PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
NON-SELECTIVE & SELECTIVE HERBICIDES (contd)
Broad & narrow spectrum herbicides
WHY ARE SOME
HERBICIDES
SELECTIVE
(contd)
Stage of crop growth. Some crops are only tolerant of recommended rates of
a herbicide at certain stages of growth. Herbicides must be applied at the
appropriate stage of the crop, eg before the crop has been planted, after the crop
has been planted but before emergence of the crop, or after the crop has
emerged (page 446).
Stage of weed growth. Many herbicides are effective only against certain
growth stages of the weed, eg roots, foliage, germinating seeds (page 447).
– Many are more effective when weeds are young and actively growing rather
than older and growing slowly.
– Those herbicides active against germinating seeds can be used amongst
established plants in orchards, arboreta and containers.
– Spray topping is the application of a sub-lethal dose of non-selective
herbicides to pastures at flowering. It is used to prevent the formation of viable
weed seeds without inducing a winter feed shortage. Spray topping can be very
effective in reducing the weed seed bank.
Herbicide application techniques., eg
– Non-selective herbicides, eg Roundup (glyphosate) may be applied as a
directed spray, spot spray or wiper application to avoid contact with desired plants.
– Placement of herbicide in the soil. Selectivity of herbicides absorbed by the
roots may be influenced by the depth of the plant root system. Herbicides may
be fixed in the top few layers of soil so that tree or shrub roots are not
damaged. Seed may be placed below a treated soil zone.
– Type of formulation. Granular herbicides may be used which bounce off the
crop onto the soil, thereby killing germinating weed seeds only.
– Addition of wetting agents (surfactants or spray oils) increases herbicide
uptake by plants, reducing selectivity resulting in reduced crop tolerance. In
general, the smaller the droplet size the greater the number of spray droplets
retained by the leaf. Wetting agents lower the surface tension of the leaf,
increase the droplet number retained on the leaf and reducing run-off.
Some herbicides are selective at low rates only. Such herbicides may
become non-selective when applied at higher rates. Generally the higher the rate a
herbicide is applied the less selective it is.
Environmental conditions. affect herbicide selectivity, eg soil moisture
and air temperature. Some are less effective at low temperatures but too effective at
high temperatures damaging crops in glasshouses. Many herbicides only work when
weeds are young and actively growing.
Weeds - Integrated Weed Management 445