Invasiveness of agronomic weed species in wheat in Western Australia

Weed seeds are introduced to agronomic systems naturally or through human-mediated seed dispersal, and introduced seeds have a high chance of being resistant to selective, in-crop herbicides. However, colonisation (invasion) rates for a weed species are usually much lower than rates of seed dispersal. The current research investigated colonisation of a winter annual wheat cropping system in Western Australia by a range of winter or summer annual weed species. The weed seeds were sown (at 100 seeds/m²) directly before seeding the crop in 2016 and allowed to grow in the following 3 years of wheat. Selective herbicides were not applied, to simulate growth of weed populations if the initial seed had been resistant to herbicide. Bromus diandrus, Hordeum leporinum, Rumex hypogaeus, Sonchus oleraceus, Polygonum aviculare, Lolium rigidum, Citrullus amarus and Tribulus terrestris colonised the crop, while Dactyloctenium radulans, Chloris truncata and Salsola australis failed to establish over 3 years. The most successful weed was B. diandrus, with a plant density of 1,170/m² by the third year and seed production of 67,740/m². The high density of B. diandrus reduced wheat density by 76% in the third year and reduced average yield by 36%. Lolium rigidum reduced average yield by 11%, and the other weed species did not affect crop yield. Further research is required on the invasiveness of these species in other regions, but it is clear that the spread of B. diandrus to new areas or the introduction of resistant B. diandrus seeds via contaminated grain should be avoided.     

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.     

Postdispersal seed predation by Teleogryllus emma (Orthoptera: Gryllidae) reduces the seedling emergence of a non‐native grass weed,Italian ryegrass (Lolium multiflorum)

The potential impact of postdispersal seed predation by the field cricket, Teleogryllus emma (Ohmachi et Matsuura), on the seedling emergence of Italian ryegrass (Lolium multiflorum Lam.), a non‐native grass weed in Japan, was determined. The relationship between cricket density and the proportion of seedling emergence was investigated by conducting manipulative experiments in duplicate, using enclosures. One‐thousand seeds of Italian ryegrass were exposed to adult crickets at different densities (zero, one, two, four and eight individuals per 2.25 m² enclosure) for 2 weeks. The proportion of seedling emergence decreased with increasing cricket density. The proportion of seedling emergence in the enclosures without the crickets was 90.9–97.2%, whereas the seedling emergence in the enclosures with eight crickets greatly decreased, to 0.5–2.5%. These results suggest that postdispersal seed predation by T. emma can substantially decrease the abundance of Italian ryegrass at an early life stage and reduce its population in environments with a high density of crickets.     

Chloris polydactyla (L.) Sw., a perennial Poaceae weed: Emergence, seed production, and its management in Brazil

Experiments were conducted to evaluate the plant emergence and seed production of Chloris polydactyla and the efficacy of herbicides for its control. The plants emerged mainly when the seeds were placed on the soil surface at ≤ 3 cm depth. Isolated plants produced a great amount of seeds. The pre-emergence herbicides, acetochlor, atrazine + simazine, s-metolachlor, alachlor, and trifluralin, were effective for C. polydactyla control. The postemergence herbicides, clodinafop-propargil, haloxyfop-methyl, clethodim, fluazifop- p -butil, tepraloxydim, sethoxydim, and quizalofop- p -tefuril showed satisfactory control of the plants at a 20 cm height with six leaves. During the flowering stage (85 cm plant height), only glyphosate was effective in controlling C. polydactyla .     

An on-farm approach to investigate the impact of diversified crop rotations on weed species richness and composition in winter wheat

Weed species diversity may benefit from organic farming due to enhanced temporal diversification of crop species in a rotation and omission of herbicide applications. However, in intensively managed conventional systems, little evidence exists as to what extent diversified crop rotations contribute to higher weed species richness. Using an on-farm approach, the effect of crop rotation (organic, conventional diverse (CD) and conventional simple (CS) crop rotations) and weed control (with vs. without) on weed species richness, cover, community composition and crop biomass, was analysed in 24 winter wheat fields. Weed species with beneficial functions for invertebrates and birds were analysed separately. Weed species richness was higher in the organic crop rotation, but did not differ between CD and CS crop rotations. Weed control treatment reduced species richness in both conventional rotations, but not in the organic one. Redundancy analyses revealed that crop rotation intensity accounted for the largest part of the explained variation in weed species composition. Results from the study indicate that the maintenance of weed species richness and conservation of species with important ecological functions requires not only temporal diversification of crop species in the rotation, but also an adjustment of weed control strategies.     

Secondary dispersal, spatial dynamics and effects of herbicides on reproductive capacity of a recently introduced population of Bromus sterilis in an arable field

An arable field was subdivided and subjected to either deep inversion ploughing or non‐inversion cultivation after viable seeds of Bromus sterilis had been sown into oilseed rape stubble. After sowing in isolated plots distributed within the field, sequences of cropping treatments for the establishment of two successive winter wheat crops were applied. Each subfield was split into an uphill and a downhill direction for soil cultivation. The field had a 10° slope. In the season following seed introduction, 2.6% of the introduced seeds had successfully germinated and established in the non‐inversion cultivation regime, when no effective graminicide was applied. Ploughing eradicated B. sterilis. Using differential global positioning system (DGPS) mapping of the whole field population, emerged plants were observed up to 8.7 m (uphill treatment) and 21.3 m (downhill treatment) of their initial source. The median distance seeds were transported was 2.3 m uphill and 4.8 m downhill. Post‐emergence application of the herbicide propoxycarbazone slightly reduced weed density and seed weight, and almost halved weed seed production. Application of fenoxaprop‐P‐ethyl was followed by higher density of plants, tillers and seeds of B. sterilis. Seed viability was unaffected by herbicide use. Thus, in the second wheat crop following seed rain, the weed population was dispersed more widely in the field, such that 20–30% of seeds were dispersed more than 5 m distance from the first year's foci of infestation. The relevance of soil cultivation to secondary dispersal of B. sterilis is discussed.     

Effects of crop management practices on Echinochloa crus-galli and Chenopodium album seed production in a maize/soyabean rotation

Seed production of residual weed populations needs to be taken into account when estimating the long-term impact of low-input agronomic practices. The objective of this study was to measure the effects and interactions of crop, weed control, tillage practice and nutrient source on the seed production of the dominant residual weed species in a maize/soyabean rotation at two sites: Echinochloa crus-galli (L.) Beauv. on a Sainte-Rosalie clay and Chenopodium album L. on a Duravin clay loam. Seed production per unit area was estimated in each experimental unit. Weed seed production was greater under mechanical weed control compared with chemical weed control. In 1997, E. crus-galli seed production reached over 326 000 seeds m^–2 in mechanical weed control treatments, but averaged less than 500 seeds m^–2 in the chemical weed control treatments. Chenopodium album produced in the range of 766 000 and 73 000 seeds m^–2 in mechanical and chemical weed control treatments respectively. Very few or no weed seeds were produced in soyabean under chemical control. Tillage intensity and nutrient source did not affect seed production of either weed species, with the exception that E. crus-galli produced more seeds in chisel than in mouldboard plough tillage in soyabean. Weed control method had more impact on seed production than tillage intensity and nutrient source in a maize/soyabean rotation.     

Weed seed predation rate in cereals as a function of seed density and patch size,under high predation pressure by rodents

Weed seed predation is an ecosystem service, influencing weed population dynamics. The impact of weed seed predation on weed population dynamics depends on how predators respond to seed patches at the field scale. Seed predation will be most effective if the proportion of seeds predated increases with increasing size and seed density of patches. Density‐dependent rodent seed predation was measured by varying seed density and patch size in four irrigated conventionally managed cereal fields in north eastern Spain. Artificial weed seed patches were created by applying a range of Lolium multiflorum seed densities from 0 to 7500 seeds m^?2 in 225 m² patches (2008) or in patches that varied in size from 1 to 9 m² (2009). Seed predation was estimated using seed cards and seed frames. The granivorous rodents Mus spretus and Apodemus sylvaticus caused high seed predation rates (92%) in three fields, whereas in a fourth field, it was lower (47%). Rodents responded in an inversely density‐dependent manner, but this had little biological meaning as even in patches seeded with the highest density, the input to the soil seedbank was reduced by 88%. For the period of time this experiment lasted, hardly any new seeds would have entered the seedbank.     

Weed populations in winter wheat as affected by crop sequence, intensity of tillage and time of herbicide application in a cool and humid climate

Summary There is a lack of information on the combined effects of preceding crop, reduced tillage (especially no-tillage) and the time of herbicide application on the development of weed populations and the efficiency of weed control in winter wheat in humid temperate climates. An experiment was conducted with a crop rotation (winter wheat – oilseed rape – winter wheat – maize) on a sandy loam and a loamy silt soil in the Swiss midlands to investigate the impact of different preceding crops and pre- and post-emergence control of weeds in conventional tillage (CT; mouldboard plough), minimum tillage (MT; chisel plough) and no-tillage (NT; no soil disturbance systems). When winter wheat was grown after maize and winter wheat was grown after oilseed rape, the ranking order of weed density in treatments without herbicide application was NT < MT < CT and CT < MT < NT respectively. Analysis of variance and canonical discriminant analysis showed that Epilobium spp., Sonchus arvensis , Myosotis arvensis and volunteer crops were more abundant in NT than in MT and CT. The efficiency of post-emergence weed control was generally better than that of pre-emergence weed control, regardless of tillage intensity.