- To improve the efficiency of herbicide
applications, each stage of the spray application
process must be considered. Two of these stages, the
process of spray deposition within plant canopies and
the influence of the form of the spray deposit on
efficacy were investigated.
The effect of droplet size, spray volume, and
droplet trajectory on the vertical distribution of
spray deposits was measured within canopies of bracken
fern (Pteridium aquilinum L. Kuhn) and greenleaf
manzanita (Arctostaphylos patula Greene). Spray
containing a fluorescent tracer was applied using
spinning disc and flat fan nozzles, and deposition was
measured on horizontal strings placed at various levels
in the canopy.
Spray deposition profiles were not significantly
affected by droplet size. With bracken, 50 1/ha
application volumes gave greater deposition than at 100
1/ha. Spray deposition was increased, particularly
with the vertically oriented manzanita foliage, when
droplets entered the canopy with a significant
horizontal component to their trajectory.
The foliage structure of bracken and manzanita
canopies was measured using a point quadrat vegetation
sampling technique. A model to predict spray
deposition profiles was then developed, and observed
deposition profiles were compared to predicted
profiles. With bracken, the modelled profile was close
to, but underestimated, the measured deposit
attenuation. With manzanita, the predicted deposit
profile overestimated deposit attenuation, suggesting
that the vertically moving droplets were reflected from
the foliage inclined at 72° from the horizontal.
The effects of droplet size, spray volume, and
herbicide rate on phytotoxicity were also investigated.
Glyphosate and fluroxypyr were applied to bracken fern
and greenleaf manzanita, respectively, using spinning
discs. Increasing glyphosate concentration or the area
of foliage wetted were equally effective in enhancing
efficacy on bracken fern. A smaller droplet size and
higher spray volume increased the efficacy of
fluroxypyr on manzanita. For both species and chemical
combinations, the addition of a surfactant, L-77,
showed the greatest potential for increasing the
efficiency of spray applications.
In general, using small droplets with a
horizontal velocity component, and a suitable
surfactant will increase spray efficiency. High
herbicide concentrations may be beneficial if localized
scorch does not occur.