Enhancing the competitive ability of crops - Ecological Management of Agricultural Weeds - page 292

Ogg & Hou,1999).Correlation between early ground cover and weed suppres-
sion has been demonstrated for barley and wheat varieties with a limited
range of heights (Richards, 1989; Richards & Whytock, 1993; Huel & Hucl,
1996).
Little work has focused on characteristics conferring competitiveness in
forage crops. Monzote, Funes & Díaz (1979) found that the tallest of 17 culti-
vars of guineagrass had the lowest infestation of weeds.Similarly,Black (1960)
found that tall-petioled varieties of subterranean clover outcompeted short-
petioled varieties in diallel competition experiments. Forcella (1987) com-
pared isogenic strains of tall fescue differing in leaf-area expansion rate and
found that faster expanding strains were more competitive. However,
Sugiyama (1998) concluded that the greater competitive ability of tetraploid
cultivars of perennial ryegrass relative to diploid cultivars could not be attrib-
uted to differences in early growth rate.
Few studies have addressed below-ground characteristics that relate to
varietal differences in competitive ability. Satorre & Snaydon (1992) found
that although root competition between Avena fatua and spring barley was
more important than shoot competition, varieties differed more in above-
ground competitive ability. Kawano, Gonzalez & Lucena (1974) found that
rice cultivars with a high responsiveness of yield to nitrogen fertilization were
poor competitors. In their experiment, traits of the 25 cultivars tested were
correlated such that cultivars were either adapted to low intensity (low N, low
density, no weeding) or to high intensity (high N, high density, weed-free)
agronomic conditions,but not to mixed conditions (e.g.,high N,no weeding).
Whether this represents inherent trade-offs between characters, or merely the
history of plant breeding in rice is unclear. Siddiqi et al . (1985) found substan-
tial differences in the potassium uptake efficiency of barley cultivars that
related to biomass accumulation under competition with Avena fatua .'Fergus'
produced a high biomass when grown at high potassium levels in monocul-
ture, even though its shoot potassium concentration was relatively low.
Related to this, potassium utilization efficiency of 'Fergus' was high when
stressed for potassium in monoculture and when competing with Avena fatua .
This apparently enabled it to attain substantially higher biomass in mixtures
under low potassium conditions than could the other cultivars. Experiments
designed to separate above- and below-ground competitive effects in herba-
ceous plants have usually found that below-ground competition was highly
important (Donald, 1958; Aspinall, 1960; Martin & Snaydon, 1982; Satorre &
Snaydon,1992).Consequently,additional work comparing crop varieties with
respect to competition for below ground resources would be useful.
Wu et al . (1999) reviewed studies showing variation in allelopathic poten-