Agriculture Reference
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Groeneveld (1992), the maximum period of infestation tolerated by the crop
was also greatly shortened by increased moisture stress. Cultivar comparisons
in rice and bean noted that shorter-statured varieties suffered greater yield
losses earlier in the season as the period of weed infestation increased (Barreto,
1970; Kwon, Smith & Talbert, 1991). Taken together these studies indicate
that factors that increase the competitive ability of weeds relative to a crop
tend to decrease the period of infestation that the crop can tolerate. However,
Weaver (1984) found that the maximum tolerated period of infestation was
shorter in narrow-row/high-density cabbage and cucumbers, probably
because competition occurred earlier at higher crop densities.
As just discussed, functions relating yield to initial weed-free period and
initial period of weed infestation depend on properties of the weeds (e.g.,
species, density), the crop (e.g., density, row spacing, cultivar), and the envi-
ronment (e.g., fertility, soil moisture, temperature). Consequently, actually
using information on the maximum tolerated infestation period and the
minimum weed-free period to determine management strategies is a complex
problem. Exploration of crop and weed phenology using mechanistic compe-
tition models offers hope for realistically addressing the complexity of inter-
acting factors (Weaver, Kropff & Groeneveld, 1992; Kropff, Lotz & Weaver,
1993).In another approach, Dunan et al .(1995) modified the weed-free period
and weed infestation period concepts to include the cost of alternative weed
control options. They then used these relationships in a model that explicitly
included weed density and competitive differences among weed species. In
addition, they controlled a major source of environmental variation affecting
the yield versus period of weed infestation relation by basing the timing of
weed removal on cumulative heat units rather than days. Their simulations
indicated that pre-emergence application of DCPA in onions was rarely cost
effective. Such models provide the principal practical application for the
weed-free period and weed infestation period concepts.
The time between the maximum tolerated period of infestation and the
minimum initial weed-free period has been referred to as the “critical period
for weed competition” (Figure 6.8) (Weaver, 1984; Dawson, 1986;
Baziramakenga & Leroux, 1994; Ghosheh, Holshouser & Chandler, 1996;
Singh et al ., 1996; Burnside et al ., 1998). It is the period during which weeds
must be controlled to prevent yield reduction. A single weeding or nonresid-
ual post-emergence herbicide application will not constitute sufficient weed
management in systems that manifest a critical period. In situations where
the tolerated period of infestation ends after the minimum initial weed-free
period,the critical period is undefined,and a single weeding may be sufficient
to avoid yield loss (Weaver, 1984; Van Acker, Swanton & Weise, 1993).
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