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Figure 4.4 Projected stable vertical seed distributions for a weed reproducing according to the population model of Doyle, 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 (1964b) 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 generally 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 importantly, emergence through compact (untilled) soil is more difficult than through loose soil (Figure 4.5) (Morton & Buchele, 1960; Mohler & Galford, 1997; Vleeshouwers, 1997).

Untilled Tilled

Unfilled Tilled

Untilled Tilled

Figure 4.5 Proportion of Chenopodium album, Amaranthus retroflexus, and Abutilon theophrasti emerging after planting into either tilled or untilled soil with an apparatus that produced minimal soil disturbance. See Mohler & Galford (1997).

Untilled Tilled

Unfilled Tilled

Untilled Tilled

Figure 4.5 Proportion of Chenopodium album, Amaranthus retroflexus, and Abutilon theophrasti emerging after planting into either tilled or untilled soil with an apparatus that produced minimal soil disturbance. See Mohler & Galford (1997).

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