Type of tillage affects weed seedling density

As explained in Chapter 2, tillage modifies the soil environment in ways that promote the germination of weed seeds. How this promotion of germination by tillage translates into weed seedling density is less than transparent, however, and the large literature on the effects of tillage on seedling density is highly contradictory. For example, some studies have found more weeds in tilled plots (Roberts & Feast, 1972), whereas others have found more without tillage (Moss, 1985b; Cardina, Regnier & Harrison, 1991; Mohler & Callaway, 1992; Stahl et al., 1999). Moreover, many studies have found the effects of tillage to vary among species (Chancellor 1964a; Pollard & Cussans,

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 populations (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 indicating the distribution of the seed bank prior to initiation of the experiment. A few exceptions include Roberts (1963), Moss (1985b), 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.

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