Mechanical management of weeds - Ecological Management of Agricultural Weeds - page 157

Figure 4.4 Projected stable vertical seed distributions for a weed reproducing
according to the population model ofDoyle,Cousens & Moss (1986) and
subjected to yearly moldboard plow or rigid tine tillage.(Redrawn from Cousens
& Moss,1990.)
the surface because the likelihood of exhaustion of nutrient reserves prior to
emergence is less (Vleeshouwers, 1997) and the period of exposure of the
upwardly growing shoot to hazards in the soil is shorter.
Chancellor (1964 b ) determined the depth from which seedlings of 19
species emerged in field conditions,and Mohler (1993) reviewed 21 studies on
seedling emergence from weed seeds placed at various depths. These and
more recent studies (MacDonald, Brecke & Shilling, 1992; Horak & Sweat,
1994; Cussans et al ., 1996; Yenish et al ., 1996; Prostko et al ., 1997; Fausey &
Renner, 1997; Grundy & Mead, 1998) show that most individuals of most
weed species arise from the top 2-4 cm of soil. However, for many species
some individuals emerge from deeper soil layers, and a small percentage of
some large-seeded species emerge from 10 cm or more (Stoller & Wax, 1973;
Horak & Sweat,1994; Cussans et al .,1996).About half of the species examined
showed a monotonic decline in emergence with decreasing depth; for the
remainder, shallow burial increased emergence (Mohler, 1993), probably by
improving water uptake. Clearly, although deep burial of weed seeds gener-
ally prevents seedling emergence, shallow incorporation into the soil affects
various species differently.
In addition to indirectly affecting emergence via seed distribution, tillage
changes soil properties that affect emergence. Cussans et al . (1996) showed
that weed species varied in their emergence response to clod size.More impor-
tantly, emergence through compact (untilled) soil is more difficult than
through loose soil (Figure 4.5) (Morton & Buchele, 1960; Mohler & Galford,
1997; Vleeshouwers, 1997).