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Seed production (seeds per plant)

Figure2.6 Distribution of estimated seed production by 231 individuals of Amaranthus retroflexus in no-till sweet corn plots. (C. L. Mohler & M. B. Callaway, unpublished data; see Mohler & Callaway, 1995.)

growing with minimal competition, though some produce 10000 to 25000 seeds per plant (Stevens, 1932; Salisbury, 1942), and a few like Salsola iberica and Echinocloa crus-galli may produce over 100000 seeds per plant (Young, 1986; Norris, 1992).Afew annuals (e.g., Veronica hederifolia) produce fewer than 100 seeds per individual (Salisbury, 1942; Boutin & Harper, 1991). Stationary perennial weeds show a similar range in seed production to annuals (Stevens, 1932; Salisbury, 1942), except that monocarpic perennials (biennials) tend to produce more seeds (Stevens, 1932), probably because the observed reproductive output is based on resources captured over more than one season of growth. Comparable data for wandering perennials are lacking, but given that they allot resources to vegetative spread, their seed production probably tends to be less on a per ramet basis. Some wandering perennials (e.g., biotypes of Cynodon dactylon) produce no viable seeds at all (Horowitz, 1972; Kigel & Koller, 1985).

Although most weed species potentially produce very many seeds per plant, the actual productivity in a crop is usually much less. C. L. Mohler & M. B. Callaway (unpublished data) found that Amaranthus retroflexus produced up to 253 000 seeds per plant, but that individuals emerging in unplanted plots in July as effects of an atrazine application dissipated averaged only 770 seeds per plant, probably due to a short growth period. Moreover, when growing with sweet corn, A. retroflexus averaged only 28 seeds per plant. Thus, cultural practices and competition from the crop act as important regulators of weed seed production (Zanin & Sattin, 1988; Mohler & Callaway, 1995; Blackshaw & Harker, 1997).

Several models have shown that including the effects of seed production on future crops lowers economic weed density thresholds by a factor of 3 to 8 relative to the effect of competition on the current crop alone (Cousens et al., 1986; Doyle, Cousens & Moss, 1986; Bauer & Mortensen, 1992). Some authors have argued that the damage inflicted on future crops by seed production is so great that certain weeds should not be allowed to reproduce at all (Abutilon theophrasti - Zanin & Sattin, 1988; Echinochloa crus-galli - Norris, 1992). Although extreme efforts to prevent spread of new, localized populations are often justified, the economic utility of a zero tolerance policy for long-established populations remains to be demonstrated.

In any case, measures should be taken to reduce seed production. Depending on the phenology of the weed relative to the crop, a substantial proportion of potential seed production can sometimes be prevented by prompt post-harvest weed control measures (Young, 1986; Kegode, Forcella & Durgan, 1999). This is particularly true for cereals and early season vegetables where harvest of the crop releases the weeds from competition at a time in the season when temperature and day length allow rapid growth and maturation of previously suppressed weeds. For example, Webster, Cardina & Loux (1998) found that killing weeds in July or August following wheat harvest controlled 70% to 95% of various weed species in maize the following spring relative to control plots in which weeds were allowed to mature.

In some grain crops, a large portion of the weed seed produced passes through the combine. For example, Ballare et al. (1987a) found that <2% of Datura ferox in soybean were shed prior to harvest, and that all three of the combines tested took up nearly all capsules. In such cases, if equipment were added to the combine to capture or destroy weed seeds rather than dispersing them with the chaff, substantial reductions in the annual addition of viable seeds to the seed bank could be achieved. Slagell Gossen et al. (1998) proposed attaching hammer mills or roller mills to grain combines to destroy weed seeds before they were returned to the field. They found that both types of mill killed a high percentage of Bromus secalinus seeds. In many crop-weed systems, however, the benefit of capturing or killing weed seeds in the combine would be small because most of the seeds disperse prior to harvest (Moss, 1983). Although seed collection and post-harvest weed control usually will not provide effective control by themselves, they can contribute substantially to integrated management of weed populations, especially if crop rotation provides some years in which the crop is removed early in the maturation period of the weed.

The capacity of wandering perennial species to produce vegetative propa-gules is also large. For example, single tubers of Cyperus esculentus planted in California and Zimbabwe grew into clones that in one year produced 6900 and 17700 tubers, respectively (Tumbleson & Kommedahl, 1961; Lapham, 1985). Unlike seed production, which is necessarily preceded by a period of vegetative growth, vegetative reproduction in wandering perennials often begins early in life. Production of new tubers in Cyperus esculentus may begin as early as 3 weeks after tubers sprout (Bell et al., 1962). Adventitious buds form on the roots of Euphorbia esula and Cirsium arvense within 1-2 and 6-8 weeks of seedling emergence, respectively (Selleck, 1958; Bakker, 1960). Consequently, the number of potential individuals produced is roughly proportional to the size of the plant, and tends to increase exponentially when interference is absent (Lapham, 1985).As a result, vigorous competition from a crop is important for reducing vegetative reproduction of wandering perennials (Hákansson, 1968; Hákansson & Wallgren, 1972). Strategies for mechanical control of wandering perennials are discussed extensively in Chapter 4.

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