Hazards of establishment

The period of establishment may be defined as the time between germination and the production of the first true leaf. This is the most poorly studied stage in the weed life cycle, except with respect to its sensitivity to herbicides. The few quantitative data available indicate that this phase of the life cycle represents a major bottleneck for some species (Boutin & Harper, 1991). Several mortality factors act on establishing weeds, including exhaustion of seed reserves, drought, seedling predation, disease, physical disturbance, and expression of morphological and genetic defects. Data on the effects of all these phenomena are scarce.

One of the most important factors is exhaustion of seed reserves during emergence. The probability of emergence for a newly germinated seedling is a function of depth of burial, the energy content of the seed, and the resistance of the soil. Although the soil in most tilled seedbeds is probably sufficiently loose to not greatly impede emergence, penetration of the shoot through compact soil requires more energy (Morton & Buchele, 1960), and this probably prevents some weed emergence in no-till systems (Mohler & Galford, 1997).

The seeds of most agricultural weeds weigh less than 2 mg and few exceed 10 mg (Table 2.6) (Stevens, 1932; Thompson, Band & Hodgson, 1993). Consequently, successful emergence requires that weed seeds germinate within a few centimeters of the soil surface (Chancellor, 1964; see also literature review and summary table in Mohler, 1993). In contrast to the many

Table 2.6. Propagule weight of annual weeds and crops

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