Effect of simulated mowing on the translocation of mefluidide in perennial ryegrass (Lolium perenne L.)

Young plants of perennial ryegrass (Lolium perenne L.) absorbed approximately 50% of an application of the grass growth retardant, mefluidide, while from 6.5 to 21.9% of the applied dose was translocated during a 2- or 7-day period respectively. After the 2-day period 93.4% of extractable radioactivity was identified as mefluidide. This potentially useful chemical was made unavailable as a result of mowing before or after spraying. Mowing before spraying increased the proportion of cut mature laminae and young laminae, neither of which translocated mefluidide to other plant organs as efficiently as intact, mature leaves. Mowing after spraying removed potentially translocatable mefluidide with the cut mature laminae and reduced mefluidide accumulation in untreated plant parts. While mowing treatments may be aesthetically desirable they probably increase the amount of mefluidide necessary for effective growth retardation.     

Competition between vegetative plants of wild oat (Avena fatua L.) and wheat (Triticum aestivum L.)

Competition between wild oat (Avena fatua L.) and wheat (Triticum aestivum L.) was studied in two experiments; a replacement series model and a technique for separation of root and shoot systems. Wild oat and wheat in association resulted in a relative yield total very close to unity showing that the two species were‘crowding for the same space’(or competing for the same resources) and were‘mutually exclusive'. Wild oat was more competitive than wheat, as shown by its aggressivity relative to wheat, relative yields, shoot dry weights and other plant attributes. The greater competitive ability of wild oat was predominantly due to its greater root competitive ability, while the two species had similar shoot competitive ability. Root competition had a much greater effect on the relative performance of the two species than did shoot competition. The effects of root and shoot competition were additive.     

Components of early competition between upland rice (Oryza sativa L.) and Brachiaria brizantha (Hochst. ex A.Rich) Stapf.

Abstract

In Latin America early weed competition is a constraint upon upland rice production, leading to intensive herbicide use. Breeding for rice competitiveness may help reduce the need for herbicides. Two consecutive screenhouse pot experiments were conducted with 13 upland cultivars grown alone or in mixtures with Brachana brizantha to identify plant characteristics for early rice competitiveness which could serve as selection criteria. Rice growth reduction from B. brizantha competition was evident in all cultivars by 30 d.a.e., and the traits most related to cultivar competitiveness were: total above‐ground plant biomass > leaf weight > stem weight > leaf area. Plant height and tillering were least affected by competition. Rice and B. brizantha partially avoided competition since the competing canopies did not fully intercept the incoming light. Traits measured in monoculture did not correlate well with the competition‐induced growth reductions. In breeding for competitiveness, selection according to the above traits should be conducted on rice growing in competition, rather than in monoculture. A measure of plant biomass or leaf area at 30 d.a.e. appears to provide the most convenient estimates of early interference.     

The uptake and translocation of 2,4,5-T in gorse (Ulex emopaeus L.)

The uptake of 2,4,5-T by spines of gorse (Ulex europaeus L.) was limited and not enhanced when picloram was added to the application solution. Translocation of 2,4,5-T in 6-month-old cuttings after treatment of a single spine or lateral branch was poor. The most significant accumulation of translocated herbicide occurred in stem tissue, with lesser amounts detected in root tissues, root nodules, stem apices and flowers. Untreated lateral branches or spines accumulated minimal amounts of herbicide. The pattern and extent of distribution of 2,4,5-T was not increased by addition of picloram. The lack of efficient control of gorse by 2,4,5-T can largely be attributed to its inadequate uptake and lack of true systemic translocation.     

Dynamics of competition between wild oats (Avena fatua L.) and winter cereals

This study examined the effects of competition on the growth of Avena fatua, winter wheat and winter barley. Plants were sampled at frequent intervals from replacement series experiments at two contrasting sites in the U.K. A. fatua was much slower to establish than the two cereals, but thereafter exhibited a faster rate of growth. In monoculture, it took a considerable time for A. fatua to reach a size equal to that of the cereals, but by the end of the experiments it was the largest of the three species. The change-over from cereal dominance in mixtures to A. fatua tiominance was rapid, and in three cases coincided with cereal flag leaf emergence. In the fourth case, it appeared to coincide with the art of canopy height extension. At one site the rder of competitiveness at anthesis was A. fatua= barley>wheat, and at the other site the order was A. fatua>wheat>barley. In order to explain and predict differences between years and sites, more studies are required on morphological development in relation to abiotic variables.     

Effects of shepherd's purse (Capsella bursa-pastoris L. Medic.) on the growth of radish (Raphanus sativus L.)

Radish (Raphanus sativus L.), a salad vegetable, and shepherd's purse (Capsella bursa-pastoris (L.) Med.), a common annual or biennial weed, are plants of similar height, leaf area and root biomass which often occur together in early summer. To assess the effects of the weed on the crop, plants were grown in well-watered and fertilized monocultures and mixtures in the greenhouse. Growth of both species in monoculture decreased, per plant, as planting density increased, particularly in radish which had a high relative monoculture response. In mixtures, at a range of planting densities, and in different proportions, radish was much the stronger competitor, its total dry matter and tuber production being affected only slightly, and not at all in some experiments, by mixture with C. bursa-pastoris. Thus the relative mixture response of radish ranged from 0.1 to 0.0, and the relative crowding coefficient of radish over Capsella was high, indicating that radish was strongly aggressive in the mixture. Although the two species had similar leaf areas in monoculture, that of C. bursa-pastoris was greatly reduced in mixture. The advantage displayed by radish in intercepting light was further enhanced when it was grown in mixtures because radish increased the height of the greatest part of its leaf area, so increasing its shading of the weed. Withholding water from plants increased dry matter partitioning to the root systems in both species, and slightly increased the competitive advantage of radish over C. bursa-pastoris. It is concluded that the weed poses little threat to the yield of radish crops in the field. Effets de la capselle bourse a pasteur (Capsella bursa-pastoris L. Medic.) sur la croissance du radis (Raphanus sativus L.) Le radis (Raphanus sativus L.) et la capselle bourse à pasteur (Capsella bursa-pastoris L. Medic) mauvaise herbe annuelle ou bisanuelle commune, sont des plantes de hauteur, surface foliaire et système racinaire semblables qui apparaissent souvent ensemble au début de l'été. Pour évaluer les effets de l'adventice sur la culture, les plantes ont été cultivées en monoculture et en culture mixte sous serre avec un bon arrosage et une bonne fertilisation. La croissance des deux espèces en monoculture a diminué par plante parallèlement à l'augmentation de la densité, en particulier chez le radis qui a une réponse à la monoculture relativement élevée. En mélanges, sur un éventail de densités de plantation et dans différentes proportions, le radis a été un compétiteur plus fort avec un poids total en matière sèche et une production tubéreuse peu affectée, et pas dans tous les essais, par le mélange avec la capselle. Ainsi, la réponse au mélange du radis a varié de 0,1 à 0,0 et le coefficient de masse relative du radis par rapport à capselle était élevé, soulignant que le radis était fortement compétiteur dans le mélange. Bien que les 2 espèces aient des surfaces foliaires semblables-en monoculture celle de C. bursa-pastoris a été considérable-ment amoindri en mélange. L'avantage que le adis tire de son interception de la lumière, a été favorisé dans la culture en mélange parce que le radis a accru la hauteur à laquelle se trouvait la majorità de sa surface foliaire, augmentant ainsi son ombrage de l'adventice. En réduisant la source en eau des plantes, on a eu une augmentation de la matière sèche au niveau des racines chez les 2 espèces avec une légère contri-bution à l'augmentation de l'avantage du radis sur la capselle. Il en est conclu que l'adventice représente une faible menace pour le rendement des cultures de radis au champ. Konkurrenz von Hirtentäschel (Capsella bursapastoris (L.) Medic, mit Radieschen (Raphanus sativus L.) Die Gemüsepflanze Radieschen (Raphanus sativus L.) und das verbreitete, ein- bis überjährige Unkraut Hirtentäschel (Capsella bursapastoris (L.) Medic.) haben eine ähnliche Höhe, Blattfläche und Wurzelmasse, sie treten im Frühsommer oft zusammen auf. Um die Wirkung des Unkrauts auf die Kulturpflanze zu untersuchen, wurden die Pflanzen im Gewächshaus in gut gewässerten und gedüngten Rein- und Mischbeständen kultiviert. Bei beiden Arten nahm das Wachstum der Einzelpflanze mit zunehmender Dichte ab, besonders beim Radieschen. In Mischbeständen mit verschiedenen Pflanzendichten und Mischungs-verhältnissen war das Radieschen der stärkere Konkurrent, dessen Gesamttrockenmasse und Rübchenertrag nur wenig und in manchen Versuchen überhaupt nicht durch das Unkraut beeinträchtigt wurden. So variierte die relative Reaktion des Radieschens in den Mischbeständen (mixture response) von 0,1 bis 0,0, und der relative Verdrängungskoeffizient des Radies-chens über Capsella war hoch, worin zum Ausdruck kommt, wie konkurrenzstark das Radieschen in den Mischkulturen war. Obwohl die beiden Arten in Reinbeständen ziemlich ähnliche Blattflächen entwickelten, war diese bei Capsella in Mischbeständen erheblich reduziert. Das Radieschen erlangte in Mischbeständen auch dadurch einen Vorteil bei der Lichtaufnahme, indem der gröte Teil seiner Blattfläche höher lag und so das Unkraut zunehmend beschattete. Bei Wassermangel stieg die Wurzeltrockenmasse bei beiden Arten an, wodurch der Konkurrenzvorteil des Radieschens gegenüber dem Hirteltäschel leicht verstärkt wurde. Diese Unkrautart übt also nur eine kleine Schadwirkung auf den Ertrag von Radieschen aus.     

SPATIAL GROWTH OF CYNODON DACTYLON (L.) PERS.*

Summary. Small sprigs of Cynodon dactylon (L.) Pers., were planted and grown without competition from weeds and crops for 2 1/2 years. The plant spread by means of above-ground creeping stolons and subterranean rhizomes which could also emerge as aerial shoots, producing in their turn new stolons and rhizomes. Initially there was linear extension of stolons and rhizomes, following which gaps in the sod were filled by stolon branching and new shoots. There was no preferential direction of stolon elongation and established sods developed an approximately circular shape and expanded concentrically. The rate of increase in sod area was similar in both years of observation but radial extension was smaller in the second year than in young plants. The mean sod area was 25 m² after 2½ years of growth, and mean monthly area increment was 0·9 m². Growth almost stopped in the cold season and exceeded 2 m² per month in the summer. The plants had extended up to 3·9 m from the plant centre after 2 1/2 years. The number of flowering culms per sod, produced only in the warm season, was directly proportional to sod area; seeds were infertile in the Newe Ya'ar area. After 2½ years of growth about 70% of the rhizome dry weight was present in the upper 20 cm of soil. Few rhizomes penetrated deeper than 40 cm. More than 60% of the rhizomes were present within 1 m radius of the plant centre and 90% were present within 2 m radius. Croissance spatiale de Cynodon dactylon (L.) Pers.     

Inhibition of seedling growth by wild myrtle (Myrtus communis L.)

Plants of wild myrtle (Myrtus communis L.) collected at full bloom contained substances inhibitory to the germination and seedling growth of perennial ryegrass. (Lolium perenne L. cv Hollandi) Tissue suspensions had a greater inhibitory activity than water extracts of the plant tissue. Boiling the plant material also increased the inhibitory activity of suspensions and extracts. Flower, and buds, fruit covers and leaves had the greatest inhibitory effect/unit weight ot tissue but inhibitory activity was also present in stems and roots at lower concentrations. The effect of myrtle leaf suspensions on germination of seed of thirteen common garden flowers was also examined: Germination of Petunia. Antirrhinum, Liaum. Cheiranthus and Lathyrus was inhibited completely or substantially depressed. Germination of Zinnia, Tagrtes. Verbena and Dianthus was not much affected, while that of Brllis. Gaillardia. and Viola was affected to an intermediate extent. These results suggest that the possible use of wild myrtle as an evergreen hedge plant needs to be examined further because of possible interference with the growth of nearby lawns and flowers.     

Effect of annual ryegrass (Lolium rigidum Gaud.) on yield of wheat

The effect of five densities of annual ryegrass (Lolium rigidum Gaud.) on the growth, moisture usage and grain yield of wheat at three sowing times was determined in the field. The presence of ryegrass reduced wheat dry matter production and grain yields by decreasing fertile tiller and fertile spikelet production. Ryegrass was more competitive in the later sown crops and its effect on wheat grain yield was directly proportional to the square root of its density. The relationship between ryegrass density and wheat yield reduction is quantitatively described.     

Competition between Silene vulgaris (Moench) Garcke and alfalfa (Medicago sativa L.)

Growth indices describing the nature of the competitive relationship between Silene vulgaris and alfalfa during crop establishment were determined from experiments conducted outdoors in southem Manitoba in 1986 and 1987. The results of the experiments indicated that alfalfa dry matter yields were influenced more by intra- than interspecific competition. In comparison S. vulgaris was affected more by interspecific competition. Replacement diagrams and aggressivity indices both demonstrated that alfalfa and S. vulgaris were the aggressor and sub-ordinate species, respectively. Relative replacement rates for alfalfa versus S. vulgaris were significantly greater than unity, indicating that alfalfa exerted a greater replacement pressure on S. vulgaris than did S. vulgaris on alfalfa. Over time alfalfa could be expected to replace S. vulgaris as the dominant species within a population. Barley was more competitive than alfalfa with S. vulgaris. Not only was S. vulgaris shoot dry weight reduced more by barley than by alfalfa during the establishment year, but fewer S. vulgaris seedlings emerged the following year in plots that had been seeded to barley than in those that had been seeded to alfalfa.     

SPATIAL GROWTH OF SORGHUM HALEPENSE (L.) PERS.*

Summary. Small plants of Sorghum halepense (L.) Pers. were planted in the field and grown without competition from weeds and crops for 2½ years. The plant spread by means of subterranean rhizomes from which aerial shoots developed at increasing distances from the plant centre, and by tillering around these aerial shoots to form clumps. No preferential direction of expansion was detected and established patches developed an approximately circular shape. Aerial growth stopped completely in the cold season. During the warm season the mean area increment amounted to 1·3 m²/month and was similar in both years of observations. After 2½ years of growth, patches had extended up to 3·4 m from the initial sprig and had a mean area of 17 m². In the second summer of growth, shoot density reached 190/m² and mean seed production was 84 g, or 28 000 seeds per plant. About 80% of rhizome dry weight was present in the upper 20 cm of soil and rhizomes did not penetrate deeper than 40 cm. About 80% of rhizome dry weight was present within 1 m radius from the plant centre. Croissance dans l'espace de Sorghum halepense (L.) Pers.     

The effect of nitrogen, plant density and competition between Bromus sterilis and three perennial grasses: the implications for boundary strip management

The competitive ability of Festuca rubra L., Hoicus lanatus L. and Poa trivialis L. when grown from seed, in monoculture and in 1:1 additive mixtures with Bromus sterilis L. was studied. B. sterilis was more aggressive when grown in additive mixtures with F. rubra than P. trivialis or H. lanatus. H. lanatus was less dominated by B. sterilis at the second harvest; visual observations suggested that this dichotomy was due to its slower initial growth rate, and its subsequent dense vegetative growth habit. There was no statistically significant difference between the Relative Yield Total (RYT) of B. sterilis in additive mixtures with F. rubra, H. lanatus or P. trivialis, indicating that they were competing for the same resources. B. sterilis produced significantly more reproductive tillers and seeds as a result of nitrogen application, and such production was accentuated in the absence of interspecific competition. The implications of sown grass strips for field margin management are discussed.     

Phytotoxic activity of Chinese violet (Asystasia gangetica (L.) T. Anderson) and two phytotoxic substances

Chinese violet (Asystasia gangetica (L.) T. Anderson) is a perennial invasive weed belonging to Acanthaceae. Leaves of this weed have been suggested to possess phytotoxic activity. However, phytotoxic substances in this weed have not yet been reported. Therefore, the present study investigated phytotoxic activity of Chinese violet extracts and phytotoxic substances. The extracts of Chinese violet leaves inhibited the root and shoot growth of cress, lettuce, alfalfa, barnyard grass, ryegrass, and timothy, where the level of inhibition increased with increasing extract concentrations. Bioassay‐guided separations of the extracts led to isolation of two phytotoxic substances, indole‐3‐carboxaldehyde and (6R,9S)‐3‐oxo‐α‐ionol. Indole‐3‐carboxaldehyde significantly inhibited the root and shoot growth of cress at concentrations ≥100 and 30 μmol L^?1, respectively, and concentrations of the substance required for 50% growth inhibition were 210 and 127 μmol L^?1 for cress roots and shoots, respectively. The other substance, (6R,9S)‐3‐oxo‐α‐ionol, was reported to have strongly inhibited cress roots and shoots. The present results suggest that Chinese violet contains two phytotoxic substances indole‐3‐carboxaldehyde and (6R,9S)‐3‐oxo‐α‐ionol, and those substances may play an important role in the phytotoxic activity of Chinese violet.     

Phytotoxic and antiphytopathogenic compounds from Thai Alpinia galanga (L.) Willd. rhizomes

The management of weeds and diseases that are caused by phytopathogenic fungi is important for preventing the loss of agricultural products. The aim of the present study was to identify phytotoxic and antiphytopathogenic agents from the Thai Alpinia galanga rhizome. Extracts of the dried rhizomes of A. galanga (Zingiberaceae) were separated and tested for phytotoxic activity against lettuce (Lactuca sativa L. cv. Great Lakes) and Italian ryegrass (Lolium multiflorum Lam. cv. Wasefudou) and for antiphytopathogenic activity against Alternaria porri, Colletotrichum gloeosporioides, Fusarium oxysporum and Phytophthora nicotianae. 1′‐Acetoxychavicol acetate ( 1 ) was identified as one of the main components, together with trans‐p‐coumaryl acetate ( 3 ) and trans‐p‐acetoxycinnamyl acetate ( 2 ). 1′‐Acetoxychavicol acetate ( 1 ) was solvolyzed with 2% EtOH to yield trans‐p‐coumaryl ethyl ether ( 6 ), trans‐p‐coumaryl acetate ( 3 ) and trans‐p‐coumaryl alcohol ( 5 ). 1′‐Acetoxychavicol acetate ( 1 ) completely inhibited the root growth of the lettuce seedlings at 50 μg mL^–1, but had a weaker inhibitory effect on the growth of Italian ryegrass. 1′‐Acetoxychavicol acetate also inhibited the growth of P. nicotianae and A. porri, with minimum inhibition concentration values of 15.6 and 31.5 μg mL^–1, respectively. The plant growth‐inhibitory activity and fungal growth‐inhibitory activity of trans‐p‐coumaryl acetate ( 3 ), trans‐p‐coumaryl ethyl ether, trans‐p‐coumaryl alcohol ( 5 ) and trans‐p‐acetoxycinnamyl acetate ( 2 ) were lower than those of 1′‐acetoxychavicol acetate. A structure–activity relationship suggested that the strong phytotoxic and antiphytopathogenic activity of 1′‐acetoxychavicol acetate relied on the 1′‐acetoxyl group.     

Simulation study of the competitive ability of erect, semi-erect and prostrate cowpea (Vigna unguiculata) genotypes

Ecophysiological simulation models provide a quantitative method to predict the effects of management practices, plant characteristics and environmental factors on crop and weed growth and competition. The INTERCOM interplant competition model was parameterised, calibrated by monoculture data for three cowpea (Vigna unguiculata) genotypes that differed in growth habit, common sunflower (Helianthus annuus) and common purslane (Portulaca oleracea), and used to simulate competition of cowpea cover crops with sunflower or purslane. The simulation results were compared with observations from field competition experiments in 2003 and 2004. INTERCOM more accurately simulated actual field data for the competition of cowpea genotypes and sunflower than companion field experiments for the competition of cowpea and purslane. The validated simulation model of cowpea and sunflower at two densities was used to study the effects of cowpea growth habit on final biomass production of cowpea and sunflower. The model suggested that erect growth habit was more competitive than semi‐erect and prostrate growth habit, when cowpea genotypes were grown with sunflower. Cowpea leaf area distribution was important to higher cowpea biomass production, while cowpea height growth was important to reduce sunflower biomass. Our simulation approach is suggested as a method for crop breeders to gauge the likely success of selection for competitive crops before undertaking expensive long‐term breeding experiments.     

Improved physical control of glyphosate‐resistant Italian ryegrass (Lolium multiflorum) on rice paddy levees in Japan

Italian ryegrass ( Lolium multiflorum Lam.) is a non‐native annual winter grass that has seriously infested rice paddy levees and wheat fields in Japan. Recently, glyphosate‐resistant Italian ryegrass was found on paddy levees in central Japan, thereby making control of the grass by using glyphosate less effective. In this study, physical control methods were tested that combined the timing and frequency of mowing in order to more effectively control glyphosate‐resistant Italian ryegrass on rice paddy levees. A 3 year field experiment was conducted from 2012 to 2014 in a western region of Shizuoka Prefecture, where glyphosate‐resistant Italian ryegrass has become dominant. Five treatments were tested: (i) mowing once before the flowering of the grass (i.e. conventional mowing measure); (ii) mowing once during flowering; (iii) mowing twice during flowering; (iv) glyphosate application before flowering (i.e. one of the conventional mowing measures); and (v) no treatment. The above‐ground biomass, seed production, soil seed bank and seedling occurrence of Italian ryegrass were measured to determine the effectiveness of these treatments. Mowing during the flowering period resulted in reduced above‐ground biomass, seed production and soil seed bank when compared with the other treatments. Additionally, mowing twice during the flowering period resulted in a lower seedling density than mowing once. The results suggest that, in this region, physical control by mowing during the flowering period would be more effective than conventional measures for controlling glyphosate‐resistant Italian ryegrass.     

禾草内生真菌与丛枝菌根互作对多年生黑麦草生长及叶斑病的影响

为明确禾草内生真菌和丛枝菌根真菌(arbuscular mycorrhizal fungi,AMF)互作对多年生黑麦草Lolium perenne生长发育及叶斑病的影响,设置禾草内生真菌处理(由带有和不带禾草内生真菌种子建立)、AMF(幼套球囊霉Claroideoglomus etunicatum和根内球囊霉Rhizophagus intraradices)单独接种和混合接种处理及不接菌处理(对照),并在植物生长6周后接种或不接种多年生黑麦草叶斑病病原菌根腐离蠕孢Bipolaris sorokiniana,测定各处理多年生黑麦草的发病率、叶绿素含量、净光合速率、水分利用效率、AMF侵染率、P含量和生物量。结果表明:接种病原菌2周后,多年生黑麦草叶斑病的发病率为25.00%～38.75%,禾草内生真菌和幼套球囊霉均在一定程度上降低了多年生黑麦草的发病率,二者共同作用时发病率显著降低了35.48%。禾草内生真菌与AMF互作能在一定程度上提高植物叶绿素含量,促进光合作用,并促进P吸收和植物生长,二者的互作效应因禾草内生真菌与AMF组合而异,但均优于二者单独使用时的效应,其中禾草内生真菌与幼套球囊霉互作对多年生黑麦草生长及叶斑病防治的效果最好。     

The preferential accumulation of picloram at sites of active growth in gorse (Ulex europaeus L.)

Application of ¹⁴C-picloram to single spines of 6 month old, soil grown, vegetative plants of gorse (Ulex europaeus L.) resulted in its translocation to regions of meristematic and metabolic activity, with significant accumulation in the stem apex and root nodules. Dissection of the root system of plants grown in nutrient solution revealed a greater mass of picloram in differentiated root tissue than in root apices, although the concentration of chemical in the latter fraction was greater than in the former. Loss of picloram to the nutrient solution was interpreted as preferential leakage from the root nodules because of the lack of picloram accumulation in this fraction. In reproductive plants bearing flowers and floral buds the picloram was diverted from the root tissue fractions to the floral organs, which constituted the primary sink.     

Cirsium arvense (L.) Scop. competition with winter wheat (Triticum aestivum L.)

A series of field experiments was conducted to evaluate the competitive effect of Cirsium arvense (L.) Scop. on winter wheat (Triticum aestivum L. ‘Norstar’) yield. Grain yield at the centre of dense C. arvense patches ranged from 28–71% of the yield in adjacent weed-free plots, based on measurements made at 11 experimental sites over a 2-year period. The mean reduction in yield was 49%. Two models were used to describe grain yield reduction in terms of C. arvense shoot density: (a) linear regression of percentage yield reduction on the square root of shoot density, and (b) non-linear regression, based on a rectangular hyperbola. Both models give similar yield loss estimates at commonly occurring shoot densities of C. arvense, but the non-linear model accounted for more of the observed variation in grain yield. The major yield component affected by C. arvense competition was spikes per unit area, with kernel weight affected to a lesser extent.     

Comparison of empirical models depicting density of Amaranthus retroflexus L. and relative leaf area as predictors of yield loss in maize (Zea mays L.)

The outcome of crop-weed competition should be predicted as early as possible in order to allow time for weed control measures. Maize grain yield losses caused by interference from Amaranthus retroflexus L. (redroot pigweed) were determined in 1991 and 1992. The performance of three empirical models of crop-weed competition were evaluated. Damage functions were calculated based on the weed density or relative leaf area of the weed. In the yield loss-weed density model, values of I (percentage yield loss at low weed density) were relatively stable for similar emergence dates of A. retroflexus across years and locations. Estimated maximum yield loss (A) was more variable between locations and may reflect environmental variation and its effect on crop-weed competition, at least in 1991. The two-parameter yield loss-relative leaf area model, based on m (maximum yield loss caused by weeds) and q (the relative damage coefficient) gave a better fit than the single-parameter version of the model (which includes only q). In both relative leaf area models, the values of q varied between years and locations. Attempts to stabilize the value of q by using the relative growth rate of the leaves of the crop and weed were successful; however, the practical application of such relative leaf area models may still be limited owing to the lack of a method to estimate leaf area index quickly and accurately.