Enhancing the competitive ability of crops - Ecological Management of Agricultural Weeds - page 293

tial among cultivars. For example, Fay & Duke (1977) screened 3000 oat acces-
sions for presence of scopoletin, a compound known to have allelopathic
effects. All accessions appeared likely to contain some scopoletin, but four
studied in detail contained three- to four-fold more scopoletin than a stan-
dard commercial cultivar. Seedlings of Brassica kaber ( Sinapis arvensis ) grown
with one of these varieties were stunted, twisted, and chlorotic. Putnam &
Duke (1974) screened 540 cucumber accessions for allelopathic affects on
Brassica hirta and Panicum miliaceum . One of these reduced P. miliaceum growth
by 87% in a controlled environment chamber. It also had a substantial but
short-lived effect on emergence and growth of several weed species in field
trials (Lockerman & Putnam,1979).Other studies have also found variation in
allelopathic potential among accessions of crop species (sunflower - Leather,
1983; soybean - Rose et al ., 1984; coffee - Waller et al ., 1986; sweet potato -
Harrison & Peterson, 1986; rice - Dilday et al ., 1991; Olofsdotter et al ., 1999).
Nicol et al .(1992) found eight-fold variation in the concentration of the allelo-
pathic compound DIMBOA among 47 cultivars of wheat.Wu et al . (1999) con-
cluded that although the potential for development of allelopathic crops is
substantial, more research on the genetic control of allelopathy is needed
before breeding programs can be initiated. Crop allelopathy is most likely to
provide effective weed management if combined with other competitive char-
acteristics (Olofsdotter et al ., 1999).
Highly competitive varieties often have multiple characteristics that con-
tribute to competitive ability. This is illustrated by a study on potato (Yip,
Sweet & Sieczka, 1974). The investigators compared four cultivars, including
'Katahdin', which was at the time the leading cultivar in the northeastern
USA. Weed control differed substantially between cultivars. In particular
'Green Mountain' was highly competitive against a range of weed types
(Figure 6.6). As a result, the yield of 'Green Mountain' with one cultivation
plus hilling was as good as with an effective herbicide plus hilling (Figure 6.7).
The greater competitiveness of 'Green Mountain' could be attributed to (i) its
more rapid emergence and canopy closure, (ii) its extensive branching and
dense foliage, and (iii) its maintenance of a closed canopy for a greater portion
of the season (Table 6.4). Root competition was unimportant (Sweet, Yip &
Sieczka, 1974). The characteristics just listed allowed 'Green Mountain' to
suppress weed growth early in the season, compete effectively with the weeds
that did establish,and maintain that competitive pressure through the critical
part of the growing season.Together they allowed 'Green Mountain' to main-
tain yield under a weed pressure that substantially reduced yields in the less
competitive cultivars (Figure 6.7). Although, 'Katahdin' produced a higher
yield in weed-free conditions (Figure 6.7), the characteristics of 'Green