Effect of herbicides on seedhead production and control of serrated tussock (Nassella trichotoma (Nees) Arech.)
M.H. CampbellA, L.G. MillerB and H.I. NicolA
A NSW Agriculture, Orange Agricultural Institute, Forest Road, Orange, NSW 2800, Australia.
B Department of Natural Resources and Environment, Geelong, Victoria 3220, Australia.
Summary
Herbicides were applied to serrated tussock (Nassella trichotoma (Nees) Arech.) just before and after the beginning of seedhead emergence in Victoria and New South Wales in 1994 and 1995 to examine their effects on seedhead production and control. The critical factor in reducing seedhead production was to apply herbicides before the beginning of seedhead emergence. Glyphosate applied at 0.225-0.45 kg a.i. ha-1 in September, October or November, 0-8 weeks before the start of seedhead emergence, reduced seedhead production by 91-99%. The optimum time of application appeared to be 2-4 weeks before the seedheads began to emerge. Because it is difficult to assess when the seedheads will begin to emerge and because the timing of this event varies from year to year, applying glyphosate when the thicker-than-normal flowering tillers are present in September, October or November is the best practical indicator of when to spray and should result in >90% reduction in seedhead production.
When applied after seedhead emergence began glyphosate reduced seedhead production (e.g. 77% reduction from 0.45 kg a.i. ha-1) much less than when applied before. It also reduced the percentage of mature seeds in seedheads but had little effect on germination of these seeds and no effect on growth of seedlings from the seeds. Flupropanate had no effect on seedhead production, seed set, germination or seedling growth when applied soon before or after the beginning of seedhead emergence. When mixtures of flupropanate and glyphosate were applied, the seedhead reduction effect of glyphosate was reduced and the effects of flupropanate in killing mature tussocks and restricting seedling regeneration were reduced. Therefore, for optimum results in large scale applications, glyphosate should be applied alone for seedhead reduction and flupropanate applied alone for killing serrated tussock and restricting seedling regeneration until research solves the problem of applying the two herbicides together. However, mixtures could be used for spot spraying. Use of glyphosate to prevent seedhead production of serrated tussock would best be applied as an interim treatment to provide time for more permanent control measures to be implemented.
Plant Protection Quarterly (1998) 13 (3) 106-110.