Relative Time Of Emergence Research Articles

Heat wave time of emergence patterns: a matter of definition

Hot extremes, such as heatwaves, have been associated with health, economic, and ecosystem-wide impacts. The timing of emergence of changes in extremes due to anthropogenic climate change is a topic of broad scientific and societal importance. While various studies have estimated the timing and impacts of heatwaves, the definitional aspect of a heatwave in determining the relative time of emergence has not been addressed. We adopt two commonly used definitions of heatwave employed in different reports of the Intergovernmental Panel on Climate Change (IPCC) to evaluate the time at which the frequency of heatwaves becomes detectably different from the historical baseline using an ensemble of 10 GCMS from the CMIP6 archives forced by the SSP2–4.5 concentration pathway. For a heatwave definition of sustained temperatures exceeding 5 °C warmer above the historical climatology, time of emergence is earliest in high latitudes over land and displays correlation with the signal (amount of warming) and noise (variability). In contrast, for a heatwave definition of sustained temperatures exceeding the 90th percentile of historical climatology, time of emergence is earliest in low latitude regions and is correlated with the signal to noise ratio. This work underscores the importance of metric choice in estimating the timing of new climate regimes and that metric selection for informing adaptation timing should thus be tailored to the regional context.

Seasonal Abundance and Population Fluctuation of Leucaena Tree Beetle, Acanthoscelides macrophthalmus (Schaeffer) (Coleoptera: Chrysomelidae: Bruchinae) and its Accompaniment Parasitoids

The data presented in this study provide some basic information regarding seasonal abundance and population fluctuation, the influence of temperature and relative humidity, monthly population and emergence percentages, time and peacks number, and emergence ratios of population density of each leucaena pods and seeds beetle, Acanthoscelides macrophthalmus and its accompaniment parasitoids (Dinarmus basalis Rondani and Lyrcus sp.) emerged from infested Leucaena leucocephala trees. Insects are abundant throughout the year. The highest monthly population of beetles recorded during November, followed by May and December, the highest population of parasitoids observed during February, followed by March and January (D. basalis) also January and August (Lyrcus sp.).Nine peaks for beetles are recoded, the higher peak observed at 4th week Nov. followed by 3rd week of May, then 4th week of January, the same number of peaks recorded for D. basalis parasitoid, the highest peak noticed at 3rd week of Jan. and March, whereas Lyrcus sp. parasitoid has 8 peaks, the higher found at 3rd week of Jan.General ratio of insect’s emergence was 1:0.384:0.047 (beetles: Dinarmus: lyrcus). Highest ratios between beetles: parasitoids noticed during February (1:2), July (1:1.774) and March. (1:1.287). The dominant weather factors (MaxTemp., Min.Temp. & RH%.) evidently showed effective changes in the population density of beetles and parasitoids during annual activity months. The correlation between parasitoids and beetles population differed during the different seasons. The current work shows that Dinarmus basalis (26.8% seasonal emergence) and Lyrcus sp. (3.3% seasonal emergence) parasitoids develop within infested leucaena seeds with A. macrophthalmus beetle. Numerous numbers of each beetle (69.9% seasonal emergence) and parasitoids (31.3% seasonal emergence) were observed throughout the year, therefore A. macrophthalmus beetles can be used as a substitute host for mass rearing of D. basalisparasitoids, which are biological agent to control stored seed bruchids.

Is deoxygenation detectable before warming in the thermocline?

Abstract. Anthropogenic greenhouse gas emissions cause ocean warming and oxygen depletion, with adverse impacts on marine organisms and ecosystems. Warming is one of the main indicators of anthropogenic climate change, but, in the thermocline, changes in oxygen and other biogeochemical tracers may emerge from the bounds of natural variability prior to warming. Here, we assess the time of emergence (ToE) of anthropogenic change in thermocline temperature and thermocline oxygen within an ensemble of Earth system model simulations from the fifth phase of the Coupled Model Intercomparison Project. Changes in temperature typically emerge from internal variability prior to changes in oxygen. However, in about a third (35±11 %) of the global thermocline deoxygenation emerges prior to warming. In these regions, both reduced ventilation and reduced solubility add to the oxygen decline. In addition, reduced ventilation slows the propagation of anthropogenic warming from the surface into the ocean interior, further contributing to the delayed emergence of warming compared to deoxygenation. Magnitudes of internal variability and of anthropogenic change, which determine ToE, vary considerably among models leading to model–model differences in ToE. We introduce a new metric, relative ToE, to facilitate the multi-model assessment of ToE. This reduces the inter-model spread compared to the traditionally evaluated absolute ToE. Our results underline the importance of an ocean biogeochemical observing system and that the detection of anthropogenic impacts becomes more likely when using multi-tracer observations.

Relative time of weed and crop emergence is crucial for managing weed seed production: A study under an aerobic rice system

Managing weed seed production may help in reducing weed emergence and biomass in successive seasons in dry-seeded rice. Field experiments were conducted in 2012 and 2013 to ascertain the effect of delayed weed emergence, i.e., 0, 14, 28, 42, 56, and 70 days after sowing (DAS) on seed production ability of Digitaria ciliaris and Echinochloa crus-galli in two rice cultivars (PR115 with 125 days duration and PR114 with 145 days duration). Emergence of E. crus-galli was similar in both the cultivars, except at 42 DAS, where the emergence was higher in PR114 than PR115 during 2012. In PR114, D. ciliaris continued to emerge up to 56 DAS, although the increase was significant up to 28 DAS; the increased emergence of this weed was noticed up to 14 DAS in PR115. Weed biomass was also higher in PR114, although the rate of the decrease with respect to emergence time was similar in both the rice cultivars. D. ciliaris, when emerged with the crop, produced 11,820 seeds per plant in PR114, but seed production reduced by 25% in PR115. However, differences in seed production of E. crus-galli were noticed when this weed emerged between 14 and 42 DAS. An exponential model predicted that, for a 99% reduction in seed production of D. ciliaris, 42–45 days of crop advancement, while, for E. crus-galli, 21–33 days of crop advancement was required. Our study suggests that delaying the weed emergence in the crop and/or adopting better competitive rice cultivars can reduce the weed seed production substantially.

Effects of Density and Relative Time of Pigweed (Amaranthus retroflexus) Emergence on Yield of Potato (Solanum tuberosum)

AbstractFor create of food security is essential increasing of efficiency and more productivity strategic products such as potato. In order to investigate the effects of density and relative time of pigweed emergence on potato, a factorial field experiment was conducted during 2014 based on randomized complete block design with three replications at the Research Station of Borojen, Chaharmahal and Bakhtiari, Iran. Treatments were combination of three different pigweed densities (5, 10 and 15 plants/m2) and relative time of pigweed emergence (4 and 8 days before potato, emerged with potato, 4 days after potato emergence). Results indicated that the effects of density and relative time of emergence of pigweed was significant on number of branches per plant, plant height, number of tubers per square meter, biological yield and tuber yield of potato. Also, the interaction of between experimental factors was significant on the biological yield and tuber yield of potato. The earlier emergence and pigweed density led to reduce number of branches per node,biological yield and tuber yield and increase plant height of potato. The presence of weeds caused reduce the size and number of potato tubers. Procedure of decreasing biological yield and tuber yield potato was different with density increasing of pigweed in various relative time of emergence treatment. The highest (39.8 t ha-1) and the least (18.4 t ha-1) tuber yield were related to density of five plants/m2and emergence at 4 days after potato and density of 15 plants/m2and emergence at 8 days before potato treatments, respectively.

Quantifying the effect of crop spatial arrangement on weed suppression using functional-structural plant modelling.

Suppression of weed growth in a crop canopy can be enhanced by improving crop competitiveness. One way to achieve this is by modifying the crop planting pattern. In this study, we addressed the question to what extent a uniform planting pattern increases the ability of a crop to compete with weed plants for light compared to a random and a row planting pattern, and how this ability relates to crop and weed plant density as well as the relative time of emergence of the weed. To this end, we adopted the functional-structural plant modelling approach which allowed us to explicitly include the 3D spatial configuration of the crop-weed canopy and to simulate intra- and interspecific competition between individual plants for light. Based on results of simulated leaf area development, canopy photosynthesis and biomass growth of the crop, we conclude that differences between planting pattern were small, particularly if compared to the effects of relative time of emergence of the weed, weed density and crop density. Nevertheless, analysis of simulated weed biomass demonstrated that a uniform planting of the crop improved the weed-suppression ability of the crop canopy. Differences in weed suppressiveness between planting patterns were largest with weed emergence before crop emergence, when the suppressive effect of the crop was only marginal. With simultaneous emergence a uniform planting pattern was 8 and 15 % more competitive than a row and a random planting pattern, respectively. When weed emergence occurred after crop emergence, differences between crop planting patterns further decreased as crop canopy closure was reached early on regardless of planting pattern. We furthermore conclude that our modelling approach provides promising avenues to further explore crop-weed interactions and aid in the design of crop management strategies that aim at improving crop competitiveness with weeds.

Effect of the relative time of emergence on the growth allometry ofGalium aparinein competition withTriticum aestivum

The impact of the relative time of emergence on the growth allometry ofGalium aparineL. (cleavers) in competition withTriticum aestivumL. (spring wheat) was studied in a greenhouse experiment. The hypothesis that changes in growth allometry, particularly in the leaf area/height ratio, caused by a shift in the relative time of emergence, could lead to plants overcoming the competition or to competitive suppression was tested. The plant height, dry weight, total leaf area and vertical distribution were analyzed during the first 83 days of growth. The late emergence ofG. aparinesubstantially influenced the growth dynamics for the dry weight and total leaf area. The dynamics of height growth were affected only following a significant delay inG. aparineemergence. The ratio of dry weight to total leaf area was almost unaffected by the competition. By contrast, the allometric relationship between the plant height and total leaf area was highly sensitive to the relative time of emergence. The results suggest that height is the most important growth trait for the plants to resist increasing competition. Even though the plants responded to competition with elongation, the physiological costs linked with “shade avoidance” caused reductions in the dry weight and leaf area growth that resulted in the total suppression of the late‐emerging individuals, in particular when their emergence was >10 days after that of the crop.

Métodos para acelerar e uniformizar a emergência de plântulas de Schizolobium amazonicum

A dormência intensa verificada em sementes de Schizolobium amazonicum dificulta o processo de formação de mudas. O objetivo deste trabalho foi testar procedimentos para acelerar e uniformizar a emergência de plântulas de paricá. Foram utilizadas sementes coletadas de diversas matrizes na Floresta Estadual do Antimary, Acre. Os tratamentos constaram de sementes intactas (testemunha), escarificação com ácido sulfúrico concentrado (imersão por 5; 10; 15; 20; 25; 30; 35; 40; 45 e 50 min), imersão em água aquecida (60 ºC, 80 ºC e 100 ºC), escarificação da porção basal da semente com lixa, escarificação com punção por meio de perfurações na maior porção lateral do tegumento, e escarificação com esmeril elétrico na lateral do tegumento. Os efeitos dos tratamentos foram avaliados por meio da emergência de plântulas (EP) e do índice de velocidade de emergência (IVE) em seis momentos de observação, além da frequencia relativa e do tempo médio de emergência nos melhores resultados. O delineamento estatístico foi o inteiramente casualizado com quatro repetições em esquema fatorial para a EP e em arranjo simples para o IVE. A máxima EP em menor tempo pode ser conseguida com uso do punção, imersão em água fervente (100 ºC) e do esmeril elétrico.

Full‐Season Retrospectives on Causes of Plant‐to‐Plant Variability in Maize Grain Yield Response to Nitrogen and Tillage

Maize (Zea mays L.) plant‐to‐plant variability at different plant densities and N fertilizer rates has been studied previously, but little attention has been devoted to consequences of different N placement and tillage management on plant variability in kernel number (KN) and grain yield. This study investigated effects of pre‐plant N placement relative to intended maize rows on the origin and magnitude of plant‐to‐plant variability in per‐plant grain weights (GW). Field studies compared two “shallow” anhydrous ammonia (NH3) placement directions (diagonal to future row vs. parallel but 15‐cm offset from the row) in both no‐till and conventional tillage systems at two N rates (145 and 202 kg N ha−1). Maize was planted with starter fertilizer (20 kg N ha−1) within 6 d following NH3 application. Aboveground growth and development was monitored on bar‐coded plants from seedling emergence to maturity. Plant‐to‐plant uniformity in GW and KN was not improved by parallel NH3 placement, conventional tillage, or a higher N rate; however, all three factors resulted in a slight shift towards higher mean GW and KN. Within‐row plant spacing and relative seedling emergence time had little influence on relative GW. Within‐row differences in silk emergence timing and estimated stem volumes were the most highly correlated parameters to per‐plant GW. Regression models confirmed that either of these mid‐silking factors explained >50% of such GW variations within most treatment combinations. Even in management systems with conventional tillage and precision N fertilizer placements, precision seed placement alone will not guarantee low variability in per‐plant GW.

Genotypic differences in salt tolerance from germination to seedling stage in peanut

Aims In the present study, the degree of salt tolerance in different peanut varieties/lines was investigated from emergence to seedling stage. Methods The seeds of peanut varieties/lines were planted in pots with salt levels of 0, 0.15%, 0.30%, and 0.45% NaCl in soils. The degree of salinity tolerance was systematically evaluated according to ten indices reflecting emergence, morphology, and biomass accumulation, etc. Important findings Statistic analysis for the 10 indices, including the relative emergence time, relative plant height, relative stem height, relative taproot length, relative shoot fresh mass, relative root fresh mass, relative shoot dry mass, relative root dry mass, and relative plant dry mass, showed large variations among the 107 varie- ties/lines; each index displayed a normal distribution under different salt stresses. Using the principal component analysis, the ten indices could be categorized into several independent factors. According to the total score, the 107 varieties/lines were divided into four groups, i.e., highly salt tolerant, salt tolerant, salt susceptible, and highly salt susceptible, by cluster analysis at salt level of 0.15%, 0.30%, and 0.45%, respectively. The 107 varieties/lines accounted for 29.0%, 39.0%, 27.5%, and 4.5%, respectively under salt level of 0.15%, and 5.5%, 34.5%, 23.5%, and 29.0%, respectively under salt level of 0.30%, in the highly salt tolerant, salt tolerant, salt susceptible, and highly salt susceptible groups. The salt tolerance in some varieties/lines varied with the level of salt stresses; some varieties/lines showed tolerance under low salt stress but susceptible under high salt stress. The number of varie- ties/lines showed the same tolerance or susceptibility under different salt stresses was 1 ('Fuhua 11'), 7 ('Huayu 28', 'Z11', 'Huayu 39', 'Huayu 32', 'HLN2', 'Yueyou 26', and 'XE019'), 0 and 1 ('D1035'), respectively, in the highly salt tolerant, salt tolerant, salt susceptible, and highly salt susceptible groups. The 10 indices reflecting emergence, morphology, and biomass accumulation, etc., differed significantly or highly significantly among the varieties/lines in the four groups. Compared to the salt tolerant groups, the salt susceptible groups had prolonged

Influence of Raphanus raphanistrum L. Density and Relative Time of Emergence on Bean Crop

The objective of this study was to evaluate the influence of emergence times and density of R. raphanistrum plants on bean cv. ‘Rubi’ development and pods production. The experimental design utilized was factorial (2 x 5) in randomized blocks with three replications. There were two relative times of emergence of wild radish in relation to bean (at the same day of bean and when the bean had the first trefoil fully expanded) and four R. raphanistrum densities (1, 2, 4 and 8 plants per pot, or 3.33, 6.66, 13.32 and 26.64 plants m-2 and a control with only one bean plant). There was no difference between the relative times of weed emergence on the bean development. The presence of R. raphanistrum from 2 plants per pot provided negative effect on the beans features dry weight of leaves, leaf area, and dry weight of pods.

Suppressing Impact of Avena fatua and Phlaris minor on the Growth and Yield in Wheat

4 Abstract: Weed-crop competition is a complex field of study. The extent of competition is governed by a number of factors including crop species, crop cultivar, crop density, weed species, weed density, the relative time of emergence of the crop and weed, the duration of the weed presence, the efficiency of weed control and soil and environmental factors. The greatest single cause of economic loss is, however, a reduction in yield due to weeds competing with the crop for available light, nutrients and moisture. The effect of Phalaris minor and Avena fatua on wheat was investigated at agronomic research area, College of Agriculture, Dera Ghazi Khan during 2007-2008. The experiment was laid out according to Randomized Complete Block Design (RCBD) having four replications and a net plot size of 5×2.5m. Plant density of 2, 4, 6 and 8 m for -2 Phalaris minor and Avena fatua was maintained. The data collected were analyzed using appropriate statistical techniques. Maximum plant density of wheat (96.500) was observed in T 1 (control). All other parameters like total number of tillers (403.25), number of fertile tillers (400.25) and maximum plant height (86.750) was also observed in T 1 (control). Growth and yield parameters like number of spike lets/spike, number of grains/spike, 1000 grain weight, grain yield and biological yield showed significant response in the presence of Phalaris minor and Avena fatua under study. Maximum grain yield (7.0500 t/ha) was obtained in T1 (control) and T2 (6.8500 t/ha) and T6 (6.8750 t/ha), respectively. Maximum biological yield (14.875 t/ha) was obtained in T1 (control) and T2 (15.750 t/ha) and T6 (15.975 t/ha), respectively. It can be conclude from the current study that presence of weeds (Phalaris minor and Avena fatua) in wheat has remarkable adverse effect on the yield and its contributing factors. As the density increases, the crop performance is gradually declined.

The Biology of Canadian Weeds. 27.Avena fatuaL. (updated)

Beckie, H. J., Francis, A. and Hall, L. M. 2012. The Biology of Canadian Weeds. 27. Avena fatua L. (Updated). Can. J. Plant Sci. 92: 1329–1357. An updated review of biological information is provided for Avena fatua. A widespread species originating in Eurasia, A. fatua is one of the 10 worst annual weeds of temperate agricultural regions of the world. Key weediness traits of this highly selfing species include fecundity, seed shatter, and a large and persistent seed bank with variable degrees of primary seed dormancy. The species occurs in all Canadian provinces and most states in the USA. In Canada, it is most troublesome as a weed in the prairies, where it has spread throughout crop areas in all climatic zones. Depending upon plant density and relative time of emergence, A. fatua competition may reduce annual crop yields by as much as 70%. First cohort emergence of A. fatua coincides with planting and emergence of spring-seeded crops, although additional cohorts can emerge throughout the growing season. Avena fatua is more abundant in zero-than intensive-tillage systems; the former regime promotes earlier and greater emergence because of a shallower and less persistent seed bank. Despite the introduction of highly efficacious herbicides in the 1970s and 1980s, abundance of the species has not declined across the Canadian prairies or elsewhere. The continual evolution of herbicide-resistant A. fatua populations, seed spread via farm machinery, and limited herbicide modes of action for its control threaten sustained annual field crop production in many temperate agricultural areas. Further adoption and integration of multiple non-herbicidal weed management practices, such as enhanced crop seeding rate, competitive crops and cultivars, and precision fertilizer placement, should help mitigate A. fatua interference. The species has some beneficial uses as an alternative feed and food constituent or industrial feedstock, as well as potential in cultivated oat (Avena sativa L.) improvement.

Effects of relative emergence time and water deficit on the timing of fruit dispersal in Raphanus raphanistrum L.

Seed dispersal is both a spatial and a temporal phenomenon, although most studies focus on spatial aspects. Seed initiation on the maternal plant may occur over a considerable period, especially in indeterminately flowering species, and thus seeds may be exposed to a wide range of environmental conditions during their development. The result is variation in the timing of seed development, the anatomy of structures related to the dispersal process, and the behaviour and fate of seeds post-dispersal. A key resource during the growth and development of summer-maturing species in most areas, and one that is thus likely to modify these processes, is water. Two experiments were therefore undertaken to describe (i) the development of Raphanus raphanistrum fruits and the timing of fruit dispersal, and (ii) the effects of water availability on the timing of fruit dispersal. Fewer seeds were produced and subsequently dispersed by later emerging plants. The duration of fruit dispersal became shorter when the plants emerged progressively later than the crop, and the time of maximum dispersal was later. For cohorts of fruits initiated at the same time, those that developed under mild and severe water deficit reached their final length sooner, and were dispersed sooner, than those receiving a plentiful supply of water. Thus, the phenology of the maternal plant and the nature of its environment can modify the timing of propagule maturity and consequently dispersal. Such information may provide an opportunity for managers to reduce weed seed return to their field or, conversely, to regulate the amount of contaminated grain or reduce dispersal to other locations.

Effects of the relative time of emergence and the density of common lambsquarters (Chenopodium album) on corn (Zea mays) yield

Common lambsquarters (Chenopodium album) is one of the world's worst weeds. In order to study the competitive potential of single‐cross 704 corn (Zea mays) in competition with common lambsquarters at different relative times of emergence and density levels of the weed, an experiment was conducted in 2006 at the farm of the Faculty of Agriculture, Ferdowsi University of Mashhad, Mashhad, Iran. This experiment was designed as a split plot based on a randomized complete block design with three replications. The emergence time of the weed was considered at three levels (7 days and 14 days earlier than corn and simultaneously with corn) as the main plot, while the density of the weed was considered at six levels (0, 4, 8, 12, 16, and 20 plants per m2) as the subplot. The results showed a decrease in the grain yield and biomass of corn, as the emergence time of corn was delayed in comparison with the weed in a way that the maximum reduction was observed at the earlier emergence of the weed, compared to corn, and also at a high density of the weed. As the weed emerged earlier than corn, the rate of yield loss resulting from the first flush of weeds was not that high. However, with every few days that the weed emerged earlier than corn, the rate of yield loss became higher as the density of the weed increased to its maximum. The maximum reduction in the yield components was observed at 14 days earlier emergence of the weed, compared to corn, and at high densities, as the corn plants were overshadowed by the weed canopy and no ear was produced.

Corn and Barnyardgrass Competition as Influenced by Relative Time of Weed Emergence and Corn Hybrid

Barnyardgrass [Echinochloa crus‐galli (L.) Beauv.] is a weed with global impacts on corn (Zea mays L.) production. Field experiments were conducted in western Greece in 2008 and 2009 to determine the influence of barnyardgrass emergence time and corn hybrid on corn yield and barnyardgrass growth and fecundity. Barnyardgrass was uniformly seeded at dates such that it emerged concurrently with corn and at the two, four, and six‐leaf stage of corn (VE, V2, V4, and V6 growth stages, respectively). Barnyardgrass biomass, tillers, canopy area and seed production were significantly affected by the date of barnyardgrass emergence. Barnyardgrass plants emerging with corn produced 1010 to 1305 seeds per plant compared to only 112 to 240 seeds from plants emerging after the V4 growth stage. Maximum corn grain yield loss ranged from 24 to 34% for early emerging barnyardgrass, and <9% yield loss occurred from barnyardgrass seedlings emerging later than the V4 growth stage. Corn hybrids with rapid initial growth rate (such as ‘Factor’ and ‘Dracma’) were more competitive than the other hybrids. The results of this study are essential in the development of an integrated weed management strategy for corn. Findings suggest that despite the high competitiveness of barnyardgrass, careful selection of a competitive corn hybrid and control of barnyardgrass through at least the V2 to V4 growth stage could dramatically reduce grain yield loss and weed seed production.

پاسخ خصوصیات رشدی ذرت (Zea mays L.) به رقابت علف هرز سلمه تره (Chenopodium album L.)

In order to investigate the effects of common lambsquarters (Chenopodium album L.) competition on corn (Zea mays L.) morphological and physiological characteristics, an experiment was conducted in 2005-2006 at the Research Station of Agricultural Faculty, Ferdowsi University of Mashhad, Iran. The experimental design was split plot based on randomized complete block design (RCBD) with three replications. The emergence time was considered in three levels: emergence of the weed 14 days earlier, 7 days earlier and concurrent with corn, respectively, as main plots and density of weed at six levels: 0, 4, 8, 12, 16 and 20 plants per m2, respectively, as subplots. Plant height, leaf area index (LAI) and dry matter (DM) of corn decreased with earlier emerging and increasing density of the common lambsquarters. In contrast, LAI and dry matter of common lambsquarters increased with earlier emerging especially at high densities. Though, the increase in density of the weed resulted in reducing measured characteristics, but there was no difference between the processes of reduction in high densities of weed. Totally, it can be stated that relative time of weed emergence compared to its density had maximum effect on corn plants and as the number of the earlier emergence days of common lambsquarters related to the corn increase, the competitive potential of corn reduces and an intense decrease is being observed in its growth and development.

Modeling With Limited Data: The Influence of Crop Rotation and Management on Weed Communities and Crop Yield Loss

Theory and models of crop yield loss from weed competition have led to decision models to help growers choose cost-effective weed management. These models are available for multiple-species weed communities in a single season of several crops. Growers also rely on crop rotation for weed control, yet theory and models of weed population dynamics have not led to similar tools for planning of crop rotations for cost-effective weed management. Obstacles have been the complexity of modeling the dynamics of multiple populations of weed species compared to a single species and lack of data. We developed a method to use limited, readily observed data to simulate population dynamics and crop yield loss of multiple-species weed communities in response to crop rotation, tillage system, and specific weed management tactics. Our method is based on the general theory of density dependence of plant productivity and extensive use of rectangular hyperbolic equations for describing crop yield loss as a function of weed density. Only two density-independent parameters are required for each species to represent differences in seed bank mortality, emergence, and maximum seed production. One equation is used to model crop yield loss and density-dependent weed seed production as a function of crop and weed density, relative time of weed and crop emergence, and differences among species in competitive ability. The model has been parameterized for six crops and 15 weeds, and limited evaluation indicates predictions are accurate enough to highlight potential weed problems and solutions when comparing alternative crop rotations for a field. The model has been incorporated into a decision support tool for whole-farm management so growers in the Central Great Plains of the United States can compare alternative crop rotations and how their choice influences farm income, herbicide use, and control of weeds in their fields.

Selection of a Barley Yield Model Using Information–Theoretic Criteria

Empirical models of crop–weed competition are integral components of bioeconomic models, which depend on predictions of the impact of weeds on crop yields to make cost-effective weed management recommendations. Selection of the best empirical model for a specific crop–weed system is not straightforward, however. We used information–theoretic criteria to identify the model that best describes barley yield based on data from barley–wild oat competition experiments conducted at three locations in Montana over 2 yr. Each experiment consisted of a complete addition series arranged as a randomized complete block design with three replications. Barley was planted at 0, 0.5, 1, and 2 times the locally recommended seeding rate. Wild oat was planted at target infestation densities of 0, 10, 40, 160, and 400 plants m−2. Twenty-five candidate yield models were used to describe the data from each location and year using maximum likelihood estimation. Based on Akaike's Information Criterion (AIC), a second-order small-sample version ofAIC(AICc), and the Bayesian Information Criterion (BIC), most data sets supported yield models with crop density (Dc), weed density (Dw), and the relative time of emergence of the two species (T) as variables, indicating that all variables affected barley yield in most locations.AIC,AICc, andBICselected identical best models for all but one data set. In contrast, the Information Complexity criterion,ICOMP, generally selected simpler best models with fewer parameters. For data pooled over years and locations,AIC,AICc, andBICstrongly supported a single best model with variablesDc,Dw,T, and a functional form specifying both intraspecific and interspecific competition.ICOMPselected a simpler model withDcandDwonly, and a functional form specifying interspecific, but no intraspecific, competition. The information–theoretic approach offers a rigorous, objective method for choosing crop yield and yield loss equations for bioeconomic models.

Evaluation of Different Empirical Models of Crop/Weed Competition to Estimate Yield and LAI Losses from Common Lambsquarters (Chenopodium album L.) in Maize (Zea mays L.)

Usefulness and validity of different empirical yield loss models at describing the effect of common lambsquarters competition in maize were evaluated in a two year experiment in Karaj during 2001 and 2002 growing seasons. Experimental factors were density (1st year: 2, 5, 10 and 15 plants m(-2); 2nd year: 6.6, 13.3 and 20 plants m(-2)) and the relative emergence time (1st year: simultaneous to, at 2-3 and 4-5 leaf stages of maize; 2nd year: simultaneous to, at 2-3 and 5-6 leaf stages of maize) of common lambsquarters. Results indicated that the highest maize yield and LAI losses were observed at simultaneous emergence of weed and maize resulted in 85 and 92% yield loss and 73 and 53% LAI loss in the first and second years of experiments, respectively. Also, delaying common lambsquarters emergence reduced its competitive ability against maize. Comparison of different empirical models revealed that the empirical yield loss models based on density and the relative time of weed emergence and the weed relative leaf area, also the rectangular hyperbolic yield loss model based on weed density were more reliable at predicting maize yield and LAI losses according to their high coefficient of determination (R2). Also, results indicated that the negative effect of the relative time of common lambsquarters emergence on maize yield loss was more than weed density, so that the rectangular hyperbolic yield loss model based on weed density was more capable at predicting yield loss at each of weed emergence time.