PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
WHEN CAN HERBICIDES BE APPLIED – STAGE OF WEED GROWTH.
Again timing is important, otherwise significant crop damage may occur and weeds may
not be controlled (check keys and books describing the growth stages of weeds).
POST-EMERGENT
HERBICIDES
Knockdown herbicides
PRE-EMERGENT
HERBICIDES
Do you know when your
weeds emerge?
No pre-emergent kills
all weed seeds
The crop or the
emerging crop must
be tolerant
Soil residual up to 6
months kills
germinating weed
seeds
Many herbicides
show more than one
type of activity, eg
post-emergent and
some pre-emergent.
RESIDUAL
ACTIVITY
IN SOIL
If residuals are
used ensure they
do not affect
subsequent crops.
Some are only
effective against
germinating seeds
(pre-emergent
herbicides), while
others control
established weeds.
Examples of soil
residual herbicides
which kill established
weeds include diuron,
Casoron (dichlobenil
Applied to the foliage of emerged existing weeds. Young actively growing weeds are
more easily controlled than older well established plants. They may need to be applied
at a certain stage, eg seedling 2-leaf stage. They may be:
Contact herbicides (non-systemic), eg Basta (glufosinate-ammonium), affect only
the part of the plant they touch; they have short duration, fairly rapid action.
Translocated (systemic), eg Glypho , Roundup , Zero (glyphosate) and are taken up
by the foliage/stems and translocated into the root system. They may be:
– Non-selective, eg Roundup (glyphosate) which kills annual and perennial weeds.
– Selective, eg MCPA controls broadleaved weeds in grass crops.
Weed seeds usually germinate in the upper
centimeter (small seeds) to 10cm (large
seeds) of soil depending on the availability
of moisture near the surface. Pre-emergents
aim to kill germinating weed seeds before
they emerge from the soil (page 458).
Dacthal Goal Surflan
Ronstar Simazine Rout
For effective application:
Pre-emergent herbicides are formulated to remain near the soil surface where
weed seeds germinate and not to move deeper where they could affect roots of
established plants. Soil compaction, solubility of herbicide, soil organic matter or clay
also affects their activity. Seedbeds must be clump-free, apply to weed-free soil.
Even incorporation ensures that pre-emergent herbicides are applied at the correct
depth to contact the roots or shoots of germinating weed seedlings. Some pre-emergents
break down in sunlight or are volatile (rapidly evaporate damaging nearby crops) and
must be incorporated within a few hours of application or will lose their effectiveness.
– Irrigation/rain incorporation can be pre-or post-plant. Some pre-emergents are
fairly soluble in soil moisture and are sufficiently mobile by themselves, or with only a
little rain or irrigation to move into the upper few centimeters of soil where they will
actively control weeds. Furrow irrigation or drippers are not suitable. Check weather,
rainfall, temperature, wind, volatilization, and photo-degradation.
– Mechanical incorporation into the soil surface of some herbicides is pre-plant only,
to avoid damaging crop seeds. They are either less soluble/mobile in the soil or must be
carried to greater depths to control larger seeded and deeper germinating weeds.
Mechanical incorporation is a major cause of damage to soil structure, not suitable for
conservation tillage (CT) systems.
Activation. Soil moisture is essential for activating and dissolving pre-emergents.
Roots or shoots of germinating weeds then take up the herbicide.
Soil disturbance can have a dramatic effect on the effectiveness of pre-emergents.
Pre-emergents may generally be:
– Non-selective, eg Surflan ® (oryzalin), Ronstar ® (oxadiazon).
– Selective, eg Goal ® (oxyfluorfen), Dual ® (metolachlor), Casoron ® (dichlobenil).
NON-RESIDUAL. HERBICIDES break down quickly in soil, eg Tryquat (diquat +
paraquat) allowing a crop to be planted soon afterwards.
RESIDUAL. HERBICIDES persist in the soil for long periods and are taken up by
the roots and shoots of germinating seedlings and roots of established weeds. They can
be selective or non-selective.
Pre-emergent herbicides at selective rates provide long term control of
weed seedlings, protecting the crop during its early growth stages when it is most
sensitive to weed competition, during high growth seasons of spring and autumn.
– Select pre-emergents and adjust application rates to leave no damaging residues in the
soil after the crop is harvested to interfere with growth of subsequent crop.
– At higher application rates some soil residual pre-emergent herbicides are non-selective
and kill all plant growth, eg simazine.
Soil sterilants used non-selectively at high rates give long-term control of all
plant growth, usually in non-crop situations, eg fire breaks and around buildings and
industrial installations. Herbicide remains in soil for a considerable time, eg
– simazine (6 months to more than 12 months)
– diuron (years)
Factors affecting residual activity in the soil include:
– Residual activity can vary from a few weeks to a year or more.
– Concentration, generally the higher the rate the longer the residual effects.
– High temperatures favour the breakdown of herbicide.
– Leaching by soil water, which will depend on rainfall, soil type, herbicide solubility.
– Ultra-violet light, which can breakdown some herbicides, eg simazine.
– Volatilization may occur under high soil temperatures and dry conditions.
– Microbial breakdown in the soil.
– Adsorption by humus and clay may render a herbicide unavailable to the plant.
– Some post-emergent herbicides may fall on soil and be active for a short time against
subsequent crops, eg glyphosate. Check label for plant-back time.
Weeds - Integrated Weed Management 447