Mechanical management of weeds - Ecological Management of Agricultural Weeds - page 154

1976; Pollard et al ., 1982; Froud-Williams, Drennan & Chancellor, 1983;
Buhler & Daniel, 1988; Buhler & Oplinger, 1990), or among sites (Wilson &
Cussans, 1972; Froud-Williams, Drennan & Chancellor, 1983; Buhler &
Mester, 1991), or among years of an experiment (Wilson, 1981, 1985; Roman,
Murphy & Swanton, 1999). Several authors have reviewed the extensive and
often confusing literature on the effects of primary tillage on weed popula-
tions (Cussans, 1975, 1976; Froud-Williams, Chancellor & Drennan, 1981;
Froud-Williams, 1987; Mohler, 1993; Buhler, 1995).
Much of the difficulty in understanding the effects of tillage on weed
density can be resolved by recognizing that the vertical distribution of seeds
in the seed bank is a critical factor affecting seed survival, germination, and
emergence (Mohler, 1993). If most seeds are near the surface and plowing
buries them deeply, then tillage will reduce seedling density. In contrast, if
most seeds are deeply buried, then plowing may increase seedling density by
bringing seeds to the surface. Although this is obvious, remarkably few
studies of the effects of tillage on weed populations give any information indi-
cating the distribution of the seed bank prior to initiation of the experiment.
A few exceptions include Roberts (1963), Moss (1985 b ), and Van Esso, Ghersa
& Soriano (1986).
The effect of tillage on the density of weeds in the subsequent crop is a
function of (i) how tillage redistributes seeds in the soil profile,(ii) the capacity
of weed species to emerge from various depths in the soil, and (iii) how depth
in the soil affects survival of weed seeds. Each of these issues is discussed
before considering their combined effect on weed density.
Redistribution of weed seeds by tillage
In the absence of tillage, seeds infiltrate into an agricultural soil via
cracks,the activities of soil fauna,and frost action.This infiltration is slow.For
example, Moss (1985 b ) found 92% of Alopecurus myosuroides seeds in the top 2.5
cm of soil 10 months after sowing. After 34 months, 78% were still in the top
2.5 cm. Other studies show similar results (Weaver & Cavers 1979; Van Esso,
Ghersa & Soriano, 1986).
Tillage implements move seeds vertically to different extents.Most studies
of seed movement have observed the distribution of seeds or beads that were
sown on the soil surface and then tilled in with one or more tillage operations
(Pawlowski & Malicki, 1968; Moss, 1988; Staricka et al ., 1990; Yenish et al .,
1996). Others have compared the distribution of natural seed populations
before and after tillage (Russel & Mehta, 1938; Roberts, 1963; Wicks &
Somerhalder, 1971; Yenish, Doll & Buhler, 1992). Hulburt & Menzel's (1953)