Enhancing the competitive ability of crops - Ecological Management of Agricultural Weeds - page 278

crop development. For example, Gruenhagen & Nalewaja (1969) showed that
at a given crop density Polygonum convolvulus decreased flax seed yield more
when the crop was fertilized, apparently because the weed was better able to
use nitrogen for plant growth than was the crop. However, at high weed
density, the yield response to increased crop density was greater under ferti-
lized than under unfertilized conditions.This result,combined with consider-
ations on the effects of weed density discussed above, indicates that conditions
favorable to the weed increase the usefulness of elevated crop density.
The effectiveness of high crop density in suppressing weeds depends on the
biology of the weed as well as that of the crop,but few systematic comparisons
are available. Håkansson (1983) found that although total biomass of all
weeds declined as cereal density increased, the species composition of the
weed community changed.The percentage of climbing species like Bilderdykia
(Polygonum) convolvulus and Galium aparine increased with crop density,
whereas Sonchus asper and Brassica napus , which begin life as rosettes, decreased
in relative biomass. Thus, some species are better than others at resisting the
effects of increased crop density.
The competitive mechanisms involved in the suppression of weeds by
increased crop density are not well explored. Competition for light is inher-
ently asymmetric: taller plants receive a disproportionate share of the light
relative to their leaf areas (Weiner & Thomas, 1986; Weiner, 1990). If the crop
is capable of overtopping the weed, then causing this to occur earlier in the
season through increased planting density will give the crop a competitive
advantage. This was the case for safflower and Setaria viridis discussed above:
safflower was taller,and the higher-density plantings achieved a given level of
light extinction earlier in the growing season (Blackshaw, 1993).
However, even when the crop is shorter in stature than the weeds, at high
density the crop occupies a greater portion of the land area at the time the two
species grow into competitive contact. This insures that at the time competi-
tion begins, the resource acquisition rate of the crop is an increasing function
of planting density, and this may lead to a greater biomass of the mature crop
at higher densities. Greater crop height due to intraspecific competition, and
greater total root density and leaf area index may also improve the crop's rela-
tive performance at higher density. Experiments in which crop density is
manipulated by transplanting before and after the onset of competition could
falsify the hypothesis that early preemption of space governs the observed
effects of crop density. Experiments in which above- and below-ground com-
petition between the crop and weed is regulated by barriers over a range of
densities could indicate the relative importance of above- and below-ground
resource capture.