Weed life history: identifying vulnerabilities - Ecological Management of Agricultural Weeds - page 60

weed against local extinction, but from the grower's point of view, it makes
weeds with seed banks highly resistant to eradication. However, complete
eradication is rarely necessary, and knowledge of seed longevity of a species
allows some predictability regarding how long perfect control of the weed is
required to reduce weed pressure by a given amount (Donald & Zimdahl,
1987). Similarly, rotation into a sod crop allows several years for mortality to
reduce the seed bank (Thurston, 1966; Warnes & Andersen, 1984). The effect
of rotation on seed banks is discussed further in Chapter 7.
Seed longevity of a species also has a large effect on its response to different
tillage regimes, and, as explained in Chapter 4, may be an important factor
contributing to the shift from broadleaf species to grasses with reduced
tillage. Finally, accumulation of high densities of seeds in the soil allows dis-
persal in soil clinging to animals, vehicles, and tillage machinery, and this is
probably an important route for dispersal of weeds between fields (see section
“Dispersal of seeds and ramets”below).
Hazards of establishment
The period of establishment may be defined as the time between ger-
mination 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 her-
bicides. 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 prob-
ably 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 litera-
ture review and summary table in Mohler, 1993). In contrast to the many