Mechanical management of weeds - Ecological Management of Agricultural Weeds - page 140

largely determine how they respond to redistribution in the soil column by
farm machinery.
A second general principle is that the timing of tillage or cultivation determines
how effective the operation is for weed management . Timing is critical in several
respects. First, obtaining a desired action on the soil and weeds requires
proper timing relative to season and weather. Second, a given weed species
will be more susceptible to a certain type of disturbance at some stages in its
development than at others. Finally, the stage of crop development affects the
degree and type of disturbance that the crop can tolerate.
A third principle is that mechanical weed management is most effective when
multiple operations are performed in a planned sequence . Disturbance can be used to
manage weeds at several points in the crop cycle.Tillage prior to planting can
bury extant vegetation and disrupt roots and rhizomes. Shallow cultivation
prior to emergence and close to young crop plants can kill small weeds before
they establish. Shallow cultivation is largely ineffective, however, unless the
soil has first been prepared by proper tillage.Deeper cultivation between rows
can dig out weeds and throw soil into the crop row to bury young weeds. By
the time the crop is large enough to stand the impact of soil thrown around
the stems, however, many weeds will be too big to bury unless early germinat-
ing weeds are suppressed, for example, by over-the-row cultivation. Finally,
tillage between harvest and the next crop can be used to suppress perennials
and flush seeds from the soil. Thus, tillage and cultivation for weed manage-
ment require conscious planning of the sequence of soil disturbances
throughout the crop cycle.
Tillage: pros and cons
Tillage prior to planting a crop can be used to meet a variety of objec-
tives, including weed control, seedbed preparation, and residue management
(Buckingham & Pauli, 1993, p. 2). From a weed management perspective,
tillage re-initiates ecological succession, allowing dominance by early succes-
sional annual crops rather than the perennial species that naturally come to
dominate undisturbed vegetation.
Tillage has been criticized as a cause of erosion and destroyer of soil tilth.
Indeed, when applied without soil conservation measures or used in inappro-
priate soil and weather conditions, some types of tillage can promote erosion
or loss of soil structure (Dickey et al ., 1984; Andraski, Mueller & Daniel, 1985;
Gebhardt et al ., 1985; Langdale et al ., 1994). When properly used, however,
tillage can enhance water infiltration (Unger & Cassel, 1991), facilitate man-
agement of soil fertility (Randall, 1984), and help warm cold soils (Johnson &
Lowery, 1985; Cox et al ., 1990; Coolman & Hoyt, 1993). It can also increase the