Mechanical management of weeds - Ecological Management of Agricultural Weeds - page 164

should provide sufficient qualitative information for choosing the most
appropriate tillage method for the situation.
Many authors have noted an increase in density of annual grass weeds with
reduced tillage (Pollard & Cussans, 1981; Froud-Williams, Drennan &
Chancellor,1983; Wrucke & Arnold,1985; Cardina,Regnier & Harrison,1991;
Teasdale, Beste & Potts, 1991; Swanton et al ., 1999). For example, S. R. Moss
(personal communication) summarized 13 comparisons from several studies
on Alopecurus myosuroides (Pollard et al ., 1982; Wilson, Moss & Wright, 1989;
Clarke & Moss,1991) and found that plowing reduced density by an average of
63% (range 17% to 98%) relative to shallow tine tillage. This shift toward
grasses with reduced tillage has been attributed to less effective grass herbi-
cides (Wrucke & Arnold, 1985), emergence response to burial depth of seeds
(Buhler, 1995), or to peculiarities of the cropping system or the particular
weed species present (Swanton, Clements & Derksen, 1993).
Although these phenomena may play a role, a major factor is the shorter
seed longevity of most grass weeds relative to many annual broadleafs (see
Chapter 2).Species with relatively short-lived seeds require regular seed input
to maintain high densities.When shed seeds are plowed under each year,most
perish before they return to the surface by subsequent tillage. For a single
cohort, the model of Mohler (1993) predicts higher density with plow tillage
in the second and subsequent years because burial enforces dormancy and
protects some seeds from near-surface causes of mortality. However, for
species with short-lived seeds, the number of emergents from these older
cohorts is low regardless of tillage regime. For species with short-lived seeds,
the most recently shed cohort of seeds largely determines weed density, and
this cohort will produce fewer emerged individuals if it is plowed under. In
contrast, for many broadleaf species with long-lived seed banks, the most
recently shed cohort may be a small fraction of the total seeds in the soil, in
which case plowing may increase weed density.
Population dynamics of annual weeds in ridge tillage
The effect of ridge tillage on weeds is distinctive from other types of
tillage in that seeds do not move randomly relative to the crop row. In ridge
tillage (sometimes referred to as till-plant) a crop is planted on ridges formed
the previous growing season (Figure 4.8). During planting, the surface of the
ridge is scraped into the inter-row valleys. Seeds that were shed onto the ridge
the previous season are thus moved to the valleys where seedlings can be
destroyed by inter-row cultivation. Finally, ridges are rebuilt for the next
season by hilling up around the crop stems during the final cultivation.
Wicks & Somerhalder (1971) sampled soil for weed seeds at various dis-