Enhancing the competitive ability of crops - Ecological Management of Agricultural Weeds - page 288

Competitive effect can be measured as reduction in weed performance (e.g.,
biomass, cover) in the presence of a given cultivar relative to a weedy check
treatment or to a standard cultivar. Competitive response can be measured as
percentage reduction in yield relative to a weed-free control treatment.
Although both concepts are important attributes of a crop's competitive
ability, neither measures the attribute of greatest interest to a grower, namely
the ability of the variety to produce a high yield despite weed competition.
Thus, for example, Nangju (1978) found that 'Jupiter' soybean at low density
allowed only 58% the weed biomass of 'Bossier' soybean (competitive effect),
and had a yield loss due to weeds of 42% compared with 'Bossier's 53% (com-
petitive response). However, 'Bossier' had a several-fold higher yield, both
with and without the presence of weeds. Although neither competitive effect
nor competitive response provides an adequate guide for choosing cultivars,
they are both useful in identifying crop characteristics that correlate with
competitive ability. Potentially, these characteristics could be bred into high-
yielding lines to create new cultivars with high yield in the presence of weeds
(Callaway & Forcella, 1993). Although competitive effect, competitive
response, and yield under competition are not always correlated, they often
are. In the discussion below, the term 'competitive ability' is used to encom-
pass all three concepts.
Crop varietal differences contributing to competitive ability (of one sort or
another) have been identified in a wide range of crops (Table 6.3). Some
studies have failed to find differences in competitive ability among cultivars
(e.g.,Staniforth,1962; Bridges & Chandler,1988; Glaz,Ulloa & Parrado,1989;
Yelverton & Coble,1991),but these all dealt with crops for which other studies
have shown differences.Thus,differences in competitive potential among cul-
tivars appear to exist for most crop species, although few data are available on
tree crops. Not surprisingly, most work has focused on major crops, especially
wheat and soybean (Table 6.3).
Variation in competitive ability of cultivars between experiments con-
ducted in different years or at different locations is often large (Fiebig,
Shilling & Knauft, 1991; Lemerle, Verbeek & Coombes, 1995; Cousens &
Mokhtari, 1998; Ogg & Seefeldt, 1999). This is a problem commonly encoun-
tered in screening accessions for any desirable characteristic. However, com-
petitive ability may be an inherently more complex characteristic than, say,
monoculture yield or resistance to a fungal disease. Not only can several dif-
ferent types of competitive ability be identified (see above), but also, competi-
tion between plants involves several different resources, and varies with the
identity of the weed species and the phenologies of the weed and crop.
Although these factors can be controlled in experiments, use of results from