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

Themeda re-establishment
McDougall (1989) reported the results of detailed experiments to determine suitable methods for the re-establishment of
Themeda triandra. The most effective method involved application of T. triandra thatch immediately after harvest in January,
followed by burning of the thatch in September when dry enough. This provided seedbed conditions favourable for T. triandra
germination and seedling establishment. A mulch of brown coal greatly improved seedling establishment from surface-sown
seed. Hebicide spraying prior to T. triandra seed germination indicated that establishment was not inhibited by low-growing
weeds. Spring burning promoted germination of T. triandra seed in the soil seed bank.
Stafford (1991) reported on long-term investigations of techniques to establish T. triandra to restrict weed invasion in secondary
grassland and woodlands. The ability to burn established T. triandra swards in spring/early summer and its tolerance to
herbicides used to control woody weeds provided the weed management advantages. His technique can be characterised as
‘hay/spray/burn’: haying with T. triandra culm thatch, spraying to suppress weeds and burning to remove the thatch and dry
weeds. Herbicidal control of weeds was critical to success, but the effective herbicides are widely active against a range of
plants, both exotic and native. High labour inputs to obtain sufficient seed were identified as a major problem. A vehiclemounted
reel-stripper was developed, but proved to have limited efficiency in gathering undamaged, chaff-free seed. It also
damaged the sward. Modification of the stripper, including addition of wire flails to the rotor, enabled harvesting of the panicles
at high efficiency. However germination of the material harvested was only 10% of that with hand-cut material. Harvesting
sufficient seed for the areas requiring rehabilitation remains an ongoing problem.
‘Spray and hay’ techniques have been developed and modified to a point where N. neesiana swards in Themeda grasslands can
be selectively removed and replaced with T. triandra when seasonal rainfall is adequate (Dare and Hocking 1997, Hocking 1998,
Mason 1998, Mason and Hocking 2002, Mason 2004, Hocking 2005b). The method, first developed for N. trichotoma (Phillips
and Hocking 1996, Mason 2004) involves mowing in late spring to reduce N. neesiana biomass, spraying of N. neesiana plants
with glyphosate the following autumn, application of seed-bearing T. triandra grass hay in winter and allowing the seeds to bury
themselves in the ground, then removal of the hay thatch by hand or by burning in early spring (Hocking 2005b). Tested
variations on the technique have involved tilling of the soil and use of other herbicides (Mason 1998 2004). Dense patches of N.
neesiana have been controlled in this way in dry and average rainfall years, with N. neesiana cover reduction from >65% to c.
10% and T. triandra cover increase from c. 5% to >85% over 3 y with c. 17 T. triandra and c. 6 N. neesiana plants m -2
established in the most successful trials (Mason and Hocking 2002, Hocking 2005b). A very similar ratio of T. triandra to N.
neesiana tussocks was found in these trial plots 5 years later, adding weight to the finding of Lunt and Morgan (2002) that
invasion of N. neesiana is restricted when there is a healthy cover of T. triandra (Hocking 2005b).
Atrazine has also been used as the herbicide in the ‘spray and hay’ method (Mason 1998). Both atrazine and acetic acid vinegar
were tested in a small trial by Mason (2005) but acetic acid treatments resulted in much reduced T. triandra seedling
establishment and control of Nassella spp. that was less than, or approximately equivalent to that provided by atrazine, 10
months after treatment.
Dare and Hocking (1997) reported an unsuccesful trial of the method, blamed on very low rainfall during summer. Appropriate
timing of the stages of the treatment is critical, and abundant, high quality seed is required (Mason and Hocking 2002). The best
time to spray when using the spray and hay method was determined by Phillips and Hocking (1996), who found that late autumn
to winter herbicide application resulted in a 3 month weed-free window, whereas February and September spraying resulted in
rapid invasions by a suite of weeds.
‘Spray and hay’ methods have generally had poor effectiveness under drought conditions (Dare and Hocking 1997, Hocking
1997, Hocking pers. comm.) and are subject to the same general constraints on rehabilitation of native grasslands as other
methods, notably the difficulty of harvesting and very limited supplies and low quality of native seed or thatch (McDougall
1989, Bedggood and Moerkerk 2002, Muyt 2005) and high labour inputs.
To circumvent some of these problems Muyt (2005) recommended sowing of other native perennial grasses at the same time as
T. triandra, including Austrodanthonia, Austrostipa, Microlaena stipoides and Elymus scabra.
Biological control
No succesful biological control programs against grass species have been undertaken and programs targetting grasses are all of
recent origin (Witt and McConnachie 2004). Grass species were once thought to lack specific arthropod herbivores because they
very rarely produce toxic secondary metabolites as defenses, and are too simple and similar in structure, physiology and ecology
for specific herbivores to have evolved (Evans 1991). However secondary metabolites that function as defensive compounds are
common in many cereals and grasses, including hordenine in barley (McDonald 1991), a specific invertebrate herbivores have
gradually begun to be discovered. Gross similarities and close taxonomic relationships between grass weeds and valuable crop
and pasture species was also thought to provide little scope for development of biological control (Witt and McConnachie 2004).
The potential for biological control of Nassella spp. in Australia was also initially considered to be low due to their close
relationship to Australian Stipeae (Wapshere 1990 1993).
An Australian project with Argentine and New Zealand collaborators to biologically control Nassella spp. with fungi has been
pursued since 1999: see below under “Pathogens”. Anderson et al. (2006) presented a recent update on progress in host
specificity testing and production of potential agents. One reason biological control remains difficult is because the geographical
source of Australian populations has not been identified: effective predators and parasites in the area of origin cannot therefore
be identifed. Another reason for slow progress is the need to demonstrate that the large number of endemic Australian Stipeae
will not be affected. Biological control using insects may be worth further consideration in the future, given increasing
recognition of the existence of host specificity amongst invertebrate grass herbivores and overemphasis of the role of plant
chemical defenses as an evolutionary driver for monophagy (Witt and McConnachie 2004) (see discussion below under
‘Predators and Pathogens’).
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