Agriculture Reference
In-Depth Information
within 1 week of application, is necessary for these to be effective. The duration
of their residual action depends on the temperature and moisture content of
the soil. For example, propachlor can persist for 6-9 weeks, but breaks down
more rapidly in warm, irrigated conditions. If crop emergence is slow, as with
spring sowing in cool regions, the application of pre-emergence residual
herbicides is best delayed until a few days before crop emergence, so that the
herbicidal effect persists for as long as possible after emergence. Soil type affects
the choice and dosage of residual herbicide: sandy and silt soils low in organic
matter are less retentive of herbicides than soils higher in organic matter.
Chlorpropham should not be used on light, sandy soils since heavy rain can
leach herbicide to the roots, causing severe crop damage; and the dose rate of
pendimethalin should be increased in moving from sandy to fine soil to
maintain effectiveness. The herbicidal surface layer of residual chemicals is
destroyed if the soil is cultivated, so such treatments must be repeated following
hoeing or other soil disturbance. All pre-emergence treatments for sown crops
are also suitable for use prior to emergence of sets and bulbs.
3.
Post-emergence or post-planting residual herbicides. The residual herbicides
propachlor, pendimethalin and chloridazon are applied post-emergence in the
UK, commonly in conjunction with a low dosage of a contact herbicide like
ioxynil (Grundy
et al.
, 2003).
4.
Post-emergence or post-planting contact herbicides. Nowadays, appli-
cations of post-emergence contact herbicides in onions begin as early as the
loop stage of seedlings (see Fig. 2.1) in the UK (Grundy
et al.
, 2003). In the early
1970s, application of this type of herbicide was delayed until after the first two
true leaves had formed (Fig. 2.1) because of herbicide damage to seedlings
younger than this. However, by lowering herbicide dose rate, onion seedling
damage can be prevented and, by repeated spraying, weed seedlings can be
destroyed while they are small and still susceptible to low herbicide doses. As a
result of the adoption of repeat, low-dose programmes the average number of
herbicide applications per onion crop in the UK increased from three to four in
1977 to nine to ten in 1999, but the increase in the total weight of herbicidal
substances applied was only about 19%. Ioxynil is the herbicide most widely
used in these repeat, low-dose applications. Onion resistance to herbicide
damage increases with plant size, and therefore crops grown from sets or
transplants are less susceptible than small seedlings. Herbicide resistance by
onions depends on the extent and integrity of the leaf-surface wax layer (Verity
et al.
, 1981). Conditions or treatments that damage or reduce this wax layer -
e.g. abrasion by blown sand, pest or disease damage, high humidity during leaf
development or strong surfactants in sprays - can increase crop susceptibility
to herbicide damage. The increase in herbicide tolerance with stage of growth
can be dramatic. For example, the dose of methazole (now no longer available)
needed to decrease onion seedling dry weight by 50% on day 14 after spraying
increased 80-fold between the loop stage and the three-leaf stage (see Fig. 2.1;
Verity
et al.
, 1981). The retention per unit leaf dry weight of the dye tartrazine,
