Enhancing the competitive ability of crops - Ecological Management of Agricultural Weeds - page 291

a vining habit that allows them to crawl over weeds and completely fill inter-
row areas relatively quickly. Indeterminate growth may also allow crops to
continue active competition with weeds through more of the season.
In contrast with row crops,height is the most frequent characteristic corre-
lated with competitive ability in cereals (e.g., Appleby, Olson & Colbert, 1976;
Ahmed & Hoque, 1981; Challaiah et al ., 1986; Balyan et al ., 1991; Garrity,
Movillon & Moody, 1992; Lemerle et al ., 1996; Hucl, 1998; Seefeldt, Ogg &
Hou, 1999). This is reasonable since cereals are often no taller than the weeds
with which they compete,and the near vertical orientation of much of the leaf
surface in these crops allows light penetration deep into the crop canopy.
Consequently, even when the taller cultivar does not completely overtop the
competing weeds, at least the more elevated distribution of leaf area causes
greater interception of light by the crop and more shade on the weeds.Greater
light interception by taller cultivars has been demonstrated in rice and wheat
(Jones, Zimmermann & Dall'Acqua, 1979; Gooding, Thompson & Davies,
1993). Unfortunately, short-statured cereal varieties often yield more. Indeed,
increasing harvest index by decreasing stature has been used to breed higher-
yielding cultivars in several cereal crops (Evans, 1980).
Nevertheless, some studies have found cereal cultivars that combine high
yield with competitive ability (Fischer, Ramírez & Lozano, 1997; Hucl, 1998).
High-yielding competitive varieties are possible because several characters
other than height also contribute to competitive ability. Several studies
suggest that high leaf area index or high biomass contribute to competitive
ability in cereals (Kawano, Gonzalez & Lucena, 1974; Challaiah et al ., 1983;
Sechniak, Lyfenko & Pika, 1985; Balyan et al ., 1991; Garrity, Movillon &
Moody, 1992; Fischer et al ., 1995; Huel & Hucl, 1996; Fischer, Ramírez &
Lozano, 1997). These characteristics are frequently correlated with plant
height, but they can also vary substantially within a height class (Sechniak,
Lyfenko & Pika,1985; Fischer,Ramírez & Lozano,1997).Tillering is one route
to high leaf area index and biomass in cereals. Several studies have found a
correlation between tillering and competitiveness (Kreuz, 1982; Challaiah et
al ., 1983, 1986; Valenti & Wicks, 1992), although others have looked for this
and found none (Kawano,Gonzalez & Lucena,1974; Moss,1985; Huel & Hucl,
1996). Challaiah et al . (1983) found that several of their highest-yielding culti-
vars also had high percentage light interception and correspondingly low
weed densities. Lanning et al . (1997) demonstrated variation in light intercep-
tion and suppression of Avena fatua among barley varieties of similar height.
Recent work has shown that, as with row crops, early growth rate and charac-
ters relating to leaf area expansion rate appear to contribute to competitive
ability in rice and wheat (Johnson et al ., 1998; Ogg & Seefeldt, 1999; Seefeldt,