance of some grasses to 2-chloro- 1,3,5-triazine herbicides.     

The relationship between soil inoculum density and plant infection as a basis for a quantitative bioassay of Verticillium dahliae

Using potato, eggplant and thorn apple as test plants, the relationship between soil inoculum density and plant infection was studied as a basis for the development of a quantitative bioassay of Verticillium dahliae. A linear relationship was demonstrated (P < 0.05) between soil inoculum density and population density on roots for all three test plants and for soil inoculum density and population density in sap extracted from stems for eggplant. Correlation coefficients were higher with densities on or in roots (R² varying from 0.45 to 0.99) than with densities in stems (R² varying from 0.04 to 0.26). With eggplant, population densities on/in root and in sap extracted from stems were significantly correlated at 20 and 25°C with Pearson's correlation coefficients of 0.41 and 0.53, respectively. For potato, root colonization was higher at 15 than at 20°C, whereas the reverse applied to eggplant. Stems of potato were less colonized than stems of eggplant. The pathozone sensu Gilligan (1985) was calculated to be <300 µm, indicating that infection was caused by microsclerotia which were located close to the roots. To assess the density of V. dahliae in plant tissue pipetting infested plant sap on solidified ethanol agar medium without salts yielded higher densities than using pectate medium or mixing sap with molten agar. A bioassay for determining effects of (a)biotic factors on development of V. dahliae in the plant is recommended with eggplants as a test plant, grown in soil infested with 300 single, viable microsclerotia g^-1 soil at a matric potential of –6.2 kPa, and incubated at 20°C for 8 weeks.     

The role ofPseudomonas spp. and competition for carbon,nitrogen and iron in the enhancement of appressorium formation byColletotrichum coccodes on velvetleaf

Colletotrichum coccodes is currently being investigated as a mycoherbicide against the weed velvetleaf (Abutilon theophrasti). Two isolates ofPseudomonas spp. (Ps2 and Ps5) reduced the percentage of germ tubes and increased appressorial formation ofC. coccodes on detached leaves of velvetleaf. A study was conducted to see whether this effect could be attributed to competition for nutrients or iron betweenC. coccodes andPseudomonas spp. Ps2 and Ps5 had no effect on early spore germination, but reduced the percentage of germ tubes at 24 and 30 h, compared to the nontreated control. This reduction was diminished by the addition of nutrients but not Fe³⁺. Ps2 and Ps5 stimulated the formation of dark-coloured appressoria without germ tubes (AWGT), but this stimulation was diminished by the addition of nutrients or Fe³⁺. Germ tube branching at 30 h was also inhibited by the bacteria, but was not diminished by the addition of nutrients or iron. EDTA stimulated conidial germination at 10 h, which was reduced by the addition of Fe³⁺. However, EDTA did not stimulate the formation of appressoria (AWGT). These results suggest that the reduction in the percentage of germ tubes and the increase in the percentage of appressoria induced by the bacteria may be due to the competition for carbon or nitrogen. Iron competition may also be involved in the stimulation of appressorial formation, but not in the reduction in germ tube percentage and branching. Phylloplane bacteria may compete for carbon, nitrogen and iron, limiting the saprophytic phase of the pathogen on the phylloplane and accelerating the development of the parasitic phase. This may enhance the field efficacy ofC. coccodes as a biocontrol agent against velvetleaf.     

Competition between winter wheat and Veronica hederifolia: influence of weed density and the amount and timing of nitrogen application

The effects at Veronica hederifolia. densities on the yield of winter wheat (Triticum aestivum L.) were studied in field trials conducted at the same location in 1992 and 1993. In 1992, nitrogen at 60 kg ha^-1 was applied at tillening followed by a further 80 kg ha^-1 at the first node of stem elongation. In 1993. four regimes of nitrogen applications were compared: a total of 140 kg ha^-1 supplied at three dales. 60 kg ha^-1 supplied at tillering, 60 kg ha^-1 Supplied al the first node of stem elongation and no nitrogen as the control. Competitive effects from V. hederifolia (ranging from 17 to 192 plants m^-22), were greatest in 1993, the year with the best early growth development of this weed. In 1993, yield losses, as described with a non-linear model changed for each regime of fertilization. In both years, V. hederifolia decreased wheat ear number per unit area for each nitrogen treatment. This is explained hy an increase in tiller mortality and a nitrogen deficiency in wheat at the stem elongation and flowering stages. Nevertheless, with late application of nitrogen, individual grain weight increased and the effect of V. hederifolia on wheat yield loss was lowest. The results are discussed in relation to the effects on all yield components, and show the importance of choosing the rieht nitrogen fertilization for specific yield targets of wheat production when infested by weeds.     

The influence of herbicides, nitrogen fertilizer, seed rate and method of sowing, on the establishment and long-term composition of a perennial ryegrass ley.

Changes in ihe species composition of a ryegrass pasture were examined over a 2 year period from a spring establishment. Seed of S23 perennial ryegrass was sown in main plots, eilher broadcast or drilled, at rates of 15, 25 or 35 kg/ha in a field experiment at Oxford. The whole area was oversown withfour weed grasses (Agrostis siolonifera.Holcus lanatus. Poa annua and P trivialis). Five cultural and three herbicide treatments were applied across the plots. These consisted of three levels of nitrogen (0. 50 and 100 kg N per ha), two companion species (Italian ryegrass and white clover) and 2.4-D, benazolin and methabenzihiazuron applications. Ten weeks after sowing, broad-leaved weeds accounted for nearly half the lotal dry-matter yield, except where controlled by the three herbicides. Grass yields were nol increased by herbicide treatment. The only cultural treatment checking broad-leaved weeds was ihe 35 kg/ha seeding rale. Seven months after sowing, the only treatment which had decreased the abundance of weed grasses was melhabenzthiazuron especially againsi A. stolonifera and Poa spp. Two years afler sowing, perennial ryegrass had only 34% ground cover and had been replaced largely by H. lanatus (38%), Poa spp, (15%) and A. stolonifera (7%), Where methabenzthiazuron had been used the ryegrass ground cover was 48%. with less P. trivialis and A. stolonifera. but H. lanatus was nol altered. White clover decreased H. lanatus cover. In two supplementary pot experiments, methabenzthiazuron sprayed pre-emergence damaged perennial ryegrass unless the seed was covered with at least 4 mm of soil. Only H. lanatmand F. rubra out of sixteen grasses were relatively resistant to methabenzihiazuron when applied at ihe tillering stage, confirming that this herbicide has potential for preventing the ingress of many unsown grasses in new leys.     

The Conductance model of plant growth and competition in monoculture and species mixtures: a review

Over the past two decades, an ecophysiological model has been developed for annual horticultural crops and weeds, which has the powerful ability to predict the growth of plants in monoculture and mixed species stands from parameter values derived from plants grown in isolation, even if the species display contrasting canopy architecture. The model can also simulate the effects of different spatial arrangements on plant growth. The purpose of the model is to describe, in simple yet mechanistically‐based terms, the effects of contrasting environments and competitive interactions on the growth of individual plants. In the simplest form of the model, growth is described by an empirical growth equation, using time calculated from an integration of the growth‐promoting effects of environmental factors. More complex versions of the model include a self‐shading component, which provides an algorithm for inter‐plant competition based on crown zone areas. This model is termed the ‘Conductance model’ and this article outlines its development, applications to date, goodness of fit to experimental data, and discusses its strengths and weaknesses and scope for further testing and application. This article, which is dedicated to the late David Aikman, also sets out how the model can be applied to simulating weed–crop competition from simple data sets.     

Competitive interactions between weedy rice and cultivated rice as a function of added nitrogen and the level of competition

The competitive outcomes between weedy rice from Malaysia (MWR), the Philippines (PWR), and Vietnam (VWR) and cultivated rice (IR64) grown in pots were evaluated in a replacement series experiment with added N (0, 50, 100, and 150 kg N ha^?1) and competition with IR64 plants (no competition, eight weedy rice plants : 0 IR64 plants; low competition, six weedy rice plants : two IR64 plants; and high competition, two weedy rice plants : six IR64 plants). The growth observations were taken at 10 weeks after sowing. When grown in a monoculture (no competition with IR64 plants), the PWR plants had a lower shoot biomass across N rates than did the MWR and VWR plants. The leaf area and shoot biomass of weedy rice across populations significantly increased with an increase in the N application rate. Each weedy rice population and the IR64 population showed linear responses of the leaf area and shoot biomass to the N rate at all levels of competition. The weedy rice and IR64 plants, when grown without competition, had a similar rate of response in the shoot biomass to the N rate. However, when grown in competition, the response to the added N varied among the weedy rice populations. The MWR plants under competition produced a similar amount of shoot biomass to the IR64 plants per unit addition of N. In contrast, the PWR and VWR populations under competition produced a greater amount of shoot biomass with each additional unit of N, compared to the IR64 population. The results illustrate that N fertilizer management might affect the outcome of weedy rice competition. This information could be incorporated into weedy rice management strategies.