Volunteer winter wheat: its effects and control in ryegrass seed production

The effects of different populations of volunteer winter wheal and their control with ethofumesate and TCA on growth, seed yield and yield components of S.24 perennial ryegrass were investigated in lield experiments in 1978 and 1979. Reductions in ryegrass seed yield due to the presence of wheat depended on the density of wheat and the number and dry weight of ryegrass tillers. The greatest percentage reduction in ryegrass seed yield occurred at high densities of wheat (300 plants m^?2) when the number of ear-bearing tillers and 1000 seed weight of ryegrass were reduced. When Ihe density of wheat was low (80 plants m^?2) a smaller reduction in ryegrass seed yield occurred and the number of live wheat plants remaining gradually decreased. Within the range of wheat densities in these experiments (0–300 plants m ^?2) a 1% loss in ryegrass seed yield occurred for every 10 wheal plants m^?2 present in the crop post winter. Both herbicides caused a reduction in number of ryegrass tillers during growth but, except where TCA was applied at 12 kg ha^?1 in November, ryegrass seed yields were not significantly reduced (in comparison with a wheat-free control) and were always greater than those obtained in the presence of wheat where no herbicides had been applied. Levels of volunteer infestation of 300 wheat plants m^?2 were controlled with minimum risk of crop damage by applications of 6 kg ha^?1 TCA in either October or November, or by application of 1–9 kg ha^?1 ethofumesate in November.     

Interference of the agrestal Helianthus annuus biotype with sunflower growth

The Helianthus annuus taxon includes the wild (ruderal), weedy (agrestal) and volunteer biotypes, which may share space and time with the sunflower crop. The agrestal biotype is one of the most harmful weeds challenging the use of modern biotechnological tools in sunflower. The interference of the agrestal biotype was studied at a range of weed densities within a sunflower crop. The area of influence of one agrestal plant was estimated as a circular area of more than 2.5 m². The agrestal biotype at 10.7 plants m^?2 reduced the sunflower seed production by 66%, the seed weight by 41% and the yield by 80%. The agrestal biotype showed intermediate vegetative stage duration between sunflower and the ruderal biotype. The interference of the agrestal biotype would be due to its high initial growth rate, its larger leaf area and plant height. Some agrestal plants showed partial dehiscence at the sunflower harvest stage replenishing the soil seedbank with over 18 000 seeds m^?2. The sunflower harvest operation and grain storage might be affected due to a fraction of agrestal plants remaining alive and moist when the crop reached harvest maturity. The results support the recommendation of preventing the entry of the agrestal biotype into crop production fields.     

Threshold level and seed production of velvetleaf (Abutilon theophrasti Medicus) in maize

Four tests were carried out in 1980 and 1981 to determine: (a) the economic threshold density of Abutilon theophrasti Medicus (velvetleaf) in maize, and (b) seed production with varying densities of infestation, both in the presence and in the absence of maize. The infestation was artificially created, and the density of the weed ranged from 0 to 80 plants m^?2. The economic threshold, calculated using the Cousens (1987) model, varied between 0?3 plants m^?2 and 2?4 plants m^?2, depending on the variables considered. The presence of maize reduced the seed-rain of A. theophrasti by 50%. This seed-rain reached its maximum level at 20–30 plants m^?2 in maize, and at 30–35 plants m^?2 in weed monoculture. However, with only 4–5 plants m^?2 in competition with maize, A. theophrasti produced 8–10 thousand seeds m^?2. The usefulness of threshold density in weed management is debatable when one considers the ecological characteristics of the A. theophrasti seed, and the great capacity of seed production of this weed.     

Interference of Sinapis arvensis L. and Chenopodium album L. in spring rapeseed (Brassica napus L.)

Interference of Sinapis arvensis L. (wild mustard) and Chenopodium album L. (lamb's-quarters) in spring rapeseed (Brassica napus L.) was studied under field conditions in 1983 and 1984. Both weed species interfered with rapeseed early in the growing season, causing significant reductions in rapeseed dry weight by June of each year. Sinapis arvensis caused greater rapeseed grain yield reductions than did C. album. With weed densities of 20–80 plants m^?2, rapeseed grain yield reductions ranged from 19 to 77% with S. arvensis but only 20 to 25% with C. album. Rapeseed yield reductions caused by mixtures of both weed species ranged from being less than to being equal to the sum of reductions caused by each weed alone, depending on the weed density and year of study. Both weed species were prolific seed producers capable of returning large quantities of seed to the soil. With weed densities ranging from 10–80 plants m^?2, S. arvensis produced 5700–30 100 seeds m^?2 while C. album produced 3100–63 600 seeds m^?2.     

Interference between velvetleaf (Abutilon theophrasti Medic.) and soybean (Glycine max (L.) Merr.) I. Growth

Three field experiments were conduced to evaluate the effect of Abutilon theophrasti on dry matter, branching and flowering node, and seed production of soybean. Low populations (2.4–4.7 plants m^?2) of the weed reduced soybean dry matter, flowering node and seed production. The effect was most pronounced on soybean seed yield, and less on flowering node and dry weight production. These effects were due to the presence of A. theophrasti with soybean, and not to changes in plant population. The effect on soybean branching node production was inconsistent between years.     

Wild oat (Avena sterilis L.) competition with winter barley: plant density effects

The competitive interactions between Avena sterilis ssp. ludoviciana (Dur.) Nyman and winter barley have been studied, taking into consideration the densities of both species. As the density of A. sterilis increased, barley yield decreased exponentially. A 10% reduction in yield was found with wild oat densities ranging from 20–80 panicles m^–2, and yield losses reached 50%, with densities of >300 panicles m^–2, Barley grain yield was reduced by wild oats through a reduction in the number of fertile tillers. Climatic conditions during the growing seasons affected the response of barley to wild oat competition. In general, barley yields were relatively unaffected by seeding rates, with similar responses observed in the presence and in the absence of wild oat infestations. However, the highest yield losses were obtained with the lowest seeding rate (100 kg ha^–1). Furthermore, low barley densities allowed the wild oat plants to produce more seeds, increasing the potential infestation during the following season.     

Crop and density effects on weed beet growth and reproduction

Weed beet populations growing in each crop of the arable rotation could be a relay for the gene flow from adjacent transgenic herbicide‐resistant sugarbeet. In this study, weed beet growth and reproduction were assessed under several conditions which could be found in the rotation: various weed beet densities (ranging from 1 to 120 plants m^?2) and various crops (winter wheat, spring barley, spring pea, sugarbeet, maize, ryegrass). Measurements were carried out both on life‐cycle dynamics (bolting time, time to flowering onset, dynamics of flower opening) and on other quantitative data (survival rate, bolting rate and pollen, flower and seed production). Increasing weed beet density resulted in decreases in bolting rate and flower and seed production per plant. In cereals, weed beet establishment and reproduction were strongly reduced, compared with bare ground as a control situation. In pea, there was no effect on establishment, but the early harvest limited seed set. In the other crops, flower and seed production were reduced to a lesser extent. Parameters of the fitted equations on the bolting and flowering progress were modified by the weed beet density and by the crop. Our data may be used in a model predicting weed beet demographic evolution according to cropping system, and in assessing gene flow.     

Herbicide-resistant crops: yield penalties and weed thresholds for oilseed rape (Brassica napus L.)

An expérimental procedure was designed to provide a simple model for types of analyses necessary to determine weed density thresholds for advantageous use of crop plants engineered for herbicide resistance. Oilseed rape (Brassica napus L., cv. Tower) biotypes resistant (RES) and susceptible (SUS) to atrazine were used as model crop plants, and wild oat (Avena fatua L.) was used as the model weed. Along a wild oat density gradient equivalent to 0–128 plants m^?2, RES plants consistently experienced biomass and yield reductions of approximately 10–20% compared to SUS plants. When atrazine was applied at 1.5 kg ha^?1 to control wild oats competing with RES plants, RES biomasses and yields were stabilized at the same level as that where 25–30 wild oats m^?2 reduce yields of SUS plants. This implies that with wild oat densities of 25–30 plants m^?2, it becomes agronomically advantageous to crop with RES plants plus atrazine rather than to crop with higher-yielding SUS plants.     

Competition of sorghum cultivars and densities with Japanese millet (Echinochloa esculenta)

Field studies were conducted at two locations in southern Queensland, Australia during the 2003–2004 and 2004–2005 growing seasons to determine the differential competitiveness of sorghum (Sorghum bicolor L. Moench) cultivars and crop densities against weeds and the sorghum yield loss due to weeds. Weed competition was investigated by growing sorghum in the presence or absence of a model grass weed, Japanese millet (Echinochloa esculenta). The correlation analyses showed that the early growth traits (height, shoot biomass, and daily growth rate of the shoot biomass) of sorghum adversely affected the height, biomass, and seed production of millet, as measured at maturity. “MR Goldrush” and “Bonus MR” were the most competitive cultivars, resulting in reduced weed biomass, weed density, and weed seed production. The density of sorghum also had a significant effect on the crop's ability to compete with millet. When compared to the density of 4.5 plants per m², sorghum that was planted at 7.5 plants per m² suppressed the density, biomass, and seed production of millet by 22%, 27% and 38%, respectively. Millet caused a significant yield loss in comparison with the weed‐free plots. The combined weed‐suppressive effects of the competitive cultivars, such as MR Goldrush, and high crop densities minimized the yield losses from the weeds. These results indicate that sorghum competition against grass weeds can be improved by choosing competitive cultivars and by using a high crop density of >7.5 plants per m². These non‐chemical options should be included in an integrated weed management program for better weed management, particularly where the control options are limited by the evolution of herbicide resistance.     

Biology of Commelina benghalensis L. in south-eastern Queensland. 1. Growth, development and seed production

Experiments conducted at Kingaroy in southeastern Queensland, investigated growth, development and seed production of Commelina benghalensis L. a weed of field crops, which produces aerial and underground flowers. Plants were grown from four seed types (large and small seeds from the two types of flowers) in pots exposed to the weather and in the field. Development was faster, and growth and seed production were greater in the field than in pots. Plants grown from aerial seeds were smaller, developed aerial flowers earlier, and produced more aerial fruits than plants grown from underground seeds. Rhizomes and underground flowers began to develop 6 weeks after emergence on plants from all seed types, prior to the development of aerial flowers. Seed production in the field was 8000 m^?2 for plants grown from underground seeds and 12000 m^?2 for plants grown from aerial seeds. Small aerial seeds represented 73–79% of the total seed production, large aerial seeds 19–22%, and underground seeds only 1–3%.     

Morphological and physiological variability in black nightshade (Solanum spp.)

Plants from two black nightshade (Solanum spp.) seed sources were compared for differences in morphology at seedling and mature plant stages, flowering and physiological responses to pre-germination seed treatment, growth responses to temperature, and response to herbicides. Differences in the morphology of the two Solanum spp. were evident from seedling to maturity. The plants grown from the Michigan (MI) seed source had a deep purple abaxial leaf surface at the three to four-leaf stage, a yellow anther column, and toothed proximal half in the mature leaf, whereas the plants grown from the California (CA) seed source had no unusual abaxial coloration or toothed proximal margin and had a dark brown anther column. Differences in response to herbicides were seen between plants grown from the two seed sources following pre-plant-incorporated, pre-emergence, and post-emergence herbicide applications. The plants from the MI seed source were up to five times more susceptible to chloramben, applied pre-plant-incorporated at 1.68 kg ha^?1, and to ethalfluralin, applied preemergence at 1.12 kg ha^?1, than plants from the CA seed source. Plants from both seed sources have been described as Solanum nigrum L. The plants from MI have been identified elsewhere as Solanum ptycanthum Dun. and those from CA as Solanum scabrum Mill. This identification could resolve conflicting reports among researchers regarding chemical control measures.     

Time of onset of competition and effects of various fractions of an Avena fatua L. population on spring barley

The growth and competition of wild oats (Avena fatua L.) emerging at different times, and the time of onset of competition by them were studied using natural populations in spring barley. In one experiment in 1972, wild oats emerging in the 0–0?5, 0?5–2?5, 2?5–4 crop leaf stages at densities of 54, 46, 15 m ^?2 respectively were allowed to compete all season with the crcp. These gave rise to 82, 17 and l% of all seed shed and caused 16%, and two non-significant yield losses respectively. In two other experiments in 1973, nearly all the wild oats that caused yield losses had emerged by the crop 2˙45 leaf stage. Where wild oats emerging up to the crop 2?5 leaf stage were removed, the later ones did not compensate by making extra growth. In one of these experiments in which densities of crop and weed were 416 and 414 m ², and in the other where they were 295 and 294 m ² respectively, grain yield losses were significant if the wild oats remained in the crop until the crop had 2?5–4?5 leaves and 4?5–6?5 leaves respectively. In a third experiment in 1973 with initial densities of 464 wild oats and 336 barley plants m ^?2, and where a top dressing of nitrogen was given at the crop 3–4 leaf stage, unlike the other two experiments in 1973 where all nitrogen was applied at sowing, no yield losses resulted unless the wild oats remained in the crop until after the crop 6 leaf stage.     

The effect of crop,cultivation and seed addition for birds on surface weed seed densities in arable crops during winter

Weed seeds present an agronomic threat, but are also an important food resource for wildlife in winter. Weed seed densities on the soil surface in winter were examined from 1999 to 2002 in 105 fields on three different farms in UK. The effect of the preceding crop, cultivation, position within the field and the application of seed for birds (bird seed) on surface seed abundance and species composition was tested. Six or fewer species comprised c. 80% of the weed seeds. By January of each study year, the densities of seeds important for farmland birds (key seeds) were 73% or 87% lower compared with early winter on two of the farms, but were stable on the third where seeds were incorporated through cultivation. At the edge and mid‐field, seed densities only exceeded 400 m^?2 in 17%, 10% and 12% of fields for total, key and dicotyledonous seeds respectively. The preceding crop only affected seed densities at one site; stubbles of winter barley had fewer seeds compared with winter wheat or spring barley. Seed densities varied between the edge and mid‐field, but trends were inconsistent between sites. The density of the larger seeds (Atriplex patula, Viola arvensis, Polygonum aviculare and Chenopodium album) were reduced in fields receiving bird seed. The objectives of weed control and conservation may not be mutually exclusive because seed return was most reduced where the ground remained uncultivated through the winter, yet this also provided the best foraging opportunities for surface feeding seed predators.     

Effects of α-difluoromethylornithine on photosynthetic oxygen evolution,respiration and polyamine concentrations in detached barley leaves

Detached barley leaves were treated with the polyamine biosynthesis inhibitor, α-difluoromethylornithine (DFMO; 1 mM ) for either 30 min or 12 h. Exposure to DFMO for 30 min led to a substantial reduction in photosynthesis, expressed either on a unit leaf-area or unit chlorophyll basis. On the other hand, photosynthesis (expressed on a unit chlorophyll basis) was increased in leaves exposed to DFMO for 12 h, although when expressed on a unit leaf-area basis, photosynthesis remained unchanged at lower photon flux densities (0-300 μmol m^?2) and was slightly reduced at higher photon flux densities (300-800 μmol m^?2 s^?1) s^?1). Respiration was unaltered in leaves exposed to DFMO for 30 min or 12 h. Exposure to DFMO for 30 min resulted in significant increases in chlorophyll concentrations, while carotenoid concentrations remained unaltered. However, chlorophyll concentrations were reduced in leaves treated with DFMO for 12 h, although no change in carotenoid concentration was observed. Polyamine concentrations were not significantly affected by exposure of detached barley leaves to DFMO for 24 h.     

Population studies on Pennyroyal Mint (Mentha pulegium L.) II. Seed banks

An investigation of seed banks found under populations of M. pulegium located within and outside pasture systems revealed that seed densities were sixteen times greater when this species occurred in pastures. The sizes of seed banks were not positively related to current seed production, but larger seed banks appeared to be associated with higher levels of soil disturbance arising from trampling by grazing stock. Densities of buried seed were very high under pasture populations (range 55 000–176 000 m^?2), which illustrates the impracticality of attempts to eliminate M. pulegium from this type of community.     

Effect of Glomus mosseae on concentrations of rosmarinic and caffeic acids and essential oil compounds in basil inoculated with Fusarium oxysporum f.sp. basilici

This study investigated the potential of the arbuscular mycorrhizal (AM) fungus Glomus mosseae to protect basil (Ocimum basilicum) against Fusarium oxysporum f.sp. basilici (Fob). It was hypothesised that G. mosseae could confer a bioprotective effect against Fob as a result of increases in leaf rosmarinic (RA) and caffeic acids (CA) or essential oil concentrations. Glomus mosseae conferred a bioprotective effect against Fob by reducing plant mortality to 20% compared to 33% in non‐mycorrhizal (NM) plants. This bioprotective effect was not related to improved phosphorus (P) nutrition, as AM and NM plants treated with Fob had similar shoot P concentrations (6 and 8 mg g^?1 dry weight (DW), respectively). Both AM and NM plants treated with Fob had similar leaf and root RA and CA concentrations. Furthermore, phenolic (40–70 mg CA g^?1 DW) or essential oil concentrations (0·1–0·6 mg g^?1 DW) were not increased in plants treated with the AM fungus and Fob. Therefore, the bioprotective effect conferred by G. mosseae was not a result of increases in the phytochemicals tested in this study. However, under the AM symbiosis, basil plants treated with Fob had lower methyleugenol concentrations in their leaves (0·1 mg g^?1 DW) than NM plants treated with the pathogen (0·6 mg g^?1 DW).     

Incidence and control of barley leaf stripe (Pyrenophora graminea) in Scottish barley during the period 1987–1992

The incidence of Pyrenophora graminea was determined in winter and spring barley seed harvested in each of the 6 years 1987 to 1992. The occurrence of P. graminea was highest in spring barley seed harvested in 1989 and 1990, with 69% of samples in 1989 and 82% in 1990 infected. The mean infection levels were 6·1% in 1989 and 7·5% in 1990. The introduction of a voluntary Code of Practice in January 1991 significantly reduced infection: in 1992 only 10% of spring barley samples were infected, and the mean infection level was less than 0·2%. In spring barley, there was a significant relationship between the level of seed infection and the percentage of diseased tillers in commercial crops ( r = 0·890, P < 0·001). However, tiller infection was consistently lower than seed infection. During the period of the study the mean annual leaf stripe infection on winter barley seed did not exceed 1%.     

Control of Achillea millefolium L. (yarrow) by rotary cultivation and glyphosate

Various control strategies for Achillea millefolium L. (yarrow) were investigated in a dense stand of the weed at Lincoln College in 1977–1978. In early spring plots were either rotary cultivated or left undisturbed. In late spring, plots of both previous treatments were either left undisturbed, rotary cultivated or sprayed with glyphosate at 1·5 kg ha^?1. The whole experiment was rotary cultivated twice 1 week later and sown with Hordeum vulgure L. cv. Zephyr (barley) at 144 kg seed ha^?1. MCPA + dicamba at 0·9+0·15 kg ha^?1 was applied to half of each plot when the second node was detectable (Zadok 32). Rotary cultivation and glyphosate both substantially reduced the regrowth of A. millefolium but glyphosate reduced regrowth by a greater proportion when applied to undisturbed plants than when applied to plants regenerating after cultivation. Both gave a more than 95% reduction compared to the control (rotary cultivation only at sowing time) in the amount of A. millefolium present in the barley stubble in the autumn. MCPA + dicamba caused seedling mortality but did not affect the numbers of primary shoots from rhizome fragments. The grain yield of the barley increased from 2·91 t ha^?1 when A. millefolium was not controlled to 4·23 t ha^?1 with good control. The barley yield appeared to be restricted by competition from regenerating A. millefolium and by a nitrogen deficiency induced in some regimes by nitrogen immobilization in decaying rhizomes.     

Interference between triazine-resistant Brassica napus and Panicum miliaceum

Atrazine carryover often limits growers to production of atrazine-tolerant crops the year following application, and allows the increase of triazine-tolerant weed species such as Panicum miliaceum L. (wild proso millet). Tiriazine-resistant Brassica napus L. cv. ‘Triton’ (oilseed rape) was tested to characterize the nature of interspecific interference with P. miliaceum. In a greenhouse study, atrazine at 2.2 kg ha^?1 depressed oilseed rape fruit (siliqua) number and fruit dry weight, and delayed flowering, but did not significantly affect height or weight of shoots, Oilseed rape fruit weight was reduced at 200 P. miliaceum plants m^?2. fruit number and shoot weight were inhibited at 400 weeds m^?2. and height was reduced and flowering delayed at 600 weeds m^?2. Number and weight of fruits were reduced by one-third after 8 weeks of interference as compared to oilseed rape grown with the weed for 4 weeks. Oilseed rape height was reduced by 29% and shoot weight by 55% by 600 weeds m^?2 and 2–2 kg ha^?1 atrazine, while fruit number and weight were reduced by 72%. Oilseed rape shoot weight was reduced by 74% by 600 weeds m^?2 for 12 weeks of interference, while fruit number and weight were reduced by 85% and 82%. respectively. In a field study, fluazifop reduced early season P. miliaceum cover by 72%, but did not increase oilseed rape cover. Mid-season P. miliaceum shoot weight was decreased by 97% by fluazifop and oilseed rape shoot weight was increased by 34%. P. miliaceum control increased oilseed rape biomass by 38% at 89 days, but biomass of oilseed rape sown at 11.2 kg ha^?1 with 2.2 kg atrazine ha^?1 was not decreased by P. miliaceum interference at 89 days.     

COMPETITION OF WILD OATS (AVENA FATUA L.) IN WHEAT AND FLAX*

Summary. This project was designed to study various aspects of wild oat competition in spring wheat and flax. From ten to forty wild oat plants/yd² were sufficient to cause significant yield reductions in wheat when grown on summerfallow land or when ammonium phosphate fertilizer was added to stubble land. However, when wheat was grown on stubble land without the benefit of a fertilizer treatment, seventy to one hundred wild oat plants/yd² were needed to suppress wheat yields significantly. This would suggest that on stubble land, soil fertility was a more important factor than moderate densities of wild oats in determining eventual crop yields. In these experiments, wild oats reduced the number of tillers per plant, but did not significantly affect the protein content of the harvested grain. Only ten wild oat plants/yd² were sufficient to reduce flax yields significantly on both summerfallow and stubble land. The only exception was in 1966, when flax grown on summerfallow land was not significantly affected until the density of wild oats reached forty plants/yd2. This confirms the general observation that flax is a poor competitor with wild oats. The results suggest that wild oat competition had already commenced prior to emergence of wheat, particularly with the higher densities of wild oats. In general, competitive effects increased with time and with wild oat density. In flax, severe competition had already taken place prior to the 2–3-Ieaf stage of the weed in 1964, but did not become severe until after the 2–3-leaf stage of wild oat growth in 1965 and 1966. Again, competitive effects increased with time and with wild oat density. Results of a final series of experiments, suggested that the optimum seeding date for flax in Manitoba is the latter part of May or the first week of June. Yield reductions due to wild oat competition became very severe as seeding dates were delayed. La compétition de la folle avoine (A vena fatua L.) avec le blé et le lin