Crop diversification for weed management - Ecological Management of Agricultural Weeds - page 347

Sheaffer & Barnes, 1997; De Haan et al ., 1997). Living mulches are used to
provide cover before, during, and after main crop production, and are often
well established at the time main crops are sown; in contrast, smother crops
are generally planted simultaneously with or after main crops, and are small
or absent during early stages of main crop growth.The key management issue
that must be addressed for both living mulches and smother crops is how they
can suppress weed recruitment and growth without functioning themselves
as weeds that compete against main crops.
When a main crop is planted into an established living mulch,competition
against the main crop frequently occurs.Kurtz,Melsted & Bray (1952) planted
maize into sods of several grass and legume species (smooth bromegrass,
timothy, lespedeza, alfalfa, birdsfoot trefoil, ladino clover, or red clover) and
found that all of the species reduced or eliminated maize grain yield, at least
in some years. Application of N fertilizer and irrigation water only partially
mitigated the problem. Other investigators working with maize and sweet
corn production systems have used low doses of herbicides,repeated mowing,
and partial incorporation into soil to manage, but not kill, grass and legume
living mulches (Echtenkamp & Moomaw, 1989; Grubinger & Minotti, 1990;
Mohler, 1991; Eberlein, Sheaffer & Oliveira, 1992; Fischer & Burrill, 1993).
The results of such approaches have been inconsistent, however, and it
appears that a considerable amount of fine-tuning will be required before
most living mulch systems can be regulated predictably.
One living mulch system that may serve as a model for the successful devel-
opment of others involves the use of subterranean clover in mixture with
maize and other warm-season annual crops. Much of the success of this
system derives from a fortuitous match between local climatic conditions and
subterranean clover's ecophysiology, and from differences in the timing of
growth between the living mulch and the main crops that prevent competi-
tive reduction of main crop growth and yield. In New Jersey, Ilnicki & Enache
(1992) observed that subterranean clover germinated during late summer,
grew vegetatively until early winter, lay dormant during winter, resumed
growth the following spring, and died in early summer after setting seed. For
most of the summer, while maize and other summer annual crops made most
of their growth, a dense mat of dead subterranean clover lay on the soil
surface; late in the summer the regeneration and growth cycle began again as
high densities of clover seedlings emerged from seeds that had been produced
in situ.
Enache & Ilnicki (1990) found that maize planted without tillage and
without herbicides into established subterranean clover produced as much or
more biomass and grain as sole-cropped maize grown with herbicides, either