Enhancing the competitive ability of crops - Ecological Management of Agricultural Weeds - page 273

Table 6.1.( cont .)

Yield response
to crop density

Weed response Weed- Weeds

Crop to crop density a,b free present c Reference

0 Teich et al .(1993)

Christensen,Rasmussen & Olesen (1994)

V( ) 0 Justice et al .(1994)

0 Doll,Holm & Søgaard (1995)

Lemerle et al .(1996)

V(0) Anderson (1997)

0 Tanji,Zimdahl & Westra (1997)

Hashem,Radosevich & Roush (1998)

V( ) Blackshaw et al .(1999)

Notes:
a Studies were included only ifweed response to crop density was measured,or yield response to
crop density was measured with and without weeds.For studies in which all treatment series
included weed control measures,the density series with the least effective weed control was
used for evaluation ofthe weed response to crop density and the yield response to density in
the presence ofweeds.

b

,the measure ofweed abundance investigated in the study (usually biomass) decreased as
crop density increased; V( ),weed response to crop density varied among treatments,years,
cultivars,sites,or experiments,but the tendency was to decrease as crop density increased; 0,
no systematic change in weed measure as crop density increased.

c

,yield increased as crop density increased; ,yield increased as crop density increased,and
percentage increase was greater than in the corresponding treatment series without weeds; ,
yield decreased as crop density increased; 0,no systematic change in yield as crop density
increased; I,yield was maximum at intermediate crop density; V,yield response varied among
treatments,years,cultivars,sites,or experiments.Vwith , ,0 or I shown in parentheses
indicates that the response varied,but that an overall tendency was reasonably unambiguous.
Ablank indicates that no information was given on crop response to density in that condition.
d Yield ofonions increased with density but percentage ofonions 7.6 cm diameter decreased
with density.
e Nutrient uptake by the crop rather than yield.
Source : Expanded from Mohler (1996).

(Figure 6.2a). Setting the density of the weed at various fixed values generates
a series of curves for the response of weed biomass to crop density (Figure
6.2b). In each case, the yield of the crop is complementary to the yield of the
weed and the biomass of the weed declines with increasing crop density.
However, note that at any given crop density, the slope of the weed biomass
curve is greater when density of the weed is high. Thus, the suppression of weeds
and increase in crop yield from an incremental increase in crop density increases with the
density of weeds .
The applicability of this conclusion to real crop-weed systems has been
confirmed by factorial experiments (Håkansson, 1983; Cousens, 1985;