Weeds and the soil environment - Ecological Management of Agricultural Weeds - page 234

The phytotoxicity of rye residue and its extracts has been particularly well
studied. Allelopathic effects of this species have been attributed to

-phenyl-

lactic acid and -hydroxybutyric acid (Shilling, Liebl & Worsham, 1985;

Shilling et al ., 1986), and to 4-dihydroxy-1,4(2H)-benzoxazin-3-one (DIBOA),
2(3H)-benzoxazolinone (BOA), and related benzoxazolinone compounds
(Barnes & Putnam, 1986, 1987; Barnes et al ., 1987; Pérez & Ormeño-Núñez,
1993). Soil microbes can transform BOA into compounds that are consider-
ably more phytotoxic than the parent chemical (Chase, Nair & Putnam, 1991;
Gagliardo & Chilton, 1992).
Sorghum residue can suppress a range of plant species, an effect attributed
to sorgoleone (a long-chain hydroquinone), dhurrin (a cyanogenic glucoside),
and several other compounds (Weston, 1996). Allelopathic potential has been
demonstrated for residues or extracts of a range of other crop species, includ-
ing alfalfa (Waller, 1989; Chung & Miller, 1995), barley (Overland, 1966; Liu &
Lovett, 1993 a , 1993 b ), berseem clover (Bradow & Connick, 1990), rapeseed
(Wanniarachchi & Voroney, 1997), crimson clover (White, Worsham & Blum,
1989; Creamer et al ., 1996 b ), hairy vetch (White, Worsham & Blum, 1989), oat
(Fay & Duke, 1977; Putnam & DeFrank, 1983; Putnam, DeFrank & Barnes,
1983), pea (Kimber, 1973; Cochran, Elliot & Papendick, 1977), red clover
(Chang et al ., 1969), sweetclover (McCalla & Duley, 1948; Guenzi & McCalla,
1962),sunflower (Leather,1983),and wheat (Liebl & Worsham,1983; Shilling,
Liebl & Worsham, 1985).
Although many species have been shown to produce allelopathic com-
pounds, demonstrating that allelopathy is responsible for weed suppression
under field conditions is technically more difficult. Residue additions may
change nutrient, temperature, moisture, and light conditions, and the pos-
sible effects of these factors must be distinguished from those of residue-
derived chemicals. Cochran, Elliot & Papendick's (1977) approach using soil
taken from beneath residue and Creamer et al .'s (1996 b ) approach using
leached and unleached residues offer potential methods for studying allelopa-
thy under field conditions.
Small-seeded weed and crop species appear especially susceptible to allelo-
chemicals.Although large-seeded crops often show susceptibility in laboratory
bioassays (Guenzi & McCalla, 1962; Kimber, 1973; Cochran, Elliot &
Papendick, 1977; Chase, Nair & Putnam, 1991), they appear to be relatively
insensitive in the field.Putnam & DeFrank (1983) found,for example,that res-
idues of several grain crops substantially reduced emergence of lettuce,radish,
tomato, and a mixture of small-seeded weed species, whereas they increased
emergence of cucumber, pea, and snap bean. The differential suppression of
smaller-seeded species by allelopathic substances released from residue may be