Life span and seed production

The potential postgermination life span of weeds in agricultural systems varies from a few months to decades. In most arable cropping systems, actual life span is rarely more than a few years due to periodic tillage. Some annuals are truly monocarpic: resources in vegetative tissues are remobilized to fill seeds and the plant senesces after seed set (e.g., Chenopodium album, Setariafaberi). However, many annuals shed seeds continuously through much of the growing season and for a substantial proportion of the weed's life span (e.g., Galinsoga ciliata, Digitaria sanguinaUs).

Continuously fruiting annuals tend to dominate the weed flora of fall-sown cereals (Figure 2.5), perhaps because most sprawling species are continuously fruiting and a sprawling habit is well adapted for surviving winter conditions. The early seed production of continuously fruiting annuals like Portulaca oleracea and Stellaria media adapts them well to cropping systems in which disturbance occurs throughout the growing season; consequently, they are common weeds in gardens and vegetable farms. In contrast, the true monocarpic annuals are more sensitive to frequent weeding or cultivation, but because they do not expend resources on reproduction early in life, they are better able to grow tall and compete with large-statured crops. Consequently, monocarpic annuals tend to dominate the weed flora of tall crops such as maize, and crops such as oat that are rarely cultivated (Figure 2.5).

The seed production capacity of weeds varies greatly both within and between species. In most populations, a few individuals produce many seeds, whereas most individuals produce far fewer (Figure 2.6) (Salisbury, 1942; Mack & Pyke, 1983). This variation in seed production is largely the result of

Life Span Annual Weed

Figure2.5 Continuous fruiting annuals as a percentage of the common annual weeds in 14 crops in New York state (data from Bridges, 1992). From top to bottom: spring grain crops (sweet corn grouped with maize grain), spring vegetable crops (roughly in decreasing order of competitive ability), winter grain crops.

Figure2.5 Continuous fruiting annuals as a percentage of the common annual weeds in 14 crops in New York state (data from Bridges, 1992). From top to bottom: spring grain crops (sweet corn grouped with maize grain), spring vegetable crops (roughly in decreasing order of competitive ability), winter grain crops.

exponential growth magnifying small differences in seed size, access to nutrients, proximity to crop plants, etc. The extreme skewness in the distribution of seed production over individuals in most weed populations indicates that hand, chemical, or mechanical killing of the largest weeds can reduce weed densities in subsequent crops even if all individuals are not destroyed. Mechanical methods for attacking the large weeds that emerge through crop canopies are discussed in Chapter 4.

Most annual weeds produce a few thousand seeds per individual when

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  • belladonna
    What is the life span of maize crops?
    1 year ago

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