Cereals processing technology

22 Cereals processing technology
2.5 Weed control
The presence of weeds will reduce the yield of all cereal crops due to their
ability to compete with the crop for water and nutrients from the soil and
intercept radiation that would otherwise be utilised by the crop canopy. The
combined effects of weed competition on crop yield is greater during the early
stages of growth; therefore adequate weed control measures are essential during
crop establishment. Some weed species can also have an indirect effect on grain
yield by increasing the likelihood of lodging and encouraging the development
of pests and diseases. The presence of weeds during the later stages of grain
growth is likely to have a detrimental effect on grain quality and often interfere
with the harvesting operations.
Intensive cereal growing systems rely heavily on the use of chemical weed
control programmes based on the use of one or more products applied either preor
post-emergence of the cereal crop. At the other end of the spectrum organic
cereal production systems are based on non-chemical control measures. Here the
options are limited to the use of mechanical weeders and the manipulation of crop
growth to allow it to become dominant and to out-compete the weeds. In future it
is likely that attention will be given to a more integrated approach to weed control
with strategic use of herbicides in combination with cultivation practices. The
preparation of a stale seed bed through shallow cultivations to encourage
germination of weeds which are then destroyed before the crop is sown is one such
technique which may be reintroduced in an attempt to reduce the cost of chemical
weed control. Further attention will also be directed to considering the weed
spectrum across the whole rotation rather than in a piecemeal, crop by crop basis.
Herbicides represent between 35% and 40% of crop protection costs for
winter sown cereals in the UK. Expenditure on weed control will often vary
from field to field depending on the weed spectrum present. For example, the
cost of controlling grass weeds will be a good deal more expensive than the cost
of controlling broad-leaved weeds such as poppy or shepherd’s purse. Selection
of the appropriate herbicide programme is dependent in the first instance on
being able to identify the weed flora present, recognising the dominant weed
species within this spectrum and having an appreciation of the likely size of the
population. A number of attempts have been made to identify weed thresholds,
below which treatment may be deemed unnecessary, but these have been
difficult to apply in practice due to the different competitive characteristics of
different weed species and their effects on crop growth.
An effective weed control strategy is also dependent on understanding the
mode of action of the active ingredient, its interaction with other chemicals with
which it may be mixed and its effect on the target weed species. In general
herbicides are more effective against weeds during their early stages of growth.
Rates of application are determined by the manufacturers, based on many
seasons of field and glasshouse evaluation trials, although in an attempt to
implement more cost effective weed control strategies growers often adopt
reduced dose rates according to prevailing crop and weather conditions.