Literature review: Impact of Chilean needle grass ... - Weeds Australia

Bourdôt and Hurrell (1987b) tested a range of herbicides for control in lucerne. Hurrell and Bourdôt (1988) tested granular
formulations of three herbicides and found that hexazinone at 16 kg ha -1 killed plants and prevented seedling establishment for at
least 7 months. Bourdôt (1988) recommended hexazinone for infestations on roadsides and waste places.
Establishment of competitive plants (pasture species or fodder crops) is generally recommended after herbicidal control on
agricultural land (e.g. Bourdôt 1988, Liebert 1996).
Natural areas
Herbicidal treatment of Nassella spp. in native vegetation has often had devastating effects on native vegetation, killing native
plants or whole populations and facilitating irreversible weed invasions (Hocking 1998). Non-selective herbicides and nonselective
application measures constitute one aspect of the problem. Uncritical adoption of recommended agricultural
applications is another. A consistent complicating factor is the low level of knowledge of the resistance and susceptibility of
native plants to particular herbicides across a range of seasons. Little detailed information appears to be available about offtarget
effects in native grasslands, perhaps because there is reluctance to admit that good intentions can produce bad results.
McDougall (1989) established that atrazine had “no direct effect” on Themeda triandra establishment and growth and suggested
it could be used to reduce weed competition in T. triandra grassland sites lacking native species that are sensitive to it. Harris
(1990) recorded that this herbicide would be a major component of N. neesiana control programs in native grasslands in the
Melbourne area, including Derrimut and Laverton North, and that trials were to be undertaken comparing high volume spot
spraying with low volume application methods such as ‘Micro-Herbi’, ‘Splatter Gun’ or ‘Gas Gun’.
Flupropanate at low rates (1.5-3 kg active ingredient ha -1 ) can be used to selectively remove Nassella trichotoma from pastures
with little effect on native grasses including T. triandra, Botriocholoa macra and Poa labillardieri, but Austrodanthonia spp. and
Microlaena stipoides are killed at less than half this application rate, and legumes may be damaged (Campbell 1997 1998).
Hocking (1998) warned that atrazine, simazine and flupropanate were likely to be highly toxic to native grassland forbs.
Bedggood and Moerkerk (2002) stated that atrazine killed N. neesiana in native grasslands with little effect on T. triandra.
Dare and Hocking (1997) tested glyphosate, atrazine, flupropanate and simazine at unspecified rates for control of N. neesiana in
Melbourne area native grasslands and found that atrazine and flupropanate provided effective kill when applied in winter, as
assessed 11 months later. Atrazine was found to act fast, providing effective kill within 10 weeks, and treated plots remained
weed free. Glyphosate and simazine were slower acting (12 weeks) and glyphosate plots were reinvaded by weeds.
Brereton and Backhouse (2003 p. 3) observed that herbicides used to control grass growth in native grasslands to reduce fire fuel
loads “usually” promoted “the establishment of exotic grasses to the detriment of the native flora”, and Williams (2007) found
that herbicide application as an alternative to burning for fuel reduction has wiped out a number of remnant native grasslands in
western Victoria. Significant herbicide damage to desirable native species has been noted in the ACT. To address this the ACT
Weeds Working Group (2002) recommended promotion of glyphosate spraying at the correct time and use of low rates of
flupropanate for seedling control, as well as the use of alternative control techniques and greater concentration on control in
buffer zones to restrict spread to conservation areas. Muyt (2005) suggested the use of either grass-selective or non-selective
herbicides from autumn to late spring in one ACT grassland. McDougall (1989) argued that fluazifop would have advantages
since is has no direct effects on establishment and growth of Themeda triandra, although it inhibits flowering, and is ineffective
at reducing competition by annual forbs.
Puhar (1996) tested the impact of glyphosate, flupropanate and atrazine on N. neesiana by measuring the root and shoot lengths
of seedlings germinated on agar plates impregnated with herbicide at various rates. Germination of dehulled seeds (i.e. lemma
removed) was unaffected by any of the herbicides at any concentration. Glyphosate had significant negatives effect on root and
shoot elongation. Atrazine had a lesser, but still significant effect, while flupropanate also caused significant negative impact, but
with higher root growth rates than for glyphosate. All four herbicides were judged to be able to kill emerging seedlings at 0.25 of
the label rate for grassland spraying, but the rapidly acting glyphosate was more damaging. Similar tests were undertaken for T.
triandra, but the susceptibility to off-target damage of the remainder of the grassland flora remains largely unknown.
Disadvantages of herbicidal management
Whatever the herbicide used, cleistogenes that have already matured but remain attached to the plant, concealed beneath leaf
sheaths are not killed (Hurrell et al. 1994). Furthermore, when a large N. neesiana seed bank is present, baring the ground with
herbicides encourages seedling recruitment and may lead to rapid re-establishment, and an ultimate increase in density and cover
(Hartley 1994, Gardener et al. 1996b, Gardener et al. 1999, Lunt and Morgan 2000, Slay 2002a, Storrie and Lowien 2003).
Herbicidal management has often resulted in the expansion of populations and exacerbation of spread due to the elimination of
competition (Slay 2001 2002a 2002c). When herbicides are applied in spring, for example, the basal cleistogenes are already
mature, there is a greater effect on potential competitor plants and the bare areas created allow for more N. neesiana seed to
germinate (Slay 2002c).
However if the seed bank is small, herbicial control may not simultaneously provide conditions for seedling establishment. Britt
(2001) found no seedling growth 6 months post-treatment. Hocking (2005b) reported on the seed bank at infestations managed
by repeated herbicide applications, mainly of glyphosate, in south-central Victoria and Hobart. In all cases the density of filled
seeds (i.e. seeds containing a grain and embryo) in the seed bank was