Biotic Factors - Agroecology: The Ecology of Sustainable Food Systems - page 158

foliage and quackgrass rhizomes, although foli-
age residue was twice as toxic as rhizomatous
material. Water extracts and incorporated resi-
dues were both phytotoxic.

subsequent crop. The compounds inhibit weeds during the
time they are actively produced by the cover crop plants, but
after the plants die or are killed through tillage, the com-
pounds quickly degrade (Mamolos and Kalburtji, 2001).
The allelopathic potential of winter rye ( Secale cere-
ale ) has been particularly well studied (Barnes et al.,
1986). Rye produces considerable biomass early in the
growing season, and has found much success as a green
manure crop in poor soils. But it is most notable for its
ability to suppress weed growth while it is actively grow-
ing, as well as after rye residues are incorporated into the
soil with tillage or left on the soil surface after cutting.
Allelopathic effects are even seen from residues left on
the soil after herbicide spraying has killed the cover.
Extensive chemical analysis has identified two benzox-
azolinones and associated breakdown products as the
probable phytotoxic agents.
The cover crop called velvet bean ( Mucuna puriens ),
used extensively in rural Tabasco, Mexico, has been shown
to inhibit weeds through allelopathy. This annual vining
legume is planted into a corn crop near the end of the
cropping cycle. It covers the open space between the corn
plants, effectively suppressing weed growth, both before
and after harvest. The weed suppression is due, in part, to
shading, but release of allelopathic compounds is also at
work. After the velvet bean plants complete their life
cycle, they are left dead on the ground, covering the soil
with nitrogen-rich mulch into which the next corn crop
will be planted. Large areas are managed in this manner
without the use of fertilizers or herbicides (Gliessman and
Garcia, 1982).
As more information is generated on the mecha-
nisms of phytotoxin release in cover crops, farmers will
be better able to optimize the use of cover crops for
weed control by maximizing the addition of the chemi-
cals into the soil and improving the timing of incorpo-
ration. Since cover crop species will vary from region
to region, an understanding is also needed of how local
climates affect the mechanism of release of the toxins
into the environment where they can impact weeds.
Proper species selection and management will vary
accordingly.

• There is some evidence that greater inhibition

is observed in the presence of soil fungi.

• Decaying quackgrass residues were shown to

produce water-soluble inhibitors, explaining the
inhibition that has been observed when quack-
grass residues are a significant part of no-till
systems.

• Inhibition of nodulation in legumes and reduc-

tion of root hair formation in other plants are
suspected as being possible mechanisms of
inhibition.

• Several compounds have been isolated and

identified from water extracts and decomposi-
tion products, and include several phenolic
acids, a glycoside, a compound known as agro-
pyrene, and a flavone tricin and related com-
pounds.

• Even when quackgrass is killed with herbicides,

the plant residues and toxins in the soil must be
allowed to degrade prior to successful establish-
ment of the succeeding crop.

The case of quackgrass demonstrates that allelopathic
interference can be very important, but it also suggests
that different plant parts may play different roles, and that
phytotoxic compounds can enter the environment through
different mechanisms and have varying impacts on crops.

A LLELOPATHIC E FFECTS OF C ROPS

Although much research has focused on the allelopathic
potential of weeds in agroecosystems, many crop plants
have been shown to release phytotoxins as well. Such
mechanisms of interaction have important possibilities for
farmers looking for alternative management practices.

Cover Crops

Cover crops are usually grown during a fallow period in
a crop field in order to protect the soil from erosion,
contribute organic matter to the soil, improve soil condi-
tions for water penetration and retention, and “smother”
weeds. Cover crops of wheat, barley, oats, rye, grain
sorghum, and sudangrass ( Sorghum sudanense ) have
been used effectively to suppress weeds, primarily annual
broadleaf species. The weed suppression ability of many
of these and other cover crops is due, at least in part, to
allelopathy (Overland, 1966).
Because the phytotoxic compounds released by cover
crops typically break down relatively quickly in the
environment, they generally have little effect on the

Organic Mulches Derived from Crops

Plant materials and crop residues can be brought to the
field and spread over the soil, serving as organic mulch.
Waste plant material from farm fields or the processing
of farm products is particularly useful for this purpose.
Such materials were already discussed for their value as
soil amendments (Chapter 8), but an important benefit of
many types of mulch that often get overlooked is their
potential for allelopathic weed control.
Like the phytotoxins produced by covercrops, the bio-
logically active compounds found in organic mulches