Enhancing the competitive ability of crops - Ecological Management of Agricultural Weeds - page 298

Figure 6.8 Effect ofinitial weed-free period and period ofweed infestation on
yield ofcabbage.(Drawn from data in Miller & Hopen,1991.)
compete effectively for light during the remainder of the crop's growing
season.The length of initial weed-free period required for negligible yield loss
depends on the initial growth rates of the crop and weed and on how the
weed's growth rate and shoot architecture change in shade (see Chapter 2).
Although the rate of uptake of water and nutrients per unit root surface may
not be equal in the crop and weed,the total uptake for each species is necessar-
ily proportional to their respective root surface areas. Providing the crop with
an initial period of development prior to germination or sprouting of the
weed thus allows proportionally more below-ground resources to flow to the
crop (Mann & Barnes, 1947). However, in most cases competition for light is
probably more important in making initial weed-free periods effective. Rates
of water and nutrient uptake per unit of root surface usually do not depend on
the relative size of the competing root systems. In contrast, a unit of leaf area
for the taller competitor is far more effective at capturing sunlight than a unit
of leaf area on the shorter species (Donald, 1963; Liebman, 1989).
Pre-emergence herbicides and most mechanical weeding methods are
aimed at giving the crop a comparatively weed-free period for initial growth.
After the herbicide dissipates, or following lay-by cultivation, weeds may
sprout, but they have a limited effect on a well-established crop. Similarly,
mulches of organic residue frequently lose effectiveness after a few weeks due