Weed-crop competition studies in swedes.

Hand-weeding experiments were conducted over a three-year period in field-grown crops of swedes in north-east Scotland. When weeds were left to grow all season they reduced the dry matter yield of swede roots by 62% in 1980 and 79% in 1981 but by only 42% in the dry year 1982, in comparison with a weed-free crop. In all three years there was at least one time when a single removal of all weeds was sufficient to prevent yield loss by weed competition. In 1980 this was 6 wk after crop sowing, but the intervals between weed removal times were too great to reveal how critical this timing was. With shorter intervals in 1981 the timing of a single removal of weeds to achieve maximum crop yield was found to be highly critical at 6 wk after sowing, but in 1982 when weed biomass was much lower a single weeding any time from 4–7 wk after sowing gave as high a yield as a crop kept weed-free all season. Earlier weeding allowed subsequently emerging weeds to become competitive and reduce crop yield, while delaying weeding until after the optimum time allowed early competition from weeds emerging with the crop to depress final yield. In all years if weed removal was delayed until 12 wk after sowing, the crop yield was only slightly or no higher than if weeds were left to grow all season. It is concluded that swedes, unlike red beet or sugar beet, are incapable of recovery from the severe restriction to growth caused by weeds competing with the crop from 6–12 wk after sowing.     

The response of spring barley and winter wheat to Avena fatua population density

Populations of Avena fatua were established in crops of spring barley and winter wheat, and relationships derived between yield and other crop parameters, and weed density. Competitive effects of A. fatua, which were similar to those found in other countries, were greatest at low crop densities. Crop head numbers were reduced proportionately less than crop yield, indicating that competition was affecting other yield components. Competition increased the proportion of thin grain of barley but not of wheat. Competition had little effect on the moisture content and the contamination of the harvested grain by A. fatua, suggesting that at threshold populations these factors are unlikely to be of economic significance. It was concluded that at average crop densities, low infestations of A. fatua are likely to result in cereal yield losses in the region of 1 % for each A. fatua plant m^-2.     

THE COMPETITION BETWEEN BARLEY AND CERTAIN WEEDS UNDER CONTROLLED CONDITIONS

Chickweed (Stellaria media) is one of the commonest annual weeds on almost all soils, and forms 20–30 % of the weed herbage on the lower greensand soils of Woburn, Beds. It occurs in all crops in this area except on very acid soils.
When barley and chickweed are planted together, with abundance of water and nutrients for both, it is found that increasing the density of planting of the barley reduces the loss due to chickweed competition, but even with very close-planted barley, the loss caused by the weed amounts to nearly two-thirds of the total fresh weight and to four-fifths of the grain yields. This is much greater than with the two annual weeds previously studied (spurrey and mayweed). In a sparse crop of barley, increase in chickweed growth may reduce the barley to less than 10 % of its growth without the weed, while the chickweed itself is relatively little affected by the presence of the barley.
There is no evidence of any specific effect of the roots of the one plant on the other; they intertwine without any sign of attraction or repulsion between them.
Comparisons are made with other annual weeds previously studied, and it is suggested that the method adopted furnishes a means of assessing the relative effectiveness of these weeds in competing with barley.     

开放式空气CO2浓度增高条件下C3作物(水稻)与C4杂草(稗草)的竞争关系

通过田间试验,研究了FACE（开放式空气CO2浓度升高）条件下C3作物水稻（Oryza sativa）和C4杂草稗草（Echinochloa crusgalli）的生长和竞争关系,结果表明,FACE条件下C3植物水稻生物量和产量增加,吉片数增加,分蘖数增加,叶面积系数（LAI）增大;而C4植物稗草相反,FACE条件下水稻和稗草中面积均减少,而净同化率（NAR）均增加;FACE条件下水稻-稗草比例为1：1时,水稻与稗草的生物量比率、产量比率、LAI比率、茎蘖比率和NAR比率均增加,水稻-稗草的竞争关系发生变化,水稻（C3植物）竞争能力增加,稗草（C4植物）竞争能力下降。     

Simulated herbivory counteracts the effects of grass competition on the invasive fireweed (Senecio madagascariensis)

Competition, herbivory and their interaction play a significant role in determining the competitive ability and survival of individual plant species. Understanding these processes and interactions can improve the efficacy of biocontrol programs against invasive weeds. Senecio madagascariensis (fireweed) is an invasive weed of South African origin that reduces pastoral productivity and poisons livestock in several countries, notably Australia. Although competitive pastures can suppress the weed’s growth in Australia, its competitive nature is poorly understood in relation to its invasion success. This greenhouse study assessed the growth and reproductive yield of fireweed growing in competition with six native and introduced grasses present in both South Africa and Australia. Since fireweed is a target for biocontrol in Australia, we examined whether its response to grass competition changed with herbivory (simulated by 40% leaf removal). The effect of grass competition and herbivory on the weed’s biomass and floral productivity was examined during a 12‐week pot trial in South Africa. Floral numbers were unaffected by both grass competition and herbivory. Biomass was used to calculate Relative Interaction Indices (RII) to quantify the weed’s competitive or facilitative response. This index compares a specific measurable trait, such as biomass, of fireweed growing alone, to fireweed growing with grass to determine the level of competitive suppression or facilitation resulting from the interaction. Despite the lack of species‐specific effects of grass competition, the presence of grass suppressed fireweed’s foliar, root and whole plant biomass the most when herbivory was absent. With herbivory, fireweed did not suffer from any measurable competitive suppression. This lack of competitive suppression may be due to an induced allelopathic response, given the levels of pyrrolizidine alkaloids common in many Senecio species. Since this result may weaken the case for biocontrol, the weed’s competitive responses should be verified in relation to actual insect herbivory.     

THE MUTUAL EFFECT OF RYEGRASS AND CLOVER WHEN GROWN TOGETHER

Italian ryegrass and a late-flowering red clover were grown together, with abundance of water and nutrients for both. It was found that even a small number of ryegrass plants reduced the growth of clover by 30%. This effect varied very little with increasing density of the clover crop.
The presence of clover reduced the ryegrass crop by an amount diminishing as the density of the ryegrass was increased. In a sparse crop of ryegrass, clover reduced the growth of the grass considerably more than did barley under comparable conditions.
There is no evidence of any specific effect of the roots of one plant on the other. When ample nitrogen is available the clover tends to take some that would otherwise be available for the grass and does not provide the grass with additional nitrogen.     

Does weed suppression by high crop density depend on crop spatial pattern and soil water availability?

Size-asymmetric competition, in which larger plants obtain a disproportionally larger share of contested resources, can be applied in agriculture to suppress weeds by increasing crop density and spatial uniformity, as these practices enhance the initial size-asymmetric competitive advantages of crop seedlings over weed seedlings early in the growing season. We do not yet know how agronomic factors influence weed suppression at high crop density. We performed a field experiment to ask how crop density, spatial pattern and irrigation interact to influence weed suppression and grain yield in semi-arid croplands. The experimental was a factorial design with 4 factors: wheat cultivar (Ningchun4, Xihan2), irrigation level (control, irrigated), sowing density (low, 196 seeds m⁻²; moderate, 400 seeds m^-2; high, 625 seeds m⁻²), and spatial sowing pattern (rows, uniform). Weed growth was effectively suppressed by increased crop density and spatial uniformity. Effects of crop density on weed suppression and grain yield were more pronounced in the uniform pattern than in crop rows. Weed biomass was 55.7% lower and grain yield increased 29.7% higher in the high density uniform pattern compared to the low density and row pattern. Crop density interacted with cultivar in determining both weed biomass and grain yield, potentially reflecting different traits regulating crop competitive ability. Irrigation and crop density had additive effects on weed biomass but interacted to influence grain yield. Our findings support the idea that increased crop density and spatial uniformity can make a valuable and environmentally friendly contribution to weed control in wheat, reducing the need for chemical or mechanical weed control. We need a better understanding of the interactions among climate, agricultural management and crop genotype to improve our ability to effectively suppress weeds with high crop density and spatial uniformity.     

耕作措施对冬小麦田杂草生物多样性及产量的影响

在连续5a秸秆全量还田的免耕、旋耕、耙耕、深松和常规耕作试验地中,设置了除草和不除草处理,研究了其对杂草总密度、优势杂草种类、生物多样性指数和冬小麦产量的影响,并分析了杂草与小麦间的竞争关系。结果表明,麦田杂草主要有7种,分别是麦蒿(Descuminia sophia (L.) Webb ex Prantl)、荠菜(Capsella bursa-pastoris (L.) Medik)、燕麦(Avena sativa L.)、田旋花(Convolvulus arvensis L.)、刺儿菜(Cirsium setosum (Willd.) Bieb.)、繁缕(Stellaria media (L.) Vill.)、麦家公(Lithospermum arvense L.)。在未除草条件下,免耕、深松的杂草总密度显著提高;而在除草条件下,杂草密度显著下降。免耕、深松、常规耕作在未除草条件下,优势杂草种类为麦蒿、荠菜,旋耕、耙耕条件下的优势杂草为麦蒿;而除草后各处理的优势杂草均只有麦蒿。耙耕、常规耕作措施在未除草条件下杂草群落具有较高的物种丰富度和均匀度。无论哪种耕作措施,除草能提高冬小麦产量,其中以深松耕作结合除草处理的小麦产量最高。在小麦抽穗期,未除草处理杂草株高接近或高于小麦株高,会造成杂草与小麦间的光竞争,对小麦的生长状况有显著影响,从而导致小麦产量降低。     

Interspecific Competition for Soil N and its Interaction with N2 Fixation, Leaf Expansion and Crop Growth in Pea–Barley Intercrops

Field experiments were carried out during three successive years to study through a dynamic approach the competition for soil N and its interaction with N₂ fixation, leaf expansion and crop growth in pea–barley intercrops. The intensity of competition for soil N varied between experiments according to soil N supply and plant densities. This study demonstrates the key role of competition for soil N which occurs early in the crop cycle and greatly influences the subsequent growth and final performance of both species. Relative yield values for grain yield and N accumulation increased with the intensity of competition for soil N. Barley competed strongly for soil N in the intercrop. Its competitive ability increased steadily during the vegetative phase and remained constant after the beginning of pea flowering. The period of strong competition for soil N (500–800 degree-days after sowing) also corresponded to the period of rapid growth in leaf area for both species and therefore an increasing N demand. For each species, the leaf area per plant at the beginning of pea flowering was well correlated with crop nitrogen status. Barley may meet its N needs more easily in intercrops (IC) and has greater leaf area per plant than in sole crops (SC). Barley having a greater soil N supply results in an even higher crop N status and greater competitive ability relative to pea in intercrop. Competition by barley for soil N increased the proportion of pea N derived from fixation. The nitrogen nutrition index (NNI) values of pea were close to 1 whatever the soil N availability in contrast to barley. However N₂ fixation started later than soil N uptake of pea and barley and was low when barley was very competitive for soil N. Due to the time necessary for the progressive development and activity of nodules, N₂ fixation could not completely satisfy N demand at the beginning of the crop cycle. The amount of N₂ fixed per plant in intercrops was not only a response to soil N availability but was largely determined by pea growth and was greatly affected when barley was too competitive.     

The conservation of arable weeds at crop edges of barley fields in northeast Spain

The effects of weed control practices and fertilisation on weed flora and crop yield were evaluated in crop edges of barley fields in northeastern Spain. The study was carried out in four organic and four conventional barley fields. In each field, four permanent plots were delimited at the crop edge, and fertilisation and weed control treatments in a factorial design were applied over 3 years. Weed composition and the aboveground biomass of weeds and barley were recorded before the crop harvest in the first and the third year. We found relatively low values of species richness per field, as well as low values of weed biomass, especially in the organic crop edges (3.9% of total biomass). Weeds were significantly reduced by herbicide applications on conventional fields and were not affected by weed harrowing on organic fields or fertilisation. These results demonstrate that specific measures are needed to enhance biodiversity at crop edges both in organic and conventional fields. Our results also suggest that under Mediterranean conditions and among impoverished weed communities, limiting the use of herbicides is crucial to enhancing arable diversity and that, contrary to findings found in previous studies in temperate climates, fertilisation and weed harrowing have little effect on weeds.     

开放式空气CO2浓度升高与作物/杂草的竞争关系

CO2浓度升高对植物的光合作用、呼吸作用和水分利用等生理过程产生直接影响,进而影响植物的生长繁殖,CO2浓度升高对于具有C3光合途径的植物较具C4光合途径的植物更为有益,由于许多重要的杂草是C4植物,而许多重要的作用是C3植物,CO2浓度升高对杂草/作物的相互关系将有重要影响,本文就全球CO2浓度升高和气候变化对杂草/作物之间竞争关系影响进行综述,同时针对目前研究现状和可持续农业的需要,提出CO2学浓度升高条件下杂草/作物之间竞争关系及未来农田杂草治理方面理论与实践中有待解决的问题。     

Competition Experiments on Alien Weeds with Crops: Lessons for Measuring Plant Invasion Impact?

Can we quantify the impact of invasive species? Here we use the per-plant competitiveness of alien weeds on crops as a model of invasive species impact in general. We reviewed 97 weed–crop competition experiments in 32 papers that included 30 alien weed and 14 crop species. The majority (68.92%) were randomised block designs where the alien weed had been either added (additive experiments) or removed (removal experiments). We propose using the relative competition index to estimate the effect of alien species in all systems, specifying in each case the density and proportion of alien and native plants essayed. We found that the impact of the weed cannot be considered independently of the crop and, thus, we should be cautious in ranking weed species according to their competition effect. A similar situation can be postulated for alien plants interfering with native species. Invaded communities are not random assemblages, and researchers tend to study the most competitive alien plants. We also found that the effect of the weed on crop yield depends on the duration of the interference and the life-history stage of the weed–crop system at which the interaction takes place. We were not able to conduct a more rigorous comparative analysis of the impacts, such as a meta-analysis. To do this would require some measure of the variation of the competition effect such as standard deviation or standard error, which we found are almost never reported.     

农林复合系统中物种间水肥光竞争机理分析与评价

低丘红壤农林复合系统被认为能通过引入树木而利用土壤深层水分及防治水土流失,从而作为亚热带地区应对季节性干旱的有效利用方式。然而,复合也可引起光能、水分和养分的竞争,导致农作物减产。通过作物生长量的测定、利用多年监测的土壤水文数据、15N微区试验及光合有效辐射的测量,综合探讨了南酸枣-花生复合系统引起的物种间水肥光的变化;通过其交互作用形成的协同,竞争关系分析,较为全面地评价了农林复合系统水肥光竞争特征。研究表明：低丘红壤上南酸枣与花生复合,促进了南酸枣生长,却减小了20％～50％的花生产量和生物量。其原因不单是南酸枣遮荫引起复合花生光合有效辐射减弱,还与水、肥竞争有关。复合系统在旱季加大利用50～100cm土层土壤水分,从而缓冲了干旱造成的影响;但南酸枣与花生间作系统也存在着一定的水分竞争。复合使得南酸枣能够利用施于花生区及淋失到60cm深处的养分,提高了养分的利用率;但同时也导致养分的竞争并影响花生的生长。在花生产量、生物量受复合南酸枣竞争影响因子中,以光最大、养分其次、水分最小。农林复合系统水肥光交互作用因其组分类型与时空配置而异,需从生态、经济、社会效益方面对复合模式加以优化。     

Modelling seedling growth rates of 18 temperate arable weed species as a function of the environment and plant traits

BACKGROUND AND AIMS: The early growth rate of seedlings in the exponential phase is an important eco- physiological trait in crop/weed competition models based on assessments of relative weed green area. An understanding of the role of various plant traits in determining early growth rate may also be useful for identifying contrasting weed strategies for establishment before canopy closure. METHODS: The response of seedling relative growth rate (RGR) to the environment was measured in outdoor sand beds in the autumn and the spring for 18 temperate annual weed species and two crops. Seedling growth was modelled using thermal time and effective day-degrees (combining the effect of temperature and radiation). The contribution of various plant traits in determining variability in RGR was investigated using regression analysis. KEY RESULTS: The effective day-degree model was more effective for describing early weed growth than thermal time. Variability in RGR measured in the autumn was largely determined by differences between the species in net assimilation rate (NAR), whereas in the spring leaf area ratio (LAR) played a larger part. There were differences between the broadleaf and grass species in the relative contribution of NAR and LAR to RGR in both seasons. RGR in the spring was negatively correlated with initial seedling size. CONCLUSIONS: The parameters derived in this study can be used to calibrate empirical models of crop yield loss based on relative weed green area to different growing seasons and assessment dates. The grass weeds, which tended to have large seeds, had a higher investment in roots in the seedling stage, potentially making them more competitive later in the season when resources become limiting.     

Influence of long-term different fertilization on soil weed seed bank diversity of a paddy soil under rice/rape rotation

A long-term fertilized paddy field under rice/ rape rotation in the Taihu Lake Region was selected to investigate the dynamics of soil weed seed diversity.Four fertilizer treatments were performed,including non-fert-ilizer (NF),chemical fertilizer only (CF),chemical fert-ilizer combined with pig manure (CMF) and chemical fertilizer plus crop stalk (CSF).We recorded the seed numbers and crop yields,estimated the weed seed bank density and identified the kinds of weed seeds in the top-soil (0-15 cm) in the study area using a stereomicroscope.Based on the records,we analyzed the effect of long-term fertilization on soil weed seed bank diversity and the rela-tionship between weed seed diversity and crop yields.Comparing the four treatments,it was found that in the cultivating seasons of both rice and rape,the density of soil weed seed bank was the lowest with the treatment of chemical fertilizer plus crop stalk.Whereas,the total num-ber of species and the weed seed bank diversity was the highest.Furthermore,the crop yields were at maximum and kept constant with this treatment.There was a def-inite correlation between fertilizer treatment and soil weed seed bank diversity and crop yields.It was concluded that balancing the fertilizer management was helpful in main-taining soil weed seed bank diversity,increasing crop yields and alleviating crop yield fluctuation.Therefore,among the four fertilizer treatments,chemical fertilizer plus rice crop stalk treatment was the best one to stimulate the productivity of agricultural ecosystems and simulta-neously protect biodiversity.     

Some influences of CO2 enrichment, nitrogen nutrition and competition on grain yield and quality in spring wheat and barley

Spring wheat and spring barley were grown in elevated atmosphericCO₂ in controlled environments. Wheat was grown in monocultureand in competition with three weed species. In monoculture,wheat had 30% more grain yield and 28% less grain nitrogen inelevated compared to ambient atmospheric CO₂- In competition,wheat had no significant increase in yield with elevated atmosphericCO₂- In competition, grain nitrogen concentration was reducedin response to CO₂ with the largest reduction occurring withthe smallest competitor and the smallest reduction occurringwith the largest competitor. Spring barley was grown in monocultureat three nitrogen fertilizer supplies. In elevated atmosphericCO₂ there were significant increases in grain yield and reductionsin grain nitrogen concentration at all levels of nitrogen supply.In both species the reductions in grain nitrogen concentrationwere large enough to affect current bread making processes. Key words: Grain nitrogen, weeds, wheat, barley     

Narrow rows reduce biomass and seed production of weeds and increase maize yield

Smallholder farmers in southern African countries rely primarily on cultural control and hoe weeding to combat weeds, but often times, they are unable to keep up with the weeding requirements of the crop because of its laboriousness, causing them to incur major yield losses. Optimisation of crop planting pattern could help to increase yield and suppress weeds and to reduce the critical period of weed control and the weeding requirements to attain maximum yield. Experiments were carried out in Zimbabwe during two growing seasons to assess the effect of maize density and spatial arrangement on crop yield, growth and seed production of weeds and to determine the critical period for weeding. Planting maize at 60 cm row distance achieved higher yields and better weed suppression than planting at 75 or 90 cm row distance. Increasing crop densities beyond the customary three to four plants m⁻² gave modest reductions in weed biomass but also diminished crop yields, probably because of increased competition for water and nutrient resources. Maize planted in narrow rows (60 cm) intercepted more radiation and suffered less yield reduction from delaying hoe weeding than those planted in wider rows (75 or 90 cm), and the duration of the weed-free period required to attain maximum grain yield was 3 weeks shorter in the narrow spacing than that in the 75- and 90-cm row spacings. Weeding was more effective in curtailing weed seed production in the narrow row spatial arrangements than in the wide row planting. The results of these studies show that narrow row spacings may reduce weeding requirements and increase yields.     

Growth and competition in annual legume—perennial grass pasture in a dry monsoonal climate

The seasonal growth characteristics of a perennial (sabi grass (Urochloa mosambicensis))-annual (Townsville stylo (Stylosanthes humilis)) pasture ecosystem in the dry monsoonal region of norther Australia are described. The phases during which the botanical composition of the ecosystem may change are dry season plant survival, germination, establishment, growth and propagation. A conceptual model with general applicability for a pasture of perennial and annual species is presented and the inter-relationships between the identified phases are indicated. For the example studied, there was a large decline in the proportion of the annual legume in the establishment phase, due to competition for restricted soil moisture. This explains the trend to grass dominance observed in sabi grass-Townsville stylo swards, and is considered to apply generally in a perennial-annual pasture in a dry monsoonal environment. Two distinct environmental periods occurred during the season and were reflected in the observed growth characteristics. During the first, the growth rate of Townsville stylo was slow and plant density fluctuated sharply. The growth rate of sabi grass also fluctuated sharply due to high temperature and radiation, and variable but limiting soil moisture. In the second period temperatures were lower and soil moisture was higher. During this period growth rate of sabi grass and tiller production declined but both the plant density of sabi grass and the growth rate of Townsville stylo increased. As sabi grass has a high initial growth rate and nitrogen and phosphorus contents which are nutritionally acceptable during the transition between the dry and wet seasons, monospecific sabi grass swards are suggested for grazing during this period.     

EFFECT OF NITROGENOUS FERTILIZER APPLIED AT DIFFERENT DATES ON TAKE-ALL, EYESPOT AND YIELD OF WINTER WHEAT GROWN ON LIGHT SANDY LOAM

In a replicated field experiment on light sandy loam at Woburn, where winter wheat is a very uncertain crop, Cappelle and Holdfast, grown after potatoes with dung, yielded 50 and 41 cwt./acre of grain, respectively, when given 6 cwt./acre of Nitro-Chalk in April, compared with 19 and 20 cwt./acre when unfertilized. The same fertilized plots yielded 29 and 19 cwt., respectively, in the second, and 25 and 17 cwt./acre, respectively, in the third year on the same land, whereas unfertilized plots of both varieties yielded only 9 and 5 cwt./acre. The decrease in mean yield from 27 cwt. in the first, to 15 and 10 cwt./acre in the second and third crops was associated with a decrease in ear number from 16.7 to 14.6 and 12.2, respectively, and with a striking decrease in weight of grain per ear, caused partly by a large increase in the proportion of small grains. Eyespot ( Cercosporella herpotrichoides Fron.) although present each year did not become prevalent; an increase in the percentage straws affected by take-all ( Ophiobolus graminis Sacc.) from 9 to 15 and 26%, respectively, and a severe increase in weed infestation ( Agrostis gigantea ) appeared to be the main factors reducing yield.
Nitro-Chalk applied in April yielded most grain every year, and wheat fertilized at this time had less eyespot and take-all than that fertilized in March. Fertilizer applied in May increased weed growth, failed to decrease take-all and yielded fewer ears, less grain, and a higher proportion of tailcorn than did earlier applications.