Ecological Bases for the Cultural Control of Annual Weeds

The ecological characteristics of weeds and crops provide a theoretical foundation for the science of cultural weed control. Many annual weeds grow more rapidly than crops in high resource regimens. However, the high resource demands needed to support rapid growth result in extreme sensitivity of young weeds to competition. Because the large seed size of most crops provides them with an initial size advantage over weeds, increased planting densities, narrow row spacing, and cultivars with rapid canopy closure can be used to improve weed control. Similarly, the large size of crop seeds allows them to be planted below the zone from which most annual weed seedlings emerge, and this allows mechanical weeding in the crop row. In‐row weeding relies on regular removal of small seedlings before they become deeply rooted, or burial of weeds which have been kept smaller than the crop. The small seed size of annual weeds relative to most agronomic crops, coupled with differences in germination cues, also allows use of crop residues for weed suppression. These considerations generate numerous researchable questions. Since such a wide variety of cultural weed management methods depend on an initial difference in size between weeds and crops, a shift in weed community composition toward perennials could inhibit reduction in herbicide use. Cultural methods are most effective when used in integrated systems. Research Question Cultural weed control relies on the design and manipulation of cropping systems to reduce weed pressure. Important components of cultural weed control include tillage, cultivation, crop rotation, choice of competitive cultivars, and adjustment of crop density and arrangement. The objective of this study was to identify key characteristics in the life history of annual weeds that allow cultural weed control methods to work. Given the current increased interest in cultural weed management methods as supplements or alternatives to herbicides, it is useful to begin to create a theoretical basis for the practice of cultural weed control. Literature Summary A substantial literature exists addressing each of the several components of cultural weed control. However, the great majority of work has (i) been specific to a particular crop, and often to the interaction of that crop with one weed species, and (ii) has often focused on a single aspect of cultural control (e.g., cultivation or crop density). Consequently, few attempts have been made to synthesize information and draw conclusions about how cultural weed management works as a general approach. Study Description I reviewed the literature on all methods of cultural weed control. I studied the ecological characteristics of annual weeds through experimentation and literature review, and related them to weed management. Characteristics studied included seed size, relative growth rate, size increase from seed to maturity, depth of emergence, and influence of competition on plant size. Common themes were identified. Applied Questions What are the key ecological characteristics of weeds and crops which allow (i) use of increased crop density for weed suppression, (ii) use of straw mulch for weed suppression, and (iii) mechanical weeding in the crop row? Seed size appears to be a critical factor in all three of these disparate control strategies. Many annual weeds grow more rapidly than crops in high resource regimens. The higher growth rate of weeds is due to higher maximum relative growth rate and slower decline in relative growth rate as the plant develops. High resource demands needed to support rapid growth result in sensitivity of young weeds to competition. Because crop seedlings have an initial size advantage over weeds, increased planting densities, narrow row spacing, and cultivars with rapid canopy closure can be used to increase the competitive pressure on weeds and thereby improve yields when weed control is less than perfect. The small seed size of weeds relative to most agronomic crops, coupled with differences in germination cues, also allows use of crop residues for weed suppression. Moreover, because the larger crop seeds are usually planted below the zone from which most weed seedlings emerge, mechanical weeding in the crop row is possible. In‐row cultivation relies on regular removal of small seedlings before they become deeply rooted, or burial of weeds which have been kept smaller than the crop.