Weed management: a need for ecological approaches - Ecological Management of Agricultural Weeds - page 27

cal controls; a common method is the combination of cultivation between
crop rows with herbicide application in narrow bands over crop rows (Buhler,
Gunsolus & Ralston, 1992; Eadie et al ., 1992; Mt. Pleasant, Burt & Frisch,
1994). By reducing reliance on more toxic materials, the input substitution
approach can reduce environmental and health hazards.It can also,in the case
of herbicide banding, decrease production costs and increase returns (Mulder
& Doll, 1993). To be used most effectively and reliably, the input substitution
approach requires monitoring weed populations and their responses to man-
agement tactics. Like the improved efficiency approach, it is responsive rather
than proactive, and tends to maintain a farmer's dependence on externally
derived curative solutions and inputs (MacRae et al ., 1990).

Agroecosystem redesign
The agroecosystem redesign approach is characteristic of ecological
weed management and involves a shift from linear, one-to-one relationships
between target weeds and a particular weed management tactic, to webs of
relationships between weeds, multiple weed management tactics, and other
farming practices (Liebman & Gallandt,1997).Emphasis is placed on prevent-
ing weed problems and reducing requirements for purchased inputs through
better use of ecological factors that stress and kill weeds. Emphasis is also
placed on integrating weed management activities with other farming prac-
tices that maintain soil productivity and crop health,minimize the impacts of
other pests and unfavorable weather, and reduce financial risks (Swanton &
Murphy, 1996).
Exner, Thompson & Thompson (1996) have described an Iowa crop and
livestock farm that uses an agroecosystem approach to manage weeds success-
fully. Most of the 125-ha farm is in a five-year rotation sequence
(maize-soybean-maize-oat

clover-and-forage-grasses-hay) that challenges
weeds with varying patterns of soil disturbance and resource competition. A
rye cover crop, planted between the maize and soybean phases of the rotation,
is used to allelopathically suppress weed emergence and growth. The ridge
tillage system used for maize and soybean production kills weeds within the
crop row at planting, but also minimizes soil disturbance and stimulation of
weed germination before the crops are sown. A rotary hoe and an inter-row
cultivator designed for high-residue conditions are also used to control weeds
mechanically. Crops are planted at higher than conventional rates to increase
crop competitive ability.
The owner-operators of this farm are willing to apply post-emergence her-
bicides if other tactics fail to provide sufficient weed control, but generally
have not needed to do so. They monitor weeds in their fields, tinker with and