Mechanical management of weeds - Ecological Management of Agricultural Weeds - page 163

near-surface seed survival was good. Seed dormancy extended the range of
seed survival and emergence ability conditions over which no-till produced
more seedlings than till.In later years,tilled regimes generally had more seed-
lings than no-till regardless of dormancy.
Systematic examination of 15 field studies in which seed return to the soil
was prevented, tillage occurred once per year, and some indication was given
as to how the seed bank was vertically distributed at the beginning of the
experiment (Bibbey, 1935; Chancellor, 1964 a ; Wilson & Cussans, 1972, 1975;
Wilson,1978,1981,1985; Lueschen & Andersen,1980; Froud-Williams,1983;
Froud-Williams, Chancellor & Drennan, 1984; Schweizer & Zimdahl, 1984;
Moss, 1985 b , 1987; Buhler & Daniel, 1988; Egley & Williams, 1990) showed
that empirical results generally paralleled model predictions (Mohler, 1993).
For example, Wilson (1981) found that the density of Avena fatua emerging
from a surface sowing of seeds decreased in the order no-till, tine tillage, plow
tillage during the initial year of treatment, but that the following year plow
tillage had a greater density than no-till or tine tillage.
This analysis leads to several suggestions for management of seed banks
with tillage.
• The high density of weeds often observed during the first year of reduced tillage is
frequently a transitory phenomena .Provided seed shed can be prevented that
year,a great reduction in weed density is likely in subsequent years.
• Change from a plow tillage regime to a reduced tillage regime is likely to be more
successful if it is preceded by at least one year of good weed control .Good weed
control before transition to reduced tillage will help insure that the
surface seed bank is relatively depleted by emergence rather than
enriched by seed shed.
• If many long-viable seeds have been mixed into the soil by past tillage, the best
strategy may be frequent, consistently shallow tillage to deplete the surface seed
bank .Ifthis approach is taken,prevention ofseed shed is important.
• Finally, if weed control fails and many seeds are shed onto an otherwise relatively
clean soil, the best strategy may be to plow as deeply as possible, and then use
shallow tillage in subsequent years to prevent returning the seeds to the surface .
This will be a particularly valuable tactic for managing species with short
to moderate seed longevity.
Thus, a flexible tillage strategy that takes into account the seed longevity
and probable distribution of critical weed species in the soil is likely to facili-
tate other means of weed management.Although analysis of seed distribution
is beyond the capacity of most growers, an understanding of the way in which
tillage implements move seeds, coupled with a history of seed shed in a field,