Survival after emergence

Rates of natural mortality due to disease, herbivory, and drought are usually low for established weeds in annual crops. In the absence of post-emergence weed control, survival rates for annual weeds from the cotyledon stage to maturity usually lie between 25% and 75% (Table 2.8) (Chancellor & Peters, 1972; Naylor, 1972; Sagar & Mortimer, 1976; Weiss, 1981; Mack & Pyke, 1983; Lapointe et al., 1984; Mohler & Callaway, 1992). Sometimes, however, rates of survival to maturity exceed 90% (Young, 1986) or approach 0% (Lindquist et al., 1995) (Table 2.8). Given the high reproductive rates of annual weeds (see the following section), their reported survival rates seem surprisingly high, and probably indicate that most weed mortality occurs prior to emergence or is due to post-emergence control measures. Mortality rate usually decreases with increasing plant size or age (Weiss, 1981; Mohler & Callaway, 1992; Buhler & Owen, 1997).

Table 2.8. Survival ofseedlings emerging in maize in July from establishment to maturity (or for some A. theophrasti, death by hard frost)





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