Populations of rigid ryegrass with resistance to glyphosate have started to become a problem on fence lines of cropping fields of southern Australian farms. Seed of rigid ryegrass plants that survived glyphosate application were collected from two fence line locations in Clare, South Australia. Dose–response experiments confirmed resistance of these fence line populations to glyphosate. Both populations required 9- to 15-fold higher glyphosate dose to achieve 50% mortality in comparison to a standard susceptible population. The mechanism of resistance in these populations was investigated. Sequencing a conserved region of the gene encoding 5-enolpyruvyl-shikimate-3-phosphate synthase identified no differences between the resistant and susceptible populations. Absorption of glyphosate into leaves of the resistant populations was not different from the susceptible population. However, the resistant plants retained significantly more herbicide in the treated leaf blades than did the susceptible plants. Conversely, susceptible plants translocated significantly more herbicide to the leaf sheaths and untreated leaves than the resistant plants. The differences in translocation pattern for glyphosate between the resistant and susceptible populations of rigid ryegrass suggest resistance is associated with altered translocation of glyphosate in the fence line populations.

BACKGROUND Glyphosate failed to control two populations of Hordeum glaucum (northern barley grass) along a fence line and around stockyards near Arthurton on the Yorke Peninsula, South Australia after more than a decade of regular use of glyphosate. These were investigated to confirm resistance and to determine resistance mechanisms. RESULTS Dose–response experiments confirmed resistance of these populations to glyphosate with resistance levels of 2.8–6.6‐fold compared with two susceptible populations. Shikimate assays confirmed resistance to glyphosate with less shikimate accumulation in the resistant populations compared with the susceptible populations. Quantitative PCR of genomic DNA showed increased gene copy number in the resistant populations with 9–11‐fold more copies of EPSPS compared with the susceptible populations, suggesting resistance is likely conferred by gene amplification. CONCLUSION This study identified the first examples of glyphosate resistance in the grass species H. glaucum with resistance associated with EPSPS gene amplification. © 2019 Society of Chemical Industry

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Establishment of young cacao trees in West Africa can be severely impeded by the onset of the dry season. To address this issue, a field experiment was conducted in Ghana to examine whether different mulch treatments and irrigation applied during the dry season combined with overhead shade could improve survival, early growth and yield of cacao. The mulch treatments used were polyethylene film and coffee husks placed around the young plants. Irrigation was used as a positive control, and no mulching or irrigation was a negative control. Three shade regimes were provided through different arrangements of Gliricidia sepium and plantains. Four different cacao clones were used in the study in a replicated split-plot design. Early growth of cacao was stimulated under the irrigation and plastic mulch treatments. Higher rates of photosynthesis during the dry season appeared to underlie these increases. Significantly higher early yields were also observed under the irrigation and coffee mulch treatments compared with the control. Plant survival varied significantly between treatments; irrigation was associated with the highest plant survival (94%), followed by the plastic mulch treatment (91%), coffee husk (82%) and the control (70%). There was also an increase in survival when more intense shading was used. Under zero mulch conditions, differences in survival were observed between clones. The clones P 30 [POS] and SCA 6 were more sensitive to drought (in terms of survival) than PA 150 and T 79/501. It is concluded that relatively simple mulching techniques or controlled irrigation in conjunction with appropriate shade management can significantly improve early establishment and cropping of cacao.

The repeated use of glyphosate to control annual ryegrass along fence lines and crop margins has resulted in the evolution of resistance to this herbicide in populations of annual ryegrass (Lolium rigidum) in cropping regions of Australia. Field trials were conducted between 2009 and 2011 at four fence-line sites in South Australia to identify suitable herbicide treatments for controlling glyphosate-resistant annual ryegrass. Annual ryegrass populations growing at all four sites were found to have 12–24-fold resistance to glyphosate compared with a standard susceptible population in dose-response experiments. Glyphosate alone (1080 g ha–1) did not effectively control glyphosate-resistant annual ryegrass at any location. Single applications of paraquat + diquat and paraquat + amitrole were effective where weed populations were low, with up to 99% reduction in seed-head production. Mixtures of paraquat + diquat + diuron, glufosinate ammonium + diuron and two applications of paraquat + diquat 14 days apart consistently provided high levels of control of glyphosate-resistant annual ryegrass at all locations, with >90% reduction in seed-head production. In 2011, glyphosate resistant individuals of annual ryegrass were identified up to 50 m inside the fields adjacent to the fence. Therefore, failure to control glyphosate-resistant annual ryegrass along crop margins risks movement of resistance into adjacent cropped fields.