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.     

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.     

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.     

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 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.     

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.     

Efficacy of Sclerotinia minor for dandelion control: effect of dandelion accession, age and grass competition

Control of Taraxacum officinale (common dandelion) and other broad-leaved weeds in temperate turfgrass has been readily achieved with phenoxy herbicides. The herbicide option has been revoked through municipal and provincial legislation in many regions of Canada, necessitating alternative approaches. We examined the effects of dandelion accessions, age and grass competition on the performance of Sclerotinia minor (IMI 344141) as a biological control for dandelion in turfgrass. Disease symptoms were identical on all 14 different accessions of dandelion and the above- and below-ground biomass were reduced by 94% and 96%, respectively, with no difference among accessions. Foliar damage and dandelion mortality caused by S. minor was affected by plant age and the presence of grass competition. Dandelions of all ages were more severely affected by S. minor in the presence of grass competition. Grass competition had greater impact on foliar biomass, whereas the fungus had a greater impact on root biomass of newly established dandelions. In addition to competition for resources, we were hypothesised that the grass sward provides a microenvironment favouring the success of S. minor as a biological control agent of dandelion. Thus, proper management of the turfgrass environment may be complementary to the efficacy of S. minor as a biocontrol for dandelion.     

The effect of nitrogen on the growth and development of giant witchweed,Striga hermonthica Benth.: effect on cultured germinated seedlings in host absence

For the first time in sterile nutrient media in the absence of the host plant, different forms and rates of nitrogen compounds were screened for their effect onS. hermonthica Benth. shoot development beyond seed germination. There was no shoot formation beyond the inoculation stages whenS. hermonthica germlings were grown in media devoid of nitrogen source. In culture media containing some nitrogen sources, healthy shoots were formed. Increasing concentrations of KNO₃, NaNO₃, Ca(NO₃)₂, Mg(NO₃)₂ and asparagine resulted in a significant increase inS. hermonthica shoot development. Inversely, increasing concentrations of (NH₄)₂SO₄, NH₄H₂PO₄, NH₄Cl and urea led to increasing significant reduction ofS. hermonthica shoot development. The amino acids, glycine and asparagine supported reduced shoot development ofS. hermonthica, indicating that organic nitrogen cannot replace inorganic nitrogen forStriga growth. The ammonium nitrogen compounds, (NH₄)₂SO₄ and NH₄H₂PO₄, suppressed further shoot elongation and total dry weight of 20 and 40 days oldStriga hermonthica plants, in sterile culture media. The organic compounds urea, allylthiourea and thiourea had an effect similar to ammonium compounds. Arginine and glycine on the other hand did not suppress the further development of the parasite. The suppressive effect of nitrogen however, was greater when the parasite was 20 days old than when it was 40 days old. This work provides data to show that some nitrogen compounds reduce the severity ofS. hermonthica attack by direct suppression ofStriga growth and development at the post-germination stage and after shoots have been formed.     

The fungicide tridemorph as a selective herbicide for the control of Holcus lanatus in ryegrass and of Bonus sterilis in barly

The potential of the fungicide tridemorph selectively to control established plants of Holcus lanatus L. in ryegrass (Lolium perenne L) and Bromus sterilis L. in barley (Hordeum vulgare L.) was examined in glasshouse trails. A dose of 4.2 kg ha^?1 tridemorph gave selectively between H. lanatus and ryegrass similar to, but more costly than, that which would be provided by 1.2 kg ha^?1 asulam. B sterilis was more sensitive to tridemorph than was barley. Two additives, glycerol and the oil-surfactant mixture ‘PF’ enhanced this selectivity when tridemorph was applied at 2.1 kg ha^?1.     

The relevance of morphological plasticity in the simulation of competition between maize and Datura stramonium

The INTERCOM model was calibrated for Datura stramonium and maize with data from monoculture plots, and used to simulate the competition between the weed and the crop. The model adequately simulated the growth (leaf area index, above-ground biomass, plant height, yield) of both species in monoculture. In competition, the growth of D. stramonium and the competitive effect of D. stramonium on the growth of maize were clearly underestimated. Simulation of competition was improved when the observed morphological adaptations of D. stramonium under competitive light stress, as reflected in dry-matter partitioning, height growth and specific leaf area (SLA) were considered. Model analysis demonstrated that the observed changes in SLA and height growth were more relevant for the increased competitive ability of D. stramonium . The modification of the model in order to simulate the effect of shading by an overtopping species in the SLA of the shaded species improved model simulation of competition.     

The effect of total carbon on microscopic soil properties and implications for crop production

Soil structure is a dynamic property affected by physical, chemical, and microbiological processes. Addition of organic matter to soils and the use of different management practices have been reported to impact soil structure and crop production. Moderation in soil temperature and increases in microbial activity and soil water retention are often suggested as reasons for the rise in crop yield when organic matter is added to the soil. Less is known about the direct effect of changes in soil structure on crop production. A field experiment was conducted to study the effect of summer cover crop and in-season management system on soil structure. The experiment was a nested design with summer cover crop as the main plot and management system as the subplot. Summer cover crop treatments included cowpea (Vigna unguiculata L. Walp.) incorporated into the soil in the fall (CI), cowpea used as mulch in the fall (CM), sudangrass (Sorghum vulgare) incorporated into the soil in the fall (S), and dry fallow or bare ground (B). Management systems were organic (ORG) and conventional (CNV) systems. Lettuce (Lactuca sativa L.) and cantaloupes (Cucumis melo L.) were cultivated in rotation in the plots for three consecutive years using the same cover crops and management systems for each plot. Disturbed and undisturbed soil cores were collected at the end of the third year and used for laboratory experiments to measure physical, chemical, and hydraulic properties. Image analysis was used to quantify soil structure properties using a scanning electron microscope on thin sections prepared from the undisturbed soil cores. We found that total soil carbon was correlated with porosity, saturation percentage, and pore roughness. Pore roughness was correlated with crop production in general and with marketable production in particular. We found that the higher the complexity of the pore space, the more water retained in the soil, which may increase soil water residence and reduce plant water stress.     

The effect of three fungicides,specific for the control of rice blast disease,on the growth and melanin biosynthesis by Pyricularia oryzae cav.

Tricyclazole (EL 291), 4,5-dihydro-4-methyltetrazolo [1,5-a] quinazolin-5-one (PP 389), and pyroquilon (CGA 49104) were studied to determine the effects on growth and melanin biosynthesis by Pyricularia oryzae in vitro. The three fungicides were essentially devoid of toxicity to P. oryzae at concentrations up to 50 ug ml^?1 but each selectively inhibited melanin biosynthesis at much lower concentrations. Inhibition of melanin biosynthesis resulted in the accumulation of 2-hydroxyjuglone and flaviolin. The study indicated that the three compounds act by a similar mechanism in P. oryzae..     

The qanats of Xinjiang:historical development, characteristics and modern implications for environmental protection

In China,qanats(kan’erjing/karez)exist only in Xinjiang Uygur autonomous region,being one form of traditional irrigation technology that made great contributions to oasis civilizations.There are different opinions on the origin and date of the qanats in Xinjiang due to a lack of adequate evidences.And uncontrolled over-exploitation of groundwater by pumping wells,as well as the expansion of surface water interventions nowadays,has caused serious impacts on not only the qanats,but also local ecological environments.There exist long-time arguments on whether the qanats should be restored or preserved.This study aimed to examine the historical development, geological conditions and characteristics,and modern implications of the qanats for oasis environmental protection, and to discuss the initiatives taken by local governments.The whole Xinjiang Uygur autonomous region was included as the study area,with a specific focus on Turpan prefecture.The materials and data were obtained from literature review and governmental documentation.Based on a systematic examination of three prevailing theories on the origination of Xinjiang’s qanats,it is suggested modern archaeological techniques could be good solutions to explore the origination in addition to historical records used normally.Although qanats fail to fully meet today’s enormous water demands,their rich historical,cultural,ecological and environmental implications in arid areas should not be underestimated.As a cultural heritage and ecologically-friendly engineering creation,qanat systems shall be kept in good condition and function by a valuable number.Recent initiatives to protect and restore target qanat systems in Xinjiang could provide positive examples for the practices in other regions of the world where the protection and restoration of qanats or similar technologies are desired.     

Wintering of the biotrophic fungus Puccinia lagenophorae within the annual plant Senecio vulgaris: implications for biological weed control

Epidemics of the obligate biotrophic fungus Puccinia lagenophorae might be used to control populations of the annual plant, groundsel, Senecio vulgaris . Insight into the mechanisms of survival of P. lagenophorae over winter may help to explain the number of inoculum sources, and their strength (assessed by number and size of pustules), present in an S. vulgaris population in spring, indicating the probability and rate of progress of a subsequent epidemic. Results of the study indicated survival of the rust as mycelium within the host over winter. Survival outside the host is unlikely, because aecidiospores lost their capacity to germinate over winter and teliospores have not been reported to be infectious. Survival of S. vulgaris plants over winter was reduced by rust infection in autumn. The mortality of S. vulgaris was 30–100% depending on the date of infection. All plants infected early in autumn died but those infected late in autumn were more likely to survive. In turn, poor survival of the host impacted on the survival of P. lagenophorae over winter. Consequently, the results of the study suggest that no inoculum sources, or only a few weak ones, are present in vulgaris populations in spring. This suggestion was supported by observations of an S. vulgaris population at a ruderal site. Therefore, research on biological weed control should focus on increasing the negative impact of P. lagenophorae on S. vulgaris populations while augmenting the probability of survival of the rust over winter to start new epidemics in spring.     

Quantifying the effect of interactions between disease control,nitrogen supply and land use change on the greenhouse gas emissions associated with wheat production

A method for calculating the effect of disease control on greenhouse gas (GHG) emissions associated with wheat production, reported previously, was developed further to account for effects of disease control on the amount of fertilizer nitrogen (N) which should be applied and on changes in land use. Data from nine randomized and replicated field experiments from the UK and Denmark showed that the economic optimum N input to winter wheat was greater if diseases were controlled by fungicides, than for untreated wheat. The GHGs associated with this additional N largely negated the benefit to emissions per tonne of grain resulting from disease control. However, the mean grain yield obtained without fungicide treatment was 6·71 t ha^?1, compared to 8·88 t ha^?1 with fungicide treatment, if N input was optimal for each situation. In the absence of disease control by fungicides, and assuming that the optimum N rate was used, an additional 481 kha of wheat would be required to maintain UK wheat production at the current level. If the additional land area came from converting temperate grassland to arable production, the GHG emissions caused by ploughing grassland would cause emissions to rise from 503 to 713 kg CO₂e per tonne of grain produced. This would result in an additional 3·15 Mt CO₂e per year to produce the typical UK annual production of 15 Mt. This analysis reinforces the importance of winning the ‘arms race’ against pathogen evolution towards fungicide insensitivity and virulence.     

The effects of seed mix and management on the abundance of desirable and pernicious unsown species in arable buffer strip communities

Buffer strips are refuges for a variety of plants providing resources, such as pollen, nectar and seeds, for higher trophic levels, including invertebrates, mammals and birds. Margins can also harbour plant species that are potentially injurious to the adjacent arable crop (undesirable species). Sowing perennial species in non‐cropped buffer strips can reduce weed incidence, but limits the abundance of annuals with the potential to support wider biodiversity (desirable species). We investigated the responses of unsown plant species present in buffer strips established with three different seed mixes managed annually with three contrasting management regimes (cutting, sward scarification and selective graminicide). Sward scarification had the strongest influence on the unsown desirable (e.g. Sonchus spp.) and unsown pernicious (e.g. Elytrigia repens) species, and was generally associated with higher cover values of these species. However, abundances of several desirable weed species, in particular Poa annua, were not promoted by scarification. The treatments of cutting and graminicide tended to have negative impacts on the unsown species, except for Cirsium vulgare, which increased with graminicide application. Differences in unsown species cover between seed mixes were minimal, although the grass‐only mix was more susceptible to establishment by C. vulgare and Galium aparine than the two grass and forb mixes. Annual scarification can enable desirable annuals and sown perennials to co‐exist, however, this practice can also promote pernicious species, and so is unlikely to be widely adopted as a management tool in its current form.