Catherine P. D. Borger

Variation in postdispersal weed seed predation in a crop field

Abstract Postdispersal weed seed predation by animals during the summer fallow period may lead to a reduction in the number of weeds that grow in the following winter cropping season. In this study, we investigated the patterns of weed seed removal, the influence of crop residue cover on seed removal, the types of granivores present and their seed preferences in a 16-ha postharvest cropping field in Western Australia during the summer months over 2 yr. Seed removal from caches was extremely variable (from 0 to 100%). Removal rates were generally highest along the edges of the field near bordering vegetation and lowest in the center of the field and within the bordering vegetation. However, there were many deviations from this general pattern. There was no change in rates of predation with different levels of residue cover. Ants or other small invertebrates were found to remove the most seeds. However, seed removal by other animals, such as rodents, was also evident. Annual ryegrass seeds were preferred over wild oat seeds, followed by wild radish pod segments. Seed harvesting was lowest in late January, peaked in February, and decreased in March. Results from this study suggest seed harvesters could reduce the number of surface seeds in the field, reducing the weed seed bank. Management options that increase the activity of the seed harvesters may lead to less variability in seed predation and could, therefore, be incorporated into an integrated weed management program. Nomenclature: Annual ryegrass, Lolium rigidum Gaudin; wild oat, Avena fatua L.; wild radish, Raphanus raphanistrum L.

Manipulating Crop Row Orientation to Suppress Weeds and Increase Crop Yield

Abstract Crop rows oriented at a right angle to sunlight direction (i.e., east–west within the winter cropping system in Western Australia) may suppress weed growth through greater shading of weeds in the interrow spaces. This was investigated in the districts of Merredin and Beverley, Western Australian (latitudes of 31° and 32°S) from 2002 to 2005 (four trials). Winter grain crops (wheat, barley, canola, lupines, and field peas) were sown in an east–west or north–south orientation. Within wheat and barley crops oriented east–west, weed biomass (averaged throughout all trials) was reduced by 51 and 37%, and grain yield increased by 24 and 26% (compared with crops oriented north–south). This reduction in weed biomass and increase in crop yield likely resulted from the increased light (photosynthetically active radiation) interception by crops oriented east–west (i.e., light interception by the crop canopy as opposed to the weed canopy was 28 and 18% greater in wheat and barley crops oriented east–west, compared with north–south crops). There was no consistent effect of crop row orientation in the canola, field pea, and lupine crops. It appears that manipulation of crop row orientation in wheat and barley is a useful weed-control technique that has few negative effects on the farming system (i.e., does not cost anything to implement and is more environmentally friendly than chemical weed control). Nomenclature: Barley, Hordeum vulgare L, canola, Brassica napus L, field pea, Pisum sativum L, lupine, Lupinus angustifolius L, wheat, Triticum aestivum L

Salsola tragus or S. australis (Chenopodiaceae) in Australia—untangling taxonomic confusion through molecular and cytological analyses

Salsola tragus sensu lato (Chenopodiaceae) is found throughout Western Australia and is considered to be a weed in both natural and agricultural ecosystems, although the current taxonomic status of this species is not clear. The taxonomic literature reports morphological variation within Australian populations of the weed, indicating that there may be genetically distinct ecotypes or unidentified subspecies present within the species. A genetic and cytological approach was used to detect variation between 22 populations of S. tragus sensu lato in the south-west of Western Australia. Out-groups used in this study included a population of S. tragus L. from the USA and Maireana brevifolia (R.Br.) Paul G.Wilson (Chenopodiaceae) from Lake Grace. Four genetically distinct groups were identified, which were not closely related to the S. tragus out-group (~60% similarity). Further, these groups and a S. australis R.Br. sample from the USA were all diploid (2n = 18), unlike the tetraploid (2n = 36) S. tragus. The predominant wheatbelt weed, group A, which was previously classified as S. tragus ssp. tragus L., was identified as S. australis. This species is probably native to Australia, given its arrival predated European invasion. Further research is required to clarify the taxonomic status of the other three possible taxa and determine their status in relation to S. australis.

Evaluating the double knockdown technique: sequence, application interval, and annual ryegrass growth stage

Applying glyphosate followed by a mixture of paraquat + diquat in the same season for pre-planting weed control may reduce the risk of developing resistance to either herbicide. Glasshouse and field experiments at Merredin and Beverly, Western Australia, were conducted over 2 seasons to determine the best herbicide application sequence, growth stage of annual ryegrass at which to apply the 2 herbicides, and application time and interval to be allowed between applications for optimum control of annual ryegrass (Lolium rigidum Gaud.). Annual ryegrass plants were treated at 3 growth stages with either glyphosate 540 g a.i./ha alone, paraquat + diquat 250 g a.i./ha alone, glyphosate followed by paraquat + diquat 250 g a.i./ha, or paraquat + diquat 250 g a.i./ha followed by glyphosate 540 g a.i./ha (the double knockdown treatment). The herbicides were applied at different times of the day, with varied intervals between herbicides when applied in sequence. The glasshouse experiment showed that herbicides in sequence more effectively killed annual ryegrass plants at the 3–6-leaf stage than a single application of either herbicide. Field experiments showed that applying glyphosate followed by paraquat + diquat provided 98–100% control of annual ryegrass plants when applied at the 3- or 6-leaf stage in 2002 and at all 3 growth stages in 2003. Generally, the sequence of paraquat + diquat followed by glyphosate was less effective than the reverse sequence, although the difference was not large. Averaged over 2 seasons, herbicides in sequence were most effective when the first herbicide was applied at the 3- or 6-leaf stage of annual ryegrass. An interval of 2–10 days between applications of herbicides was more effective than 1 day or less. The application time did not significantly affect the efficacy of double knockdown herbicides on annual ryegrass plants under field conditions.

Occurrence of Summer Fallow Weeds within the Grain Belt Region of Southwestern Australia

Abstract Field surveys were conducted on 319 sites of the Western Australian grain belt in 2006 to determine the occurrence and distribution of summer fallow weed species. Sites were located across five growing season regions (north, north central, central, south central, and south) and three annual rainfall zones (high, medium, and low). A total of 51 species (or species groups) from 18 families were identified, with the large majority of species (35%) belonging to the Poaceae family. The most prevalent species found, being present at more than 10% of all sites, were wheat, “melons” (weedy watermelon and paddymelon), rigid ryegrass, capeweed, clover, mintweed, wild radish, fleabane, windmill grass, and rolypoly. Correspondence analysis revealed that the north, central, and southern regions of the grain belt could be predominately segregated according to dominant weed species occurrence; however, no segregation by rainfall zone was apparent. This study has given an overview of summer fallow weed occurrence in the Western Australian grain belt and highlights those weed species that are common and yet lack sufficient research into their ecology and management. Nomenclature: Wheat, Triticum aestivum L.; weedy watermelon, Citrullus lanatus (Thunb.) Matsumura & Nakai; paddymelon, Cucumis myriocarpus E. Mey. ex Naud.; rigid ryegrass, Lolium rigidum Gaudin; capeweed, Arctotheca calendula (L.) Levyns; clover, Trifolium spp.; mintweed, Dysphania pumilio R. Br.; wild radish, Raphanus raphanistrum L.; fleabane, Conyza spp.; windmill grass, Chloris truncata R. Br.; rolypoly, Salsola australis R. Br.

Increased Carrier Volume Improves Preemergence Control of Rigid Ryegrass (Lolium rigidum) in Zero-Tillage Seeding Systems

Abstract PRE herbicides are less effective in the zero-tillage system because of increased residual crop stubble and reduced soil incorporation. However, since weeds are not physically controlled in the zero-tillage system, reliance on efficacy of PRE herbicides is increased. This research investigated the impact of carrier volume and droplet size on the performance of PRE herbicides (in wheat crops at four sites in 2010) to improve herbicide efficacy in conditions of high stubble biomass in zero-tillage systems. Increasing carrier volume from 30 to 150 L ha−1 increased spray coverage on water-sensitive paper from an average of 5 to 32%. Average control of rigid ryegrass by trifluralin (at Cunderdin and Merredin sites) and trifluralin or pyroxasulfone (at Wickepin and Esperance sites) improved from 53 to 78% with increasing carrier volume. Use of ASABE Medium droplet size improved spray coverage compared with ASABE Extremely Coarse droplet size, but did not affect herbicide performance. It is clear that increased carrier volume improves rigid ryegrass weed control for nonwater-soluble (trifluralin) and water-soluble (pyroxasulfone) PRE herbicides. Western Australian growers often use low carrier volumes to reduce time of spray application or because sufficient high-quality water is not available, but the advantages of improved weed control justifies the use of a high carrier volume in areas of high weed density. Nomenclature: Pyroxasulfone; trifluralin; rigid ryegrass; Lolium rigidum Gaudin; wheat; Triticum aestivum L. Resumen Los herbicidas PRE son menos efectivos en sistemas de labranza cero debido a su menor incorporación en el suelo y la mayor cantidad de residuos de cultivo. Sin embargo, como las malezas no son controladas físicamente en los sistemas de labranza cero, la dependencia en la eficacia de herbicidas PRE es mayor. Se investigó el impacto del volumen de aplicación y el tamaño de gota en el desempeño de los herbicidas PRE (en cultivos de trigo en cuatro localidades en 2010) para mejorar la eficacia de herbicidas en condiciones de alta biomasa de residuos de cultivo en sistemas de labranza cero. El incrementar el volumen de aplicación de 30 a 150 L ha−1 aumentó la cobertura de la aplicación, medida con papel sensible al agua, de 5 a 32%. El control promedio de Lolium rigidum con trifluralin (en las localidades Cunderdin y Merredin) y trifluralin o pyroxasulfone (en Wickepin y Esperance) mejoró de 53 a 78% al incrementar el volumen de aplicación. El uso de gotas ASABE de tamaño mediano mejoró la cobertura de la aspersión al compararse con gotas ASABE extremadamente grandes, pero no afectó el desempeño del herbicida. Está claro que el incrementar el volumen de aplicación mejoró el control de L. rigidum con herbicidas PRE insolubles en agua (trifluralin) y solubles en agua (pyroxasulfone). Los productores del Oeste de Australia usan frecuentemente volúmenes bajos de aplicación para reducir el tiempo de aplicación o porque no hay suficiente agua de alta calidad disponible, pero las ventajas del mayor control de malezas justifica el uso de altos volúmenes de aplicación en áreas con alta densidad de malezas.

Emergence, survival, biomass production, and seed production of Chloris truncata (windmill grass) in the Western Australian wheatbelt

Chloris truncata is a C4 grass species, native to Australia. Within the wheatbelt of Western Australia (WA), it is a weed of grain cropping systems and a beneficial forage species within pasture systems. Plant emergence, density, survival, biomass production, seed production, and seed germinability were investigated, in pasture or cropping systems, at two trial sites (in Merredin, WA) over two years (from 2007 to 2009). Chloris truncata predominantly emerged and set seed during spring and early summer. This species is usually referred to as a short-lived perennial, and could survive for >14 months, but predominantly grew as a spring/summer annual in the WA wheatbelt. Maximum plant density, biomass, and seed production were, respectively, 4.2–28.2 plants/m2, 8.3–146.1 g dry biomass/m2, and 3325–61 383 seeds/m2, depending on location. Cohorts emerging in spring produced more seeds than those that emerged during other seasons. Average seed germinability reached a maximum of 62%, following an initial 3–4-month period of dormancy. There are few herbicides to control plants growing within the winter/spring annual grain crops, and so further research into increased crop competitive ability is required to reduce growth of spring cohorts and potentially reduce seed set. However, the biomass produced by C. truncata (range 0–1460 kg/ha) can be used as forage in a pasture system, or over the summer/autumn feed gap in a cropping system.

Carrier volume is more likely to impact trifluralin efficiency than crop residue

Abstract PRE herbicides are generally less effective in conservation farming systems because of high levels of crop residue. However, performance can be improved if the herbicides are applied with a high carrier volume. This research investigated the interaction of carrier volume and row spacing or height of crop residue on the control of rigid ryegrass with trifluralin, at Cunderdin and Wongan Hills Western Australia. To create plots with varying residue row spacing in 2011, wheat was seeded in 2010 using a narrow row spacing (25 or 22 cm at Cunderdin and Wongan Hills), wide spacing (50 or 44 cm), or not planted to wheat. Narrow or wide row spacing or no crop plots had an average residue biomass of 4480, 3560, and 2430 kg ha−1 at Cunderdin and 1690, 1910, and 1030 kg ha−1 at Wongan Hills. To vary residue height, the wheat was harvested to produce tall, medium, or short crop residue (22, 13, and 5 cm at Cunderdin and 27, 22, and 17 cm at Wongan Hills). Rigid ryegrass seeds were broadcast onto each site in 2011 and trifluralin was sprayed using 50, 75, or 100 L ha−1 carrier volume (directly prior to seeding). Increased carrier volume increased spray coverage at both sites (average cover of 9, 15, and 26% at 50, 75, and 100 L ha−1), leading to improved control of rigid ryegrass (68, 75, and 82% control at Cunderdin and 23, 41, and 68% control at Wongan Hills). Reduced crop residue height or increased row spacing led to reduced rigid ryegrass density at Cunderdin but had no impact at Wongan Hills. Therefore, carrier volume has a more consistent impact on the performance of trifluralin than crop residue row spacing or height. Nomenclature: Trifluralin; rigid ryegrass, Lolium rigidum Gaudin; wheat, Triticum aestivum L. Resumen Los herbicidas PRE son generalmente menos efectivos en sistemas de producción con conservación de suelos debido al alto nivel de residuos de cultivo. Sin embargo, se puede mejorar el desempeño de los herbicidas si estos son aplicados usando altos volúmenes. Esta investigación estudió la interacción entre el volumen de aplicación y la distancia entre hileras y la altura del residuo del cultivo sobre el control de Lolium rigidum con trifluralin, en Cunderdin y Wongan Hills en el oeste de Australia. Para crear las parcelas con diferentes distancias entre hileras de residuos en 2011, se sembró trigo en 2010 usando una distancia entre hileras corta (25 ó 22 cm a Cunderdin y Wongan Hills), una distancia larga (50 ó 44 cm), o no se sembró trigo del todo. Las distancias entre hileras corta, larga, y sin cultivo tuvieron un promedio de residuos de biomasa de 4480, 3560, y 2430 kg ha−1 en Cunderdin y 1690, 1910, y 1030 kg ha−1 en Wongan Hills. Para variar la altura del residuo, el trigo se cosechó de tal forma que se generaron residuos de cultivo altos, medianos, o cortos (22, 13, y 5 cm en Cunderdin y 27, 22, y 17 cm en Wongan Hills). La semilla de L. rigidum se esparció sobre el área experimental en cada localidad en 2011 y se aplicó trifluralin usando 50, 75, ó 100 L ha−1 de volumen de aplicación (directamente antes de la siembra). El aumentar el volumen de aplicación incrementó la cobertura de la aplicación en ambas localidades (cobertura promedio de 9, 15, y 26% a 50, 75, y 100 L ha−1 ), lo que mejoró el control de L. rigidum (68, 75, y 82% de control en Cunderdin, y 23, 41, y 68% de control en Wongan Hills). Una menor altura en los residuos de cultivo o una mayor distancia entre hileras resultó en una menor densidad de L. rigidum, en Cunderdin, pero no afectó en Wongan Hills. De esta forma, el volumen de aplicación tiene una impacto más consistente en el desempeño de trifluralin que la distancia entre hileras o la altura del residuo del cultivo

Emergence, survival and seed production of Enteropogon ramosus in a pasture–wheat rotation or continuous pasture rotation in the wheatbelt of Western Australia

Enteropogon ramosus is a native, perennial, C4 grass species found within the wheatbelt of Western Australia. Emergence, survival, seed production and seed dormancy of E. ramosus was investigated in a continuous pasture rotation, a pasture–minimum tillage wheat rotation, and a pasture–minimum tillage wheat rotation where a cultivation event at the beginning of the pasture year was used to kill all E. ramosus plants. The results indicated that E. ramosus could germinate throughout the year, although plant density (ranging annually from 0 to 17 plants m−2) was lowest in conditions of low rainfall (summer–autumn drought). Seed production (estimated from seed head production, r = 91.7, P < 0.001) ranged from 0 to 2274 m–2 and was greatest in spring, in the continuous pasture rotation. Seed germinability reached 80–89%, following an initial 3 months of dormancy directly after seed production. Cultivation at the beginning of the pasture-crop rotation killed all plants, reduced emergence and prevented seed production for the 2-year period of the experiment. Soil disturbance from minimum tillage crop sowing reduced but did not eliminate E. ramosus plants. As a result, E. ramosus grew throughout the year in the minimum tillage cropping system. Further research is required to determine the competitive effect of E. ramosus on crop growth.

Weed Seed Wizard: A tool that demonstrates the value of integrated weed management tactics such as harvest weed seed destruction

Abstract Harvest weed seed destruction (HWSD) can be included in an integrated weed management (IWM) program to help control weed populations and combat herbicide resistance. However, it may not be possible or practical to use this technique in every year. This research utilised a computer model, the Weed Seed Wizard, to investigate the impact of employing HWSD every second year. Data from a long term trial in Merredin Western Australia demonstrated the value of residue burning (an early method of HWSD) compared to residue retention on the destruction of Lolium rigidum seed from 2003 to 2013. The agronomic practices utilised in this trial, soil type and rainfall at the field site, and crop yield data were used to parametrise two scenarios in the model. Scenario 1 was based on data from the field trial plots where residue was retained and scenario 2 was based on data from the plots where residue was burnt (i.e. HWSD). A third hypothetical scenario was based on scenario 2, but only incorporated HWSD in every second year. The model gave reasonable predictions of L. rigidum seed production each year (when compared to the actual seed production in the field trial), and accurately predicted when L. rigidum seed numbers would reach very low levels in scenario 2, due to annual HWSD. Scenario 3 indicated that HWSD in every second year could not reduce L. rigidum seeds to the same extent as annual HWSD, but the L. rigidum population was reduced to and maintained at less than one plant m −2 at harvest within four years. The model indicated that the total cost of weeds ranged from