ALBERT

Notes: In this paper we analyse competition between soybean and Sorghum halepense through evaluation of canopy attributes. Field experiments were conducted during three years. An additive design was used, combining different densities of S. halepense tillers with a single density of soybean. Monocultures of both species were also established. There was a significant linear relationship between the relative leaf area of the weed (Rw) and its relative contribution to the total biomass of the mixture, measured early in the crop growth cycle. Crop and weed canopies in monocultures developed simultaneously. The crop and the weed started to compete at a thermal time 230–350°C.days after sowing, coincident with the onset of active net growth of S. halepense biomass. The relationship between crop yield and Rw was always linear and the slope varied according to crop sowing date. The results suggest that the evaluation of Rw may be a simple and non-destructive way of predicting soybean yield losses. Utilization de caractéristiques des couverts végétaux pour évaluer la concurrence entre Sorghum halepense (L.) Pers. et la sojaCet article analyse la concurrence entre le soja et Sorghum halepenseà partir de caractéristiques des couverts végétaux, mesurées lors d'expériences au champ conduites pendant trois ans. Nous avons utilisé un modèle additif qui combinait différentes densités de talles de S. halepense avec une densité unique de soja. Des monocultures de chaque espèce étaient aussi établies. Il existait une relation linéaire significative entre l'aire relative des feuilles de la mauvaise herbe (Rw) et sa contribution relative à la biomasse totale du mélange, mesurée précocement lors du cycle de croissance de la culture. Les couverts de la culture et des mauvaises herbes en monocultures se développaient simultanément. La culture et la mauvaise herbe entraient en concurrence 230–350°C.jours après le semis, alors que la biomasse de S. halepense commençait à croître de manière importante. La relation entre le rendement de la culture et Rwétait toujours linéaire et la pente variait en fonction de la date de semis de la culture. Ces résultats suggèrent que la mesure de Rw peut constituer une méthode simple et non-destructive pour prédire les pertes de rendement du soja. Untersuchung der Konkurrenz zwischen Sorghum halepense (L.) Pers. und Sojabohne anhand von BeschattungsmerkmalenZur Untersuchung der Konkurrenz zwischen Sorghum halepense und Sojabohne anhand von Beschattungsmerkmalen wurden über drei Vegetationsperioden Feldversuche durchgeführt. Neben Reinbeständen der beiden Arten wurde eine additive Versuchsanordnung mit verschiedenen Dichten von S. halepense gewählt, die Dichte der Sojabohne wurde nicht variiert. Im frühen Entwicklungsstadium der Kulturpflanze gab es eine signifikante lineare Beziehung zwischen der relativen Blattoberfläche des Unkrauts (Rw) und seines relativen Anteils an der Gesamtbiomasse. Bei der Kulturpflanze und dem Unkraut ergab sich in den Reinbeständen eine jeweils gleiche Beschattung. Die Konkurrenz zwischen den beiden Arten begann bei 230–350 Temperaturtagen nach der Aussaat und fiel mit dem Beginn der Netto-Biomassebildung von S. halepense zusammen. Die Beziehung zwischen den Ertrag der Sojabohne und Rw war immer linear, die Steigung hing vom Sätermin ab. Die Ergebnisse lassen darauf schließen, daß die Bestimmung des Rw eine einfache und nichtdestruktive Methode zur Bestimmung der Ertragsverluste bei der Sojabohne sein kann.

Notes: Cynodon dactylon is a low-growing C4 weed that is highly sensitive to shading. This species shows severe biomass reductions and highly plastic morphological changes in response to resource availability. The objective of this study was to analyse the effect of shading on the spatial and biomass growth of C. dactylon. Two experiments were carried out exploring different environmental conditions. Six treatments were applied on C. dactylon patches. Daylight and 18%, 41%, 48%, 69% and 85% shading of daylight photosynthetic active radiation (PAR) were imposed in Expt 1 and 0%, 22%, 44%, 50%, 74% and 87% shading of daylight PAR in Expt 2. Patches were slightly elliptical in shape and grew 3.2 and 1.5 cm d−1 with 0% shading, on average, during the whole studied periods in Expts 1 and 2 respectively. Biomass was significantly reduced from 41% shading onwards in Expt 1 and from 50% shading in Expt 2. However, patch extension rate was only significantly reduced from 85% and 74% shading in Expts 1 and 2 respectively. Biomass partitioning was modified by shading in both experiments. A functional analysis of C. dactylon patches showed that the patch extension rate diminished linearly when patch biomass growth rate was lower than 1.66 g d−1; above this value, the extension rate remained constant.

Notes: The invasive ability of Cynodon dactylon is dependent on self dispersal and on cultivation practices. Tillage can seriously change patch biomass and spatial structure, spreading vegetative propagules of the weed. The objectives of this study were: (i) to quantify the effect on non-inversion tillage on dispersal, establishment and colonization of C. dactylon and (ii) to propose a simple model considering soil cultivation effects and light availability on spatial growth of weed patches. Two experiments were carried out, exploring different soils and environmental conditions. Spatial distribution of vegetative units differed when tillage was conducted with different non-inversion implements and could be described by simple functions. A minimum patch biomass seems necessary before vegetative structures are vulnerable to movement by cultivation. Only a small proportion of the biomass dispersed from original patches was able to establish. However, simulation showed that the area colonized by C. dactylon mostly increased by means of tillage dispersal, both with and without crop competition, in one growing cycle. It appears sensible to consider changing cultivation practices to reduce weed dispersal and to use crop competition for light to create unsuitable habitats limiting weed colonization.

Notes: Bud-sprouting dynamics, sprouting base temperature and reserve mobilization of rhizomes and stolons of Cynodon dactylon (L.) Pers. were characterized in a laboratory experiment under dark conditions. In addition, the dynamics of sprout emergence were determined during 2 years under field conditions. In the laboratory experiment, buds from the vegetative structures did not sprout at temperatures below 7^C. The rate of sprouting increased with temperature within the range 11–33^C, and a base temperature of 7.71^C was determined for bud sprouting of rhizome and stolons of the weed. An exponential model is proposed relating the percentage of sprout emergence in the field to accumulated thermal units, calculated as the sum of the daily mean air temperature above the base temperature of bud sprouting. This model satisfactorily predicted the dynamics of C. dactylon sprout emergence in sunflower (Helianthus annuus L.) crop fields. The efficiency of carbohydrate reserve use during sprout growth was highly dependent on temperature and type of vegetative structure The efficiency was maximum at 20^C and was higher for rhizomes than for stolons. The main factors influencing bud sprouting and early weed establishment are discussed.

Notes: Patteras of seed dispersal and seedling recruit ment of Sorghum halepense (L.) Pers. were compared in abandoned and cultivated fields in Argentina. In both the abandoned and cultivated plots, the greatest concentration of seed fell below, or only a short distance, from the parent plants. A maize combine harvester greatly increased the distance of dispersal of a small part of the seed population. The availability of seed for dispersal, the direction and velocity of wind and the combine harvester were important factors determining the temporal and spatial patterns of seed distribution. The ratio of recruited seedlings to seeds shed was only 1% under the parent plant, but increased to about 100% at a distance of 1.25 m. Soil tilling, which is required for seedling recruitment, and seed density were important factors in determining seedling density as the distance from the parent plants increased. The rate of seedling recruitment was higher in areas where most of the seeds were dispersed late in the season than in those where dispersal occurred early. Dispersion des graines, distribution et reprise des plantules de Sorghum halepense (L.) Pers.Les schémas de dispersion des graines et de reprise des plantules de Sorghum halepense (L.) Pers. ont été comparés dans des champs abandonnés et d'autres cultivés en Argentine. Tant dans les parcelles abandonnées que cultivées, la plus grande concentration de graines tombe sous ou seulement à une faible distance des plantes parentes. Une moisonneuse à maïí a grandement augmenté la distance de dispersion d'une petite partie de la population grainière. La disponibilité des graines pour la dispersion, la direction et la vitesse du vent, et la moissonneuse ont été d'importants facteurs de détermination pour les schémas de distribution spatiale et temporaire. Le ratio de reprise de plantule était seulement de 1% sous les plantes parentes mais augmentait jusqu'à près de 100%à une distance de 1, 25 m. Le travail du sol, qui est nécessaire pour la reprise des plantules, et la densité grainière étaient d'importants facteurs dans la détermination de la densité des plantules quand la distance des plantes parentes augmente. Le taux de reprise des plantules était plus élevé dans les zones où la majorité des graines était dispersée tard en saison que dans celles où la dispersion avait eu lieu tôt. Ausbreitung der Samen sowie Verbreitung und Entwicklung der Keimpflanzen von Sorghum halepense (L.) Pers.Ausbreitung der Samen und Entwicklung der Keimpflanzen von Sorghum halepense (L.) Pers. wurden in Argentinien auf brachliegenden und kultivierten Feldern untersucht. In beiden Situationen fanden sich die größten Samendichten unter oder nahe der Mutterpflanzen. Mit einem Mais-Mähdrescher wurde ein kleiner Teil der Samenpopulation erheblich ausgebreitet. Die Verfügbarkeit von Samen für die Ausbreitung, die Windrichtung und-geschwindigkeit und der Mähdrusch waren wichtige Faktoren für das zeitliche und räumliche Verbreitungsmuster, Unter den Mutterpflanzen keimten die Samen nur zu 1%, aber 1,25 m entfemt zu fast 100%. Bodenbearbeitung und Samendichte bestimmten die Keimpflanzendichte mit zunehmender Entfernung von der Mutterpflanze erheblich. Die Entwichlungsrate spät ansgebreiteter Samen war höher als die früh ausgebreiteter.

Notes: The dynamics of sprout emergence and rhizome populations of Sorghum halepense (L.) Pers. were followed in two field experiments. In addition, the herbicide dalapon was applied at various stages during rhizome development and the degree of control was evaluated. Rhizome biomass oscillated during the growth cycle, whereas the number of rhizome sprouts increased asymptotically until mid-summer. Observed values were compared against those predicted from a thermal calendar model. The dynamics of the rhizome population was predicted accurately in both experiments, but only in one of them was the prediction of the sprout emergence satisfactory. The degree of control differed markedly between herbicide applications at various intervals during population development. Best results were obtained from applications when rhizome biomass was at a minimum. The time of minimum biomass was predicted by a thermal calendar model. The advantages of using the thermal calendar model to determine the time of herbicide application, and the possible influence of the distribution of the rhizome population in the soil profile on the predictions of the model, are discussed.

Notes: Two cultivars of each of three spring cereals (wheat, barley and oats) were grown with Avena fatua in a box experiment, where the effects of root and shoot competition were separated using soil and aerial partitions. Measures of resource complementarity (RYT) and of the relative severity of competition indicated that competition for soil resources, particularly nitrogen, was more severe than competition for aerial resources, i.e. light. When both root and shoot competition occurred, oats and barley were generally more competitive than wheat, but there were significant differences in competitive ability between cultivars of all three cereals. Although competition between cereals and A. fatua occurred predominantly below-ground, the various cultivars only differed slightly in root competitive ability against weeds. Nitrogen fertilizer did not significantly change the ranking of competitive abilities of the cultivars. Although shoot competition had less effect than root competition, the various cereals differed markedly in shoot competitive ability. The two oat cultivars, and the barley cultivar Egmont, had higher shoot competitive abilities than the two wheat cultivars or the barley cultivar Goldmarker. There was little evidence that differences in shoot competitive ability were due to differences in plant height. Une comparaison de la competition racinaire et aerienne entre les cereales de printemps et Avena fatua L.Deux cultivars de chacune des 3 céréales de printemps (blé, orge et avoine) ont été cultivés avec de l'Avena fatua dans des boites expérimentales ou les effets de la compétition aérienne et ceux de la compétition racinaire étaient séparées en utilisant des partitions de sol et d'air. Des mesures de complémentarité de ressources (RYT) et de la sévérité relative de la competition ont montré que la compétition pour les ressources du sol, en particulier l'azote, était plus forte que la compétition pour les ressources aériennes comme la lumière. Quand à fois on avait une compétition aérienne et racinaire, l'avoine et l'orge étaient généralement plus compétitives que le blé, mais il y avait des différences significatives dans l'aptitude à la compétition entre les cullivars des 3 céréales. Bien que la compétition enlre céréales et A. fatua soit apparue pour la majeure partie sous le sol, les différents cultivars ont seulement légèrement différé dans leur aptitude à la competition contre les adventices. Les engrais azotés n'ont pas changé de façon significative le niveau de la competitivité des cullivars. Bien que la compétition aérienne soit moins forte que la racinaire, les différentes céréales ont exprimé des différences nettes dans leur aptitude à la compétition aérienne. Les 2 avoines et l'orge ‘Egmont’ ont une plus grande capacité de compétition que les 2 blés et l'orge ‘Goldmaker’. II n'est pas ressorti que les différences dans la compétitivé aérienne soient liées à des différences de hauleur de plantes. Konkurrenz im Wurzel- und im Sproßbereich zwischen Sommergetreide und Avena fatua L.Je 2 Sorten von Sommerweizen, Sommergerste und Hafer wuchsen mit Avena fatua in einem Topfversuch, wo durch Trennwände die Wirkung der Wurzel- und der Sproßkonkurrenz unterschieden werden konnte. Messungen der Nährstoffaufnahme und der relativen Stärke der Konkurrenz zeigten, daß die Konkurrenz um Nährstoffe im Boden, besonders Stickstoff, heftiger war als die Konkurrenz im Sproßbereich, z. B. um Licht. Lag sowohl Wurzel- wie Sproßkonkurrenz vor, waren Hafer und Gerste allgemein konkurrenzstärker als Weizen, aber es gab bei allen 3 Getreidearten signifikante Unterschiede zwischen den Sorten. Obwohl die Konkurrenz zwischen den Getreiden und A. fatua vor allem im Wurzelbereich lag, unterschieden sich die verschiedenen Sorten da in ihrer Konkurrenzkraft nur wenig, was auch durch Stickstoffdüngung nicht signifikant geändert werden konnte. Trotz der geringeren Bedeutung der Konkurrenz im Sproßbereich unterschieden sich die Getreidearten hier in ihrer Konkurrenzkraft deutlich: Die beiden Hafersorten und die Gerstensorte ‘Egmont’übertrafen hierin die beiden Weizensorten oder die Gerstensorte ‘Goldmaker’. Es gab keine Hinweise darauf, daß die Unterschiede der Konkurrenzkraft im Sproßbereich auf die Halmlänge zurückzuführen sei.

Notes: A model of crop:weed competition, based on parameters from the logistic biomass growth of both crop and weeds in monocultures, was validated under several sets of conditions. Model inputs were (i) dry matter per unit area at 50% of seedling emergence (Bi) for each species; (ii) maximum total biomass of each species in monocultures (K ); (iii) intrinsic growth rate of each species in monocultures (r); (iv) the accumulated thermal time required for each species to reach 50% emergence; and (v) the total accumulated thermal time during the growing season considered in the simulation. The model adequately described the dynamics of the competition between species when the relative yield total of the mixture was close to one, i.e. the slope and intercept of the regression between observed and simulated values was not significantly different from one and zero respectively. Sensitivity analysis indicated that final crop biomass was particularly affected by changes in the relative growth rate of the species. The occurrence of a critical period for weed control was explored using the model. The simplicity of the model validated here would allow its use as a tool to predict the outcome of competition and species relative importance, particularly when parameters needed to run more complex models are not available.

Notes: The dynamics of Sorghum helpense (L.) Pers. rhizome sprout emergence was followed in three different crop systems. A simple exponential model relating the number of rhizome sprouts m−2 to accumulated thermal units, calculated as the sum of the daily mean air temperatures above 15°C, provided a good estimate of cumulative shoot emergence populations (r2= 0·93) over all sampling dates between June and January. A good description of the dynamics of shoot emergence in an independent experiment was obtained from the model.The possible influence of the crop structure and soil water supply in relation to the model fitness is discussed.