Ecology and management of Brome grass (Bromus rigidus Roth and Bromus diandrus Roth) in cropping systems of Southern Australia.
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Date
2013
Authors
Kleemann, Samuel George Lloyd
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Advisors
Gill, Gurjeet Singh
Diggle, Art
Diggle, Art
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Abstract
Brome grass species Bromus rigidus (rigid brome) and B. diandrus (great brome) are winter annual grasses that have proliferated in recent years to become serious weeds of crops and pastures in southern Australia. Until recently there had been few studies on the population ecology of B. rigidus and B. diandrus and the research that had been done tended to focus on populations that had naturalised in Western Australia. Increased knowledge of the behaviour of B. rigidus and B. diandrus under current farming systems in southern Australia and the impact of management strategies on population ecology and seedbank dynamics would facilitate development of more effective weed control programs. A field survey was undertaken in 2003 on the Yorke and Eyre Peninsulas of South Australia to determine relative distribution of these two brome grass species. Bromus rigidus was found more frequently and at higher densities in South Australian crops than B. diandrus. Field populations of both spp. were shown to possess much longer seed dormancy than what had been previously reported in Australian literature. Germination of dormant seeds of B. rigidus and B. diandrus was overcome with the addition of gibberellic acid (0.001 M GA₃) rather than upon removal of the husk (i.e. lemma and palea) protecting the seed; indicating that dormancy is most likely under hormonal control within the embryo. Dormant populations of B. diandrus from cropping fields were highly responsive to cold stratification (i.e. chilling), a process which has been shown to increase GA synthesis within the seed. Populations of B. diandrus from cropping fields also showed much longer seed dormancy than those collected from adjacent fence-lines. The large differences in germination pattern between these cropping and adjacent fence-line populations provide some evidence to suggest that management practices being used by growers in crop production are selecting for increased dormancy in B. diandrus. In cropping fields, there could be considerable adaptive value of dormancy mechanisms (i.e. cold stratification requirement, light inhibition) that delay germination and seedling emergence until after pre-sowing weed control tactics have been used. Dormant populations of both B. rigidus and B. diandrus also showed strong inhibition of seed germination when exposed to light. This is the first Australian study to report the inhibitory effect of light on seed germination of Bromus spp. and provides a possible explanation for their increasing prevalence in southern Australia since the adoption of no-till farming. Selection of greater dormancy in Bromus spp. is likely to contribute to the development of a more persistent seedbank. A 3-year field experiment undertaken at Lock on the Eyre Peninsula of South Australia from 2003 to 2005 showed that about 20% of B. rigidus seedbank can persist from one season to the next. In this field study, management strategies that combined effective herbicides (ACCase-inhibitors and imidazolinone Clearfieldtechnology) and crop competition over consecutive years provided effective control of B. rigidus population and depleted its seedbank to low levels (from 1748 to <5 seeds m⁻²) within 3 years. In field studies, metribuzin, and metribuzin plus pendimethalin incorporated by sowing provided safe and effective control of B. rigidus (>75%) in barley. Post-emergent applications of imidazolinone herbicides, imazapyr, imazapyr plus imazapic and imazapyr plus imazamox, to imidazolinone-tolerant wheat (Clearfield™) also provided consistent and high levels of B. rigidus control (≥87%). Effective management of Bromus spp. will require a major change in cropping systems used by growers in southern Australia and will involve combining effective herbicide technologies (i.e. Clearfield™) with more competitive crops and more diverse rotations, where possible.
School/Discipline
School of Agriculture, Food and Wine
Dissertation Note
Thesis (Ph.D.) -- University of Adelaide, School of Agriculture, Food and Wine, 2013
Provenance
Copyright material removed from digital thesis. See print copy in University of Adelaide Library for full text.