Agriculture Reference
In-Depth Information
Fig. 5.3.
The effects of the time for which weed competition is allowed to persist
on the yield of irrigated bulb onions in Colorado, USA. The points and solid lines
show observed yields and 95% confidence bands, respectively, and the broken line
shows the predictions from the simulation model outlined in the text. The time of
weed removal is expressed as thermal time (HU - heat units = °C-days from sowing
with 7.2°C base temperature) (from Dunan
et al
., 1999. Courtesy of the
Journal of
the American Society for Horticultural Science
).
these mechanistic competition models can be quite complex and may themselves
need further simple descriptive models of the responses found to summarize their
predictions (Baumann
et al.
, 2002). The latter authors used their parameters for
growth and light interception of leeks, celery and groundsel, grown separately, to
model the consequences of growing leeks and celery as an intercrop on the yields
of each crop species and on seed production by the weed. Thereby they were able
to specify the optimum densities at which to interplant leeks with celery so that
the stronger competitive ability of the latter could supplement the rather weak
competitiveness of leeks and ensure good suppression of the weed while main-
taining leek quality (i.e. ensuring leeks were of marketable size) and maximizing
financial return per unit area of field. Weed suppression was expressed both in
terms of the reduction in biomass of the weed and also the consequent reduction
in the number of seeds shed by the weed. The fewer seeds a weed sheds the less
seed it leaves in the soil weed seed-bank to infest future crops, a consideration of
particular importance when managing weeds over the whole rotational cycle of
crops, particularly in organic production (see below). These complex interactions
are summarized graphically in Fig. 5.4.
Weed control
Herbicidal weed control
The availability of herbicides for allium vegetable crops starting in the 1940s and,
becoming fully established from the 1960s onwards, enabled their production, in
