Agriculture Reference
In-Depth Information
The choice of appropriate herbicides and application rates depends on the
following factors:
1. Weed species present. Tables indicating which weeds are destroyed by
which herbicide, and what dose rates are required can be found in advisory
literature on herbicides and allium production, much of which is available
online (e.g. Beck et al. , 2004; University of California, 2007); the principles
involved are also discussed in Grundy et al. (2003). Both residual and post-
emergence contact herbicides are frequently applied in combinations in order
to control the whole spectrum of weeds present. Ioxynil is applied along with
cyanazine to control Polygonums; fluroxypyr to control volunteer potatoes and
cleavers; bentazone to control mayweeds and clopyralid to control thistles and
other Compositae (Grundy et al. , 2003). Grass weeds - including volunteer
cereals and rows of barley deliberately planted to provide shelter and protection
from wind erosion damage to onion seedlings - are controlled by the
graminicides fluazifop-P-butyl or propaquizafop.
2. Soil type. The type and dose of residual herbicides appropriate vary with
the texture and organic matter content of soil (see 'Pre-emergence residual
herbicides', above). Concern about the contamination of groundwater with
herbicide residues has been a major driver of increased restriction and regu-
lation of their use. To avoid leaching the more soluble herbicides into the
groundwater, their dose rates may need lowering on light, sandy soils.
3. Crop growth stage. This determines the extent of herbicide tolerance (see
above) and, therefore, the extent of selectivity between crop and weeds.
4. Weather conditions. Weather can affect herbicide tolerance and persistence.
For example, the tolerance of onion to ioxynil is less in conditions of bright sun
and high humidity. Residual herbicides degrade faster as soil temperature and
moisture content increase, provided aeration is adequate.
5. Herbicides permitted by the regulatory authorities. The herbicide types
approved for use on allium crops vary between countries (e.g. see references
under point 1 above), although the countries of the European Community (EC)
are harmonizing their herbicide regulation and approval systems. It is now very
expensive to test a herbicide for use on a crop so as to fulfil the stringent
regulatory demands of developed countries. Such testing is uneconomic for
agrochemical producers for all except major arable crops. The value of the
herbicide market for crops grown on a smaller area, like most vegetable crops, is
too small for manufacturers to recover their costs of regulatory testing. This is
causing problems for allium producers because few new herbicides are being
tested and approved. Moreover, in Europe and the USA, concerns about the
environmental damage from certain herbicides, notably groundwater pollution,
have led to a tougher regulation of their use. Some of the older compounds have
been required to fulfil newer, more rigorous criteria than applied when they
were originally brought on to market, and approval for their use has been
withdrawn, because either they cannot meet the new criteria or manufacturers
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