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Fig. 5.2. The effect of the intensity and duration of weed competition on yields of
irrigated bulb-onions in Colorado, USA. The duration of competition is expressed
as thermal time units (TTU) above a base temperature of 7.2°C from sowing, and
weed loads were calculated from the population densities of the weeds present
weighted by a competitiveness index for each species (see text). Yields were
expressed relative to the maximum (weed-free) yield in each field (from Dunan et
al ., 1996. Courtesy of Weed Research ).
describe relative growth rate and its response to temperature, and the
partitioning of photosynthate into leaf area) into a prediction of how a
competitive mixture of species will behave. These models can be used to simulate
the effects of weeds or weed removal treatments on yields (see Fig. 5.3).
Models can be combined with economic information on the value of yield
likely to be lost to competition from the weed species emerging in a crop and the
cost of herbicide or hand-weeding treatments, thereby providing growers with
a decision aid for weed control (Dunan et al. , 1999). By using such a decision
aid, growers can maximize their economic returns from weed control treat-
ments and apply herbicides only when there is economic benefit, thus avoiding
unnecessary environmental pollution.
Providing models are shown adequately to simulate reality, they can then be
used to predict outcomes form a wide range of competitive scenarios much more
economically than could be done by field experimentation. The predictions from
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