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

Slashing and mowing
Slashing can reduce the production of panicle seed and stem cleistogenes but has little other benefit, apart from stimulating
regrowth that is more palatable to grazing livestock (Snell et al. 2007). Britt (2001) found that plants slashed to 1 cm in early
May at Greenvale had failed to produce flowering stems by late October, whereas 60% of unslashed plants had produced
flowering heads by that time. Hocking (2005b) found that mowing in spring had no major effect on the number of mature
tussocks and failed to significantly reduce tussock size. Slay (2001) incorrectly attributed to N. neesiana the findings of Mulham
and Moore (1970) about mowing Austrostipa swards. Those authors found that mowing in late September prevented seed
maturation and that late mowing could allow seed maturation on cut stems. Slay (2001) found that mowing twice at early seed
head emergence (Feekes stages 10-10.4) totally prevented the formation of viable panicle seed and stem cleistogenes and
resulted in regrowth acceptable for livestock grazing. Grech (2007a) also studied slashing impact in pastures and quantified some
benefits in terms of palatable regrowth that could be exploited by grazing. Mowing before emergence of the panicle may result
in the growth of more reproductive tillers, so repeated treatments are generally required. The best time to slash or mow is before
flowering when 25-75% of the heads have emerged from the flag leaf sheath (Slay 2002c), or at the flowering stage (Grech
2007a, Snell et al. 2007). Slay (2002c) noted that the resultant reduction in shade may facilitate the survival of seedlings.
Mowing can encourage the formation of prostrate swards, so may also reduce the production of stem cleistogenes (Snell et al.
2007).
Fire
Published information on the value of fire as a managment method are equivocal. According to Bourdôt (1989) the New Zealand
experience was that repeated fires favour the grass, and when tried as a means of control in Marlborough appeared to have
“eliminated other species resulting in pure stands”. Muyt (2001 p. 73) stated that fire “stimulates vigorous regrowth”, often
promotes spread, but is useful for improving access and effectiveness of herbicide application by removal of litter and dead
material. However he also noted that fire “generates bare ground and reduces immediate competion, conditions that are ideal for
seed germination” (Muyt 2001 p. 73) so should therefore should be followed by herbicide application (Muyt 2005). In pastures,
Slay (2002c) recommended spraying with glyphosate to dry out the grass before burning and follow-up herbicidal control to kill
seedlings that grow from cleistogenes in burnt tussocks.
Snell et al. (2007) advocated the use of fire to prevent seed set, burn off standing seed and to stimulate growth of seed from the
soil seed bank, but advised integration with other selective control techniques. They also noted its usefulness in enabling a
clearer indication of intensity and pattern of infestations.
In T. triandra grasslands burning in spring is recommended, so competing cover establishes quickly (Muyt 2001). Hocking
(2005b) found that burning in spring in a native grassland under drought conditions resulted in major reduction of large, mature
tussocks and proliferation of small tussocks, probably derived from the large. The total area occupied by tussocks was decreased
by >75% without decreasing the density of the population. Late spring burns reduced seed production by half, and did not lead to
major seedling recruitment. He suggested that fire was therefore useful to weaken the stand, making it more susceptible to
subsequent control activities of a different type and in limiting seed production, and could play a role in a containment strategy
and integrated management. Hocking (2007) noted that fires kill few plants and that seed production resumes within a year. Snell
et al. (2007) agreed that burning usually does not result in much kill of mature plants. However reductions in density of c. 90%
have been reported after annual November burning for 5 years at Plenty Gorge Parklands, Victoria, however N. neesiana was
replaced by Phalaris aquatica (Snell et al. 2007 p. 34), which may have even worse biodiversity impacts.
The effects of repeated periodical burning and of fires in autumn do not appear to have been adequately assessed by scientific
tests. In particular better knowledge of the effects of fire on the litter and near-surface soil seed banks would be desirable.
Cultivation and cropping
The objective of cultivation and cropping is too reduce the soil seed bank before re-establishment of a vigorous pasture that can
suppress any seedlings that may subsequently appear. Cultivation stimulates seed germination and shallow cultivations are
supposedly superior in depleting the seed bank because most of the seed is close to the surface (Snell et al. 2007). Herbicides
applied by boom spray are recommended as the intitial treatment, followed by a series of shallow cultivations interspersed with
dense, uniform sowing of annual fodder crops (Bourdôt 1988, Duncan 1993, Slay 2002a 2002c). Shallow cultivation (no greater
than 5 cm) is recommended so that seed will not buried and the maximum amount will germinate. Slay (2002a) warned that the
cultivation technique and timing were critical to minimise dispersal of cleistogenes. Duncan (1993) recommended winter
cropping for two years followed by pasture establishment, and in less arable areas herbicide spraying, direct drilling of fodder
crops, and fertiliser application. Bourdôt (1988) recommended three years of such management before sowing of ‘permanent’
pasture. Storrie and Lowien (2003) broadened the range of recommended crops to include summer forage species and summer
grains, and the aerial application of herbicide, seed and fertiliser to treat steep, rocky, inaccessible areas. Establishment of
lucerne after glyphosate spraying, with residual grass herbicide application and winter spraying with grass-selective paraquat and
metribuzin is an alternative approach (Bourdôt 1988).The best methods, including crop and pastures to sow, depend on the
specific characteristics of the infestation, regional agronomic factors, etc. (Slay 2002c).
Grazing
Overgrazing is likely to favour N. neesiana, due to its lower palatability (Bourdôt 1988) at least during its reproductive period
(Grech 2007a). Crude protein, metabolisable energy and digestible dry matter contents of N. neesiana peak in winter and early
spring and decline markedly from September to December, corresponding with the onset of reproductive phase (Grech 2007a).
Strategic grazing in pastoral ecosystems to best utilise the green feed produced by N. neesiana in winter and spring has been
intensively studied (Gardener 1998, Grech 2005a 2007a). Intense grazing pressure over a short period is required to suppress
seed production (Storrie and Lowien 2003) but in practice is very difficult to achieve and probably not feasible in most grazing
enterprises (Slay 2002a, Grech 2007a). In addition there is a risk that overgrazing will encourage further N. neesiana
establishment (Slay 2002a). Set stocking with sheep exacerbates the problem by reducing the cover of desirable pasture grasses
and creating more bare ground suitable for further N. neesiana establishment (Grech 2007a, Snell et al. 2007). Preliminary
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